Module 1 Networking

MODULE I MCA-402 Computer Networks ADMN 2011-14

Dept. of Computer Science And Applications, SJCET, Palai Page 1

BASIC COMMUNICATION MODEL

Communication is the conveyance of a message from one entity, called the source or transmitter, to another, called the destination or receiver, via a channel of some sort. To give a

very basic example of such a communication system is conversation; people commonly exchange

verbal messages, with the channel consisting of waves of compressed air molecules at frequencies,

which are audible to the human ear. Another example is the exchange of voice signals between two

telephones over the same network

The only way that a message source can be certain that the destination properly received the

message is by some kind of acknowledgment response from the destination. Conversing people might

say "I understand" or nod their head in response to a statement made by their peer. This

acknowledged form of dialogue is the basis of reliable communications - somehow the source must

get feedback that the destination correctly received the message.

(Fig:1.1) Basic Communication Model

The key elements of a communication model are:

Source: This device generates the data to be transmitted; examples are telephones and personal

computers.

Transmitter: Usually, the data generated by a source system are not transmitted directly in the

form in which they were generated. Rather, a transmitter transforms and encodes the information in

such a way as to produce electromagnetic signals that can be transmitted across some sort of

transmission system. For example, a modem takes a digital bit stream from an attached device such as



a personal computer and transforms that bit stream into an analog signal that can be handled by the

telephone network.

Transmission system: This can be a single transmission line connecting the two systems

communicating or a complex network to which numerous communicating systems are connected.

Receiver: The receiver accepts the signal from the transmission system and converts it into a form

that can be handled by the destination device. For example, a modem will accept an analog signal

coming from a network or transmission line and convert it into a digital bit stream.

Destination: Takes the incoming data from the receiver

(Fig:1.2) Simple Data Communication Model

COMMUNICATIONS MODEL TASKS

Some of the Key tasks to be performed by a Communications System are listed below:

Transmission System Utilization

Interfacing

Signal Generation

Synchronization

Exchange Management

Error detection and correction

Addressing and routing



Recovery

Message formatting

Security

TRANSMISSION SYSTEM UTILIZATION: refers to the need to make efficient use of transmission

facilities that are typically shared among a number of communicating devices. Various techniques

(referred to as multiplexing) are used to allocate the total capacity of a transmission medium among a

number of users.

INTERFACE:- A device must interface with the transmission system in order to communicate.

SIGNAL GENERATION: All the data that are transmitted over the transmitting system propagate as

Electromagnetic signals. Hence the communicating device must be able to generate and receive these

signals. Signal generation should be such that the resultant signal is capable of being propagated

through the transmission medium and interpretable as data at the receiver.

SYNCHRONIZATION: Unless the receiver and transmitter are in Synchronization the receiver will

not be able to make sense out of received signals. Receiver should know when the transmission of

data starts, when it ends.

EXCHANGE MANAGEMENT: For meaningful data transaction there should be some kind

management of data being exchanged. Both the transmitter and receiver should adhere to some

common convention about the format of data, amount of data that can be sent at a time and so

on. This requires a prior definition of message formatting.

ERROR DETECTION AND CORRECTION: In any communication system transmitted data is prone

to error. Either it is because of transmitted signal getting distorted in the transmission medium

leading to misinterpretation of signal or errors introduced by the intermediate devices. Error

detection and Correction is required in cases where there is no scope for error in the data

transaction. We can think of file transfer between two computers where there is a need for this. But

in some cases it may not be very important as in the case of telephonic conversation.

ADDRESSING AND ROUTING: When more than two devices share a transmission facility, a source

system must indicate the identity(or address) of the intended destination. The transmission system

must assure that the destination system, and only that system, receives the data. Further, the

transmission system may itself be a network through which various paths may be taken. A specific

route through this network must be chosen.

RECOVERY is a concept distinct from that of error correction. Recovery techniques are needed in

situations in which an information exchange, such as a database transaction or file transfer, is

interrupted due to a fault somewhere in the system. The objective is either to be able to resume

activity at the point of interruption or at least to restore the state of the systems involved to the

condition prior to the beginning of the exchange



MESSAGE FORMATTING has to do with an agreement between two parties as to the form of the

data to be exchanged or transmitted, such as the binary code for characters.

SECURITY: It is important to provide some measure of security in a data communications system.

The sender of data may wish to be assured that only the intended receiver actually receives the data

and the receiver of data may wish to be assured that the received data have not been altered in

transit and that the data actually come from the purported sender

NETWORK MANAGEMENT: Data communications facility is a complex system that cannot create

or run itself. Network management capabilities are needed to configure the system, monitor its

status, react to failures and overloads, and plan intelligently for future growth.

DATA COMMUNICATION MODEL

Data Communication is a process of exchanging data or information between two devices via

some form of transmission medium such as a wire cable. The word data refers to any information

which is presented in a form that is agreed and accepted upon by its creators and users. For data

communication to occur, the communicating devices should be part of a communication system made

up of a combination of hardware and software. The hardware part involves the sender and receiver

devices and the intermediate devices through which the data passes. The software part involves

certain rules which specify what is to be communicated, how it is to be communicated and when. It is

also called as a Protocol.

The effectiveness of any data communications system depends upon the following four fundamental

characteristics:

DELIVERY: The data should be delivered to the correct destination and correct user.

ACCURACY: The communication system should deliver the data accurately, without introducing any

errors. The data may get corrupted during transmission affecting the accuracy of the delivered data.

TIMELINESS: Audio and Video data has to be delivered in a timely manner without any delay; such a

data delivery is called real time transmission of data.

JITTER: It is the variation in the packet arrival time. Uneven Jitter may affect the timeliness of data

being transmitted.

There may be different forms in which data may be represented. Some of the forms of data used in

communications are as follows:

Text: Text includes combination of alphabets in small case as well as upper case. It is stored as a

pattern of bits. Prevalent encoding system : ASCII, Unicode



Numbers: Numbers include combination of digits from 0 to 9. It is stored as a pattern of bits.

Prevalent encoding system : ASCII, Unicode

Images: In computers images are digitally stored. A Pixel is the smallest element of an image. To

put it in simple terms, a picture or image is a matrix of pixel elements. The pixels are represented

in the form of bits. Depending upon the type of image (black n white or color) each pixel would

require different number of bits to represent the value of a pixel. The size of an image depends

upon the number of pixels (also called resolution) and the bit pattern used to indicate the value of

each pixel.

Example: if an image is purely black and white (two color) each pixel can be represented by a

value either 0 or 1, so an image made up of 10 x 10 pixel elements would require only 100 bits in

memory to be stored. On the other hand an image that includes gray may require 2 bits to

represent every pixel value (00 - black, 01 dark gray, 10 light gray, 11 white). So the same 10

x 10 pixel image would now require 200 bits of memory to be stored. Commonly used Image

formats : jpg, png, bmp, etc

Audio: Data can also be in the form of sound which can be recorded and broadcasted. Example:

What we hear on the radio is data or information. Audio data is continuous, not discrete.

Video: Video refers to broadcasting of data in form of picture or movie

DATA COMMUNICATION NETWORK

A communication network, in its simplest form, is a set of equipment and facilities that

provides a communication service: the transfer of information between users located at various

geographical points. Examples of such networks include telephone networks, computer networks,

television broadcast networks, cellular telephone networks, and the Internet. The ability of

communication network to transfer information at extremely high speeds allows users to gather

information in large volumes, nearly instantaneously and, with the aid of computers, to almost

immediately exercise action at a distance. These two unique capabilities form the basis for many

existing services and an unlimited number of future network-based services

In its simplest form, data communication takes place between two devices that are directly

connected by some form of point-to-point transmission medium. Often, however, it is impractical for

two devices to be directly, point-to-point connected. This is so for one (or both) of the following

contingencies:

The devices are very far apart. It would be inordinately expensive, for example, to string a

dedicated link between two devices thousands of miles apart.



There is a set of devices, each of which may require a link to many of the others at various times.

Examples are all of the telephones in the world and all of the terminals and computers owned by

single organization. Except for the case of a very few devices, it is impractical to provide a

dedicated wire between each pair of devices.

The solution to this problem is to attach each device to a communications network like Wide Area

Network (WAN) or Local Area Network(LAN).

COMPUTER NETWORKS

A computer network is simply a collection of computers or other hardware devices that are

connected together, either physically or logically, using special hardware and software, to allow them

to exchange and share information. Networking is the term that describes the processes involved in

designing, implementing, upgrading, managing and otherwise working with networks and network

technologies. Three criteria necessary for an effective and efficient network are:

PERFORMANCE: Performance of the network depends on number of users, type of transmission

medium, the capabilities of the connected h/w and the efficiency of the s/w.

RELIABILITY: Reliability is measured by frequency of failure, the time it takes a link to recover from

the failure and the networks robustness in a catastrophe.

SECURITY: Network security issues include protecting data from unauthorized access, protecting

data from damage and development, and implementing policies and procedures for recovery from

breaches and data losses.



CLASSIFICATION OF NETWORKS

Computers connected to a network are broadly categorized as servers or workstations.

Servers are generally not used by humans directly, but rather run continuously to provide

"services" to the other computers (and their human users) on the network. Services provided can

include printing and faxing, software hosting, file storage and sharing, messaging, data storage and

retrieval, complete access control (security) for the network's resources, and many others.

Workstations are called such because they typically do have a human user which interacts

with the network through them. Workstations were traditionally considered a desktop, consisting

of a computer, keyboard, display, and mouse, or a laptop, with integrated keyboard, display, and

touchpad. Every computer on a network should be appropriately configured for its use. Depending

upon the geographical area covered by a network, it is classified as:

Local Area Network (LAN)

Metropolitan Area Network (MAN)

Wide Area Network (WAN)

LOCAL AREA NETWORK

A Local Area Network (LAN) is a network that is confined to a relatively small area. It is

generally limited to a geographic area such as a writing lab, school, or building. LANs interconnect

computers and peripherals over a common medium in order that users might share access to host

computers, databases, files, applications, and peripherals. LANs in addition to linking the computer

equipment available in a particular premises can also provide a connection to other networks either

through a computer, which is attached to both networks, or through a, dedicated device called a

gateway. The main users of LANs include business organizations, research and development groups in

science and engineering, industry, educational institutions.

The most common use of LANs is for linking personal computers within a building or office to

share information and expensive peripheral devices such as laser printers. Most local area networks

are built with relatively inexpensive hardware such as Ethernet cables, network adapters, and hubs.

The defining characteristics of LANs, in contrast to WANs (wide area networks), include their higher

data transfer rates, smaller geographic range, and lack of a need for leased telecommunication lines

METROPOLITAN AREA NETWORK

The term Metropolitan Area Network (MAN) is typically used to describe a network that spans

a citywide area or a town. It is confined to a larger area than a LAN and can range from 10km to a

few 100km in length. MANs are larger than traditional LANs and predominantly use high-speed

media, such as fiber optic cable, for their backbones. MANs are common in organizations that need

to connect several smaller facilities together for information sharing. This is often the case for

hospitals that need to connect treatment facilities, outpatient facilities, doctor's offices, labs, and

research offices for access to centralized patient and treatment information



WIDE AREA NETWORK

A Wide Area Network (WAN) covers a significantly larger geographic area than LANs

or MANs. WAN can range from 100krn to 1000krn and the speed between cities can vary from l.5

Mbps to 2.4 Gbps. Typically, a WAN consists of two or more local-area networks (LANs) or MANs.

They can connect networks across cities, states or even countries. Computers connected to a wide-

area network are often connected through public networks, such as the telephone system. They can

also be connected through leased lines or satellites.

Typically, a WAN consists of a number of interconnected switching nodes. Transmission from

any one device is routed through these internal nodes to the specified destination device. These

nodes (including the boundary nodes to which the devices are connected) are not concerned with

the content of the data; rather, their purpose is to provide a switching facility that will move the data

from node to node until they reach their destination. Traditionally, WANs have been implemented

using one of two technologies: circuit switching and packet switching. More recently, frame

relay and ATM networks have assumed major roles.

In Circuit Switching a dedicated communications channel is established between sender and

receiver for the duration of a given transmission. This works like a normal telephone line works for

voice communication. Packet switched Networks use a networking technology that breaks up a

message into smaller packets where each packet carries the destination address and a sequence

number. Here no dedicated line is being provided for data transmission. So packets may travel

different routes to the destination and they may reach out of sequence or experience different types

of delays.

Frame Relay was developed at a time when digital long-distance transmission facilities exhibited

a relatively high error rate compared to today's facilities. As a result, there is a considerable amount

of overhead built into packet-switched schemes to compensate for errors. The overhead includes

additional bits added to each packet to introduce redundancy and additional processing at the end

stations and the intermediate switching nodes to detect and recover from errors. But with modern

high-speed telecommunication systems, the rate of errors has been dramatically lowered. Frame relay

was developed to take advantage of these high data rates and low error rates. Frame relay puts data

in a variable-size unit called a frame and leaves any necessary error correction (retransmission of

data) up to the end-points, which speeds up overall data transmission.

Asynchronous Transfer Mode (ATM), sometimes referred to as cell relay, is a culmination

of all of the developments in circuit switching and packet switching over the past 25 years. ATM can

be viewed as an evolution from frame relay. The most obvious difference between frame relay and

ATM is that frame relay uses variable-length packets, called frames, and ATM uses fixed-length

packets, called cells. As with frame relay, ATM provides little overhead for error control, depending

on the inherent reliability of the transmission system and on higher layers of logic in the end systems

to catch and correct errors. By using a fixed-packet length, the processing overhead is reduced even



THE INTERNET

Internet is evolved from the ARPANET, which was developed in 1969 by the Advanced

Research Projects Agency (ARPA) of the U.S. Department of Defense. It was the first operational

packet-switching network whose main aim was to connect stand-alone research computers. It was

the first operational packet-switching network whose main aim was to connect stand-alone research

computers. The Internet is an example of a network that connects many WANs, MANs, and LANs

into the world's largest global network. Internet Service Providers (ISPs) are responsible for

maintaining the integrity of the Internet while providing connectivity between WANs, MANs, and

LANs throughout the world. ISPs provide customers with access to the Internet through the use of

points-of-presence (POP), also called network access points (NAP), in cities throughout the world.

Customers are provisioned access to POPs from their own WANs, MANs, and LANs to Internet

access to their users.

PROTOCOLS

In computer networks, communication occurs between entities in different systems. An entity is

anything capable of sending or receiving information. However, two entities cannot simply send bit

streams to each other and expect to be understood. For communication to occur, the entities must

agree on a protocol. A protocol is a set of rules that govern data communications. A protocol defines

what is communicated, how it is communicated, and when it is communicated. The key elements of a

protocol are syntax, semantics, and timing.

SYNTAX :

The term syntax refers to the structure or format of the data, meaning the order in which they are

presented. For example, a simple protocol might expect the first 8 bits of data to be the address of

the sender, the second 8 bits to be the address of the receiver, and the rest of the stream to be the

message itself.

SEMANTICS:

The word 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 interpretation? For example,

does an address identify the route to be taken or the final destination of the message?

TIMING:

The term timing refers to two characteristics: when data should be sent and how fast they can 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 the receiver and some data will be lost.



NEED FOR PROTOCOL ARCHITECTURE

A computer network must provide general, cost effective, fair, and robust connectivity among a

large number of computers. Networks must also evolve to accommodate changes in both the

underlying technologies upon which they are based as well as changes in the demands placed on them

by application programs.

When computers, terminals, and/or other data processing devices exchange data, the procedures

involved can be quite complex. Consider, for example, the transfer of a file between two computers.

There must be a data path between the two computers either directly or via a communication

network

Typical tasks to be performed are as follow:

The source system must either activate the direct data communication path or inform the

communication network of the identity of the desired destination system.

The source system must ascertain that the destination system is prepared to receive data.

The file transfer application on the source system must ascertain that the file management

program on the destination system is prepared to accept and store the file for this particular

user.

If the file formats used on the two systems are different, one or the other system must

perform a format translation function.

It is clear that there must be a high degree of cooperation between the two computer

systems. Instead of implementing the logic for this as a single module, the task is broken up into

subtasks, each of which is implemented separately. In protocol architecture, the modules are

arranged in a vertical stack. Each layer in the stack performs a related subset of the functions

required to communicate with another system. It relies on the next lower layer to perform more

primitive functions and to conceal the details of those functions. Ideally, layers should be defined so

that changes in one layer do not require changes in other layers. A logical communication may exist

between any two computers through the layers of the same level. Layer-n on one computer may

converse with layer-n on another computer. There are rules and conventions used in the

communication at any given layers, which are known collectively as the layer-n protocol for the nth

layer.

The architecture is considered scalable if it is able to accommodate a number of layers in

either large or small scales. For example, a computer that runs an Internet application may require all

of the layers that were defined for the architectural model. The depth and functionality of this stack

differs from network to network. However, regardless of the differences among all networks, the



purpose of each layer is to provide certain services (job responsibilities) to the layer above it,

shielding the upper layers from the intricate details of how the services offered are implemented.

Data are not directly transferred from layer-n on one computer to layer-n on another

computer. Rather, each layer passes data and control information to the layer directly below until the

lowest layer is reached. Below layer-1 (the bottom layer), is the physical medium (the hardware)

through which the actual transaction takes place. Logical communication is shown by a broken-line

arrow and physical communication by a solid-line arrow.

Between every pair of adjacent layers is an interface. The interface is a specification that

determines how the data should be passed between the layers. ]t defines what primitive operations

and services the lower layer should offer to the upper layer. One of the most important

considerations when designing a network is to design clean-cut interfaces between the layers. To

create such an interface between the layers would require each layer to perform a specific collection

of well understood functions. A clean-cut interface makes it easier to replace the implementation of

one layer with another implementation because all that is required of the new implementation is that,

it offers, exactly the same set of services to its neighboring layer above as the old implementation did.

A protocol architecture is the layered structure of hardware and software that supports the

exchange of data between systems. At each layer of a protocol architecture, one or more common

protocols are implemented in communicating systems. Each protocol provides a set of rules for the

exchange of data between systems. It acts as a blue print that guides the design and implementation

of networks and there by enables to divide the workload and to simplify the systems design. The



specification of architecture must contain enough information to allow an implementer to write the

program or build the hardware for each layer so that it will correctly obey the appropriate protocol.

Neither the details of the implementation nor the specification of the interfaces is part of the

architecture because these are hidden away inside the machines and not visible from the outside.

THE OSI REFERENCE MODEL

This model is based on a proposal developed by the International Standards Organization

(ISO) as a first step toward international standardization of the protocols used in the various layers.

It was revised in 1995. The Open Systems Interconnection (OS1) reference model describes how

information from a software application in one computer moves through a network medium to a

software application in another computer. The OSI reference model is a conceptual model composed

of seven layers each specifying particular network functions and into these layers are fitted the

protocol standards developed by the ISO and other standards bodies. The principles that were

applied to arrive at the 7 layers can be summarized as follows:

A layer should be created only when an additional level of abstraction is required.

Each layer should perform a well-defined function.

The function of each layer should be chosen with the goal of defining internationally

standardized protocols.

The number of layers should be large enough to enable distinct functions to be separated, but

few enough to keep the architecture from becoming unwieldy.

The OSI model divides the tasks involved with moving information between networked

computers into seven smaller, more manageable task groups. A task or group of tasks is then

assigned to each of the seven OSI layers. Each layer is reasonably self-contained so that the tasks

assigned to each layer can be implemented independently. This enables the solutions offered by one

layer to be updated without affecting the other layers. The seven layers of OSI model are:

Application

Presentation

Session

Transport

Network

Data link

Physical



Although, each layer of the OSI model provides its own set of functions, it is possible to

group the layers into two distinct categories. The first four layers i.e., physical, data link, network, and

transport layer provide the end-to-end services necessary for the transfer of data between two

systems. These layers specify the protocols associated with the communications network used to link

two computers together. Together, these are communication oriented. The top three layers i.e., the

application, presentation, and session layers provide the application services required for the

exchange of information. That is, they allow two applications, each running on a different node of the

network to interact with each other through the services provided by their respective operating

systems. Together, these are data processing oriented.

A message begins at the top application layer and moves down the OSI layers to the

bottom physical layer. As the message descends, each successive OSI model layer adds a header to it.

A header is layer-specific information that basically explains what functions the layer carried out.

When formatted data passes through physical layer it is transformed into appropriate signals and

transmitted. Upon reaching destination signal is transformed back into digital format. Data then

moves up back through the layers and at each layer the headers and trailers are stripped off and the

actions appropriate to that layer are taken. When data reaches top layer it is in a form appropriate to

application and is made available to the recipient. On every sending device, each layer calls upon the

service offered by the layer below it. On every receiving device, each layer calls upon the service

offered by the layer above it.



PHYSICAL LAYER

The data units on this layer are called bits. This layer defines the mechanical and electrical

definition of the network medium (cable) and network hardware. The physical layer is responsible for

passing bits onto and receiving them from the connecting medium. This layer gives the data-link layer

(layer 2) its ability to transport a stream of serial data bits between two communicating systems; it

conveys the bit that moves along the cable. It is responsible for ensuring that the raw bits get from

one place to another, no matter what shape they are in, and deals with the mechanical and electrical

characteristics of the cable.

The main network device found the Physical layer is a repeater. The purpose of a

repeater (as the name suggests) is simply to receive the digital signal, reform it, and retransmit the

signal. This has the effect of increasing the maximum length of a network, which would not be

possible due to signal deterioration if, a repeater were not available. Each layer, with the exception of

the physical layer, adds information to the data as it travels from the Application layer down to the

physical layer. This extra information is called a header. The physical layer does not append a header

to information because it is concerned with sending and receiving information on the individual bit

level.

The physical layer is also concerned with the following:

REPRESENTATION OF BITS: The physical layer is concerned with transmission of signals from one

device to another which involves converting data (1s & 0s) into signals and vice versa. It is not

concerned with the meaning or interpretation of bits.

DATA RATE: The physical layer defines the data transmission rate i.e. number of bits sent per

second. It is the responsibility of the physical layer to maintain the defined data rate.

SYNCHRONIZATION OF BITS: To interpret correct and accurate data the sender and receiver

have to maintain the same bit rate and also have synchronized clocks.

PHYSICAL TOPOLOGY: The physical layer defines the type of topology in which the device is

connected to the network. In a mesh topology it uses a multipoint connection and other topologies it

uses a point to point connection to send data.

TRANSMISSION MODE: The physical layer defines the direction of data transfer between the sender

and receiver. Two devices can transfer the data in simplex, half duplex or full duplex mode

On the sender side, the physical layer receives the data from Data Link Layer and encodes it into

signals to be transmitted onto the medium. On the receiver side, the physical layer receives the

signals from the transmission medium decodes it back into data and sends it to the Data Link Layer



DATA LINK LAYER

The data link layer is concerned with the reliable transfer of data over the communication

channel provided by the physical layer. To do this, the data link layer breaks the data into data

frames, transmits the frames sequentially over the channel, and checks for transmission errors by

requiring the receiving end to send back acknowledgment frames. Responsibilities of the data link

layer include the following:

FRAMING: On the sender side, the Data Link layer receives the data from Network Layer and

divides the stream of bits into fixed size manageable units called as Frames and sends it to the physical

layer. On the receiver side, the data link layer receives the stream of bits from the physical layer and

regroups them into frames and sends them to the Network layer.

PHYSICAL ADDRESSING: The Data link layer appends the physical address in the header of the

frame before sending it to physical layer. The physical address contains the address of the sender and

receiver. In case the receiver happens to be on the same physical network as the sender; the receiver

is at only one hop from the sender and the receiver address contains the receivers physical address.

In case the receiver is not directly connected to the sender, the physical address is the address of the

next node where the data is supposed to be delivered.

FLOW CONTROL: The data link layer makes sure that the sender sends the data at a speed at

which the receiver can receive it else if there is an overflow at the receiver side the data will be lost.

The data link layer imposes flow control mechanism over the sender and receiver to avoid

overwhelming of the receiver.

ERROR CONTROL: The data link layer imposes error control mechanism to identify lost or

damaged frames, duplicate frames and then retransmit them. This is achieved by specifying error

control information is present in the trailer of a frame.



NETWORK LAYER

The network layer is concerned with the routing of data across the network from one end to

another. To do this, the network layer converts the data into packets and ensures that the packets

are delivered to their final destination, where they can be converted back into the original data. In

order to route the data through multiple networks, network layer relies on two things: Logical

Addressing & Routing

LOGICAL ADDRESSING: The network layer uses logical address commonly known as IP address to

recognize devices on the network. The header appended by the network layer contains the actual

sender and receiver IP address. The network layer of intermediate nodes checks for a match of IP

address in the header. If no match is found the packet passes to the data link layer and it is forwarded

to next node

ROUTING: The network layer divides data into units called packets of equal size and bears a

sequence number for rearranging on the receiving end. Each packet is independent of the other and

may travel using different routes to reach the receiver hence may arrive out of turn at the receiver.

Hence every intermediate node which encounters a packet tries to compute the best possible path

for the packet. The best possible path may depend on several factors such as congestion, number of

hops, etc. This process of finding the best path is called as Routing. It is done using routing

algorithms.

When a packet has to travel from one network to another to get to its destination, many

problems can arise. The addressing used by the second network may be different from the first one.

The second one may not accept the packet at all because it is too large. The protocols may differ,



and so on. It is up to the network layer to overcome all these problems to allow heterogeneous

networks to be interconnected.

TRANSPORT LAYER

The aim of the transport layer is to isolate the upper three layers from the network, so

that any changes to the network equipment technology will be confined to the lower three layers. It

provides a network independent, reliable message interchange service to the top three application-

oriented layers. This layer acts as an interface between the bottom and top three layers. The lower

data link layer (layer 2) is only responsible for delivering packets from one node to another, where as

the transport layer is responsible for overall end-to-end validity and integrity of the transmission i.e.,

it ensures that data is successfully sent and received between two computers. A logical address at

network layer facilitates the transmission of data from source to destination device. But the source

and the destination both may be having multiple processes communicating with each other. To

ensure process to process delivery the transport layer makes use of port address (also known as

Service Point Address) to identify the data from the sending and receiving process.

At the sending side, the transport layer receives data from the session layer, divides it into units

called segments with a sequence number. These numbers enable the transport layer to reassemble

the message correctly upon arriving at the destination. At the receiving side, the transport layer

receives packets from the network layer, converts and arranges into proper sequence of segments

and sends it to the session layer.



The transport layer also carries out flow control and error control functions; but unlike data link

layer these are end to end rather than node to node. The data can be transported in a connection

oriented or connectionless manner. In connection oriented transmission, the receiving devices sends

an acknowledgement back to the source after a packet or group of packet is received. It is slower

transmission method. In Connectionless Transmission the receiving devices does not sends an

acknowledgement back to the source. It is faster transmission method.

SESSION LAYER

Session layer has the responsibility of beginning, maintaining and ending the communication

between two devices, called session. It establishes a session between the communicating devices

called dialog and synchronizes their interaction. The session layer at the sending side accepts data

from the presentation layer adds checkpoints to it called syn bits to allow for fast recovery in the

event of a connection failure. The checkpoints or synchronization points is a way of informing the

status of the data transfer. At the receiving end the session layer receives data from the transport

layer removes the checkpoints inserted previously and passes the data to the presentation layer.

PRESENTATION LAYER

Unlike lower layers, which are mostly concerned with moving bits around, the presentation layer

is concerned with the syntax and semantics of the information transmitted. It is also called syntax

layer The main services provided by presentation layer are: Translation, Compression and

Encryption.

TRANSLATION: The sending and receiving devices may run on different platforms (hardware,

software and operating system). Hence it is important that they understand the messages that are

used for communicating. Presentation layer provides a translation service which converts the

message into a common format supported by both sender and receiver.



COMPRESSION: Data compression reduces the number of bits contained in the information and

there by ensures faster data transfer. The data compressed at sender has to be decompressed at the

receiving end, both performed by the Presentation layer.

ENCRYPTION: It is the process of transforming the original message to change its meaning before

sending it. The reverse process called decryption has to be performed at the receiving end to

recover the original message from the encrypted message. The encryption and decryption services

which ensures privacy of sensitive data

The presentation layer at sending side receives the data from the application layer adds

header which contains information related to encryption and compression and sends it to the session

layer. At the receiving side, the presentation layer receives data from the session layer decompresses

and decrypts the data as required and translates it back as per the encoding scheme used at the

receiver.

APPLICATION LAYER

The application layer is concerned with the semantics of data, i.e., what the data means to

applications. It provides an interface for the end user operating a device connected to a network.

This layer is what the user sees, in terms of loading an application (such as Web browser or e-mail);

that is, this application layer is the data the user views while using these applications. The application

layer provides standards for supporting a variety of application-independent services. In other words

application layer provides a variety of protocols that are commonly needed by users. One widely

used application protocol is HTTP (Hyper Text Transfer Protocol), which is the basis for the World

Wide Web. When a browser wants a Web page, it sends the name of the page it wants to the server

using HTTP. The server then sends the page back. Some of the functionalities provided by

application layer are:

File access and transfer: It allows a use to access, download or upload files from/to a

remote host.

Mail services: It allows the users to use the mail services.

Remote login: It allows logging into a host which is remote

World Wide Web (WWW): Accessing the Web pages is also a part of this layer



TCP/IP REFERENCE MODEL

TCP/IP originated out of the investigative research into networking protocols that the US

Department of Defense (DoD) initiated in 1969. In 1968, the DoD Advanced Research Projects

Agency (ARPA) began researching the network technology that is called packet switching.

The original focus of this research was to develop a network that is able to survive loss of subnet

hardware, with existing conversations not being broken off. In other words, DoD wanted

connections to remain intact as long as the source and destination nodes were functioning, even if

some of the machines or transmission lines in between were suddenly put out of operation. The

network that was initially constructed as a result of this research was meant to provide a

communication that could function in wartime, then called ARPANET, gradually became known as

the Internet. The TCP/IP protocols played an important role in the development of the Internet. In

the early 1980s, the TCP/IP protocols were developed. In 1983, they became standard protocols for

ARPANET. The protocols within the TCP/IP Suite have been tested, modified, and improved over

time. Because of the history of the TCP/IP protocol suite, it's often referred to as the DoD

protocol suite or the Internet protocol suite.

TCP/IP Reference Model is named from two of the most important protocols in it

The Transmission Control Protocol (TCP) and the Internet Protocol (IP).TCP handles reliable

delivery for messages of arbitrary size, and defines a robust delivery mechanism for all kinds of data

across a network and IP manages the routing of network transmissions from sender to receiver,

along with issues related to network and computer addresses.Some of TCP/IP Ref Model goals are:

To support multiple, packet-switched pathways through the network so that transmissions

can survive all conceivable failures

To permit dissimilar computer systems to easily exchange data

To offer robust, reliable delivery services for both short- and long-distance communications

The TCP/IP model follows a layered architecture very similar to the OSI reference model. Based

on the protocol standards that have been developed, we can organize the communication task for

TCP/IP into four relatively independent layers:

Application layer

Transport layer

Internet layer

Network access layer

NETWORK ACCESS LAYER

This is the lowest layer of the TCP/IP Reference Model, responsible for placing TCP/IP

packets on the network medium and receiving TCP/IP packets of the network medium. TCP/IP was

designed to be independent of the network access method, frame format, and medium. In this way,



TCP/IP can be used to connect differing network types. The Network Interface Layer encompasses

the Data Link and Physical layers of the OSI Model. Within the TCP/IP protocol suit the network

access layer is commonly viewed as a single layer with two sub layers: The Media Access Control (MAC)

Sub layer and The Physical Sub layer. The MAC sub layer prepares data for transmission and obtains

access to the transmission medium in shared access systems. The Physical sub layer encodes data and

transmits it over the physical network media. It operates with data in the form of bits transmitted

over a variety of electrical and optical cables, as well as radio frequencies. The responsibilities of this

layer include:

Formatting the data into a unit called a frame and converting that frame into the stream of

electric or analog pulses that passes across the transmission medium.

Checking for errors in incoming frames.

Adding error-checking information to outgoing frames so that the receiving computer can check

the frame for errors.

Acknowledging receipt of data frames and resending frames if acknowledgment is not received.

Network Access Layer protocols must know the details of the underlying network (its packet

structure, addressing, etc.) to correctly format the data being transmitted to comply with the

network constraints. The core protocols are:

PPP(Point to Point Protocol): commonly used to establish a direct physical connection between

two nodes and facilitates the transmission of data packets. PPP is used over many types of physical

networks including serial cable, phone line, specialized radio links, and fiber optic links

SLIP(Serial Line Interface Protocol):Older, simpler serial line protocol that only supports TCP/IP-

based communications. Its main functionality is framing of data for transmission

INTERNET LAYER

The internet layer provides services that are roughly equivalent to the OSI Network layer. The

primary concern of the protocol at this layer is to manage the connections across networks as

information is passed from source to destination. It is at this layer logical addressing, packetization of

data and routing are handled. The various functions provided by Internet layer are:

Translation between logical addresses and physical addresses

Routing from the source to the destination computer

Managing traffic problems, such as switching, routing, and controlling the congestion of data packets



Maintaining the quality of service requested by the transport layer

The primary protocols that function at the TCP/IP Internet layer are:

Internet Protocol(IP): connectionless protocol that is primarily responsible for addressing and

routing packets between network devices. It is unreliable because packet delivery is not guaranteed

and also the sender or receiver is not informed when a packet is lost or out of sequence. IP is also

responsible for fragmenting and reassembling packets

Address Resolution Protocol (ARP): Network devices must know each others hardware address in

order to communicate on a network. Address resolution is the process of mapping a hosts IP

address to its hardware address. The Address Resolution Protocol (ARP) is responsible for obtaining

hardware addresses of TCP/IP devices on networks. The source will broadcast an ARP request

containing destination IP address to find the intended destinations MAC address. Only the

destination device will respond to the ARP request

Internet Control Message Protocol(ICMP): provides a set of error and control messages to help

track and resolve network problems. ICMP is used to send a destination unreachable message

when there is an error somewhere in the network that is preventing the frame or packet from being

forwarded to the destination device. It includes a type of message, called an Echo Request, which can

be sent from one host to another to see if it is reachable on the network. If it is reachable, the

destination host will reply with the ICMP Echo Reply message.

TRANSPORT LAYER

It is designed to allow peer entities on the source and destination hosts to carry on a

conversation, just as in the OSI transport layer. From Application to Transport Layer, the application

delivers its data to the communications system by passing a Stream of data bytes to the transport

layer along with the socket of the destination machine. Its functions include:

Sequencing and Transmission of packets

Acknowledgment of receipts

Error control

Flow control

The transport layer is implemented by mainly two protocols: Transmission Control Protocol(TCP )

and the User Datagram Protocol (UDP).

TCP: TCP provides a one-to-one, connection-oriented, reliable communications service. TCP is

responsible for the establishment of a TCP connection, the sequencing and acknowledgment of

packets sent, and the recovery of packets lost during transmission. TCP is Slower compared to UDP



because of additional error checking being performed. It also adds features such as flow control,

sequencing, error detection and correction

UDP: UDP provides a one-to-one or one-to-many, connectionless, unreliable communications

service. UDP is used when the amount of data to be transferred is small (such as the data that would

fit into a single packet), when the overhead of establishing a TCP connection is not desired, or when

the applications or upper layer protocols provide reliable delivery.It is commonly used in Video and

Audio Casting.

APPLICATION LAYER

The top layer of the protocol stack is the application layer. The Application Layer is equivalent to

the top three layers, (Application, Presentation and Session Layers), in the OSI model. It refers to the

programs that initiate communication in the first place. TCP/IP includes several application layer

protocols for mail, file transfer, remote access, authentication and name resolution. These protocols

are embodied in programs that operate at the top layer just as any custom made or packaged

client/server application would.

The most widely known Application Layer protocols are those used for the exchange of user

information, some of them are:

The HyperText Transfer Protocol (HTTP) is used to transfer files that make up the Web

pages of the World Wide Web.

The File Transfer Protocol (FTP) is used for interactive file transfer.

The Simple Mail Transfer Protocol (SMTP) is used for the transfer of mail messages and

attachments.

Telnet, is a terminal emulation protocol, and, is used for remote login to network hosts.

The process by which a TCP/IP host sends data can be viewed as a five-step process:

Step 1 Create and encapsulate the application data with any required application layer headers.

Step 2 Encapsulate the data supplied by the application layer inside a transport layer header. For end-

user applications, a TCP or UDP header is typically used.

Step 3 Encapsulate the data supplied by the transport layer inside an Internet layer (IP) header. IP

defines the IP addresses that uniquely identify each computer.

Step 4 Encapsulate the data supplied by the Internet layer inside a data link layer header and trailer.

This is the only layer that uses both a header and a trailer. The physical layer encodes a signal onto

the medium to transmit the frame.



COMPARISON OF THE OSI AND TCP/IP REF. MODELS

The OSI and TCP/IP reference models have much in common. Both are based on the

concept of a stack of independent protocols. Also, the functionality of the layers is roughly similar.

For example, in both models the layers up through and including the transport layer are there to

provide an end-to-end, network-independent transport service to processes wishing to

communicate. These layers form the transport provider. Again in both models, the layers above

transport are application-oriented users of the transport service.

Despite these fundamental similarities, the two models also have many differences. Three

concepts are central to the OSI model: Services, Interfaces and Protocols. Probably the biggest

contribution of the OSI model is to make the distinction between these three concepts explicit. Each

layer performs some services for the layer above it. The service definition tells what the layer does,

not how entities above it access it or how the layer works. It defines the layer's semantics. A layer's

interface tells the processes above it how to access it. It specifies what the parameters are and what

results to expect. The peer protocols used in a layer are the layer's own business. It can use any

protocols it wants to, as long as it gets the job done (i.e., provides the offered services). It can also

change them at will without affecting software in. higher layers. The TCP/IP model did not originally

clearly distinguish between service, interface, and protocol, For example, the only real services

offered by the internet layer are As a consequence, the protocols in the OSI model are better hidden

than in the TCP/IP model and can be replaced relatively easily as the technology changes.

The OSI reference model was devised before the corresponding protocols were

invented. This ordering means that the model was not biased toward one particular set of protocols,

a fact that made it quite general. The downside of this ordering is that the designers did not have

much experience with the subject and did not have a good idea of which functionality to put in which

layer. With TCP/IP the reverse was true: the protocols came first, and the model was really just a

description of the existing protocols. There was no problem with the protocols fitting the model.

They fit perfectly

Turning from philosophical matters to more specific ones, an obvious difference between

the two models is the number of layers: the OSI model has seven layers and the TCP/IP has four

layers. Both have (inter) network, transport, and application layers, but the other layers are different.

Another difference is in the area of connectionless versus connection oriented

communication. The OSI model supports both connectionless and connection oriented

communication in the network layer, but only connection-oriented communication in the transport

layer, where it counts (because-the transport service is visible to the users). The TCP/IP model has

only one mode in the network layer (connection less) but supports both modes in the transport

layer, giving the users a choice



A CRITIQUE OF THE OSI MODEL AND PROTOCOLS

Neither the OSI model and its protocols nor the TCP/IP model and its protocols are perfect.

OSI model and its protocols did not take over the world and push everything else out of their way

because of:

Bad timing

Bad technology

Bad implementations

Bad policies

BAD TIMING

The time at which a standard is established is absolutely critical to its success. David Clark from

the MIT has developed the following theory regarding publishing a standard at the right time.

As shown in the figure, in the life cycle of a standard, there are 2 principal peaks of

activity: the research carried out in the field covered by the standard, and the industrial investments

for the implementation and deployment of the standard. These two peaks are separated by a off-peak

of activity that actually appears to be the ideal moment for the publication of the standard

When the subject is first discovered, there is a burst of research activity in the form of

discussions, papers, and meetings. After a while this activity subsides, corporations discover the

subject, and the billion-dollar wave of investment hits. It is essential that the standards be written in

the trough in between the two "peaks. If the standards are written too early, before the research is

finished, the subject may still be poorly understood; the result is bad standards. If they are written

too late, so many companies may have already made major investments in different ways of doing

things that the standards are effectively ignored. If the interval between the two curves is very short

(because everyone is in a hurry to get started), the people developing the standards may get crushed



It now appears that the standard OSI protocols got crushed. The competing TCP/IP

protocols were already in widespread use by research universities by the time the OSI protocols

appeared. While the billion-dollar wave of investment had not yet hit, the academic market was large

enough that many vendors had begun cautiously offering TCP/IP products. When OSI came around,

they did not want to support a second protocol stack until they were forced to, so there were no

initial offerings. With every company waiting for every other company to go first, no company went

first and OSI never happened.

BAD TECHNOLOGY

The second reason that OSI never caught on is that both the model and the protocols

are flawed. The choice of seven layers was more political than technical, and two of the layers

(session and presentation) are nearly empty, whereas two other ones (data link and network) are

overfull. The OSI model, along with the associated service definitions and protocols, is extraordinarily

complex. They are also difficult to implement and inefficient in operation. In addition to being

incomprehensible, another problem with OSI is that some functions, such .as addressing, flow

control, and error control, reappear again and again in each layer.

BAD IMPLEMENTATIONS

Given the enormous complexity of the model and the protocols, it will come as no

surprise that the initial implementations were huge, unwieldy, and slow. It did not take long for

people to associate "OSI" with "poor quality." Although the products improved in the course of time,

the image stuck. In contrast, one of the first implementations of TCP/IP was quite good (not to

mention, free). People began using it quickly, which led to a large user community, which led to

improvements, which led to an even larger community.

BAD POLICIES

On account of the initial implementation, many people, especially in academia, thought of

TCP/IP as part of UNIX.OSI, on the other hand, was widely thought to be the creature of the

European telecommunication ministries, the European Community, and later the U.S. Government.

This belief was only partly true, but the very idea of a bunch of government bureaucrats trying to

shove a technically inferior standard down the throats of the poor researchers and programmers

down in the trenches actually developing computer networks did not help much.

CRITIQUE OF THE TCP/IP REFERENCE MODEL

The TCPI/IP model and protocols have their problems too. First, the model does not

clearly distinguish the concepts of service, interface, and protocol. Good software engineering

practice requires differentiating between the specification and the implementation, something that

OSI does very carefully, and TCPI/IP does not. Consequently, the TCPI/IP model is not much of a

guide for designing new networks using new technologies.



Second, the TCPI/IP model is not at all general and is poorly suited to describing any

protocol stack other than TCPI/IP. Third, the TCP/IP model does not distinguish (or even mention)

the physical and data link layers. These are completely different. The physical layer has to do with the

transmission characteristics of copper wire, fiber optics, and wireless communication. The data link

layer's job is to delimit the start and end of frames and get them from one side to the other with the

desired degree of reliability. A proper model should include both as separate layers. The TCP/IP

model does not do this.

Finally, although the IP and TCP protocols were carefully thought out and well

implemented, many of the other protocols were ad hoc, generally produced by a couple of graduate

students hacking away until they got tired. The protocol implementations were then distributed free,

which resulted in their becoming widely used, deeply entrenched, and thus hard to replace.

NOVEL NETWARE

Novell NetWare is the most popular network system in the PC world. It provides transparent

remote file access and numerous other distributed network services, including printer sharing and

support for various applications such as electronic mail transfer. NetWare was developed by Novell,

Inc., and introduced in the early 1980s.It was derived from Xerox Network Systems (XNS), which

was created by Xerox Corporation in the late 1970s.NetWare runs on virtually any kind of

computer system, from PCs to mainframes

Novell Networks are based on the client/server model in which at least one computer functions

as a network file server, which runs all of the NetWare protocols and maintains the networks shared

data on one or more disk drives. File servers generally allow users on other PCs to access

application software or data files i.e., it provides services to other network computers called clients.

NOVEL NETWARE PROTOCOL SUITE

Novell provides a suite of protocols developed specifically for NetWare. The five main protocols

used by NetWare are:

Media Access Protocol.

Internetwork Packet Exchange/Sequenced Packet Exchange (IPX/SPX).

Routing Information Protocol (RIP).

Service Advertising Protocol (SAP).

NetWare Core Protocol (NCP).

These protocols wh It defines the connection control and service request encoding that

make it possible for clients and servers to interact.



This is the protocol that provides transport and session services.

NetWare security is also provided within this protocol. ich are associated with Novel Network

follows an enveloping pattern. More specifically, the upper-lever protocols (NCP, SAP, and RIP) are

enveloped by IPX/SPX.A Media Access Protocol header and trailer then envelop IPX/SPX. . The

following figure shows a Comparison between NetWare and OSI reference models

Media Access Protocols: The NetWare suite of protocols supports several media-access protocols,

including Ethernet/IEEE 802.3, Token Ring/IEEE 802.5, Fiber Distributed Data Interface (FDDI), and

Point-to-Point Protocol (PPP)

IPX(Internetwork Packet Exchange protocol):Routing and networking protocol at Network layer.

When a device to be communicated with is located on a different network, IPX routes the

information to the destination through any intermediate networks. It datagram-based, connectionless,

unreliable protocol that is equivalent to the IP

SPX(Sequenced Packet Exchange protocol): Control protocol at the transport layer (layer 3) for

reliable, connection-oriented datagram transmission. SPX is similar to TCP in the TCP/IP suite.

Routing Information Protocol (RIP): Facilitate the exchange of routing information on a NetWare

network. In RIP, an extra field of data was added to the packet to improve the decision criteria for

selecting the fastest route to a destination

Service Advertisement Protocol (SAP): It is an IPX protocol through which network resources,

such as file servers and print servers, advertise their addresses and the services they provide.

Advertisements are sent via SAP every 60 seconds. This SAP packet contains information regarding

the servers which provide services. Using these SAP packets, clients on the network are able to

obtain the internetwork address of any servers they can access

NetWare Core Protocol (NCP): It defines the connection control and service request encoding that

make it possible for clients and servers to interact. This is the protocol that provides transport and

session services. NetWare security is also provided within this protocol.

DATA LINK LAYER

In data communication, physical layer deals with transmission of signals over different

transmission medium. While sending data, the signals may get impaired due to the noise encountered

during transmission. The data flow rate between the source and destination also should be kept

under control. Therefore in order to achieve an efficient and reliable communication a data flow

control mechanism needs to be implemented. Data link layer deals with frame formation, flow

control, error control and addressing and ensures error free transfer of bits from one device to

another. For the effective data communication data link layer needs to perform a number of specified

functions.



Services provided to network layer: The main functionality of this layer is to transfer data from the

network layer on source machine to the network layer on destination machine

Flow Control: The source machine should sent data at a rate faster than the destination machine can

accept them

Framing: The bits to be transmitted is broken down into discrete frames. A frame contains user data

and control fields.

Error Control: All the frames should be delivered from source to the destination. The errors made

in bits during transmission must be detected and corrected

Addressing: On a multipoint line, such as many machines connected together, identity of individual

machines must be specified while transmitting data frames

FRAME

Frame is a data structure used in transmissions at DLL. The data link layer takes the packets it

gets from the network layer and encapsulates them into frames for transmission. Each frame contains

a frame header with fields for addressing and is located at the beginning of the frame, a payload field

for holding the packet and a frame trailer. The trailer contains fields are used for error detection and

mark the end of the frame.

FRAME SYNCHRONIZATION

Frame synchronization or simply framing is the process of defining and locating frame boundaries

(start and end of the frame) on a bit sequence. Converting the bit stream into frames is a tedious

process. The frame format is designed in a way that enables the receiver to always locate the

beginning of a frame and its various fields and should be able to separate the data field. To identify a

frame and its various fields, field identifiers are incorporated. These are unique symbols that indicate

by their presence the beginning and end of a frame. Four methods can be used to mark the start and

end of each frame:



Character count

Flag bytes with byte stuffing

Starting and ending flags, with bit stuffing

Physical layer coding violations

CHARACTER COUNT

Character count, uses a header field to specify the number of characters in the frame. The

Data Link Layer at the destination checks the header field to know the size of the frame and hence,

the end of frame. The process is shown in the following figure for a four frame of size 5, 5,8 and 8

respectively.

However, problems may arise due to changes in character count value during transmission. For

example, in the second frame if the character count 5 changes to7, the destination will receive data

out of synchronization and hence, it will not be able to identify the start of the next frame.

FLAG BYTES WITH BYTE STUFFING

Byte Stuffing also known as Character Stuffing is one of the earliest schemes adopted for

delimiting packets containing character data. This method employees three special control characters

in ASCII for the purpose of framing: DLE -Data Link Escape, STX - Start of Text and ETX -End of

Text. The pattern DLE STX denotes the beginning of each frame and DLE ETX specifies the end of

each frame.



However, there is a still a problem we have to solve. It may happen that the flag byte occurs in the

data. For example, if a DLE occurs in the middle of the data and interferes with the data during

framing then, sender stuffs an extra DLE into the data stream just before each occurrence of an

accidental DLE in the data stream. The data link layer on the receiving end discards the first DLE

and the second DLE is regarded as data.

BIT STUFFING

Bit Stuffing is similar to the Byte Stuffing, except that, the method of bit stuffing allows

insertion of bits instead of the entire character (8 bits). Here frames can contain an arbitrary number

of bits made up of units of any size. Each frame begins and ends with a special bit pattern, 01111110

or 0x7E in hexadecimal. Whenever the sender's data link layer encounters five consecutive 1s in the

data, it automatically stuffs a 0 bit into the outgoing bit stream. This bit stuffing is analogous to byte

stuffing.

When the receiver sees five consecutive incoming 1 bits, followed by a 0 bit, it

automatically removes the 0 bit. Just as byte stuffing is completely transparent to the network layer in

both computers, so is bit stuffing. With bit stuffing, the boundary between two frames can be

unambiguously recognized by the flag pattern. Thus, if the receiver loses track of where it is, all it has

to do is scan the input for flag sequences, since they can only occur at frame boundaries and never

within the data.



With both bit and byte stuffing, a side effect is that the length of a frame now depends on the

contents of the data it carries. For instance, if there are no flag bytes in the data, 100 bytes might be

carried in a frame of roughly 100 bytes. If, however, the data consists solely of flag bytes, each flag

byte will be escaped and the frame will become roughly 200 bytes long. With bit stuffing, the increase

would be roughly 12.5% as 1 bit is added to every byte.

FRAMING BY ILLEGAL CODE (CODE VIOLATION)

A fourth method is based on any redundancy in the coding scheme. In this method, we simply

identify an illegal bit pattern, and use it as a beginning or end marker. For example, in Manchester

Encoding 1- can be coded into two parts i.e., high to low = 1 0 and can be coded into two parts i.e.,

low to high = 0 1.Codes of all low (000) or all high (111) arent used for the data and therefore, can

be used for framing.

Many data link protocols use a combination of a character count with one of the other

methods for extra safety. When a frame arrives, the count field is used to locate the end of the

frame. Only if the appropriate delimiter is present at that position and the checksum is correct is the

frame accepted as valid. Otherwise, the input stream is scanned for the next delimiter.

FLOW CONTROL

Another important issue for the data link layer is dealing with the situation which occurs

when the sender transmits frames faster than the receiver can accept or process them. This situation

can easily occur when the sender is running on a fast computer and the receiver is running on a slow

machine. The sender keeps pumping the frames out at a high rate until the receiver is completely

swamped. Even if the transmission is error free, at a certain point the receiver will simply be unable

to handle the frames as they arrive and will start to lose some. To prevent this situation during

transmission, an approach is introduced called the Flow Control.

Flow Control is a set of procedures that tells the sender how much data it can transmit

before it must wait for an acknowledgment from the receiver. The flow of data should not be

allowed to overwhelm the receiver. Receiver should also be able to inform the transmitter before its

limits (this limit may be amount of memory used to store the incoming data or the processing power

at the receiver end) are reached and the sender must send fewer frames. Hence, Flow control refers

to the set of procedures used to restrict the amount of data the transmitter can send before waiting

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