Computer Literacy - II

Lecture #12Physical Topology in Network Design

What is Physical Topology? The physical topology of a network refers to

the arrangement or physical layout of the cables, computers, and other peripherals, connected to the network.

"Topology" is the standard term that most network professionals use when they refer to the network's basic design.

There are two types of topologies1. Physical2. Logical

We discuss only physical topologies

Virtual Structure A topology is a network's virtual shape or

structure. This shape does not necessarily correspond

to the actual physical layout of the devices on the network.

For example, the computers on a home LAN may be arranged in a circle in a family room, but it would be highly unlikely to find a ring topology there.

Main Types of Physical Topologies1. Bus 2. Star 3. Ring4. Tree/ Hybrid5. Mesh

1. Bus Topology Bus networks use a common backbone to connect

all devices. A single cable work as backbone Each terminal (computer) has a unique address. A device wanting to communicate with another

device on the network sends a broadcast message onto the wire.

Each message carries the address of the destination terminal.

When a message passes through the cable, each terminal check its address.

If the address matches the address of the terminal, it read the message and remove the message from the cable.

Working of Bus Topology

Cont… Bus network are typically maintained by a method

called contention. If a line is unused, a terminal or device can

transmit its message. But if two or more terminals initiate message

simultaneously, they must stop and transmit again at different intervals.

In bus topology there is no central computer or server, but it may be organized as client-server or peer-to-peer.

Bus Topology

Advantages1. Simple, easy to connect a computer or additional

peripheral to a bus. 2. Requires less cable length than a star topology. 3. Failure of a terminal does not effects the network and

all the other terminals keep on working.Disadvantages4. Entire network shuts down if there is a break in the

main cable. 5. Difficult to identify the problem if the entire network

shuts down. 6. If the terminals are closed, there signal interfere. The

minimum distance maintained between terminals to avoid this problem.

2. Star Topology In a star network topology each terminal is directly

connected to the central unit through an individual cable.

This means a central unit provides a link through which a group of smaller computers and devices is connected.

This central unit may be hub, switch, router or computer.

If the central unit is a computer then it is called a host computer.

A host computer is usually a large computer such as a minicomputer or a mainframe or a file server.

Star Topology

Cont… In the star network, all interactions between

different computers in the network travel through the host computer.

Star network are typically maintained by a method called poling system.

Poling means that central computer “polls” or asks each device in the network if it has a message to send and then allows each in turn to transmit a message.

The star network is appropriate for systems that demand centralized control.

Also it represent a very popular form of configuration for time-sharing systems.

Working of Star Topology

Advantages1. Easy to install and wire. 2. No disruptions to the network then connecting or

removing devices. 3. Easy to detect faults and to remove parts. 4. If any terminal become defective it has no effect on any

other terminal.5. Data security is high because central computer control

flow of data.Disadvantages6. Requires more cable length than a bus topology. 7. If the central unit shutdown, the whole network come to

disabled. 8. More expensive because each terminal is connected

with a separate cable to the central unit.

3. Ring Topology In a ring topology network each computer is

connected to the next two other computers in a closed loop.

No single central computer exists in the ring configuration.

Ring network are typically maintained by a method called token passing.

Electronic messages are passed around the ring until they reach the right destination.

Working of Ring Topology

Advantages1. Message flow in only one direction that’s why no danger

of data collision.2. Performs better than a star topology under heavy

network load. 3. Can create much larger network using Token Ring 4. If a terminal becomes defective, it can be bypassed

without effecting the network.5. Can span larger distance than other types of network.

Disadvantages6. Much slower than the other network topologies.

Ring Topology

4. Tree/Hybrid Topology

Tree topology is a combination of star, ring, and bus network.

Groups of star-configured networks are connected to a linear bus backbone.

This bus/star hybrid approach supports future expandability of the network much better than a bus (limited in the number of devices due to the broadcast traffic it generates) or a star (limited by the number of hub connection points) alone.

Tree Topology

Advantages1. Point-to-point wiring for individual segments.2. Supported by several hardware and software venders.3. Tree topologies allow for the expansion of an existing

network, and enable organizations to configure a network to meet their needs.

Disadvantages4. Overall length of each segment is limited by the type of

cabling used. 5. If the backbone line breaks, the entire segment goes

down. 6. More difficult to configure and wire than other

topologies.

5. Mesh Topology

In mesh topology there are at least two pathways to each terminal.

In a mesh topology, computers are connected to each other by point-to-point circuits.

messages sent on a mesh network can take any of several possible paths from source to destination.

One or more computers usually become switching centers, means interlink computers with others.

Mesh Topology

Advantages1. Highly reliable. If a computer or cable become

defective it create no effect on other terminals and network remain operative.

Disadvantages2. A the number of terminals in the network

increases, this topology become more and more complicated.

3. It is more expensive than any other topology.