Introduction to Data Communications

Chapter 1

DATA COMM• Development of PC

– Tremendous changes in sciences, industry ,education etc– No more domain of technical personnel only– Requirement of transfer of data quickly

• TELECOMM– Tele means far in Greek– Tele includes Telephony, Telegraphy & TV– Local Comm– Remote Comm

• DATA COMM– Data means facts, concepts & instructions– Exchange of data between 2 devices– Transfer takes place as 0s & 1s– Effectiveness depends upon

• Delivery: To correct destination• Accuracy: Not to be altered• Timeliness must be delivered in time

Data Communication System Components

Networks• Set of devices connected by media links.

– E.g. Computer / Printer– Links connecting them also knows com. channels

• Network Criteria: Performance, Reliability, Security• Performance measured by

– Transit Time – Response Time– Depends upon

• No of users• Type of Transmission medium• Hardware• Software• Throughput and Delay

Reliability• Frequency of failure• Recovery time of Network after failure• Catastrophe• Security

– Unauthorized access• Sensitive data must be protected• Lowest level, user name and password• At higher level encryption methods

– Viruses

• Line configuration• Topology• Transmission mode• Categories of networks• Internetworks

Basic Concepts

Line Configuration

• Two or more comm devices attached to a link• Link is a physical communicating path to transfer data

Point – to – Point

• Dedicated link b/w two devices• Entire capacity reserved between two devices• Normally actual path line of wire but microwaves & satellites

links also possible– E.G Remote of a TV

Multi Point Configuration• Also called multidrop• More than two devices share common link• Capacity of channel shared• If devices use link simultaneously

– Spatially shared-in terms of space– Time shared-devices take turns

Topology• Way a Network is laid out physically or logically• Two or more devices connect to a link• Two or more links form a topology• Geometrical representation of relationship of all links

Possible Relationships

• Peer to Peer : Devices share link equally

– Ring

– Mesh• Primary-Secondary :One device controls traffic

& other must transmit through it

– Star

– Tree

Mesh Topology

• Every device connected with every device through dedicated link

• n(n-1)/ 2 - Physical channels• Every device on NW must have (n-1) I/O ports• Each connection carry its own data • Robust , Privacy & Security• Fault identification and isolation easy but Too many cables • Installation difficult and HW requirement expensive

Mesh Topology

Star Topology

• Pt – to - Pt link to a central controller called “hub”• Does not allow direct traffic but through hub• Less expensive• Robustness - if one device fails only it is effected,

all other remains active• Since each node must be linked to hub more

cabling is required• If hub fails whole network goes down.

Star Topology

Tree Topology• Variation of star• Not every device connects to central hub• Majority of devices connect to secondary hub which

connects to central hub• Central hub is active hub. It contains a repeater that

regenerates the received bit pattern before sending them out.

• Secondary hub may be active or passive. Passive hubs are simple physical connector.

Tree Topology

Bus Topology• Multipoint, Backbone - to link all the devices• Drop lines to connect the nodes to back bone• Tap is a connector that splices into main cable• Attenuation

– Limit on number of taps a bus can support and distance between two taps

Bus Topology

Bus• Advantages

– Easy installation– Less cabling required

• Disadvantages– Fault isolation difficult– Difficult to add new devices– Fault in bus stops all transfers

Ring Topology• Pt – to - pt link• Each device has devices on either sides• Easy to install• Signal is passed in one direction• Each device incorporates repeater• Disadvantages

– Media and traffic consideration– Main ring length– Number of devices– Break in unidirectional ring can disable entire ring

Ring Topology

HYBRID TOPOLOGY

SIMPLEXInformation flows in only one direction

HALF-DUPLEXInformation flows in both directions, but only in one direction at a time.

FULL-DUPLEXInformation flows in both directions at the same time

LOCAL AREA NETWORK

Local Area Networks• Smaller scope

– Building or small campus. – Usually owned by same organization that owns

the attached devices• Network maintenance is solely user responsibility• Data rates are much higher & usually in packets• Medium shared - only one node transmit at a time• Usually broadcast systems rather switching

approach • High speed switches are being introduced making

Higher data rates possible (Gigabits)• Run, installed by network administrators

LOCAL AREA NETWORK

MAN• Designed to extend over a city• May be a single Network such as cable TV

Network or no of LANs• Owner

– Private company or – Public such as Local Tel Company

METROPOLITAN AREA NETWORK

WIDE AREA NETWORK

Wide Area Network• Across a continent (10km to 10000 km) or large

geographical area• In contrast to LANs may utilize public, leased or

private communication devices• WAN wholly owned by service providers around

the globe• Data Rate:

• Typically1.5 Mbits/sec to 2500 Mbits/sec• Run, installed by telephone companies (in most

cases)

INTERNETWORK (INTERNET)

The Internet

• Loosely administered network of networks• Agreed procedures for access and

intercommunication• Internetworking uses gateways, routers and

firewalls• Gateways: convert data traffic from one network

format to another. They link LANs to WANs and WANs to WANs

Protocols • Set of rules that govern data communications. It

defines – What is communicated?– How it is communicated?– When it is communicated?

• key elements– Syntax - Structure or format of data– Semantics – Meaning of each section of bits– Timing - When data should be sent

• How fast it should be sent

Standards• A standard provides model for development

– Ensure product to work regardless manufacturer– Essential for competition

• Data comm standards

Defacto Standards• Defacto: Not approved by organized body but

adopted through widespread use• Proprietary

– Originally invented by commercial org for its products

– Also called closed standards• Nonproprietary

– Produced by groups and given to public domain– Also called open standards

• Dejure: Legislated by officially recognized body

Standards Organizations• ISO• ITU-T• ANSI• IEEE• EIA• IETF• Regulatory Agencies (PTA in Pakistan)