COMMUNICATION NETWORK A communication network, consists of a set of equipment and facilities that provide a service, the transfer of information between users located at various geographical points. For example tel ephone net wor ks provides tel epho ne ser vic e, comput er net wor ks, television broadcast networks, cellular networks and Internet. Communic at io n ne twor k and tr ansporta ti on networ ks have become es se nt ia l infrastructure in every society. The capacity to trans fer high volume of data for a long distanc e almos t immediate ly are the fe at ur es of networ k- based services. For example Internet pr ovides e- ma il , information search and retrieval, various forms of electronic service. EVOLUTION OF NETWORK ARCHITECTURE AND SERVICES A communication service which is used of transfer of information. Different services differ in the details of how and in what form information is transferred. Let us consider three example networks i) Telegraph networks ii) Telephone networks iii) Computer networks TELEGRAPH NETWORKS AND MESSAGE SWITCHING In the year 1987 Samue l B. Morse demonstr ated a practical tele graph that provi ded the basis for telegram service, the transmission of text message over long distance. Here the text was encoded into sequence of dots and dashes. Each dot and dash which is used to be represented by short and long pulses of electrical current over a copper wire. By relying on two signals, tele graphy made use of digital tra nsmis sion syst em. In Morse code the pause between letter was 3 d ots and for word is 5 dots. In store- and-forwa rd pro ces s, the mes sage is comple tel y rec eived and sto red at an intermediate point and then forward to the next node depending on the availability oflink. The process of selecting an appropriate link is known as routing. If the information is message, then it is known as mess age switching. In simpler t erms, message switching is a switching method that sends data from point-to-point with each intermediate node storing the data , waiting for a free transmission channel and forwarding the data to the next point until the destination is reached. In the beginning communication channel used to carry information sent by one person. Naturally transmission rate (in bits/second) was less. To increase the transmission rate multiplexing was developed. Multiplexing is a technique used to place multiple signals on a single communication channel. Multiplexing partitions a channel into many separate
A communication network, consists of a set of equipment and facilities that provide aservice, the transfer of information between users located at various geographical points.For example telephone networks provides telephone service, computer networks,
television broadcast networks, cellular networks and Internet.
Communication network and transportation networks have become essentialinfrastructure in every society.
The capacity to transfer high volume of data for a long distance almost immediately arethe features of network-based services. For example Internet provides e-mail,information search and retrieval, various forms of electronic service.
EVOLUTION OF NETWORK ARCHITECTURE AND SERVICES
A communication service which is used of transfer of information. Different servicesdiffer in the details of how and in what form information is transferred.
Let us consider three example networksi) Telegraph networksii) Telephone networksiii) Computer networks
TELEGRAPH NETWORKS AND MESSAGE SWITCHING
In the year 1987 Samuel B. Morse demonstrated a practical telegraph that provided the
basis for telegram service, the transmission of text message over long distance. Here thetext was encoded into sequence of dots and dashes. Each dot and dash which is used to berepresented by short and long pulses of electrical current over a copper wire. By relyingon two signals, telegraphy made use of digital transmission system. In Morse code the
pause between letter was 3 dots and for word is 5 dots.
In store-and-forward process, the message is completely received and stored at anintermediate point and then forward to the next node depending on the availability of link. The process of selecting an appropriate link is known as routing. If the informationis message, then it is known as message switching. In simpler terms, message switchingis a switching method that sends data from point-to-point with each intermediate node
storing the data , waiting for a free transmission channel and forwarding the data to thenext point until the destination is reached.
In the beginning communication channel used to carry information sent by one person. Naturally transmission rate (in bits/second) was less. To increase the transmission ratemultiplexing was developed. Multiplexing is a technique used to place multiple signalson a single communication channel. Multiplexing partitions a channel into many separate
channels, each capable of transmitting its own independent signal, thereby enabling manydifferent transmission over a single medium.
One approach of multiplexing involves modulation. Binary symbols can be transmitted by sending a sinusoidal signal of a given frequency for a given period of time. Multiple
sequence of binary symbols could be transmitted simultaneously by using multiple pairsof frequencies for the different telegraphy signals. This concept is used in Modems.
We know characters can be represented by ASCII ( American Standard Code for Information Interchange) , is a 7 bit code. The equipments can communicate with ASCIIcode.
The telegraph service involves the transmission of text messages between geographicallyfar places. To provide the service , the architecture of the telegraph network contains thefollowing key elements or functions.
i) Digital transmission takes in the network that is information is representedeither 0 or 1. It can make use of ASCII code also. The transmission mediumcan be copper wire and radio.
ii) To identify the messages exactly, messages are covered by beginning andending by sequence of characters. This is known as framing.
iii) There should be destination address that is to whom it want to deliver.Routing procedure determines the path that a message follows across anetwork of telegraph stations interconnected by digital lines
TELEPHONE NETWORKS AND CIRCUIT SWITCHING
In 1876 Alexander Graham Bell developed a device that can transmit voice signals. Thedevice that is known as Telephone. The telephone network provides a two-way, real-timetransmission of voice signals across a network.
Telephone service became popular due to its voice service and expert operator withknowledge of Morse code is not required like in the case of telegraph. Here voice signalsare converted into equivalent electrical signal and passed through cable.
Communication is the process of transmitting information. It’s a big, vague, fuzzy wordand broadly applied. The “Communications Department “ at a university may includespeech teachers, public relations specialists, writers and broadcasters, who have nothingto do with hardware. By that definition , a writer is in the communication business.
Evolution of communications
We the human beings are communicating from early days that is by drawing the pictureson cave walls. It can be how to attack the animal or how to the save the life from thethreats or it may be a route to go to another cave.
The earlier long-distance communications was by signal fires that relayed simpleinformation. The signal used to represent “yes” or “no” – the question being asked had to
be known beforehand. It’s only a single bit information, not a detailed message.
A written letter or a human messenger could carry more information. The first systemswe might call telecommunications were series of hilltop towers, built by French engineer Claude Chappe in the 1790s. The towers had to be in sight of each other, with anoperator in each one. The operator relayed a message by moving arms on top of thetower. The operator of the next tower looked through a telescope to watch the armsmove, reading the message, recording it, then moving the arms of his tower to relay it to
the next tower. Chappe invented a code that used arm position to indicate letters or certain common words. His system was called an optical telegraph.
Samuel Morse’s electrical telegraph eventually replaced Chappe’s optical one. It alsorequired operators, but they used keys that completed an electrical circuit, sending burstsof currents through a wire. The Morse code was a series of dots an dashes ( short andlong pulses) that represented letters.
The electrical telegraph spread across the continents and in 1866 across the Atlantic. Itswires formed a network running between major cities. Telegraphers received signals andeither sent them for local delivery or relayed them to more distant stations. We can think
of each dot and dash of the telegraph as a bit of information.
Later Alexander Graham Bell developed telephone. He borrowed some principles fromthe telegraph , but instead of transmitting dots and dashes, it sent a continuously changingelectrical current that represented a speaker’s voice. The telephone network was bigger and more complex than the telegraph network.
After that radio waves are used to carry signals through air. Radio first carried telegraphsignals. Wires worked fine on land, but only radio could send telegrams to ships at seaand relay urgent messages such as request for help. It was radio rescue calls that sentships to help the survivors of the sinking of the Titanic. Radio communications startedfor ships and airplanes in the world war II.
Radio communication started at low frequencies, but gradually moved to higher ad higher frequencies as electronics improved. The higher the frequency the more information thesignal can carry. Pictures need more transmission capacity than sound alone, so televisionare broadcast at higher frequencies than audio radio.
By the 1970s , satellites were beaming radio signals around the globe. The telephonesystem had become global and known popularly “telecommunication network” . Wecould make phone calls to anywhere in the world.
After that fiber optics arrived. Fiber provides high bandwidth. It carries high capacity
information at faster rate. Now it is a integral part of telecommunication network.
Signals and systems
Information is the one transmitted via communication. It may be very simple messagelike “yes” or “no”. It can be huge and complex message, such as he files containing the
book or a television broadcast of a movie. It doesn’t matter if the “information” doesn’tcontain anything a critic might agree was “information”.
A signal transmits that information. Signals may take many forms, such as optical,acoustic, electronic or radio-frequency. Signals may be converted from one form intoanother and still contain the same information. When we make a long-distance telephonecall, the sound waves from our mouth are converted to an electrical signal at thetelephone, that electrical signal is converted to optical form at the telephone switchingoffice, back to electrical form at the local switching office on the other end and back tosound waves at the other person’s telephone.
A system is the equipment that performs a task, like transmitting signals. We often speak of telephone system as if it is the whole network of telephone equipment from our phoneto the high-capacity fiber-optics cables that carry long-distance calls. However, parts of the telephone system also may be called systems, such as a switching systems that directs
phone calls. We can assume a system which is a collection of stuffs designed to work together.
Basic components of data communication system
A data communication system is made up of five components.
Message - the message is the information to be communicated
Sender – The sender is the device that sends the data message. It can be computer,workstation, telephone etc.,Receiver – The receiver is the device that receives the message. It can be a computer ,workstations, telephone, fax machineMedium – The transmitting medium is the physical path by which a message travels fromsender to receiver . It can consists of twisted pair wire, coaxial cable, fiber optic cable or radio waves.Protocol – A protocol is a set of rules that govern data communication. It represents anagreement between the communicating devices. Without a protocol two devices may beconnected but not communicating , just as a person speaking Malayalam cannot beunderstood by a person who speaks only Kannada.
Protocols and standards
The Communication occurs between entities in different systems. An entity is anythingcapable of sending or receiving information. But two entities cannot just send bit streamsto each other and expect to be understood. For communication to occur the entities mustagree on a protocol. The key elements of a protocol are syntax, semantics and timing.
Syntax : Syntax refers to the structure or format of the data, meaning the order in whichthey are presented. For example , a simple protocol might expect the first eight bits of data to be the address of the sender , the second eight bits to be the address of receiver and rest of the stream to be the message itself.
Semantics : Semantics refers to the meaning of each section of bits. How is a particular pattern to be interpreted and what action is to be taken based on that information.
Timing : Timing refers to two characteristics. When data should be sent and how fast itcan be sent. For example , if a sender produces data at 100 Mbps but the receiver can
process data at only 1 Mbps, the transmission will overload and receiver and data will belargely lost.
Standards
Standards are essential in creating and maintaining an open and competitive market froequipment manufacturers and in guaranteeing national and international operationalability of data and telecommunication technology and processes. They provideguidelines to manufacturers, vendors, government agencies and other service providers toensure the king of interconnectivity necessary in today’s marketplace and in internationalcommunications.
Data communications standards fall into two categories
1. de facto ( by fact)
2. de jure ( by law)De jure standards are those that have been legislated by an officially recognized body.
De facto standards are often established originally by manufacturers seeking thefunctionality of a new product or technology.
Standards organization
Standards are developed by cooperation among standard creation committees forums andgovernment regulatory agencies.
i) The International Standards Organization (ISO)ii) The International Telecommunication Union (ITU-T)iii) ANSI (American National Standards Institute)iv) The Institute of Electrical and Electronics Engineer (IEEE)v) The Electronic Industries Association (EIA)
The ISO is an organization dedicated to worldwide agreement on internationalstandards in variety of fields. ITU-T is an international standards organization relatedto the United Nations that develops standards for telecommunications. Two popular standards developed by ITU-T are the V series and X series. ANSI , a nonprofitorganization is the United States, voting representative to both the ISO and ITU-T.The IEEE is the largest national professional group involved in developing standardsfor computing , communication, electrical engineering and electronics. It sponsoredan important standard for Local Area Network (LAN) called project 802. EIA is anassociation of electronics manufacturers in the United States. It is responsible for developing the EIA-232-D and EIA-530 standards.
Communications systems serve many functions. It is possible to divide into four categories on functions.
a) Point-to-point
b) Broadcastingc) Switchedd) Networked
Point-to-point
A point-to-point configuration provides a dedicated link between two devices. The entirecapacity of the channel is reserved for transmission between those two devices. Most
point-to-point line configuration use an actual length of wire or cable to connect the twoends.
Broadcasting
A broadcast system send the same signal to everyone who receives it. In its usual form ,transmission is one way, from the signal source to the individual. Local radio andtelevision transmission are good examples of pure one-way broadcast systems.
Switched systems
A switched system makes temporary connections between terminals so they canexchange information. The telephone is a good example.
Networking
A network connects many terminals that can communicate with each other. For exampleLocal Area Network is belongs to this category.
Services
We know the operation of modern communication networks is a very complex processthat involves the interaction of many systems. A communication network , is a set of equipment and facilities that provides a service, the transfer of information between userslocated at various geographical points. The ability of communication networks to transfer communication at extremely high speeds, allows users to gather information in largevolumes almost instantly with the help of computers for long distances.
Radio and television broadcasting are probably the most common communicationservices. Various stations transmit an ensemble of signals simultaneously over radio or cable distribution networks.
Telephone service is the most common real-time service provided by a network. Two people are able to communicate by transmitting their voices across the network. Theservice is considered as connection-oriented in the sense that users must first interact withthe network to set up a connection
Telephone networks provide a broad class of call management services that use theoriginating number or the destination number to determine the handling of a call. For example , in call return the last originating number is retained to allow it to beautomatically called by the destination user at a later point in time. Caller ID allows theoriginating number and sometimes name, of the originating call to be displayed to thedestination user when the receiving device is display capable. Voice mail allows adestination user to have calls forwarded to a message receiving device when thedestination user is not available.
Cellular telephone service extends the normal telephone service to mobile users who are
free to move within a regional area covered by an interconnected array of smaller geographical areas called cells. Each cell has a radio transmission system that allows it tocommunicate with users in its area. Cellular system should handle “handoff” when user moves from one area to another area.
Electronic mail (e-mail) is another popular network service. The user provides a textmessage and a name and/or address to a mail application. The application interacts with alocal mail server, which in turn transmits the message to a destination server across acomputer network. The destination user retrieves the message by using a mailapplication. E-mail is not a real-time service , where large delays can be tolerated. Heresecurity and privacy is matter of concern.
Many applications that involve an interaction between processes running in twocomputers may be characterized by client/server interaction. Internet is an example of distributed system. The World Wide Web (WWW) is accessed through a browser
program that displays the document and allows the user to access other documents byclicking the links. Each link provides the browser with a uniform resource locator (URL)that specifies the name of the machine where the document is located as well as the nameof the file that contains the requested document.
Video on demand characterization another type of interactive service. The objective of the service is to provide access to a video library. The user initiates the service byaccessing a menu from which a selection is made. Video which requires lot of bandwidthto pass from source computer to the destination.
Streamed audiovisual services over the internet provide an example of a service withsome of the feature of video on demand. An application like “real player “ can be used toaccess a channel that provides an audio visual stream to the client machine.
Another class of service is audio-visual conferencing. Here both audio and video aretransmitted over a distance. This service requires real-time response requirement. Thisservice is being used by many companies for training, client interaction from a site, groupdiscussion etc.
We know, in information technology era, sharing of resources and easy communicationare acting as a backbone of any network. Popular example is our Internet. With the helpof internet we are able to exchange information and share resources. Hence in thischapter we study different advantages of the network and different forms of the network and their features.
The learning objectives of this chapter are to knowWhat is a computer network ?How network is advantageous ?
What are different forms of networks ?What is a e-mail ?
5.1 Networking of computers and its advantages.
Computer network is defined as an interconnection of autonomouscomputer. Here autonomous means, there is no master and slave relationship. Allcomputers are equal. Computer network enables to share the resources. Computer networking also refers to connecting computers to share data, application software andhardware devices. Networks allow sharing of information among various computers and
permit users to share files. For example a students accesses compilers sitting at one place,where compiler may be stored on the other machine. The students takes printout with the
help of one printer connected to the network. The printer can be shared among manystudents.
Network offers the opportunity to communicate more efficiently withothers through electronic mail. Networks allow companies to share software and
peripherals such as printers, plotters, scanners and so on. With networking all thecomputers in an office can be connected to a single printer and scanner. It also helps inusing storage devices efficiently.
Computer network acts as a very powerful communication medium. Itmeans people exchange their information. When compared to mainframe computers,
network of computers saves money.
5.2 Types of networks
Depending the nature of the distances, protocols ( the set of rules used for communication) the network can be classified into LAN (Local Area Network ), MAN(Metropolitan Area Network ) and WAN ( Wide Area Network).
Local Area Network (LAN)A LAN (local area network) is a group of computers and network devices connectedtogether, usually within the same building. By definition, the connections must be high-
speed and relatively inexpensive (e.g., token ring or Ethernet ). They function to link computers together and provide shared access to printers, file servers, and other services .
Any individual computer connected to a network is called workstation. A workstationmay not need a floppy disk or hard disk. A LAN or local area network connectscomputers and peripherals in a limited area. LAN requires cables to connectworkstations. For example LAN is used in a hall or within a building. Figure 5.1 showsLocal Area Network , where various departments are connected.
Fig 5.1 Local Area Network
Metropolitan Area Network (MAN)
A MAN ( Metropolitan Area Network) is used to connect computers to cover the city or town. The range may be approximately 50 Kilometers. Normally cables and fiber opticcables are used to connect computers. The routing of the messages are fast. Normallycentral library in a city may be connected by a MAN, so users can access the information.Figure 5.2 shows a typical view of Metropolitan Area Network.
Wide Area Network (WAN)A WAN (Wide Area Network) covers large distance like state, country or
continents. The WAN uses the fiber optics, cables and even satellites also. Herecommunication circuits are connected with the help of hardware device called routers.Routers forward small pieces of information called packets from one to another. Internetis the popular one comes under WAN. Some of the examples makes use of internet arereservation of airplane tickets, railway tickets and even cinema tickets. Another facilitycalled e-commerce, where business is carried out through internet. Here people can buy
books, articles and so on through registering their wants through the internet. Figure 5.3
shows a typical view of Wide Area Network.
Fig 5.3 Wide Area Network
5.3 InternetThe internet is a massive wide area network ,connecting thousands of computer
networks around the world. The internet is a world wide “network of networks”. It is acollection of thousands of smaller networks in different countries around the world. Itlinks thousands of academic government, military and public computers, enablingmillions of people to share information and other resources.
Internet pathways are used to exchange digitized computer data. The basicservices that form the foundation of the internet are e-mail , telnet and FTP. With internetwe can easily exchange electronic mail with friends and family anywhere in the world.
Telnet allows you to connect to a remote computer. We can access any of the
public services or tools and library databases at the remote site. FTP(File Transfer Protocol) provides for transferring files from one computer to another across the internet.
Internet has many uses. For individuals, the most important uses of internet are e-mail and surfing the Web. One can read the topics of interest like sports, a hobby, acountry or any place of interest.
E-mail: The e-mail stands for electronic mail. One of the major features of computer networking is that messages can be sent electronically to various terminals on thenetwork. The messages are sent very quickly and accurately. E-mail uses the concept of
Storing and forwarding messages. It saves a lot of money for the users. Here user registers his/her account with one of the providers. The e-mail account normally containsusername and the provider name. For example abc @ yahoo.com , represents abc is thename of the user , who is registered in yahoo provider.
5.4 Summary:• Computer network is defined as a interconnection of autonomous computers.• Computer networking enables us to connect computers to share data, application
software and hardware devices.• Networks allow sharing of information among various computers and permit
users to share files.• A LAN or local area network connects computers and peripherals in a limited
area.• A MAN covers the city or town.• A Wide Area Network covers a large area.• The internet is a massive wide area network, connecting thousands of computer
networks around the world.• E-mail is a cost effective communication
5.5 Quiz questions
1. Computer network defined as a interconnection of ---------- computers2. Computer network allows user to ----- the resources3. -------- network confined to hall or building.4. ------- is one example of WAN.5. ------ business is carried our through internet.6. Small piece of information called ---- are forwarded by router in Internet.
2. share3. Local area4. Internet5. E-commerce6. packets
5.6 Exercise
1. What is a computer network ? What are the advantages of computer network ?2. Briefly explain the Local area network, Metropolitan area network and wide area
network.3. What e-mail ? How e-mail works ?4. What are the uses of Internet ?
Communication network must support wide range of services. Normally people usenetworks to communicate, send e-mails, transfer of files and so on. Industry people
use communication network for transfer of funds, update information about the product and so on. Hence, to provide support for current service and future services, acomplete plan is required. This necessitates developing a complete flexibility innetwork architecture.
communications functions are grouped into the following tasks• The transport across a network of data from a process in one machine to the
process at another machine.• The routing and forwarding of packets across multiple hops in a network • The transfer of a frame of data from one physical interface to another.
To reduce their design complexity, most networks are organized as a series of layersor levels, each one built upon its predecessor. The number of layers, the name of theeach layer, the contents of each layer and the function of each layer differ fromnetwork to network.
Interaction between the layers must be defined precisely. Interaction is done withdefinition of the service provided by each layer and to the layer above. Interface
between layers through which a service is requested and through which results areconveyed. New services that build on existing services can be introduced even at thelater stage. The layered approach accommodates incremental changes readily.
We know , in all networks, the purpose of each layer is to offer certain services to thehigher layers. The entities comprising the corresponding layers on different machinesare called peer processes. Between each pair of adjacent layers there is an interface.The interface defines which primitive operations and services the lower layer offers tothe upper one. The set of layers and protocols is called the network architecture.
A protocol is a set of rules that governs how two or more communicating devices areto interact. HTTP protocol enables retrieval of web pages and TCP protocol enablesthe reliable transfer of streams of information between computers.
HTTP
Let us consider a client/server architecture, a server process in a computer waits for incoming requests by listening to a port. Port is an address that identifies which
process is to receive a message that is delivered to a given machine. The server provide response to the requests. The server process always runs a process in the background called daemon. httpd refers to server daemon for HTTP. The documentsare prepared using Hyper Text Markup Language (HTML) which consists of text,
graphics and other media are interconnected by links that appear within thedocuments. The www is accessed through a browser program that displays thedocuments and allows the user to access other documents by clicking one of theselinks. Each link provides the browser with a uniform resource Locator (URL) thatspecifies the name of the machine where the document is located and the name of the
file that contains the requested document. The HTTP ( Hyper Text Transfer Protocol )specifies rules by which the client and server interacts so as to retrieve a document.
In HTTP, we use two-way connection that transfer a stream of bytes in correctsequential order and without errors. The TCP protocol provides this type of communication service between two processes in two machines connected to anetwork. Each HTTP inserts its messages into a buffer and TCP transmits thecontents of the buffer to the other TCP in blocks of information called segments.Each segment contains port number information in addition to the HTTP messageinformation. The following figure shows how communication is carried between
HTTP client and HTTP server.
Courtesy : Data communication networks by Alberto Leon - Garcia
First of all, client needs to perform a DNS query to obtain the IP addresscorresponding to the domain name. This step is done by sending a message to a DNS
server. The Domain Name System (DNS) is a distributed database that resides inmultiple machines on the Internet and is used to convert between names andaddresses and to provide e-mail routing information. Each DNS machine maintains itsown database and acts as a DNS server that other query machine can query. Initiallyrequesting machine accesses a local name server. In case if it not possible to resolvenames then it will be sent to the root server. When the root server is unable to resolve, it will be sent to authoritative name server. Every machine on the Internet is requiredto register with at least two authoritative name servers.
Steps of DNS query
1. Application requests name to address translation.2. Resolver composes query message.3. Resolver sends UDP datagram encapsulating the query message4. DNS server looks up address and prepares response5. DNS sends UDP datagram encapsulating the response message.
DNS query and response messages are transmitted by using the communicationservice provided by the User Datagram Protocol (UDP). Port 53 is used for communication. The UDP service is connectionless, no connection setup is requiredand the datagram is sent immediately.
SMTP
Simple Mail Transfer Protocol is used for sending mails. The user prepares an e-mailmessage that includes the recipient’s e-mail address, a subject line and a body. User clicks send button to send the information. Let us consider the steps are used insending the e-mail.
• The mail application establishes a TCP connection (port 25) to its local SMTPserver.
• SMTP daemon issues the message to the client, indicating that it is ready toreceive mail
• Client send s a HELO message and identifies itself.• SMTP daemon issues a 250 message, indicating the client may proceed.• Client sends sender’s address• If successful, SMTP daemon replies with a 250 message• Client sends recipient’s address.• A 250 message is returned• Client sends a DATA message requesting permission to send the mail message
• The daemon sends a message giving the client permission to send• Client sends the actual text• Daemon indicates that the message is accepted for delivery. A message ID is
returned.• Client indicates that mail session is over.
• Daemon confirms the end of the session.
SMTP works best when the destination machine is always available, for this reason,users in a PC environment usually retrieve their e-mail from a a mail server using thePost Office Protocol version 3 (POP 3).
TCP and UDP transport Layer services
UDP provides connectionless transfer of datagrams between processes in hostsattached to the Internet. UDP provides port numbering to identify the source anddestination processes in each host. UDP is simple and fast, but provides no guarantees
in terms of delivery or sequence addressing.TCP provides for reliable transfer of a byte stream between processes in hostsattached to the Internet. TCP involves the establishment of a connection between thetwo processes. To provide their service, the TCP entities implements error detectionand retransmission as well as flow control algorithms. TCP implements congestioncontrol, which regulates the flow of segments into the network.
Peer-to-peer file sharing
File sharing applications such as Napster and Gnutella are popular for sharing theMP3 audio and other files. Here clients can also acts as transient server while theapplication is activated. When a peer is interested in finding a certain file, it sends aquery. The response provides a list of peer s that have the file and additionalinformation such as the speed of each peer’s connection to the Internet. Therequesting peer can then set up a TCP connection to one of the peers in the list and
proceeded to retrieve the file.
The OSI reference model
The model proposed by ISO OSI ( Open System Interconnection) reference model because it deals with connecting open systems that is systems that are open for communication with other systems. The OSI model has seven layers.
The concepts related to layers arei) A layer should be created where a different level of abstraction is needed.ii) Each layer should perform a well defined functionIii) The function of each layer should be chosen with an eye toward defininginternationally standardized protocols.
Iv) The layer boundaries should be chosen to minimise the information flow acrossthe interface.v) The number of layers should be larger enough that distinct functions need not bethrown together in the same layers out of necessity and small enough that thearchitecture does not become unwieldy.
The following figure shows OSI model.
To remember the layers in sequence, we can use the sentence like
Please Do Not Touch Sita’s Pet Animal , Where the beginning letter of each wordrepresent the layer name. Like P in Please word stands for Physical layer, D letter inDo word stands for Data link layer and so on. Let us see the functions of the layers.
Physical layerThe physical layer deals with the transfer of bits over a communication channel.
Courtesy : Data communication networks by Alberto Leon - Garcia
Physical layer concerned with the representation of signals. How to represents 1 andhow represent 0. Which type of encoding mechanism should be used for representingthe bits.This layer is concerned with the procedures to set up and release the physicalconnections.
Data link layerThe Data link layer provides the frames (blocks of information) across a transmissionlink that directly connects two nodes. Data link layer inserts a framing information inthe sequence of transmitted bits to handle boundaries. Data link layer inserts controland address information in the header and check bits to enable recovery fromtransmission errors and flow control.
High level data link control (HDLC) and Point-to-point protocol (PPP) are the twostandard data link protocol that are wide in use. A flat addressing space is used toenable machines to listen and recognize frames that are destined to them.
Network layerThe network layer provides for the transfer of data in the form of packets across acommunication network. It uses the hierarchical way of addressing scheme. Routing
protocol is the procedure that is used to select paths across a network. The nodes inthe network must work together to perform the routing effectively. Network layer isresponsible for dealing with congestion that occurs from time to time in the network.When the two machines are connected to the same packet-switching network, singleaddress space and routing procedure are used.
If two machines are connected to the different networks, the transfer of data musttraverse two or more networks and hence internetworking protocols are necessary toroute the data between gateways/routers that connects the intermediate networks.
The internetworking protocols must understand the differences in addressing anddifferences in the size of the packets that are handled within each network.
Transport layerThe transport layer is responsible for the end-to-end transfer of messages from a
process in the source machine to a process in the destination machine. The transportlayer protocol accepts messages from its higher layers and prepares blocks of information called segments or datagram's for transfer between end machines.
The transport layer provides number of services• Transport layer may provide a connection-oriented service that involves the error-
free transfer of a sequence of bytes or messages.• The associated protocol carries out error detection and recovery and sequence and
flow control.• The transport layer is responsible for setting up and releasing connections across a
Session layerThe session layer can be used to control the manner in which data are exchanged. It ismainly concerns with establishing the sessions and termination the session. Certainapplications require half-duplex dialog . Certain applications requires the introductionof synchronization points that can be used to mark the progress of an interaction and
can serve as points from which error recovery can be initiated.
Presentation layerThe presentation layer provide the application layer with independence fromdifference in the representation of data. Presentation layer should convert themachine-dependent information provided by application A into a machine-dependentform suitable for application B, because different computers use different codes for representing characters and integers.
Application layerThe application layer is to provide services that are frequently required by
applications that involve communications. For example browser application useHTTP application layer protocol to access a WWW document. Application layers protocols have been developed for file transfer, virtual terminal, electronic mail, nameservice, network management and other applications.
In general each layer adds a header and possibly a trailer to the block of information itaccepts from the layer above.
Summary of the layers
The following table shows the different functions carried out by different layers OSImodel.
The development of OSI reference model leads the world to the development of unifiedview of layers, protocols and services. In each layer a process on one machine carries outa conversation with a peer process on the other machine across a peer interface. This isshown below.
Layer Description Examples
Application Protocols that are designed to meet the communication requirements of specific applications, often defining the interface to a service.
HTTP,FTP , SMTP,
CORBA IIOPPresentation Protocols at this level transmit data in a network representation that is
independent of the representations used in individual computers, which maydiffer. Encryption is also performed in this layer, if required.
Secure Sockets(SSL),CORBA DataRep.
Session At this level reliability and adaptation are performed, such as detection of failures and automatic recovery.
Transport This is the lowest level at which messages (rather than packets) are handled.Messages are addressed to communication ports attached to processes,Protocols in this layer may be connection-oriented or connectionless.
TCP, UDP
Network Transfers data packets between computers in a specific network. In a WANor an internetwork this involves the generation of a route passing throughrouters. In a single LAN no routing is required.
IP, ATM virtualcircuits
Data link Responsible for transmission of packets between nodes that are directlyconnected by a physical link. In a WAN transmission is between pairs of routers or between routers and hosts. In a LAN it is between any pair of hosts.
Ethernet MAC,ATM cell transfer,PPP
Physical The circuits and hardware that drive the network. It transmits sequences of binary data by analogue signalling, using amplitude or frequency modulationof electrical signals (on cable circuits), light signals (on fibre optic circuits)or other electromagnetic signals (on radio and microwave circuits).
The processes at layer n are referred to as layer n entities. Layer n entitiescommunicate by exchanging protocol data units (PDUs). Each PDU contains header,which contains protocol control information and usually user information. Thecommunication between peer processes is usually virtual in the sense that no directcommunication link exists between them.
For communication to take place, the layer n+1 entities make use of the services provided by layer n. The transmission of the layer n+1 PDU is done by passing a block of information from layer n+1 to layer n through a software port called thelayer n service access point (SAP) across a service interface. SAP is identified by aunique identifier. The block of information passed between layer n and layer n+1entities consists of control information and a layer n service data unit (SDU), whichis the layer n+1 PDU itself. The layer n SDU, which is the layer n+1 PDU, isencapsulated in the layer n PDU. The service provided by layer n typically acceptinga block of information from layer n+1, transferring the information to its peer
process.
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The service provided by the layers can be connection oriented or connectionless.A connection-oriented service has three phases.Step 1: Establishing a connection between two layer n SAPs. The setup involvesnegotiating parameters like sequence numbers, flow control.Step 2: Transferring n-SDUs using the layer n protocol.Step 3: Tearing down the connection and releasing the various resources allocated tothe connection.
In connectionless service which does not require a connection setup and each SDU istransmitted directly through the SAP. Suppose a layer n SDU is too large to behandled by the layer n-1 and so segmentation and reassembly are applied. The layer nSDU is segmented into multiple layer n PDUs that are then transmitted using theservices of layer n-1. The layer n entity at the other side must reassemble the originallayer n SDU from the sequence of layer n PDUs it receives. It is also possible that thelayer n SDUs are so small as to result in inefficient use of the layer n-1 services andso blocking and unblocking applied. The layer n entity may block several layer nSDUs into a single layer n PDU . The layer n entity on the other side must thenunblock the received PDU into the individual SDUs.
Multiplexing involves the sharing of a layer n services by multiple layer n+1 users.The N+1 layer users passes its SDUs for transfer using the services of a single layer n entity. Demultiplexing is carried out by the layer n entity at the other end.
Courtesy : Data communication networks by Alberto Leon - Garcia
Courtesy : Data communication networks by Alberto Leon - Garcia
The tcp/ip network architecture is a set of protocols that allows communication acrossmultiple diverse networks. Tcp/ip provides robustness with regard to failures in thenetwork and on flexibility in operating over heterogeneous networks. We know
Internet consists of different types computer systems and networks . Tcp/ip whichconsists of 4 layers and OSI model consists of 7 layers. The following figure showsthe TCP/Ip architecture.
Application layerThe application layer provides services that are used by the applications. Examplesare remote login, e-mail, ftp, network management.The TCP/IP application layer incorporates the functions of the top three OSI layers. HTTP is a TCP/IP applicationlayer protocol.
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The TCP/IP application layer programs are directly run through transport layer.Two basic types of services are provided by transport layer.
The first one is connection-oriented , which is provided by Transmission ControlProtocol (TCP).The second service is best-effort connectionless transfer of individual messages,which is provided by the user datagram protocols (UDP). Here in connection-lessservice no mechanism for error recovery or flow control.UDP is used for applications that require quick but not necessarily reliable delivery.TCP/IP model does not require strict layering. It means the application layer has theoption of bypassing intermediate layers.
Internet layer
The Internet layer handles the transfer of information across multiple networksthrough the use of gateways/routers . The internet layer corresponds to the network layer of OSI , it mainly concerns with the transfer of packets between machines thatare connected to different networks. Internet layer deals with deal with the routing of
packets from router to router across the networks. Internet layer uses a concept of globally unique addresses for machines that are attached to the Internet. The internetlayer provides a single service that is best-effort connectionless packet transfer. IP
packets are exchanged between routers without a connection setup, the packets arerouted independently. So packets may travel in different paths. Hence IP packets arealso called datagrams. The connectionless approach makes the system robust, if failure occurs in the network, the packets are routed around the points of failure, thereis no need to set up the connections again .
Network interface layerThe network interface layer is concerned with the network-specific aspects of the transfer of packets. It is similar to part of OSI network and data link layer services. The network interface layer is particularly concerned with the protocols that access the intermediatenetworks. At each gateway the network access protocol encapsulates the IP packet into a
packet or frame of the underlying network or link. The IP packet is recovered at the exitgateway of the given network. This gateway must then encapsulates the IP packet into a
packet or frame of the type of the next network or link.
Hence, a clear separation of the internet layer from the technology –dependent network interface layer. This allows the internet layer to provide a data transfer service that istransparent in the sense of not depending on the details of the underlying the network.The following figure shows protocols of the TCP/IP protocol suite.
• OSIThree concepts are central to OSI model services, interfaces, protocols.Protocols in the OSI model are better hidden than in the TCP/IP model and can bereplaced relatively easily as the technology changes.Model first and then next protocol.OSI has seven layers.OSI supports both connectionless and connection oriented communication in thenetwork layer.
• TCP/IPServices, interface and protocols are not distinguished properly.
Protocols comes first and model next.TCP/IP has four layers.TCP/IP model has connectionless in the Internet layer and both modes in the transportlayer.
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Let us consider a network , which consists of server, workstation, router and personalcomputer. The router which is used to connect two networks. The ordered pair (1,1)which represent network 1 and the host id 1. Router which contain two interfaces one
is connected to network 1 and other is connected to pc via ppp link. It constitutes theother network.
From the point of view of IP , the Ethernet LAN and the point-to-point link constitutetwo different networks, shown in the following figure.
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Each host in the Internet is identified by a globally unique addresses. IP addressidentifies the host’s network interface rather than the host itself. A node that isattached to two or more physical networks is called the router. Router attaches to twonetworks with each network interface assigned to a unique IP address.
An IP address is divided into two parts
• A network id• Host id
The network id must be obtained from an organization authorized to issue IP address.On a LAN, the attached device to the network is often identified by a physicaladdress. The format of the Physical address depends on the particular type of thenetwork. Ethernet MAC address is 48 bits.Each Ethernet network interface card(NIC) is issued a globally unique medium access control (MAC) or physical address.
Sending and receiving IP datagrams
Let us consider a case, in which the workstation wants to send an IP datagram to theserver. The IP datagram has the workstation’s IP address and the server’s IPaddresses in the IP packet header. Let us assume, IP address of the server is known.The IP entity in the workstation looks at its routing table to see whether it has anentry for the complete IP address. Workstation finds the server is directly connectedto the same network and the server has physical address s. The IP datagram is passedto the Ethernet device driver, which prepares an Ethernet frame as shown in the nextfigure.
The header in the frame contains the source physical address, w, and the destination physical address s. The Ethernet frame is then broadcast over the LAN. The server’s NIC recognizes that the frame is intended for its host, so the card captures the frameand examines it. The NIC finds that the protocol type field is set to IP and therefore
passes the IP datagram up to the IP entity. Now let us see how the server sends IPdatagram to the personal computer. Suppose the server knows the IP address of thePC and that the IP addresses on either side of the link were negotiated when the link
was set up. Otherwise, IP entity then checks to see whether it has a routing table entrythat matches the network id portion of the IP portion of the IP address of the PC.Suppose that the IP entity does not find such an entry. The IP entity then the checks tosee whether it has an entry that specifies a default router that is to be used when noother entries are found. Suppose that entry exists and that it specifies the router withaddress (1,3). The IP datagram is passed to the Ethernet device driver, which preparesan Ethernet frame. The header in the frame contains the source physical address, s,and the destination physical address, r. IP datagram in the frame contains thedestination IP address of the PC (2,2), not the destination IP address of the router. TheEthernet frame is then broadcast over the LAN. The router’s NIC captures the frameand examines it. The routing tables at the router show that the machine with address
(2,2) is connected directly on the other side of the point-to-point link. The router encapsulates the IP datagram in a PPP frame that is similar to the Ethernet frame. ThePPP receiver at the PC receives the frame, checks the protocol type filed and passesthe IP datagram to its IP entity.
Application layer protocols are high level that provide services to user applications.Application protocols are user written. The popular protocols are Telnet, File transfer
protocol (FTP), HTTP and SMTP.
Telnet
Telnet is a TCP/IP protocol that provides a means of accessing resources on a remotemachine where the initiating machine is treated as local to the remote host. Telnet
protocol is based on the concept of a network virtual terminal (NVT), which is animaginary device that represents a lowest common denominator terminal. Eachmachine initially maps its characteristics to that of an NVT and negotiates options for changes to the NVT or other enhancements, such as changing the character set. Telnetuses one TCP connection. A TCP connection is identified by a pair of port numbers, aserver is capable of supporting more than one Telnet connections at a time.
FTP
File transfer Protocol (FTP) is another commonly used application protocol. FTP provides for the transfer of a file from one machine to another. FTP requires two TCPconnections to transfer a file. One is the control connection that is established on port21 at the server. The second TCP connection is a data connection used to perform afile transfer. A data connection must be established for each file transferred. Dataconnections are used for transferring a file in either direction or for obtaining lists of files in either direction or for obtaining lists of files or directories from the server tothe client. FTP works across different systems because it can accommodate severaldifferent file types and structures. FTP commands are used to specify informationabout the file and how it will be transmitted. Three types of information must bespecified. File type – FTP supports ASCII, EBCDIC, image (binary) or local.
HTTP and the WWW
The world wide web provides a framework for accessing documents and resourcesthat are located in computer connected to the Internet. The HTML is used to preparethe documents. Each link provides the browser with a uniform resource locator (URL) that specifies the name of the machine where the document is located as wellas the name of the file that contains the requested document HTTP is a stateless
protocol in that it does not maintain any information ( state) about the clients. TheHTTP server handles each request independently of all other requests.The following are the few commands are usedGETRetrieve information (object) identified by the URLPOST – Send information to a URL and retrieve result, used when a user fills out afrom in a browser.PUT – Store information in location named by URL
TRACE – Trace HTTP forwarding through proxies, tunnels
Cookies and web sessions
It was indicated that the HTTP protocol is stateless and does not maintain information
about the prior requests from a given client. The use of cookies makes it possible tohave web sessions where a user interacts with a web site in a manner that takes intoaccount the user’s preferences. Cookies are data that are exchanged and stored byclients and servers and transferred as headers lines in HTTP messages. The header lines provide context for each HTTP interaction. When a client first accesses a webserver that uses cookies, the server replies with response message that includes a Set-cookie header line. This header line includes a unique ID number for the given client.If the client software accepts cookies, the cookies is added to the browser’s cookiefile. Each time the client software accepts cookies, the cookie is added to the
browser’s cookie file. Each time the client makes a request to the given site, itincludes a Cookie header line with the unique ID number in its requests. The server
maintains a separate cookie database where it can store which pages were accessed atwhat date and time by each client.
IP utilities
A number of utilities are available to help in finding out about IP hosts and domainsand to measure Internet performance. We discuss utilities like ping, traceroute,ipconfig, netstat, tcpdump.
Ping
Ping is a utility which is used to determine whether a host is reachable or not. Pingmakes use of Internet control message Protocol (ICMP) messages. The purpose of ICMP is to inform sending hosts about errors encountered in Ip datagram processingor other control information by destination hosts or by routers. Ping sends one or more ICMP echo messages to a specified host requesting a reply. Ping is often tomeasure the round-trip delay between the hosts.The syntax is ping <hostname>
The round-trip delay is indicated and time-to-live value. The TTL is the maximumnumber of hops an IP packet is allowed to remain in the network. Each time an IP
packet passes through a router, the TTL is decreased by 1. When the TTL reaches 0,the packet is discarded.
Traceroute
Traceroute utility allows users to determine the route that a packet takes from thelocal host to a remote host, as well as latency and reachability from the source to eachhop. Traceroute is used as a debugging tool by network administrator. Traceroutemakes use of both ICMP and UDP.
IpconfigThe ipconfig utility available on windows operating system. This utility displays theTCP/IP information about a host. It displays host IP address, subnet mask, defaultgateway for the host. The utility can also be used to obtain information for each IP
network interface fro the host, DNS hostname, Ip address of DNS servers, physicaladdress of the network card, IP address for the network interface and whether DHCPis enabled for automatic configuration of card’s Ip address.
netstat
The netstat queries a host about its TCP/IP network status. netstat can be used to findthe status of the network drivers and their interface cards, such as the number of
packets, out packets, error packets and so on. It is also be used to find out the state of the routing table in a host, which TCP/IP server processes are active in the host aswell as the which TCP connection are active.
Tcpdump
The tcpdump program can capture and observe IP packet exchanges on a network interface. The program usually involves setting an Ethernet network interface cardinto a active mode so that the card listens and captures every frame that traverses theEthernet broadcast network. A packet filter is used to select the IP packets that are of interest in a given situation. These IP packets and their higher-layer contents can then
be observed and analyzed. Tcpdump utility can be used as a protocol analyzer.Protocol analyzer are extremely useful in teaching the operation of protocols by
providing a means of examining traffic from a live network. Network protocolanalyzer give the ability to capture all packets in a LAN and in doing so provide anopportunity to gain unauthorized access to network information. These tools should
Communication is the process of transmitting information. It’s a big, vague, fuzzy wordand broadly applied. The “Communications Department “ at a university may includespeech teachers, public relations specialists, writers and broadcasters, who have nothingto do with hardware. By that definition , a writer is in the communication business.
Evolution of communications
We the human beings are communicating from early days that is by drawing the pictureson cave walls. It can be how to attack the animal or how to the save the life from thethreats or it may be a route to go to another cave.
The earlier long-distance communications was by signal fires that relayed simpleinformation. The signal used to represent “yes” or “no” – the question being asked had to
be known beforehand. It’s only a single bit information, not a detailed message.
A written letter or a human messenger could carry more information. The first systemswe might call telecommunications were series of hilltop towers, built by French engineer Claude Chappe in the 1790s. The towers had to be in sight of each other, with anoperator in each one. The operator relayed a message by moving arms on top of thetower. The operator of the next tower looked through a telescope to watch the arms
move, reading the message, recording it, then moving the arms of his tower to relay it tothe next tower. Chappe invented a code that used arm position to indicate letters or certain common words. His system was called an optical telegraph.
Samuel Morse’s electrical telegraph eventually replaced Chappe’s optical one. It alsorequired operators, but they used keys that completed an electrical circuit, sending burstsof currents through a wire. The Morse code was a series of dots an dashes ( short andlong pulses) that represented letters.
The electrical telegraph spread across the continents and in 1866 across the Atlantic. Itswires formed a network running between major cities. Telegraphers received signals and
either sent them for local delivery or relayed them to more distant stations. We can think of each dot and dash of the telegraph as a bit of information.
Later Alexander Graham Bell developed telephone. He borrowed some principles fromthe telegraph , but instead of transmitting dots and dashes, it sent a continuously changingelectrical current that represented a speaker’s voice. The telephone network was bigger and more complex than the telegraph network.
After that radio waves are used to carry signals through air. Radio first carried telegraphsignals. Wires worked fine on land, but only radio could send telegrams to ships at seaand relay urgent messages such as request for help. It was radio rescue calls that sentships to help the survivors of the sinking of the Titanic. Radio communications startedfor ships and airplanes in the world war II.
Radio communication started at low frequencies, but gradually moved to higher ad higher frequencies as electronics improved. The higher the frequency the more information thesignal can carry. Pictures need more transmission capacity than sound alone, so televisionare broadcast at higher frequencies than audio radio.By the 1970s , satellites were beaming radio signals around the globe. The telephonesystem had become global and known popularly “telecommunication network” . Wecould make phone calls to anywhere in the world.After that fiber optics arrived. Fiber provides high bandwidth. It carries high capacityinformation at faster rate. Now it is a integral part of telecommunication network.
Information is the one transmitted via communication. It may be very simple messagelike “yes” or “no”. It can be huge and complex message, such as he files containing the
book or a television broadcast of a movie. It doesn’t matter if the “information” doesn’tcontain anything a critic might agree was “information”.
A signal transmits that information. Signals may take many forms, such as optical,acoustic, electronic or radio-frequency. Signals may be converted from one form intoanother and still contain the same information. When we make a long-distance telephonecall, the sound waves from our mouth are converted to an electrical signal at thetelephone, that electrical signal is converted to optical form at the telephone switchingoffice, back to electrical form at the local switching office on the other end and back tosound waves at the other person’s telephone.
A system is the equipment that performs a task, like transmitting signals. We often speak of telephone system as if it is the whole network of telephone equipment from our phone
to the high-capacity fiber-optics cables that carry long-distance calls. However, parts of the telephone system also may be called systems, such as a switching systems that directs phone calls. We can assume a system which is a collection of stuffs designed to work together.
Message - the message is the information to be communicatedSender – The sender is the device that sends the data message. It can be computer,workstation, telephone etc.,Receiver – The receiver is the device that receives the message. It can be a computer ,workstations, telephone, fax machine
Medium – The transmitting medium is the physical path by which a message travels fromsender to receiver . It can consists of twisted pair wire, coaxial cable, fiber optic cable or radio waves.Protocol – A protocol is a set of rules that govern data communication. It represents anagreement between the communicating devices. Without a protocol two devices may beconnected but not communicating , just as a person speaking Malayalam cannot beunderstood by a person who speaks only Kannada.
Protocols and standards
The Communication occurs between entities in different systems. An entity is anythingcapable of sending or receiving information. But two entities cannot just send bit streamsto each other and expect to be understood. For communication to occur the entities mustagree on a protocol. The key elements of a protocol are syntax, semantics and timing.
Syntax : Syntax refers to the structure or format of the data, meaning the order in whichthey are presented. For example , a simple protocol might expect the first eight bits of data to be the address of the sender , the second eight bits to be the address of receiver and rest of the stream to be the message itself.
Semantics : Semantics refers to the meaning of each section of bits. How is a particular pattern to be interpreted and what action is to be taken based on that information.
Timing : Timing refers to two characteristics. When data should be sent and how fast itcan be sent. For example , if a sender produces data at 100 Mbps but the receiver can
process data at only 1 Mbps, the transmission will overload and receiver and data will belargely lost.
Standards are essential in creating and maintaining an open and competitive market froequipment manufacturers and in guaranteeing national and international operationalability of data and telecommunication technology and processes. They provideguidelines to manufacturers, vendors, government agencies and other service providers to
ensure the king of interconnectivity necessary in today’s marketplace and in internationalcommunications.
Data communications standards fall into two categories
3. de facto ( by fact)4. de jure ( by law)
De jure standards are those that have been legislated by an officially recognized body.
De facto standards are often established originally by manufacturers seeking thefunctionality of a new product or technology.
Standards organization
Standards are developed by cooperation among standard creation committees forums andgovernment regulatory agencies.
vi) The International Standards Organization (ISO)vii) The International Telecommunication Union (ITU-T)viii) ANSI (American National Standards Institute)ix) The Institute of Electrical and Electronics Engineer (IEEE)x) The Electronic Industries Association (EIA)
The ISO is an organization dedicated to worldwide agreement on internationalstandards in variety of fields. ITU-T is an international standards organization relatedto the United Nations that develops standards for telecommunications. Two popular standards developed by ITU-T are the V series and X series. ANSI , a nonprofitorganization is the United States, voting representative to both the ISO and ITU-T.The IEEE is the largest national professional group involved in developing standardsfor computing , communication, electrical engineering and electronics. It sponsoredan important standard for Local Area Network (LAN) called project 802. EIA is anassociation of electronics manufacturers in the United States. It is responsible for developing the EIA-232-D and EIA-530 standards.
Communications systems serve many functions. It is possible to divide into four categories on functions.e) Point-to-pointf) Broadcastingg) Switchedh) Networked
Point-to-point
A point-to-point configuration provides a dedicated link between two devices. The entire
capacity of the channel is reserved for transmission between those two devices. Most point-to-point line configuration use an actual length of wire or cable to connect the twoends.
Broadcasting
A broadcast system send the same signal to everyone who receives it. In its usual form ,transmission is one way, from the signal source to the individual. Local radio andtelevision transmission are good examples of pure one-way broadcast systems.
Switched systems
A switched system makes temporary connections between terminals so they canexchange information. The telephone is a good example.
Networking
A network connects many terminals that can communicate with each other. For exampleLocal Area Network is belongs to this category.
Services
We know the operation of modern communication networks is a very complex processthat involves the interaction of many systems. A communication network , is a set of equipment and facilities that provides a service, the transfer of information between userslocated at various geographical points. The ability of communication networks to transfer communication at extremely high speeds, allows users to gather information in largevolumes almost instantly with the help of computers for long distances.
Radio and television broadcasting are probably the most common communicationservices. Various stations transmit an ensemble of signals simultaneously over radio or cable distribution networks.
Telephone service is the most common real-time service provided by a network. Two
people are able to communicate by transmitting their voices across the network. Theservice is considered as connection-oriented in the sense that users must first interact withthe network to set up a connection
Telephone networks provide a broad class of call management services that use theoriginating number or the destination number to determine the handling of a call. For example , in call return the last originating number is retained to allow it to beautomatically called by the destination user at a later point in time. Caller ID allows theoriginating number and sometimes name, of the originating call to be displayed to thedestination user when the receiving device is display capable. Voice mail allows a
destination user to have calls forwarded to a message receiving device when thedestination user is not available.
Cellular telephone service extends the normal telephone service to mobile users who arefree to move within a regional area covered by an interconnected array of smaller geographical areas called cells. Each cell has a radio transmission system that allows it tocommunicate with users in its area. Cellular system should handle “handoff” when user moves from one area to another area.
Electronic mail (e-mail) is another popular network service. The user provides a textmessage and a name and/or address to a mail application. The application interacts with alocal mail server, which in turn transmits the message to a destination server across acomputer network. The destination user retrieves the message by using a mailapplication. E-mail is not a real-time service , where large delays can be tolerated. Heresecurity and privacy is matter of concern.
Many applications that involve an interaction between processes running in twocomputers may be characterized by client/server interaction. Interne is an example of distributed system. The World Wide Web (WWW) is accessed through a browser
program that displays the document and allows the user to access other documents byclicking the links. Each link provides the browser with a uniform resource locator (URL)that specifies the name of the machine where the document is located as well as the nameof the file that contains the requested document.
Video on demand characterization another type of interactive service. The objective of the service is to provide access to a video library. The user initiates the service byaccessing a menu from which a selection is made. Video which requires lot of bandwidthto pass from source computer to the destination.
Streamed audiovisual services over the internet provide an example of a service withsome of the feature of video on demand. An application like “real player “ can be used toaccess a channel that provides an audio visual stream to the client machine.
Another class of service is audio-visual conferencing. Here both audio and video are
transmitted over a distance. This service requires real-time response requirement. Thisservice is being used by many companies for training, client interaction from a site, groupdiscussion etc.
Computer networks
Early computers were expensive, so techniques were developed to allow them to be
shared by many users. In the 1960s tree-topology terminal-oriented networks weredeveloped to allow user terminals to connect to a single central shared computer. As thecost of computers were reduced , it became necessary to connect to different computersfor different applications. Here dumb terminals are replaced with intelligent terminals. Sotechniques were developed so that networks are more flexible and provide
communication among many computers .The ARPANET (Advanced Research projects Agency of U.S. Department of Defense)was the first network to interconnect computers over a wide geographical area. Here, theyused intelligent computer to process and the resources were available to storeinformation. Later Internet protocol was developed , that enabled communications
between computers were attached to different networks. IP made it possible to transfer information in the form of packets across many dissimilar networks.
Protocol
A protocol is a set of rules that governs how two or more communicating parties are tointeract. Let us consider an example an example of a protocol
HyperText Transfer Protocol (HTTP) specifies how a web client and server are tointeract.File Transfer Protocol for the transfer of filesSimple Mail Transfer Protocol (SMTP) for e-mailInternet Protocol (IP) for the transfer of packets.Domain Name System (DNS) for IP address lookup.
In the figure above figure a, shows the arrangement that allows a number of terminals to share a host computer. Terminal can be teletype printer or a videoterminal is connected by a set of wires. Later Modems allowed the devices for transmitting digital information so that terminals could access the host computer viathe telephone network. This is shown in the figure b.Medium access control methods were developed to allow a number of terminals atdifferent locations to communicate with a central computer using the sharedcommunication line. The access to the line, it has to be controlled so that differentdevices do not interfere with each other by transmitting at the same time.This arrangement is also called master/slave. Here central computer sends a pollmessage to a specific terminal on the outgoing line. All terminals listen to the
outgoing line, but only the terminal that is polled replies by sending any informationthat it has ready for transmission on the incoming line.
Statistical multiplexers/concentrators provided another method for sharingcommunication line among terminals. Messages from a terminal are encapsulatedinside a frame that consists of a header in addition to the user message. The header
provides an address that identifies the terminal. The messages from the variousterminals are buffered by the multiplexer ordered into a queue and transmitted one ata time over the communication line to the central computer. The central computer sorts out the messages from each terminal, carries out the necessary processing andreturns the result inside a frame.
Early data transmission systems that made use of telephone lines had to deal witherrors in transmission systems that made use of telephone lines had to deal witherrors in transmission arising from a variety of sources. It may be thermal nose or electromagnetic noise. So Error-control techniques were developed to detect theerrors and correct the errors. Hence some redundant bits called check bits areappended to the frame. If the data is changed, due to check bit it was possible todetect and receiver is used to request retransmission.
Finally elements of the terminal-oriented networks areA digital transmission system to transfer binary information. The transmission systemcan consists of a telephone system and specialized data transmission equipment.A method for the transmission of frames of information between terminals and centralcomputers. The frames contain address information to identify terminals, check bitsto enable error control. To access the same communication line, the systems need amedium access control to coordinate the transmissions of information from terminalsinto the shared communication line.
Computer-to-computer networks
The basic service provided by computer networks is the transfer of messages fromany computer connected to the network to any other computer connected to thenetwork. A switching technique in which messages are partitioned into smaller unitscalled packets, which contain addressing information as well as sequence numbers.Packets are then sent to the destination node one at a time, at any time and notnecessarily in a specific order. The channel used to transmit a packet is occupied onlyfor the duration of the packet’s transmission.
To identify computers, NETWORK INTERFACE CARD (NIC) which is used tocontain globally unique address. Broadcasting technique was used in LAN. Amedium access control protocol was used to coordinate access to the transmissionmedium in order to prevent collisions between frames.
Local area networksA variety of topologies can provide the broadcasting feature required by LANs,including ring and tree networks. In LAN, which involves the transmission over a bustopology coaxial cable. Stations with messages to transmit would first sense the cablefor the presence of ongoing transmissions. If no there is no carrier , the stationwould proceed to transmit its message encapsulated inside a frame. The station wouldcontinue to monitor the cable in an attempt to detect collisions.In star topology, computers are connected by copper wires , the computers transmit
packets using the random access procedure. The collisions may occur at the hub,where the wires converge. A LAN provides a message-transfer service betweencomputers and other devices that are attached to the LAN.
The elements of LAN architecture involveA high-speed 10 megabits/second or above , digital transmission system that support
broadcast transmissions.An addressing structure that provides station with a unique address and supports
broadcast transmissions.A frame structure to delineate individual transmission and a medium access control
procedure to coordinate transmissions into the shared broadcast mediu.
The Internet
An internet is the interconnection of multiple networks into a single large network.The Internet Protocol (IP) was developed to provide for the connectionless transfer
packets called datagram across an internetwork. In IP the component networks areinterconnected by special packet switches called gateways or routers. Routers adoptto change of the network and directs the transfer of IP packets across the internet.
After a routing decision is made, the packets are placed in a buffer to awaittransmission over the next network. Packets from different users are statisticallymultiplexed in these buffers. The underlying networks are responsible for transferringthe packets between routers.. IP provides best-effort service. It means IP makes everyeffort to deliver the packets but takes no additional actions when packets are lost,corrupted, delivered out of order or even misdelivered. Hence IP sometimes calledunreliable.IP uses a hierarchical address space that has “grouping “ information embedded in thestructure. IP addresses consists of 4 bytes usually expressed in dotted-decimalnotations. IP addresses consists of two parts• A network ID• Host ID
The internet also provides a name space to refer to machines connected to the Internetfor example nie.ac.in . Automatic translation of names to addresses is provided by theDomain Name System (DNS)The User Datagram Protocol (UDP) allows applications to transfer individual blocksof user information using datagrams. UDP takes the user information, appends
appropriate information that identifies the application in the destination host and thenuses IP to transfer the datagram across an internet. UDP may not be suitable for someapplication.The transmission Control Protocol (TCP) was developed to provide reliable transfer of stream information over the connectionless IP. TCP operates in a pair of end hostsacross an IP internet. TCP provides for error and flow control on an end-to-end basisthat can deal with the problems that can arise due to lost, delayed or misdelivered IP
packets.Examples that work on TCP are FTP, HTTP and SMTP.Finally, to provide communication modern network architecture consists of Digital transmission lines of the transfer of streams of binary information.
Exchange of frames of information between adjacent equipment. Frames contain dataas well as check bitsMedium access control regulate the transmission of frames from multiple users to ashared broadcast medium.Addresses to identify points of attachment to a network or internetwork.Exchange of packets of information between packet switches in network. Routingtables in packet switches are used to select the path of each packet as it traverses thenetwork.Dynamic calculation of routing tables at the packet switches in response to changes innetwork traffic and topology.Congestion control mechanism may be used to prevent congestion inside the network Internetworking provides connectivity across multiple, possibly dissimilar networks
by using gateways or routers.Reassembly of messages into packets are carried.
Communication network must support wide range of services. Normally people usenetworks to communicate, send e-mails, transfer of files and so on. Industry people
use communication network for transfer of funds, update information about the product and so on. Hence, to provide support for current service and future services, acomplete plan is required. This necessitates developing a complete flexibility innetwork architecture.
communications functions are grouped into the following tasks• The transport across a network of data from a process in one machine to the
process at another machine.• The routing and forwarding of packets across multiple hops in a network • The transfer of a frame of data from one physical interface to another.
To reduce their design complexity, most networks are organized as a series of layersor levels, each one built upon its predecessor. The number of layers, the name of theeach layer, the contents of each layer and the function of each layer differ fromnetwork to network.
Interaction between the layers must be defined precisely. Interaction is done withdefinition of the service provided by each layer and to the layer above. Interface
between layers through which a service is requested and through which results areconveyed. New services that build on existing services can be introduced even at thelater stage. The layered approach accommodates incremental changes readily.
We know , in all networks, the purpose of each layer is to offer certain services to thehigher layers. The entities comprising the corresponding layers on different machinesare called peer processes. Between each pair of adjacent layers there is an interface.The interface defines which primitive operations and services the lower layer offers tothe upper one. The set of layers and protocols is called the network architecture.
A protocol is a set of rules that governs how two or more communicating devices areto interact. HTTP protocol enables retrieval of web pages and TCP protocol enablesthe reliable transfer of streams of information between computers.
HTTP
Let us consider a client/server architecture, a server process in a computer waits for incoming requests by listening to a port. Port is an address that identifies which
process is to receive a message that is delivered to a given machine. The server provide response to the requests. The server process always runs a process in the background called daemon. httpd refers to server daemon for HTTP. The documentsare prepared using Hyper Text Markup Language (HTML) which consists of text,
graphics and other media are interconnected by links that appear within thedocuments. The www is accessed through a browser program that displays thedocuments and allows the user to access other documents by clicking one of theselinks. Each link provides the browser with a uniform resource Locator (URL) thatspecifies the name of the machine where the document is located and the name of the
file that contains the requested document. The HTTP ( Hyper Text Transfer Protocol )specifies rules by which the client and server interacts so as to retrieve a document.
In HTTP, we use two-way connection that transfer a stream of bytes in correctsequential order and without errors. The TCP protocol provides this type of communication service between two processes in two machines connected to anetwork. Each HTTP inserts its messages into a buffer and TCP transmits thecontents of the buffer to the other TCP in blocks of information called segments.Each segment contains port number information in addition to the HTTP messageinformation. The following figure shows how communication is carried between
HTTP client and HTTP server.
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First of all, client needs to perform a DNS query to obtain the IP addresscorresponding to the domain name. This step is done by sending a message to a DNS
server. The Domain Name System (DNS) is a distributed database that resides inmultiple machines on the Internet and is used to convert between names andaddresses and to provide e-mail routing information. Each DNS machine maintains itsown database and acts as a DNS server that other query machine can query. Initiallyrequesting machine accesses a local name server. In case if it not possible to resolvenames then it will be sent to the root server. When the root server is unable to resolve, it will be sent to authoritative name server. Every machine on the Internet is requiredto register with at least two authoritative name servers.
Steps of DNS query
6. Application requests name to address translation.7. Resolver composes query message.8. Resolver sends UDP datagram encapsulating the query message9. DNS server looks up address and prepares response10. DNS sends UDP datagram encapsulating the response message.
DNS query and response messages are transmitted by using the communicationservice provided by the User Datagram Protocol (UDP). Port 53 is used for communication. The UDP service is connectionless, no connection setup is requiredand the datagram is sent immediately.
SMTP
Simple Mail Transfer Protocol is used for sending mails. The user prepares an e-mailmessage that includes the recipient’s e-mail address, a subject line and a body. User clicks send button to send the information. Let us consider the steps are used insending the e-mail.
• The mail application establishes a TCP connection (port 25) to its local SMTPserver.
• SMTP daemon issues the message to the client, indicating that it is ready toreceive mail
• Client send s a HELO message and identifies itself.• SMTP daemon issues a 250 message, indicating the client may proceed.• Client sends sender’s address• If successful, SMTP daemon replies with a 250 message• Client sends recipient’s address.• A 250 message is returned• Client sends a DATA message requesting permission to send the mail message
• The daemon sends a message giving the client permission to send• Client sends the actual text• Daemon indicates that the message is accepted for delivery. A message ID is
returned.• Client indicates that mail session is over.
• Daemon confirms the end of the session.
SMTP works best when the destination machine is always available, for this reason,users in a PC environment usually retrieve their e-mail from a a mail server using thePost Office Protocol version 3 (POP 3).
TCP and UDP transport Layer services
UDP provides connectionless transfer of datagrams between processes in hostsattached to the Internet. UDP provides port numbering to identify the source anddestination processes in each host. UDP is simple and fast, but provides no guarantees
in terms of delivery or sequence addressing.TCP provides for reliable transfer of a byte stream between processes in hostsattached to the Internet. TCP involves the establishment of a connection between thetwo processes. To provide their service, the TCP entities implements error detectionand retransmission as well as flow control algorithms. TCP implements congestioncontrol, which regulates the flow of segments into the network.
Peer-to-peer file sharing
File sharing applications such as Napster and Gnutella are popular for sharing theMP3 audio and other files. Here clients can also acts as transient server while theapplication is activated. When a peer is interested in finding a certain file, it sends aquery. The response provides a list of peer s that have the file and additionalinformation such as the speed of each peer’s connection to the Internet. Therequesting peer can then set up a TCP connection to one of the peers in the list and
proceeded to retrieve the file.
The OSI reference model
The model proposed by ISO OSI ( Open System Interconnection) reference model because it deals with connecting open systems that is systems that are open for communication with other systems. The OSI model has seven layers.
The concepts related to layers arei) A layer should be created where a different level of abstraction is needed.ii) Each layer should perform a well defined functionIii) The function of each layer should be chosen with an eye toward defininginternationally standardized protocols.
Iv) The layer boundaries should be chosen to minimise the information flow acrossthe interface.v) The number of layers should be larger enough that distinct functions need not bethrown together in the same layers out of necessity and small enough that thearchitecture does not become unwieldy.
The following figure shows OSI model.
To remember the layers in sequence, we can use the sentence like
Please Do Not Touch Sita’s Pet Animal , Where the beginning letter of each wordrepresent the layer name. Like P in Please word stands for Physical layer, D letter inDo word stands for Data link layer and so on. Let us see the functions of the layers.
Physical layerThe physical layer deals with the transfer of bits over a communication channel.
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Physical layer concerned with the representation of signals. How to represents 1 andhow represent 0. Which type of encoding mechanism should be used for representingthe bits.This layer is concerned with the procedures to set up and release the physicalconnections.
Data link layerThe Data link layer provides the frames (blocks of information) across a transmissionlink that directly connects two nodes. Data link layer inserts a framing information inthe sequence of transmitted bits to handle boundaries. Data link layer inserts controland address information in the header and check bits to enable recovery fromtransmission errors and flow control.
High level data link control (HDLC) and Point-to-point protocol (PPP) are the twostandard data link protocol that are wide in use. A flat addressing space is used toenable machines to listen and recognize frames that are destined to them.
Network layerThe network layer provides for the transfer of data in the form of packets across acommunication network. It uses the hierarchical way of addressing scheme. Routing
protocol is the procedure that is used to select paths across a network. The nodes inthe network must work together to perform the routing effectively. Network layer isresponsible for dealing with congestion that occurs from time to time in the network.When the two machines are connected to the same packet-switching network, singleaddress space and routing procedure are used.
If two machines are connected to the different networks, the transfer of data musttraverse two or more networks and hence internetworking protocols are necessary toroute the data between gateways/routers that connects the intermediate networks.
The internetworking protocols must understand the differences in addressing anddifferences in the size of the packets that are handled within each network.
Transport layerThe transport layer is responsible for the end-to-end transfer of messages from a
process in the source machine to a process in the destination machine. The transportlayer protocol accepts messages from its higher layers and prepares blocks of information called segments or datagram's for transfer between end machines.
The transport layer provides number of services• Transport layer may provide a connection-oriented service that involves the error-
free transfer of a sequence of bytes or messages.• The associated protocol carries out error detection and recovery and sequence and
flow control.• The transport layer is responsible for setting up and releasing connections across a
Session layerThe session layer can be used to control the manner in which data are exchanged. It ismainly concerns with establishing the sessions and termination the session. Certainapplications require half-duplex dialog . Certain applications requires the introductionof synchronization points that can be used to mark the progress of an interaction and
can serve as points from which error recovery can be initiated.
Presentation layerThe presentation layer provide the application layer with independence fromdifference in the representation of data. Presentation layer should convert themachine-dependent information provided by application A into a machine-dependentform suitable for application B, because different computers use different codes for representing characters and integers.
Application layerThe application layer is to provide services that are frequently required by
applications that involve communications. For example browser application useHTTP application layer protocol to access a WWW document. Application layers protocols have been developed for file transfer, virtual terminal, electronic mail, nameservice, network management and other applications.
In general each layer adds a header and possibly a trailer to the block of information itaccepts from the layer above.
Summary of the layers
The following table shows the different functions carried out by different layers OSImodel.
The development of OSI reference model leads the world to the development of unifiedview of layers, protocols and services. In each layer a process on one machine carries outa conversation with a peer process on the other machine across a peer interface. This isshown below.
Layer Description Examples
Application Protocols that are designed to meet the communication requirements of specific applications, often defining the interface to a service.
HTTP,FTP , SMTP,
CORBA IIOPPresentation Protocols at this level transmit data in a network representation that is
independent of the representations used in individual computers, which maydiffer. Encryption is also performed in this layer, if required.
Secure Sockets(SSL),CORBA DataRep.
Session At this level reliability and adaptation are performed, such as detection of failures and automatic recovery.
Transport This is the lowest level at which messages (rather than packets) are handled.Messages are addressed to communication ports attached to processes,Protocols in this layer may be connection-oriented or connectionless.
TCP, UDP
Network Transfers data packets between computers in a specific network. In a WANor an internetwork this involves the generation of a route passing throughrouters. In a single LAN no routing is required.
IP, ATM virtualcircuits
Data link Responsible for transmission of packets between nodes that are directlyconnected by a physical link. In a WAN transmission is between pairs of routers or between routers and hosts. In a LAN it is between any pair of hosts.
Ethernet MAC,ATM cell transfer,PPP
Physical The circuits and hardware that drive the network. It transmits sequences of binary data by analogue signalling, using amplitude or frequency modulationof electrical signals (on cable circuits), light signals (on fibre optic circuits)or other electromagnetic signals (on radio and microwave circuits).
The processes at layer n are referred to as layer n entities. Layer n entitiescommunicate by exchanging protocol data units (PDUs). Each PDU contains header,which contains protocol control information and usually user information. Thecommunication between peer processes is usually virtual in the sense that no directcommunication link exists between them.
For communication to take place, the layer n+1 entities make use of the services provided by layer n. The transmission of the layer n+1 PDU is done by passing a block of information from layer n+1 to layer n through a software port called thelayer n service access point (SAP) across a service interface. SAP is identified by aunique identifier. The block of information passed between layer n and layer n+1entities consists of control information and a layer n service data unit (SDU), whichis the layer n+1 PDU itself. The layer n SDU, which is the layer n+1 PDU, isencapsulated in the layer n PDU. The service provided by layer n typically acceptinga block of information from layer n+1, transferring the information to its peer
process.
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The service provided by the layers can be connection oriented or connectionless.A connection-oriented service has three phases.Step 1: Establishing a connection between two layer n SAPs. The setup involvesnegotiating parameters like sequence numbers, flow control.Step 2: Transferring n-SDUs using the layer n protocol.Step 3: Tearing down the connection and releasing the various resources allocated tothe connection.
In connectionless service which does not require a connection setup and each SDU istransmitted directly through the SAP. Suppose a layer n SDU is too large to behandled by the layer n-1 and so segmentation and reassembly are applied. The layer nSDU is segmented into multiple layer n PDUs that are then transmitted using theservices of layer n-1. The layer n entity at the other side must reassemble the originallayer n SDU from the sequence of layer n PDUs it receives. It is also possible that thelayer n SDUs are so small as to result in inefficient use of the layer n-1 services andso blocking and unblocking applied. The layer n entity may block several layer nSDUs into a single layer n PDU . The layer n entity on the other side must thenunblock the received PDU into the individual SDUs.
Multiplexing involves the sharing of a layer n services by multiple layer n+1 users.The N+1 layer users passes its SDUs for transfer using the services of a single layer n entity. Demultiplexing is carried out by the layer n entity at the other end.
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The tcp/ip network architecture is a set of protocols that allows communication acrossmultiple diverse networks. Tcp/ip provides robustness with regard to failures in thenetwork and on flexibility in operating over heterogeneous networks. We know
Internet consists of different types computer systems and networks . Tcp/ip whichconsists of 4 layers and OSI model consists of 7 layers. The following figure showsthe TCP/Ip architecture.
Application layerThe application layer provides services that are used by the applications. Examplesare remote login, e-mail, ftp, network management. The TCP/IP application layer incorporates the functions of the top three OSI layers. HTTP is a TCP/IP applicationlayer protocol.
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The TCP/IP application layer programs are directly run through transport layer.Two basic types of services are provided by transport layer.
The first one is connection-oriented , which is provided by Transmission ControlProtocol (TCP).The second service is best-effort connectionless transfer of individual messages,which is provided by the user datagram protocols (UDP). Here in connection-lessservice no mechanism for error recovery or flow control.UDP is used for applications that require quick but not necessarily reliable delivery.TCP/IP model does not require strict layering. It means the application layer has theoption of bypassing intermediate layers.
Internet layer
The Internet layer handles the transfer of information across multiple networksthrough the use of gateways/routers . The internet layer corresponds to the network layer of OSI , it mainly concerns with the transfer of packets between machines thatare connected to different networks. Internet layer deals with deal with the routing of
packets from router to router across the networks. Internet layer uses a concept of globally unique addresses for machines that are attached to the Internet. The internetlayer provides a single service that is best-effort connectionless packet transfer. IP
packets are exchanged between routers without a connection setup, the packets arerouted independently. So packets may travel in different paths. Hence IP packets arealso called datagrams. The connectionless approach makes the system robust, if failure occurs in the network, the packets are routed around the points of failure, thereis no need to set up the connections again .
Network interface layerThe network interface layer is concerned with the network-specific aspects of the transfer of packets. It is similar to part of OSI network and data link layer services. The network interface layer is particularly concerned with the protocols that access the intermediatenetworks. At each gateway the network access protocol encapsulates the IP packet into a
packet or frame of the underlying network or link. The IP packet is recovered at the exitgateway of the given network. This gateway must then encapsulates the IP packet into a
packet or frame of the type of the next network or link.
Hence, a clear separation of the internet layer from the technology –dependent network interface layer. This allows the internet layer to provide a data transfer service that istransparent in the sense of not depending on the details of the underlying the network.The following figure shows protocols of the TCP/IP protocol suite.
• OSIThree concepts are central to OSI model services, interfaces, protocols.Protocols in the OSI model are better hidden than in the TCP/IP model and can bereplaced relatively easily as the technology changes.Model first and then next protocol.OSI has seven layers.OSI supports both connectionless and connection oriented communication in thenetwork layer.
• TCP/IPServices, interface and protocols are not distinguished properly.
Protocols comes first and model next.TCP/IP has four layers.TCP/IP model has connectionless in the Internet layer and both modes in the transportlayer.
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Let us consider a network , which consists of server, workstation, router and personalcomputer. The router which is used to connect two networks. The ordered pair (1,1)which represent network 1 and the host id 1. Router which contain two interfaces one
is connected to network 1 and other is connected to pc via ppp link. It constitutes theother network.
From the point of view of IP , the Ethernet LAN and the point-to-point link constitutetwo different networks, shown in the following figure.
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Each host in the Internet is identified by a globally unique addresses. IP addressidentifies the host’s network interface rather than the host itself. A node that isattached to two or more physical networks is called the router. Router attaches to twonetworks with each network interface assigned to a unique IP address.
An IP address is divided into two parts
• A network id• Host id
The network id must be obtained from an organization authorized to issue IP address.On a LAN, the attached device to the network is often identified by a physicaladdress. The format of the Physical address depends on the particular type of thenetwork. Ethernet MAC address is 48 bits.Each Ethernet network interface card(NIC) is issued a globally unique medium access control (MAC) or physical address.
Sending and receiving IP datagrams
Let us consider a case, in which the workstation wants to send an IP datagram to theserver. The IP datagram has the workstation’s IP address and the server’s IPaddresses in the IP packet header. Let us assume, IP address of the server is known.The IP entity in the workstation looks at its routing table to see whether it has anentry for the complete IP address. Workstation finds the server is directly connectedto the same network and the server has physical address s. The IP datagram is passedto the Ethernet device driver, which prepares an Ethernet frame as shown in the nextfigure.
The header in the frame contains the source physical address, w, and the destination physical address s. The Ethernet frame is then broadcast over the LAN. The server’s NIC recognizes that the frame is intended for its host, so the card captures the frameand examines it. The NIC finds that the protocol type field is set to IP and therefore
passes the IP datagram up to the IP entity. Now let us see how the server sends IPdatagram to the personal computer. Suppose the server knows the IP address of thePC and that the IP addresses on either side of the link were negotiated when the link
was set up. Otherwise, IP entity then checks to see whether it has a routing table entrythat matches the network id portion of the IP portion of the IP address of the PC.Suppose that the IP entity does not find such an entry. The IP entity then the checks tosee whether it has an entry that specifies a default router that is to be used when noother entries are found. Suppose that entry exists and that it specifies the router withaddress (1,3). The IP datagram is passed to the Ethernet device driver, which preparesan Ethernet frame. The header in the frame contains the source physical address, s,and the destination physical address, r. IP datagram in the frame contains thedestination IP address of the PC (2,2), not the destination IP address of the router. TheEthernet frame is then broadcast over the LAN. The router’s NIC captures the frameand examines it. The routing tables at the router show that the machine with address
(2,2) is connected directly on the other side of the point-to-point link. The router encapsulates the IP datagram in a PPP frame that is similar to the Ethernet frame. ThePPP receiver at the PC receives the frame, checks the protocol type filed and passesthe IP datagram to its IP entity.
Application layer protocols are high level that provide services to user applications.Application protocols are user written. The popular protocols are Telnet, File transfer
protocol (FTP), HTTP and SMTP.
Telnet
Telnet is a TCP/IP protocol that provides a means of accessing resources on a remotemachine where the initiating machine is treated as local to the remote host. Telnet
protocol is based on the concept of a network virtual terminal (NVT), which is animaginary device that represents a lowest common denominator terminal. Eachmachine initially maps its characteristics to that of an NVT and negotiates options for changes to the NVT or other enhancements, such as changing the character set. Telnetuses one TCP connection. A TCP connection is identified by a pair of port numbers, aserver is capable of supporting more than one Telnet connections at a time.
FTP
File transfer Protocol (FTP) is another commonly used application protocol. FTP provides for the transfer of a file from one machine to another. FTP requires two TCPconnections to transfer a file. One is the control connection that is established on port21 at the server. The second TCP connection is a data connection used to perform afile transfer. A data connection must be established for each file transferred. Dataconnections are used for transferring a file in either direction or for obtaining lists of files in either direction or for obtaining lists of files or directories from the server tothe client. FTP works across different systems because it can accommodate severaldifferent file types and structures. FTP commands are used to specify informationabout the file and how it will be transmitted. Three types of information must bespecified. File type – FTP supports ASCII, EBCDIC, image (binary) or local.
HTTP and the WWW
The world wide web provides a framework for accessing documents and resourcesthat are located in computer connected to the Internet. The HTML is used to preparethe documents. Each link provides the browser with a uniform resource locator (URL) that specifies the name of the machine where the document is located as wellas the name of the file that contains the requested document HTTP is a stateless
protocol in that it does not maintain any information ( state) about the clients. TheHTTP server handles each request independently of all other requests.The following are the few commands are usedGETRetrieve information (object) identified by the URLPOST – Send information to a URL and retrieve result, used when a user fills out afrom in a browser.PUT – Store information in location named by URL
TRACE – Trace HTTP forwarding through proxies, tunnels
Cookies and web sessions
It was indicated that the HTTP protocol is stateless and does not maintain information
about the prior requests from a given client. The use of cookies makes it possible tohave web sessions where a user interacts with a web site in a manner that takes intoaccount the user’s preferences. Cookies are data that are exchanged and stored byclients and servers and transferred as headers lines in HTTP messages. The header lines provide context for each HTTP interaction. When a client first accesses a webserver that uses cookies, the server replies with response message that includes a Set-cookie header line. This header line includes a unique ID number for the given client.If the client software accepts cookies, the cookies is added to the browser’s cookiefile. Each time the client software accepts cookies, the cookie is added to the
browser’s cookie file. Each time the client makes a request to the given site, itincludes a Cookie header line with the unique ID number in its requests. The server
maintains a separate cookie database where it can store which pages were accessed atwhat date and time by each client.
IP utilities
A number of utilities are available to help in finding out about IP hosts and domainsand to measure Internet performance. We discuss utilities like ping, traceroute,ipconfig, netstat, tcpdump.
Ping
Ping is a utility which is used to determine whether a host is reachable or not. Pingmakes use of Internet control message Protocol (ICMP) messages. The purpose of ICMP is to inform sending hosts about errors encountered in Ip datagram processingor other control information by destination hosts or by routers. Ping sends one or more ICMP echo messages to a specified host requesting a reply. Ping is often tomeasure the round-trip delay between the hosts.The syntax is ping <hostname>
The round-trip delay is indicated and time-to-live value. The TTL is the maximumnumber of hops an IP packet is allowed to remain in the network. Each time an IP
packet passes through a router, the TTL is decreased by 1. When the TTL reaches 0,the packet is discarded.
Traceroute
Traceroute utility allows users to determine the route that a packet takes from thelocal host to a remote host, as well as latency and reachability from the source to eachhop. Traceroute is used as a debugging tool by network administrator. Traceroutemakes use of both ICMP and UDP.
IpconfigThe ipconfig utility available on windows operating system. This utility displays theTCP/IP information about a host. It displays host IP address, subnet mask, defaultgateway for the host. The utility can also be used to obtain information for each IP
network interface fro the host, DNS hostname, Ip address of DNS servers, physicaladdress of the network card, IP address for the network interface and whether DHCPis enabled for automatic configuration of card’s Ip address.
netstat
The netstat queries a host about its TCP/IP network status. netstat can be used to findthe status of the network drivers and their interface cards, such as the number of
packets, out packets, error packets and so on. It is also be used to find out the state of the routing table in a host, which TCP/IP server processes are active in the host aswell as the which TCP connection are active.
Tcpdump
The tcpdump program can capture and observe IP packet exchanges on a network interface. The program usually involves setting an Ethernet network interface cardinto a active mode so that the card listens and captures every frame that traverses theEthernet broadcast network. A packet filter is used to select the IP packets that are of interest in a given situation. These IP packets and their higher-layer contents can then
be observed and analyzed. Tcpdump utility can be used as a protocol analyzer.Protocol analyzer are extremely useful in teaching the operation of protocols by
providing a means of examining traffic from a live network. Network protocolanalyzer give the ability to capture all packets in a LAN and in doing so provide anopportunity to gain unauthorized access to network information. These tools should
Circuit switching networks provide dedicated circuits that provide the flow of information between users. Multiplexing in the physical layer involves the sharing of
transmission systems by several connections or information flows. Multiplexing isrequired when the bandwidth of individual connections is smaller than the bandwidthof the available transmission system. Multiplexing helps to use bandwidth in aefficient way and also reduces cost incurred in using the separate connections.
Frequency-division multiplexing
In Frequency division multiplexing technique we partition the available transmissionfrequency range into narrower bands ( subfrequencies), each of which is a separatechannel. FDM based transmissions are parallel in nature. Examples of FDM are
broadcast radio and broadcast and cable television, where each station has an
assigned frequency band.Time division multiplexing
In Time division multiplexing technique that assigns to each node connected to achannel an identification number and a small amount of time in which to transmit.TDM based transmissions are serially sequenced. TDM was started in the telephonenetwork in the early 1960s. The T-1 carrier system that carries 24 digital telephoneconnections. The T-1 system uses a transmission frame that consists of 24 slots of eight bits each. Each slot carries one PCM sample for a single connection. The
beginning of each frame is indicated by a single framing bit.
The T-1 carrier system was used to carry the traffic between telephone central offices.Later growth of telephone network traffic and the advances in digital transmission ledto the development of a standard digital multiplexing hierarchy.In North America and Japan, the digital signal 1 (DS1) , which corresponds to theoutput of T-1 multiplexers, became the basic building block. The DS2 signal isobtained by combining 4 DS1 signals and then adding 136 kilobits of synchronizationinformation. The DS3 signals is obtained by combining 7 DS2 signals and adding 522kilobits of synchronization.
A multiplexing method used with fiber-optic cables. Involves the simultaneoustransmission of light sources over a single fiber-optic channel. Light sources of different wavelengths are combined by a WDM multiplexer and transmitted over asingle line. When the signals arrive, a WDM demultiplexer separates them andtransmits them to their respective destination receivers. Normally speed of
information is in terms of tens of Gbps. Fiber provides a high bandwidth.
SONET
The advances in optical transmission technology led to the development of SONET( Synchronous Optical Network). An ANSI physical layer standard that provides aninternational specification for high-speed digital transmission via optical fiber.Initially developed in North America. Later CCITT (Consultative Committee for International Telegraphy and Telephony) developed a corresponding set of standardscalled Synchronous Digital Hierarchy ( SDH). Current backbone networks in US areSONET and in Europe are SDH.
The SONET simplifies the handling of lower-level digital signals and reduces theoverall cost of multiplexing in the network. In SONET , incorporation of overhead
bytes in the frame structure for use in monitoring the signal quality, detecting faultsand signaling among SONET equipment to identify rapid recovery from faults.
The SONET standard uses a 51.84 Mbps electrical signal, known as Synchronoustransport signal level – 1 (STS-1), as a building block to extend the digitaltransmission. Hierarchy into the multigigabit/ second stage.
A SONET system is divided into three layersSection , lines and Paths.A Section refers to the span of fiber between two adjacent devices, such as tworegenerators. The section layer deals with the transmission of an STS-n signal acrossthe physical medium. A line refers to the span between two adjacent multiplexers andtherefore in general encompasses several sections. Lines deals with the transport of anaggregate multiplexed stream and the associated overhead.
A path refers to the span between the two SONET terminals at the endpoints of thesystem and in general encompasses one or more lines. Adjacent multiplexersexchange information using frames.
A frame consisting of a rectangular array of bytes arranged in 9 rows by 90 columnsis repeated 8000 times a second. Thus each byte in the array corresponds to a bit rateof 64 kbps and the overall bit rate of the STS-1 is• 8 * 9 * 90 * 8000 = 51.84 MbpsThe first three columns of the array are allocated to section and line overhead. Theoverhead provide framing, error monitoring and other section related managementfunctions. For example A1 and A2 indicates the beginning of the frame. The fourth
byte B1 carries parity checks of the transmitted signal and is used to monitor the biterror rate in a section.
The last three bytes of the section overhead are used to provide a datacommunications channel between regenerators that can be used to exchange alarms,
control, monitoring and other administrative messages. The line overhead is used for synchronization and multiplexing, performance monitoring, line maintenance, as wellas protection-switching capability in case of faults. The byte B2 is used to monitor the
bit rate in a line. The K1 and K2 bytes are used to trigger recovery procedure in caseof faults.
The remaining 87 columns of the frame constitute the information payload thatcarries the path layer of user information.The bit rate of the information payload is
8 * 9 * 87 * 8000 = 50.122 Mbps.
Let us see how information is organized at the path level.The SONET terminal equipment takes the user data and the path overhead and mapsinto a synchronous payload envelope (SPE), which consists of nine rows and 87columns.
The path overhead uses the first columns of this array. This SPE is then inserted intothe STS-1 frame. The first two bytes of the line overhead are used as a pointer thatindicates the byte within the information payload where the SPE begins. The use of
pointer makes it possible to extract a selected signal from the multiplexed signal. Thisfeature gives SONET multiplexers an add-drop capability, which represent theselected signal can be dropped and individual signal can be added without todemultiplex the entire signal.
The pointer structure consisting of the H1, H2 and H3 bytes maintainssynchronization of frames and SPEs is situation where their clock frequencies differ slightly. If the payload stream falls behind the payload stream. To allow the frame tocatch up, an extra SPE byte is transmitted in a frame from time to time. This extra
byte, which is represented by H3 clears the backlog. When the payload stream isslower than the frame stream, the number of SPE bytes transmitted in a frame needsto be reduced by one byte from time to time.
TRANSPORT NETWORKS
A transport network provides high bit rate connections to clients at differentlocations. The clients of a transport network are routers, large telephone switches or even other networks.
Routers are interconnected for form an ISP backbone network and the telephoneswitches are interconnected to form a telephone network. The failure of a single
connection in a transport network can be disastrous because each connection carriesheavy traffic.
SONET Networks
In the beginning, asynchronous multiplexing concept was used. In that bit stuffingwas used to deal with slips and hence the complete multiplexed stream to bedemultiplexed to access a single tributary.Transit tributaries would then have to be remultiplexed onto the next hop. Back-to-
back multiplexers are an unnecessary expense in situations where most of the trafficis transit and only a few tributaries need to be dropped. SONET which enables inreduction in cost by enabling add-drop multiplexers (ADM) that can insert and extracttributary streams without disturbing tributary streams that are in transit. ADMs are arranged in linear fashion to interconnect various SONET terminals. Theseterminals can be part of other equipment, they could be interfaces to a large router.
From the point of view of the terminals, they are arranged in the fully connectedmesh. The SONET standards also defines automatic protection switching (APS)schemes that provide linear protection against failures at the line layer.
Here, protection at the line level applies to a multiplexed signal while it traverses theline between two multiplexers. Two SONET multiplexers connected in a 1+1 (one
plus one ) configuration using two SONET lines, a working line and protection line.Two lines carries the same information. A selector in the downstream node picks the
better signal based on the information provided by the two monitors and does notneed to coordinate with the upstream node. Recovery from failures are fast because itis done by the hardware.
A 1:1 ( one for one) APS arrangement is shown (refer ppt), Here, signal istransmitted in the working line during normal operation. The optical signal that isreceived is monitored and in case if the signal is degraded, then a request is sent.Once again, upstream node switches the signal to the protection line.The 1:1 takes more time to recover from failure than 1 + 1 APS.The 1:1 scheme is efficient in terms of the bandwidth.
In the other arrangement 1:1 APS scheme can be generalized to a 1:n APS schemewhere one protection line protects n working lines. The scheme assumes that theworking lines are unlikely to fail at the same time.
The SONET linear APS specification says that fault recovery be completed within 50milliseconds in all of the schemes discussed previously.
In the figure above, a, b and c represents the three sites and are connected by threeadd-drop multiplexers. At node b, two STS-n tributaries are inserted destined for nodec and for node a, respectively. The first tributary ends at node c and second tributaryflows across node c and terminates at node a.
SONET cross-connect and mesh networks
SONET ring networks provide good service even in case of faults. SONET networkssuffers from the drawback where scalability issue is concerned. As networks carriesmore traffic , certain spans in a ring will become congested. To increase the capacityof a single span in a single ring network, all the ADMs need to be upgraded at thesame time. This upgrade involves very high expense and effort.The drawback can be eliminated by building multiple parallel ring networks.
This approach is compatible with WDM transmission system, which provides parallelrings using a single fiber channel. At each node, the signal carried in each wavelengthmust be connected to a separate ADM. To exchange traffic between ADMs at anode, the traffic must first be dropped from a source ADM, connected to a switch andthen added into the destination ADM.
In SONNET cross-connect system can take SONNET OC-n optical signals at itsinputs, decompose these signals into its components STS-1 or other tributaries, switchthese component signals to output ports and combine the component signals intooutgoing SONET OC-n signals. Mesh networks are constructed by interconnectingcross-connect systems with SONNET optical lines .
Mesh networksWhen certain segments become congested, mesh networks have the advantage thatonly the capacity of the affected segments have to be upgraded. Mesh networksrequire less protection bandwidth than ring networks.
Optical transport networks
Optical transport networks provide optical wavelength connections between attachedclients. Optical add-drop multiplexers (OADM) have been used for WDM systems.An OADM takes a multiwavelength signal arriving in an input fiber, drops one or
more pre-selected wavelength at a site and adds one or more pre-selected wavelengthsinto multiwavelngth signal that exists in an output fiber.OADM can be arranged in linear and ring topologies.chain of optical add-drop multiplexers in which a single fiber connects adjacentmultiplexers. Each fiber contains a set of four wavelengths that are removed andadded to provide a one-directional communication link from source to destination.
The advantage of all optical networks is that they can carry signals transparently. InWDM each wavelength is modulated separately, so each wavelength need not carryinformation in the same format. Optical mesh networks based on optical cross-connect and optical fiber switching systems are used as backbone of transportnetworks. The use of fiber switch is to transfer entire multiwavelength signals frominput ports to output ports without demultiplexing.
Figure shown above, optical signals that carry WDM signals arrive in individual
fibers to a node. Optical signals that carry signals destined for the node are switchedto the local wavelength cross-connect. The wavelength signals are destined for thenode are then dropped. Locally generated wavelengths signals are added through thecross-connect. Next the outputs from the cross-connect are fed to WDM multiplexersthat produce composite optical signals that are then inserted into the fiber switch andtransferred out in an outgoing fiber.
Optical transport networksThe previous mentioned arrangement makes the overall cost incurred is low withrespect to increasing the traffic.
The function of a circuit switch is to transfer the signal that arrives at a given input toan appropriate output. The interconnection of a sequence of transmission links andcircuit switches enables the flow of information between inputs and outputs in thenetwork.
Space-division switches provide a separate physical connection between inputs andoutputs so the different signals are separated in space. Crossbar switch is an examplefor this category.
The crossbar switch consists of an N x N array of crosspoints that can connect anyinput to any available output. When a request comes in from an incoming line for anoutgoing line, the corresponding crosspoint is closed to enable information to flowfrom the input to the output. The crossbar switch is nonblocking switch means
requests are never denied because of lack of connectivity resources that iscrosspoints. Connections requests are denied when only outgoing line is already inuse.
The complexity is cross bar switch is N * N. Huge number of crosspoints cannot beimplemented.
In the figure shows a multistage switch that consists of three stages of smaller spacedivision switches. The N inputs are grouped into N/n groups of n input lines. Eachgroup of n input lines enters a small switch in the first stage that consists of n x k array of crosspoints. Each input switch has one line connecting it to each of k intermediate stage N/n x N/n switches. Each intermediate switch has one lineconnecting to each N/n switches in the third stage. The next switches are k x n. Eachset of n input lines shares k possible paths to any one of the switches at the last stage.
CLOS Nonblocking switching fabric
Here , we determine when a multistage switch becomes nonblocking. The worst casefor the desired input is when all the other inputs in its group have already connected.Similarly it is same for the output , where all the other outputs in its group havealready been connected. The set of routes which uses the intermediate switchesalready in use shown in the next figure.
Each existing connection uses a different intermediate switch. The maximum number of intermediate switches not available to connect the desired input to the desiredoutput is 2 (n-1). Suppose k = 2n -1 , then k paths are available from any input groupto any output group, because 2 (n -1) of these paths are already in use, it follows asingle path remains available to connect the desired input to the desired output.Hence, the multistage switch k = 2n -1 is nonblocking.
The number of crosspoints required in a three stage switch is the sum of the following
components• N/n input switches x nk crosspoints/input switch• K intermediate switches x (N/n)^2 crosspoints/intermediate switch• N/n output switches x nk crosspoints/output switch.Total number of crosspoints is 2 Nk + k (N/n)^2. The number of crosspoints requiredto make the switch nonblocking is
2 N(2n -1) + (2n-1)(N/n)^2.
Time division switches
We know in TDM a slot within a frame refers to a single connection. The time-slotinterchange (TSI) technique replaces the crosspoints in a space switch with thereading and writing of a slot into a memory. Assume we have many speakers want tohave conversation. The speech of the speaker are sampled at 8000 bytes/second andalso bytes are placed on T-1 carrier shown in the next figure.
In the figure above , first pair of speakers are assigned slots 1 and 23. For thespeakers to hear each other, we need to route slots 1 and 23 in the incoming frames toslots 23 and 1 in the outgoing frames. The octets in each incoming frame are storedinto a register. The call setup procedure has a set a permutation table that controls theorder in which the contents of the register are read out. Hence the outgoing the frame
begins by reading the contents of slot 23, followed by slot 24 and so on until slots 1and 2 are read.
A hybrid switch, TSI switches are used at the input and output stages and a crossbar space switch is used at the intermediate stage. These switches are called time-space-time switches. We replace n x k switch in the first stage of a multistage space switch
by an n x k TSI switch , shown in the next figure.
Each input line to the switch corresponds to a slot, so the TSI switch has input framesof size n slots. The output frame from the TSI switch has k slots. In essence, n slotsfrom the incoming frame and reading them out in a frame of size k, according tosome preset permutation table. The first slot corresponds to the first output line out of each of the first stage switches. First line out of the first stage switch is connected to
the first intermediate switch. Hence, first slot in each intermediate frame will bedirected to intermediate switch 1 shown in the next figure.
The second slot in a frame is directed to crossbar switch 2 and all other intermediateswitches are idle. It thus becomes apparent that only one of the crossbar switches isactive during any given timeslot. This makes it possible, to replace the k intermediatecrossbar switches with a single crossbar switch that is time-shared among the k slotsin a frame shown in the next figure.
To replace the k intermediate original crossbar switches, the time-shared crossbar switch must be reconfigured to the interconnection pattern of thecorresponding original switch at every time slot. This approach to sharing a spaceswitch is called time-division multiplexing.
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