01 Overview Chapter 1

Chapter 1 Data Communications and Networks Overview

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Data Communications • Communications Model • Data Communications • Data Communication Networking 2

Communications Model • Is the xchange of data b/w 2 parties • Is a communication b/w a workstation & a server over a public telephone network • Can also be the xchange of voice signals b/w 2 telephones over the same n/w • Key elements in this model include : a.Source – telephones & PCs b.Transmitter – performs transforming & encoding c.Transmission system – single line / complex network d.Receiver – modem/transmission line

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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 4

Communications Tasks Transmission system utilization Interfacing

Addressing

Signal generation

Recovery

Synchronization

Message formatting

Exchange management

Security

Error detection and correction Flow control

Network management

Routing

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Contd… • 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 like multiplexing are used to allocate the total capacity of a transmission medium among a number of users 6

Contd… • Interface- to communicate, a device must interface with the transmission system. once an interface is established, signal generation is required for communication • Signal has 2 properties : form & intensity • Form refers to capability of being propagated thru the transmission system • Intensity refers to interpretable as data at the receiver 7

Contd… • Signals should be conformed to the requirements of the transmission system & receiver • Synchronization has to be b/w transmitter & receiver • If data has to be exchanged, two parties involve & must cooperate in which timing & nature of signals is important • Error detection & correction are required in circumstances where errors cannot be tolerated 8

Contd…

• Flow control is required to assure that the source does not overwhelm the destination by sending data faster than they can be processed & absorbed • Addressing refers that a source system must indicate the identity of the destination in the form of its address • The transmission system must ensure that the destination can receive the data. For this some route on the network help out which is routing 9

Contd… • Recovery is needed in situations in which an information exchange, such as database transaction or file transfer is interrupted due to a fault in the system • Message formatting has to check with the agreement b/w 2 parties with the data to be exchanged or transmitted such as binary code for characters • Security provides assurance of data reaching to the destination in correct time • Network Management deals with the configuration of the system, monitor, react to failures, overloads & plans for future growth 10

Simplified Communications Model - Diagram

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Data Communications using email as an example

1)Input information m 2)Input data g(t) 3)Transmitted signal s(t) 4)Received signal r(t) 5)Output data g´(t) 6)Output information m´

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Contd… • Source - this device generates the data to be transmitted • Transmitter – this transforms & encodes the information to produce electro-magnetic signals that can be transmitted across some sort of transmission system • Transmission system – can be single transmission line or a complex network connecting source & destination • Receiver – accepts signal from transmission system & converts into a form that can be handled by the destination • Destination – takes the incoming data from the receiver 13

Contd… • In the above figure shown, consider that a User of the PC wishes to send a message m to another user • User activates email package on PC & enters the message via the keyboard • The char string is briefly buffered in main memory say g bits • PC is connected to some transmission medium like Local nw/phone line/modem/IO device etc • I/P data are transferred to the transmitter as a sequence of voltage shifts g(t) 14

Contd… • The transmitter is connected directly to the medium & converts the incoming stream g(t) into a signal s(t) • S(t) is subjected to a number of impairments before it reaches the receiver & received signal r(t) is generated • Receiver attempts to estimate s(t) based on r(t) & produces g´(t) • These bits are sent to the output PC as g´ • Destination system attempts to check for errors & then presents the data on to the o/p device • Message m´ is viewed by the user just like m. 15

Data Communication Networking • Very often two communicating devices can be directly point-to-point connected • Likewise devices can be separate, or very far apart • Solution to such is to attach a communication network. • Wide Area Network (WAN) Metropolitian Area Network (MAN) Local Area Network (LAN) 16

Introduction to Computer Networks

LAN, MAN & WAN Network in small geographical Area (Room, Building or a Campus) is called LAN (Local Area Network)

Network in a City is call MAN (Metropolitan Area Network)

Network spread geographically (Country or across Globe) is called WAN (Wide Area Network)

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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) 18

Wide Area Network

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Internetwork (Internet)

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Wide Area Networks • Relation between hosts on LANs and the subnet.

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Wide Area Networks (2) • A stream of packets from sender to receiver.

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Contd… • Cover a large geographical area • Provides a common carrier • Consists of a number of a number of interconnected switching nodes • Transmission from any one device is routed thru these internal nodes to the specified destination device • Data moves b/w until reaches the destination 23

Contd… • Implemented using any one technology : • Circuit switching – dedicated comm.path is established & is connected with sequence of physical links b/w nodes • Packet switching – data is in small chunks called packets & these are transmitted from node to node until reaching the dest • Frame relays – extension to packet switching to deal with errors in the packet The packet is of variable size & maintained with a extra bit that checks for errors • ATM relays – also called as cell relays that is a combination of circuit & packet switching that reduces the overhead in error detection & correction since the packet is of fixed size 24

Circuit Switching • Dedicated communications path established for the duration of the conversation • e.g. telephone network

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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

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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

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Asynchronous Transfer Mode • • • • • •

ATM 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 28

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 29

LAN Configurations • Switched – Switched Ethernet • May be single or multiple switches

– ATM LAN – Fibre Channel

• Wireless – Mobility – Ease of installation 30

Local Area Network

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Local Area Network

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Local Area Networks

• Two broadcast networks • (a) Bus • (b) Ring

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Metropolitan Area Networks • • • • •

MAN Middle ground between LAN and WAN Private or public network High speed Large area

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Metropolitan Area Network

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Metropolitan Area Networks • A metropolitan area network based on cable TV.

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Networking Configuration

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Introduction to Computer Networks

Computer Networks  Computer network connects two or more autonomous computers.

 The computers can be geographically located anywhere. 38

Introduction to Computer Networks

Applications of Networks Resource Sharing Hardware (computing resources, disks, printers) Software (application software) Information Sharing Easy accessibility from anywhere (files, databases) Search Capability (WWW) Communication Email Message broadcast  Remote computing  Distributed processing (GRID Computing)

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Applications  E-mail  Searchable Data (Web Sites)  E-Commerce  News Groups  Internet Telephony (VoIP)  Video Conferencing  Chat Groups  Instant Messengers  Internet Radio

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