DATA COMMUNICATION

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Definition• Data-communication is the combination of data-processing and communication. It includes the processing of data of program's running on computer-systems, and the communication over great distance where the information is transported by using of electrical-conductivity, radio-waves, light-signals, etc. With data-communication it is possible to communicate over great distances from terminals connected on the communication network.

Three Components of Data Communication

Ddata Aanalog: Continuous value data (sound, light,

temperature) Ddigital: Discrete value (text, integers, symbols)

signal Aanalog: Continuously varying electromagnetic wave Ddigital: Series of voltage pulses (square wave) Transmission Analog: Works the same for analog or digital signals Digital: Used only with digital signals

1. Data

• Voice

• Images

• Digital data

• Analog data

• Text

• Digitized voice or images

ElectroMagnetic Signals

Function of timeAnalog (varies smoothly over time)Digital (constant level over time, followed by a change to another level)

Function of frequency (more important)Spectrum (range of frequencies)Bandwidth (width of the spectrum)

BandWidth

Width of the spectrum of frequencies that can be transmittedif spectrum=300 to 3400Hz, bandwidth=3100HzGreater bandwidth leads to greater costsLimited bandwidth leads to distortion

BandWidth on a Voice Circuit

Human hearing ranges from about 20 Hz to about 14,000 Hz (some up to 20,000 Hz). Human voice ranges from 20 Hz to about 14,000 Hz.The bandwidth of a voice grade telephone circuit is 0 to 4000 Hz or 4000 Hz (4 KHz).Guardbands prevent data transmissions from interfering with other transmission when these circuits are multiplexed using FDM.

Bandwidth on a Voice Circuit

Data Transmissions

Analog Transmission of Analog Data Telephone networks (PSTN) Digital Transmission of Digital Data A computer system Analog Transmission of Digital Data Uses Modulation/Demodulation (Modem) Digital Transmission of Analog Data Uses Coder/Decoder (CODEC)

Advantages of Digital Transmission

The signal is exactSignals can be checked for errorsNoise/interference are easily filtered outA variety of services can be offered over one lineHigher bandwidth is possible with data compression

Why Use Analog Transmission?

Already in placeSignificantly less expensiveLower attenuation ratesFully sufficient for transmission of voice signals

Analog Encoding of Digital Data

Data encoding and decoding technique to represent data using the properties of analog wavesModulation: the conversion of digital signals to analog formDemodulation: the conversion of analog data signals back to digital form

Methods of Modulation

Amplitude modulation (AM) or amplitude shift keying (ASK)Frequency modulation (FM) or frequency shift keying (FSK)Phase modulation or phase shift keying (PSK) Differential Phase Shift Keying (DPSK)

Analog Channel Capacity: BPS vs. BaudBaud=# of signal changes per second.

BPS=bits per second

In early modems only, baud=BPS. The bit rate and the symbol rate (or baud rate) are the same only when one bit is sent on each symbol.

Each signal change can represent more than one bit, through complex modulation of amplitude, frequency, and/or phase

Digital Transmission of Analog Data

Codec = Coder/DecoderConverts analog signals into a digital form and converts it back to analog signalsWhere do we find codecs?Sound cardsScannersVoice mailVideo capture/conferencing

Codec vs. Modem

Codec is for coding analog data into digital form and decoding it back. The digital data coded by Codec are samples of analog waves.Modem is for modulating digital data into analog form and demodulating it back. The analog symbols carry digital data.

Digital Encoding of Analog DataPrimarily used in retransmission devicesThe sampling theorem: If a signal is sampled at regular intervals of time and at a rate higher than twice the significant signal frequency, the samples contain all the information of the original signal.Pulse-code modulation (PCM)8000 samples/sec sufficient for 4000hz

Pulse Code Modulation (PCM)

Analog voice data must be translated into a series of binary digits before they can be transmitted.With Pulse Code Modulation (PCM), the amplitude of the sound wave is sampled at regular intervals and translated into a binary number.The difference between the original analog signal and the translated digital signal is called quantizing error.

Pulse Code Modulation (PCM)

Analog voice data must be translated into a series of binary digits before they can be transmitted.With Pulse Code Modulation (PCM), the amplitude of the sound wave is sampled at regular intervals and translated into a binary number.The difference between the original analog signal and the translated digital signal is called quantizing error.

PCM

PCM

PCM

PCM

PCM uses a sampling rate of 8000 samples per second.

Each sample is an 8 bit sample resulting in a digital rate of 64,000 bps (8 x 8000).

Converting Samples to Bits

QuantizingSimilar concept to pixelizationBreaks wave into pieces, assigns a value in a particular range8-bit range allows for 256 possible sample levelsMore bits means greater detail, fewer bits means less detail

Transmission Timing - Asynchronous vs. Synchronous

Sampling timing – How to make the clocks in a transmitter and a receiver consistent?Asynchronous transmission – sending shorter bit streams and timing is maintained for each small data block.Synchronous transmission – To prevent timing draft between transmitter and receiver, their clocks are synchronized. For digital signal, this can be accomplished with Manchester encoding or differential Manchester encoding.

Digital Interfaces

The point at which one device connects to anotherStandards define what signals are sent, and howSome standards also define physical connector to be used

Generic CommunicationsInterface Illustration

DTE and DCE

DTE DTE

host com puter term inal

in terface in terface

m odem m odem

DCE

Transmission Efficiency: Multiplexing

Several data sources share a common transmission medium simultaneouslyLine sharing saves transmission costsHigher data rates mean more cost-effective transmissionsTakes advantage of the fact that most individual data sources require relatively low data rates

Multiplexing Diagram

Alternate Approaches to Terminal Support

Direct point-to-point links Multidrop lineMultiplexer Integrated MUX function in host

Direct Point-to-Point

Multidrop Line

Multiplexer

Integrated MUX in Host

Frequency Division Multiplexing

Requires analog signaling & transmissionTotal bandwidth = sum of input bandwidths + guardbandsModulates signals so that each occupies a different frequency bandStandard for radio broadcasting, analog telephone network, and television (broadcast, cable, & satellite)

Frequency Division Multiplexing (FDM)

Synchronous Time-Division Multiplexing (TDM)

Used in digital transmissionRequires data rate of the medium to exceed data rate of signals to be transmittedSignals “take turns” over mediumSlices of data are organized into framesUsed in the modern digital telephone systemUS, Canada, Japan: DS-0, DS-1 (T-1), DS-3 (T-3), ...Europe, elsewhere: E-1, E3, …

TDM

Statistical Time Division Multiplexing (STDM)

“Intelligent” TDMData rate capacity required is well below the sum of connected capacityDigital only, because it requires more complex framing of dataWidely used for remote communications with multiple terminals

STDM

*Transmission Efficiency: Data Compression

Reduces the size of data files to move more information with fewer bitsUsed for transmission and for storageCombines w/ multiplexing to increase efficiencyWorks on the principle of eliminating redundancy

Codes are substituted for compressed portions of dataLossless: reconstituted data is identical to original (ZIP, GIF)Lossy: reconstituted data is only “perceptually equivalent” (JPEG, MPEG)

Computer Network

• An interconnected collection of autonomous computers.

• Two computers are said to be interconnected if they are able to exchange information.

• A system with one control unit and many slaves is not a network.

Computer Network (Cont.)Distributed Systems Computer

Network

The existence of multiple autonomous computers is transparent to the user.

User must explicitly do everything. 

Allocation of jobs to processor and files to disks and all other system functions must be automatic.

 

Distributed system is a software system built on top of a network.

 

Overlap between distributed systems and Computer Network Example:More files around System can involve the User movement. 

 

Computer Network (Cont.)Uses of Computer Network

Companies People Social Issues

Resource Sharing Access to remote information

News-groups

Geography Person To Person communication & e-mail

Bulletin Boards

High reliability: replication

Interactive Entertainment

 

Saving money on the flow    

Client-server model    

Scalability: Ability to increase system performance gradually as the workload grows.

   

A Communications Model

• Source– Generates data to be transmitted

• Transmitter– Converts data into transmittable signals

• Transmission system– Carries data

• Receiver– Converts received signal into data

• Destination– Takes incoming data

Simplified Communications Model - Diagram

Key Communications Tasks• Transmission system utilization

• Interfacing

• Signal generation

• Synchronization

• Exchange management

• Error detection and correction

• Addressing and routing

• Recovery

• Message formatting

• Security

• Network management

Network Hardware Transmission Technology

Broadcast Network Point – To – Point Network

Single communication channel that is shared by all the machines on the network.

Many connections between individual pairs of machines 

All the others receive “Packets” in certain contexts, sent by any machine. 

A packet may have to visit one or more intermediate machine. 

An address field within the packet specifies for whom it is intended. 

Routing algorithms play an important role in PTP networks. 

Multicasting: transmission to a subnet of the machines.

 

Simplified Data Communications Model

Networking• Point to point communication not usually

practical– Devices are too far apart– Large set of devices would need impractical

number of connections

• Solution is a communications network

Simplified Network Model

Local Area Networks

• Smaller scope– Building or small campus

• Usually owned by same organization as attached devices

• Data rates much higher• Usually broadcast systems• Now some switched systems and ATM are

being introduced

Local Area Networks (Cont.)NETWORKS

LAN MAN WAN INTERNET

LAN CHARACTERISTICS

Size Transmission Technology Topology

 Restricted in Size

 Single Cable

10 to 100 Mbps

Low delay (ms)

Very few Errors

Megabits/Sec. (Unit)

 BUS (Ethernet) Ring (Token ring)  

MAN• Metropolitan Area Network • Support data and voice• No switching elements • Standard: DQDB

(Distributed Queue Dual Bus) • Two unidirectional buses to which all the computers are

connected. • Each bus has a head-end, a device that initiates

transmission activity. • Traffic that is destined for a computer to the right of the

sender uses the upper bus, traffics to the left uses the lower one.

Wide Area Networks

• Large geographical area• Crossing public rights of way• Rely in part on common carrier circuits• Alternative technologies

– Circuit switching– Packet switching– Frame relay– Asynchronous transfer mode (ATM)

Wide Area Networks (Cont.)

• Host (end system).

• Subnet (communication subnet).

• WANs typically have irregular topologies.

WAN CONSISTS OF

 Transmission Lines:- Circuits, Channels or Tanks  

 Switching Elements:- Specialized computers used to connect two or more transmission lines.

Internet

• Collection of interconnected networks.

• Example: A collection of LAN’s connected by a WAN.

• WAN : (router + hosts).

• SUBNET : (only routers).

Circuit Switching

• Dedicated communications path established for the duration of the conversation

• E.G. Telephone network

Packet Switching

• Data sent out of sequence

• Small chunks (packets) of data at a time

• Packets passed from node to node between source and destination

• Used for terminal to computer and computer to computer communications

Frame Relay

• Packet switching systems have large overheads to compensate for errors

• Modern systems are more reliable

• Errors can be caught in end system

• Most overhead for error control is stripped out

Asynchronous Transfer Mode

• ATM (cell relay)• Evolution of frame relay• Little overhead for error control• Fixed packet (called cell) length• Anything from 10mbps to Gbps• Constant data rate using packet switching

technique• Offers a constant data rate channel

Integrated Services Digital Network• ISDN• Designed to replace public telecom system• Wide variety of services• Entirely digital domain• First generation ( narrowband ISDN )

– 64 kbps channel is the basic unit– Circuit-switching orientation– Contributed to frame relay

• Second generation ( broadband ISDN )– 100s of mbps– Packet-switching orientation– Contributed to ATM ( cell relay )

Protocols• Used for communications between entities in a

system• Must speak the same language• Entities

– User applications– E-mail facilities– Terminals

• Systems– Computer– Terminal– Remote sensor

Protocol Hierarchies • Organized as a series of layers or levels.• The purpose of each layer is to offer certain services to

the higher layers.• Layer n on one-machine carries on a conversation with

layer n on another machine.• Protocol: is an agreement between the communicating

parties on how communication is to proceed.• Peers communicate using the protocol.• In reality, no data directly transferred from layer n on one

machine to layer n on another machine.

Protocol Hierarchies (Cont.)• Each layer passes data and control information to the

layer immediately below it.• Between each pair of adjacent layers there is an

“interface”.• The design of layers helps in:

– Minimizing the amount of information that must be passed between layers

– Make it simpler to reduce the implementation of one layer with a completely different one

• Protocol stack: A list of protocol used by a certain system, one protocol

per layer.

Key Elements of a Protocol

• Syntax– Data formats– Signal levels

• Semantics– Control information– Error handling

• Timing– Speed matching– Sequencing

Design Issues for the Layers• Addressing.• Data transfer.

– Simplex communication.– Half-duplex communication.– Full-duplex communication.

• Number and priorities of the logical connection channels. Many networks provide at least two logical channels per connection, one for normal data and one for urgent data.

• Error control.– Error detecting code.– Error correcting code.

Design Issues (Cont.)

• How to receive data in order (sequence no.).

• How to keep a fast sender from swamping a slow receiver with data (flow control).

• Size of the message: disassembling >transmitting >reassembling messages.

• Routing: multiple paths between source and destination.

Protocol Architecture

• Task of communication broken up into modules

• For example file transfer could use three modules– File transfer application– Communication service module– Network access module

Simplified File Transfer Architecture

A Three Layer Model

• Network access layer

• Transport layer

• Application layer

Network Access Layer

• Exchange of data between the computer and the network

• Sending computer provides address of destination

• May invoke levels of service

• Dependent on type of network used (LAN, packet switched etc.)

Transport Layer

• Reliable data exchange

• Independent of network being used

• Independent of application

Application Layer

• Support for different user applications

• e.g. e-mail, file transfer

Interfaces and Services• Active elements in each layer are called ENTITIES.• Entity.

– Software [example: process.].

– Hardware [example: intelligent I/O chip.].

• The entities in layer n implement a service used by layer n+1.

• Layer n called service provider.• Layer n + 1 called service user.• Services are available at sap’s (service access points).• Each SAP has an address that uniquely identifies it.

Interfaces and Services (Cont.) – IDU: interface data unit.

– ICI: interface control info.

– SDU: service data unit.

• At a typical interface, the layer n + 1 entity passes an IDU to the layer n entity through the SAP.

• In order to transfer the SDU, the layer n entity may have to fragment it into several pieces, each of which is given a header and send to as a separate PDU (protocol data unit) such as a packet.

Addressing Requirements

• Two levels of addressing required

• Each computer needs unique network address

• Each application on a (multi-tasking) computer needs a unique address within the computer– The service access point or SAP

Protocol Architectures and Networks

Protocols in Simplified Architecture

Protocol Data Units (PDU)

• At each layer, protocols are used to communicate• Control information is added to user data at each

layer• Transport layer may fragment user data• Each fragment has a transport header added

– Destination SAP– Sequence number– Error detection code

• This gives a transport protocol data unit

Network PDU

• Adds network header– Network address for destination computer– Facilities requests

SERVICES

Connection Oriented Connectionless

Modeled after the telephone system

Modeled after posted system

Establish a connectionUse the ConnectionRelease the connection

 

Acts like a tube: receive data by the same order was sent

Messages could be received in different order than it was sent with

Reliable connection oriented service

Unreliable connectionless service (not acknowledged)  

Request reply service

• Sender transmits a single datagram containing a request, the reply contains the answer.

• Used to implement communication in the client-server model.

Operation of a Protocol Architecture

Service Primitives• A service is formally specified by a set of primitives

(operations) available to a user or other entity to access the service.

• Primitive tells the service to– Perform some action OR

– Report an action by a peer entity.

• Example: Connection oriented service with 8 service primitives.– CONNECT.request – Request a connection to be established.

– CONNECT.indication – Signal the called party.

Example (Cont.)– CONNECT.response – Used by the caller to accept/reject calls.– CONNECT.confirm – Tell the caller whether the call was

accepted.– DATA.request – Request the data be sent.– DATA.indication – Signal the arrival of data.– DISCONNECT.request – Request that a connection be released.– DISCONNECT.indication – Signal the peer about the request.– Service Could be.

• Confirmed (Example: CONNECT).• Unconfirmed (Example: DISCONNECT).

Relationship of Services to Protocols

• Service: is a set of primitives (operations) that a layer provides to the layer above it.

• Protocol.– A set of rules governing the format and meaning of the frames,

packets, or messages that are exchanged by the peer entities within a layer.

– Entities use protocols in order to implement their service definitions.

– Entities are free to change their protocols, provided they do not change the service visible to their users.

REFERENCE MODELS

OSI References Model TCP/IP Reference Model

TCP/IP Protocol Architecture• Developed by the US defense advanced research

project agency (DARPA) for its packet switched network (ARPANET).

• Used by the global internet.• No official model but a working one.

– Application layer.– Host to host or transport layer.– Internet layer.– Network access layer.– Physical layer.

Physical Layer• Physical interface between data

transmission device (e.G. Computer) and transmission medium or network

• Characteristics of transmission medium

• Signal levels

• Data rates

• Etc.

Network Access Layer

• Exchange of data between end system and network

• Destination address provision

• Invoking services like priority

Internet Layer (IP)

• Systems may be attached to different networks

• Routing functions across multiple networks

• Implemented in end systems and routers

Transport Layer (TCP)

• Reliable delivery of data

• Ordering of delivery

Application Layer

• Support for user applications• e.g. http, SMPT

TCP/IP Protocol Architecture Model

OSI Model

• Open systems interconnection

• Developed by the international organization for standardization (ISO)

• Seven layers

• A theoretical system delivered too late!

• TCP/IP is the de facto standard

OSI References Model

• International Standards Organization.

• OSI (Open Systems Interconnection).

• Reference model: deals with connecting open systems that are; Open for communication with other systems.

Principles

• A layer should be created where a different level of abstraction is needed.

• Each layer should perform a well-defined function.• The function of each layer should be chosen with an eye

toward defining internationally standardized protocols.• The layer boundaries should be chosen to minimize the

information flow across the interfaces.• The number of layers should be large enough that distinct

functions need not be thrown together on the same layer out of necessity.

OSI Layers

• Application

• Presentation

• Session

• Transport

• Network

• Data link

• Physical

The Physical Layer

• Deals with transmitting raw bits over a communication channel.

• How many volts for 1 or 0.

• How many microseconds a bit lasts.

• Mechanics, electrical and procedural interfaces.

Data link Layer

• Break the input data up into data frames.• Process the acknowledgement frames sent back by the

receiver.• Insert the frame delimiter.• Solve the problems caused by damaged, lost and duplicate

frames.• Flow control.• Full duplex transmission (piggybacking)• Medium access sub layer deals with how to control access

to the shared channel in broadcast networks.

Network Layer

• Routing packets from source to destination.

• Routes can be static or dynamic

• Bottleneck, congestion

• Connect heterogeneous networks (different addressing method, larger packet service).

• In broadcast networks, routing problem is simple, so the network layer is thin.

Transport Layer

• Accept data from the session layer, split it up into smaller units if needed, pass these to the network layer and ensure that the all pieces arrive correctly at the other end

• Under normal conditions, the transport layer creates a distinct network connection for each transport connection required by the session layer

• If the transport connection requires a high throughput, the transport layer might create multiple network connections, dividing the data among the network connections to improve throughput

Transport Layer (Cont.)• Transport layer determines what type of service to provide

the session layer with and ultimately, the users of the entire network

• The transport layer is a true end-to-end layer, from source to destination

• Multiple connections will be entering and leaving each host. There is a need to tell which message belongs to which connection (transport header)

• Establishing and deleting connections across the network• Flow control between hosts (as oppose between routers)

so fast host cannot overrun a slow one

Session Layer • Allows users on different machines to establish sessions

between them• A session might be used to allow a user to log into a

remote timesharing system or to transfer a file between two machines

• Example: token management. Only the side holding the token may perform the critical operation.

• Synchronization: insert a checkpoint.– Example: sending file for 20 hours. After a crash the portion

after the checkpoint will be resend again.

Presentation Layer

• Concerned with the syntax and semantics of the information transmitted.

• A typical example of a presentation service is encoding data in a standard agreed upon way. [Character strings, integers, floating-point numbers…].

Application Layer

• The application layer contains a variety of protocols that are commonly needed.

• Example: incompatible terminal type.• One way to solve this problem is to define an abstract

network virtual terminal that editor can be written to deal with. To handle each terminal type, a piece of s/w must be written to map the functions of the network virtual terminal onto the real terminal.

• Other application is file transfer(ftp).

TCP/IP and OSI Protocol Architectures

Example Of Networks

• Novell NETWARE.– Client-server model.– IPX/SPX.– Network layer runs IPX (internet packet exchange).– IPX uses 10 byte address (IP uses 4 bytes) flat addressing.– Transport protocol.

• NCP (network core protocol).• Transport service & other services.• SPX (sequenced packet exchange):• Just transport service.

Example Of Networks (Cont.)

• The application can choose between NCP & SPX– Transport control field counts how many networks the packet

has traversed.– About once a minute, each server broadcasts a packet giving its

address and telling what services it offers.– SAP (Service Advertising Protocol) is used for broadcasting– Routers run some kind of special agent processes to construct

databases of which servers are running.– When a client is booted, it sends a request for a server. The

agent on the local router machine sees this request, and matches up the request with the best server.

Example Of Networks (Cont.)• The APRANET.

– Packet switched network, consisting of subnet and host computers.– IMPS (interface message processors) connected by transmission

lines.– Each IMP would be connected to at least two other imps.– Each node consists of IMP and a host.– Host sends messages of up to 8063 bits to its IMP.– IMP breaks the message into packets of at most 1008 bits and

forwards them independently toward the destination.– 56-kbps lines leased from telephone companies interconnect the

IMPS.– By 1990, the ARPANET had been overtaken by newer networks.

Example Of Networks (Cont.)

• NSFNET– By 1984 NSF Fig 1.26(the U.S. national science

Foundation) began designing a high-speed successor to the ARPANET that would be open to all university research groups.

– By 1995 the NSFNET backbone was no longer needed to interconnect the NSF regional networks because numerous companies were running commercial IP Networks.

Example Of Networks (Cont.)

• The Internet. In 1992, the internet society was set up, to

promote the use of the internet.• Four main applications.

– Email.– News.– Remote login: telnet, rlogin.– File transfer: FTP.

Example Of Networks (Cont.)• Gigabit TESTBEDS.

– The backbones operate at megabit speeds.– Gigabit networks provide better bandwidth but not always

much better delay.– Example: sending a 1-kbit packet from NYC to san Francisco at

(1 mbps) take.– 1 msec to pump the bits out and 20 msec for the delay, for a

total of 21 msec. A 1-Gbps network can reduce this to 20.001 msec.

– For some applications, bandwidth is what counts, and these are the applications for which gigabit networks will make a big difference.

– Examples:- telemedicine & virtual meeting.

THANKS

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