Thefundamentalpurposeofacommunicationssystemistheexchangeofdata Between Two Parties

UNIT-1

Datacommunications

Thefundamentalpurposeofacommunicationssystemistheexchangeofdata between two parties.

Datacommunicationsaretheexchangeofdatabetweentwodevicesviasome formoftransmissionmediumsuchasawirecable.Fordatacommunicationsto occur,thecommunicatingdevicesmustbepartofacommunicationsystemmade upofacombinationofhardware(physicalequipment)andsoftware(programs). Theeffectivenessofadatacommunicationssystemdependsonfourfundamental characteristics:delivery,accuracy,timeliness,and jitter.

1.Delivery.Thesystem mustdeliverdatatothecorrectdestination. Datamust be receivedbythe intendeddevice or user andonlybythatdeviceoruser.

2. Accuracy.Thesystemmustdeliverthedataaccurately.Datathathavebeen

altered in transmissionand leftuncorrectedareunusable.

3. Timeliness.Thesystemmustdeliverdatainatimelymanner.Datadeliveredlate are useless. In the case of video and audio, timely delivery means deliveringdataastheyareproduced,inthesameorderthattheyareproduced, and without significant delay. This kind of delivery is called real-time transmission.

4. Jitter.Jitterreferstothevariationinthepacketarrivaltime.Itistheuneven delayinthedeliveryofaudioorvideopackets.Forexample,letusassumethatvideopacketsaresentevery30-ms.Ifsomeofthepacketsarrivewith3D-ms delayandotherswith40-msdelay,an uneven quality in the videoistheresult.

COMPONENTS:

A data communication system has five components:

a)Message:

-It is the information(data) to be communicated.

-Popular forms : text, numbers, images, audio, video.

b)Sender:

-It is a device that sends the data message.

-It can be a computer, a work station , video camera and so on.

c)Receiver:

-It is a device that receives the data message.

-It can be a computer, a work station , video camera and so on.

d)Transmission Medium:

-It is a physical path by which a message travels from sender to receiver.

-Examples : twisted-pair cable, co-axial cable, fiber-optic cable and radio waves.

e)Protocol:

-It is a set of rules that govern data communications.

-It represents an agreement between the communicating devices.

-Without a protocol, two devices may be connected but not communicating.

Figure(A) Fivecomponentsofdata communication

Data representation

Informationtodaycomesindifferentforms suchastext,numbers, images,audio, andvideo.

•Text

-Indatacommunications,textisrepresentedasabitpattern,asequenceofbits(0s or1s).

-Differentsetsofbitpatternshavebeendesignedtorepresenttextsymbols.

-Eachsetiscalledacode,andtheprocessofrepresentingsymbolsiscalledcoding.

•Numbers

-Numbersarealsorepresentedbybitpatterns.

• Images

-Imagesarealsorepresentedbybitpatterns.Initssimplestform,animageis composedofamatrixofpixels(pictureelements),whereeachpixelisasmalldot. Thesizeofthepixeldependsontheresolution.

-Each pixel is assigned a bit pattern.

-If an image is made of pure black and pure white dot, a 1-bit pattern is used to represent a pixel.

-If an image is not made of pure white and pure black pixels, increase the size of bit pattern.

-There are several methods represent color images:

i)RGB(Red, Green, Blue)

ii)YCM (Yellow, Cyan, Magenta)

The intensity of each color is measured and a bit pattern is assigned to it.

•Audio

-Audio refers to the recording or broadcasting of sound or music.

-Audio is by nature different from text, numbers, or images.

-It is continuous, not discrete.

-Even when we use a microphone to change voice or music to an electric signal, we create a continuous signal.

•Video

-Video refers to the recording or broadcasting of a picture or movie.

-Video can either be produced as a continuous entity (e.g., by a TV camera), or it can be a combination of images, each a discrete entity, arranged to convey the idea of motion.

DATA FLOW

Communicationbetweentwodevicescanbesimplex,half-duplex,orfull-duplex

•Simplex:

-Thecommunicationisunidirectional,asonaone-waystreet.

-Onlyoneofthetwodevicesonalinkcantransmit;theothercanonlyreceive.

-Examples:Keyboardsandtraditionalmonitors.

-Thekeyboardcanonlyintroduceinput;themonitorcanonlyacceptoutput.

-The simplexmodecanusetheentirecapacityofthechanneltosenddatainone direction.

•Half-Duplex:

-Eachstationcanbothtransmit andreceive, butnotatthe same time.

-Whenonedeviceis sending, theother can only receive, andvice versa.

-Examples: Walkie-talkies, Radios.

•Full-Duplex(alsocalledduplex) :

-Bothstationscantransmitandreceive simultaneously.

-Example : Telephone network.

• When two people are communicating by a telephone line, both can talk and listen at the same time.

NETWORK CRITERIA

A network must be able to meet certain number of criteria. The most important of these are:

a)Performance

-It is measured in many ways including Transit time and Response time.

-Transit time : The amount of time required for a message to travel from one device to another.

-Response time :The elapsed time between inquiry and a response.

-Other factors of Performance :

Number of users

Type of transmission medium

Capabilities of hardware

Efficiency of software

-Performance is evaluated by 2 networking metrics:

Throughput and delay

We often need more throughput and less delay.

If we try to send more data to the network, it may increase the throughput but increases the delay because of traffic congestion in the network.

b)Reliability

-It is measured by the frequency of failure, the time it takes a link to recover from failure.

c)Security

-Security issues include:

a)Protecting data from unauthorized access

b)Protecting data from damage

c)Implementing policies and procedures for recovery from breaches and data losses.

TYPES OF CONNECTION

For communication to occur , two devices must be connected to the same link at the same time.

There are two possible types of connections:

i)Point-to-point

ii)Multi-point

Point-to-point :

-It provides a dedicated link between two devices.

-The entire capacity of the link is reserved for transmission between two devices.

-It uses actual length of the wire or cable to connect the two ends.

-Example : changing television channels by infrared remote control.

Figure (a)

Figure (b)

Multi-point : (also called Multidrop)

-More than two specific devices share a single link.

-The entire capacity of the link is shared either spatially or temporally.

-Spatially shared: Several devices uses the link simultaneously

-Temporally shared(time shared) : Several users uses the link.

TOPOLOGY

The term topology refers to the way in which a network is laid out physically. Two or more devices connect to a link, to form a topology.

The topology of a network is the geometric representation of the relationship of all links and linking devices to one another.

There are fourbasic topologies

i)Mesh

ii)Star

iii)Bus

iv)Ring

Mesh:

-In this type of topology, every device has a dedicated point-to-point link to every other device.

-The term ‘dedicated’ means the link carries traffic only between the two devices it connects.

-Fully connected mesh network of ‘n’ nodes consist of n(n-1) physical links.

-If the physical link is in duplex mode, it needs n(n-1)/2 links.

Advantages / Disadvantages
1. / With the use of dedicated links, Each connection can carry its own data load thus eliminating traffic problems. / Installation and reconnection are difficult, because every device is connected to every other device.
2. / Robust – if one link becomes unusable, it doesn’t affect the entire system. / The bulk of wiring is greater than the available space (in walls, ceilings or floors)
3. / Privacy or Security – As the message travels along a dedicated line, only the intended recipient sees it. Other users cannot access the message / The Hardware required to connect each link is expensive.
4. / Point-to-point links make Fault identification and fault Isolation easy.

For the above mentioned demerits, mesh topology is usually implemented in a limited fashion. i.e., as a backbone connecting main computers of hybrid network.

Practical Example : Connection of telephone regional offices.

Star:

-In this type of topology, each device has a dedicated point-to-point link only to a central controller, usually called Hub.

-The device are not directly linked to one another.

-Star topology doesn’t allow traffic between devices.

-Hub, the controller acts as an exchange :

If one device wants to send data to another, it sends the data to the controller, which then relays the data to the other connected device.

Advantages / Disadvantages
1. / Less expensive than mesh topology. / The dependency of the whole topology is on the single point, the Hub.
If the Hub goes down, the whole system is dead.
2. / Easy to install and reconfigure – each device needs only one link and one I/O port to connect to any number of devices. / Although star requires less cabling than mesh, each node must be linked to a central hub.
For this reason, more cabling is required in star than other topology like Bus or ring.
3. / Less cabling needs to be housed. i.e., for addition and deletion involve only one connection(between the device and the Hub)
4. / Robustness : If one link fails , only that link is affected. All other nodes remain active.
5. / Easy Fault Identification and isolation.

Bus:

-One of the first topologies used in the design of early LAN.

-A Bus topology has a multi-point connection.

-A long cable acts as a backbone cable to link all the devices in a network.

-Nodes are connected to the bus cable by droplines and Taps.

-A Dropline is a connection running between the device and the main cable.

-A Tap is a connector that either splices into the main cable to create a contact with the metallic core.

-As the signal travels along the backbone, some of its energy is transformed into heat. Hence, it becomes weaker and weaker as it travels farther and farther. For this reason, there is limit on the number of taps a bus can support and on the distance between them.

Advantages / Disadvantages
1. / Ease of installation. / Reconnection and Fault Isolation
2. / Uses less cabling than Mesh or star topologies. / Adding new devices requires modification or replacement of the backbone cable.
3. / Less cabling needs to be housed. i.e., for addition and deletion involve only one connection(between the device and the Hub) / Signal reflection at the taps causes degradation in quality.
Degradation can be controlled by limiting the number & spacing of devices connected to the cable.
4. / Robustness : If one link fails , only that link is affected. All other nodes remain active. / Fault or break in the bus cable stops all transmission
5. / Easy Fault Identification and isolation.

Ring:

-In this type of topology, each device has a dedicated point-to-point connection with only two devices on either side of it.

-A signal is passed along the ring in one direction, from device to device until it reaches its destination.

-Each device in the ring incorporates a Repeater.

-When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along.

-Generally, a signal is circulating at all times . If one of the devices doesn’t receive the signal within a specified period, it can issue an alarm , which alerts the network operator to the problem and its location.

-This topology was prevalent when IBM introduced its Local area network Token ring.

Advantages / Disadvantages
1. / Ease to install and reconfigure. / Unidirectional traffic
2. / Addition or deletion of a device requires the change of only two connections. / A break in the ring disables the entire network.
Solution : Usage of dual ring or a switch capable of closing off the break.
3. / Fault isolation is simplified. / A break in the ring disables the entire network.

Hybrid:

-A network can be hybrid.

-Eg : A main star topology with each branch connecting several stations in a bus topology

CATEGORIES OF NETWORKS :

The three broad categories of network based on its size, its ownership, its coverage and its physical architecture.

i)Local Area Network (LAN)

ii)Metropolitan Area Network (MAN)

iii)Wide Area Network (WAN)

Local Area Network (LAN) :

-It is privately owned and links the devices in a single office, building or campus.

-Size : limited to few kilometers.

-It allows resources like hardware(Eg :Printer) and software(Eg :Application program) to be shared between the devices.

-Size of the LAN may be determined by the licensing restrictions on the number of users per copy of the software.

-LANs are distinguished from other types of networks by their transmission media and technology

-In general, a LAN will use only one type of transmission medium.

-Most common LAN topologies are : Bus, Star , Ring.

-Early LAN data rates : 4 to 16 Mbps, today it is 100 to 1000 Mbps.

-Newest Evolution : Wireless LANs

Metropolitan Area Network (MAN) :

-It is a network with size between a LAN and a WAN

-It normally covers an area inside a town or city.

-It is designed for the customers who need a high-speed connectivity, normally to the Internet , and have endpoints spread over a city or part of a city.

-Example : part of the telephone company network that can provide a high-speed DSL line to the customer.

-Most common LAN topologies are : Bus, Star , Ring.

-Early LAN data rates : 4 to 16 Mbps, today it is 100 to 1000 Mbps.

-Newest Evolution : Wireless LANs

Wide Area Network (WAN) :

-It provides long distance transmission of data, that may comprise a country, continent or even the whole world.

-It can be as complex(Switched WAN) as the backbone that connects the Internet or as simple(Point-to-point WAN) as a dial-up line that connects a home computer to the Internet.

-Example of Switched WAN : ATM (Asynchronous Transfer Mode)

PROTOCOLS AND STANDARDS :

Protocols –rules
Standards – agreed upon rules

Protocols :

An entity is anything capable of sending or receiving information.

For communication to occur, the entities must agree on a protocol

A protocol defines ,

- what is communicated

- how it is communicated

- when it is communicated

Key elements of Protocol :

a)Syntax :

-It refers to the structure or format of the data

-Eg : a simple protocol might expect first 8 bits to be the sender’s address , second 8 bits to be the receiver’s address and the rest of the stream to be the original message.

b)Semantics:

-Meaning of each section of bits

-Also based on actions taken based on interpretation

-Eg : The receiver’s address may identify either the route to be taken or the final destination of the message.

c)Timing :

-refers 2 characteristics :

i)When the data should be sent

ii)How fast they can be sent

-Eg : If a sender produces data at 100 Mbps but the receiver can process data only at 1Mbps , the transmission will overload the receiver and some data will be lost.

Standards :

Data communication standards fall in two categories :

i)De facto (by fact)

-Standards that have not been approved by an organized body , but adopted as standards through widespread use.

-It is established originally by the manufacturers who defines their new product or technology.

ii)De jure (by law)

-Standards that have been legislated by an officially recognized body.

Standards Organizations :

Standards are developed through the co-operation of,

-Standard creation committees

-Forums

-Regulatory Agencies

Standard Creation committees:

ISO (International Organization for Standardisation)

-It is a multinational body

-It is active in developing co-operation in the realms of scientific, technological and economic activity

ITU-T (International Telecommunication- Telecommunication Standards Sector):

-CCITT(Consultative Committee for International Telegraphy and Telephony) devoted to the research and establishment of standards for telecommunication especially for phone and data systems.

-Then , in March 1,1993 the name CCITT was changed to ITU-T.

ANSI (American National Standard Institute )

-It is a private, nonprofit corporation.

-Its activities are undertaken with the welfare of United States and its citizens occupying primary importance.

IEEE (Institute of Electrical and Electronics Engineers):

-It is the largest professional engineering society in the world.

-It aims to advance theory , creativity and product quality in the fields of electrical engineering, electronics and as well as in all related branches of Engineering.

EIA (Electronic Industries Association)

-It is a non profit organization devoted to the promotion of manufacturing concerns.

-It has made significant contributions by defining physical connection interfaces and electronic signaling specifications for data communication.

Forums:

-Many special-interest groups have developed forums made upof representatives from interested corporations to facilitate the standardization process.

-The Forums present their conclusions to the standard bodies.

Regulatory agencies:

-Its purpose is to protect the public interest by regulating radio, television and wire communications.

MULTIPLE ACCESS :

-Data link layer has 2 sublayers:

-The upper sub layer that is responsible for flow and error control is called Logical Link Control(LLC) Layer.

-The Lower sub layer that is responsible for multiple access resolution is called Media Access Control (MAC) Layer.

-If the channel is dedicated, MAC sub layer is not needed.

-Many formal protocols have been devised to handle access to a shared link. They are categorized in to three groups:

RANDOM ACCESS :

-In random access or contention methods, no station is superior to another station and none is assigned the control over another.

-No station permits, or does not permit, another station to send.

-At each instance, a station that has data to send uses a procedure defined by the protocol to make a decision on whether or not to send.

-This decision depends on the state of the medium(idle or busy)

-Each station has the right to the medium without being controlled by any other station.

-Two features gives the method its name:

• There is no schedule time for a station to transmit: transmission is random among stations.
• Stations compete with one another to access the medium

Contention method.

-Collision: an access conflict occurs when more than onestation tries to send, as a result the frame will be either destroyed or modified.

-Each station follows a procedure that answer the following questions to avoid collision:

When can the station access the medium?

What can the station do if the medium is busy?

How can the station determine the success or failure of the transmission?

What can the station do if there is an access conflict ( collision)?

Evolution of random-access methods :

ALOHA : usesMA(Multiple Access)No carrier sense

CSMA:Carrier Sense Multiple Access

CSMA/CD :Collision Detection

CSMA/CA Collision Avoidance

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Random Access Methods:

ALOHA

Carrier Sense Multiple Access ; CSMA

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)

Pure ALOHA :

-The earliest random access method developed at the University of Hawaii in the early 1970s

-Designed for a radio (wireless) LAN

-Simple method. Each station sends a frame whenever it has a frame to send.

-Since there is only one channel to share, there is the possibility of collision between frames from different stations.

-Each station sends a frame whenever it has a frame to send.

-It relies on acknowledgments from the receiver.

-If the ACK dose not arrive after a time-out period, the station resend the frame.