Chapter 1

Data Communications and Networks - Overview

Data Communications deals with the transmission of signals in a reliable and effective manner. Topics covered include signal transmission, transmission media, signal encoding, interfacing, data link control, and multiplexing.

Networking deals with the technology and architecture of the communication networks used to interconnect communicating devices (entities). Topics include LANs, WANs.

Data Communications between two devices

Networks provide networking services between two entities

The communications between two end-points uses Protocol

1.1 A Communications Model Source

Generates (binary) data to be transmitted

TransmitterConverts data into transmittable electromagnetic signals

Transmission systemThis can be a single transmission line or a complex network

ReceiverConverts received signal into data

DestinationTakes incoming data

pp. 10-11

Simplified Communications Model - Diagram

p. 11

Communications Tasks

Transmission system utilization

Addressing

Interfacing Routing

Signal generation Recovery

Synchronization Message formatting

Exchange management Security

Error detection and correction

Network management

Flow control

p. 12

1.2 Data Communications

p. 13

1.3 Data Communication Networking Point to point communication not usually

practicalDevices are too far apartLarge set of devices would need impractical number of connections

Solution is a communications networkWide Area Network (WAN)Local Area Network (LAN)

p. 14

Wide Area Networks Large geographical area Crossing public rights-of-ways Rely in part on common carrier circuits Alternative technologies

Circuit switchingPacket switchingFrame relayAsynchronous Transfer Mode (ATM)

pp. 15-16

Circuit Switching Dedicated communications path

established for the duration of the conversation

Data transmission is rapid, without delay This kind of switching is good for real-time

services The typical network which adopts this kind

of switching is telephone network

p. 15

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 At each node, the entire packet is

received, stored briefly, and then transmitted to the next node

Used for terminal to computer and computer to computer communications

There are two kinds of packet switching: Virtual circuit and datagram

p. 15

Packet switching systems have large

overheads to compensate for errors Modern systems are more reliable Errors can be caught in the end system Most overhead for error control is stripped

out Developed to take advantage of high data

rates and low error rates

p. 15

Frame Relay

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

technique

p. 16

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

p. 16

LAN Configurations Switched

Switched Ethernet• May be single or multiple switches

ATM LANFibre Channel

WirelessMobilityEase of installation

pp. 16-17

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

p. 17

1.4 NetworkingConfiguration

p. 18

Standards Required to allow for interoperability

between equipments Advantages

Ensures a large market for equipment and softwareAllows products from different vendors to communicate

DisadvantagesFreeze technologyMay be multiple standards for the same thing

pp. 4-5

Standards Organizations Internet Society IEEE 802 ISO ITU-T (formally CCITT) ATM forum

p. 5

Further Reading Stallings, W. [2003] Data and Computer

Communications (7th edition), Prentice Hall, Upper Saddle River NJ, chapter 1

Web site for Stallings bookhttp://williamstallings.com/DCC7e.html