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

SIMPLEX Information flows in only one direction

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

FULL-DUPLEX Information 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 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)