UNIT -1. DATA COMMUNICATIONS The term telecommunication means communication at a distance. The word data refers to information presented in whatever form.

UNIT -1

DATA COMMUNICATIONSDATA COMMUNICATIONS

The term The term telecommunicationtelecommunication means communication means communication at a distance. at a distance.

The word The word datadata refers to information presented in refers to information presented in whatever form is agreed upon by the parties creating whatever form is agreed upon by the parties creating and using the data. and using the data.

Data communicationsData communications are the exchange of data are the exchange of data between two devices via some form of transmission between two devices via some form of transmission medium such as a wire cable. medium such as a wire cable.

DC- is the exchange of data between two devices by means of any transmission medium.

Characteristics:

1.Delivery 2. Accuracy 3. Timeliness 4.Jitter

1. The data must be delivered to the correct destination.2. The data must be delivered accurately. i.e. without

alteration.3. The system must deliver data in a timely manner.

e.g. Real time application.4. Variation in the packet arrival time.

Five components of data communication System

Components of data Communication:Message : It is the data to be communicated. It consists of text, numbers, pictures, sound, or video or any combination of these.

 Sender : It is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera. Receiver : It is the device that receiver the message. It can be a computer, workstation, telephone, and television. Medium : Transmission medium is the physical path by which a message travels from sender to receiver. Example it consists of twisted pair wire, co axial cable, fiber optical, laser or radio waves. Protocol: It is a set of rules that govern data communication. Without a protocol two devices are connected but not communicating.

Protocol :

It is defined as a set of rules and regulations used for

communication.

The key elements are given below.

1.Syntax: Refers to the structure of data, meaning the order in

which they are presented.

2. Semantics: The meaning of each section of bits.

3.Timing: Refers to two characteristics: - When data should be

sent and how fast they can be sent.

Data flow (simplex, half-duplex, and full-duplex)

NETWORKSNETWORKS

A A networknetwork is a set of devices ( is a set of devices (nodesnodes) connected by ) connected by communication communication linkslinks. .

A node can be a A node can be a computer, printer, or any other devicecomputer, printer, or any other device capable of sending and/or receiving data generated capable of sending and/or receiving data generated by other nodes on the network.by other nodes on the network.

Types of connections: point-to-point and multipoint

NetworksNetworks

Physical Topology The way a network is laid out physicallyTwo or more links form a topologyThe topology of a network is the geometric

representation of the relationship of all the links and linking devices (nodes) to one another.

Four topologies : Mesh, Star, Bus, and Ring

Physical TopologyPhysical Topology

Physical TopologyPhysical Topology

MeshEvery link is dedicated point-to-point linkThe term dedicated means that the link carries

traffic only between the two devices it connects

Physical TopologyPhysical Topology

MeshTo link nn devices fully connected mesh has:

n ( n - 1) / 2n ( n - 1) / 2 physical channels (Full-Duplex)

Every Device on the network must have n - 1n - 1 ports

Physical TopologyPhysical Topology

MeshExample:

8 devices in mesh has links: n(n-1) / 2

number of links = 8 (8-1)/2 = 28

number of ports per device = n – 1 = 8 –1 = 7

Physical TopologyPhysical Topology

MeshAdvantages

Each connection carry its own data load (no traffic problems)

A mesh topology is robustPrivacy or securityFault identification and fault isolation

Physical TopologyPhysical Topology

Mesh:Disadvantages

Big amount of cablingBig number of I/O portsInstallation and reconnection are difficultSheer bulk of the wiring can be greater than the

available spaceHardware connect to each I/O could be expensive

Mesh topology is implemented in a limited fashion; e.g., as backbone of hybrid network

Physical TopologyPhysical Topology

Star:Dedicated point-to-point to a central controller

(Hub)No direct traffic between devicesThe control acts as an exchange

Physical TopologyPhysical Topology

StarAdvantages

Less expensive than mesh(1 Link + 1 port per device)

Easy to install and reconfigureLess cablingAdditions, moves, and deletions required one

connectionRobustness : one fail does not affect othersEasy fault identification and fault isolation

Physical TopologyPhysical Topology

StarDisadvantages

Dependency of the whole topology on one single point (hub)

More cabling than other topologies ( ring or bus)

Used in LAN

Physical TopologyPhysical Topology

BusIt is multipoint One long cable acts as a backboneUsed in the design of early LANS, and Ethernet

LANs

Physical TopologyPhysical Topology

BusNodes connect to cable by drop lines and tapsSignal travels along the backbone and some of

its energy is transformed to heatLimit of number of taps and the distance

between taps

Physical TopologyPhysical Topology

BusAdvantages

Ease of installationLess cables than mesh, star topologies

DisadvantagesDifficult reconnection and fault isolation ( limit of

taps)Adding new device requires modification of

backboneFault or break stops all transmissionThe damaged area reflects signals back in the

direction of the origin, creating noise in both directions

Physical TopologyPhysical Topology

RingEach device has dedicated point-to-point connection

with only the two devices on either side of itA signal is passed along the ring in one direction from

device to device until it reaches its destinationEach devices incorporates a Repeater

Physical TopologyPhysical Topology

RingAdvantages

Easy of install and reconfigureConnect to immediate neighborsMove two connections for any moving (Add/Delete)Easy of fault isolation

DisadvantageUnidirectionalOne broken device can disable the entire network.

This weakness can be solved by using a dual ring or a switch capable of closing off the break

Physical TopologyPhysical Topology

Hybrid TopologyExample: having a main star topology with each

branch connecting several stations in a bus topology