Course Instructor : Sehrish Rafiq

Department Of Computer Science

University Of Peshawar

Data Communications & NetworkingLecture-03

Lecture overview

Topology Mesh Star Tree Ring Bus

Topology The term topology refers to the way a network is

laid out, either physically or logically. Two or more devices connect to a link. Two or more links form a topology. The topology of a network is the geometric

representation of the relationship of all the links and linking devices(usually called nodes) to each other.

The topology describes that how the devices in a network are interconnected rather than their physical arrangement.

Topology continued… A consideration when choosing a topology is the relative

status of the devices to be linked. Two relationships are possible. Peer-to-Peer: where the devices share the link equally. Primary-Secondary: where one device controls traffic and

the others can transmit through it. Five basic topologies Bus Star Mesh Tree Ring Note: Ring & Mesh topologies are more convenient for Peer-to-Peer

transmission while star & Tree topologies are better for primary–secondary transmission. A bus topology is convenient for either.

Mesh topology

Mesh topology

In mesh topology every device has a dedicated point-to-point link to every other device.

The term dedicated means that the link carries traffic only between the two devices it connects.

A fully connected mesh network has n(n-1)/2 physical channels to link n devices and every device must have n-1 I/O ports.

Mesh topology continued… Advantages: Each connection can carry its own data load. This topology is robust. If one link becomes unusable, it does

not affect the entire system. Privacy or security Easy fault identification. Disadvantages: Large amount of cabling & I/O ports are required. Installation & reconfiguration are difficult because every device

must be connected to every other device. The bulk of wiring can be greater than the available space( in

walls, ceilings or floors) can accommodate. The hardware(I/O ports & cabling) required to make a fully

connected mesh network can be prohibitively expensive.

Star Topology Each device has a dedicated point-to-point link only to a

central controller usually called a hub. The devices are not directly connected to each other. Like mesh a star topology does not allow direct traffic

between devices.

Star Topology Continued… If one device wants to send data to another, It sends

data to the controller which then relays the data to the other connected device.

Usually two types of Hubs are available. Passive Hub Active Hub

Types of Hubs

Passive Hub: A passive Hub is used to connect computers in a broadcast

network. The signal sent to a passive hub is sent to all workstations

without any regeneration or amplification. Passive hubs do not require any external power source. Active Hub: An active hub uses an external power source and

regenerates the signal, therefore greater cable distances are allowed.

A switched hub directs the signal directly to the recipient. Active hubs also contain diagnostic features to aid in

network trouble shooting. Active Hub is more expensive than a passive Hub.

Advantages of using star topology

A star topology is less expensive than a mesh topology. Each device needs only one cable and one I/O port to

connect with any number of devices. Easy to install and easy to reconfigure. Far less cabling is required therefore space problems are

rare. Addition, deletions and moves involve only one

connection between that device and hub. If a link fails only that link is affected. Easy fault identification & fault isolation because of

central Hub.

Disadvantages of using star topology

More cabling than any other topology(such as bus, ring or tree ) except Mesh because every device must be connected to the central hub.

Active hub is expensive.

A central hub failure can damage the whole network.

Tree Topology

Tree topology A variation of star topology.

As in star, nodes in a tree are linked to a central Hub that controls the traffic to the Network.

However, not every device is directly plugged in to the central Hub.

The majority of the devices connect to a secondary Hub that in turn is connected to the central Hub.

The central Hub in the tree is an active Hub.

Tree topology continued… An active Hub contains a repeater which is a hardware

device that regenerates the received bit patterns before sending them out.

Repeating strengthens transmissions and increases the distance a signal can travel.

The secondary Hubs may be active or passive hubs.

A passive Hub provides a simple physical connection between the attached devices.

Advantages & Disadvantages of using Tree

The advantages and disadvantages of tree are almost the same to that of star.

However, because of the addition of secondary Hubs more devices can be connected with the central Hub and can increase the distance a signal can travel.

It allows the network to isolate and prioritize communications from different computers.

By means of prioritizing, the network designers and operator can guarantee that time-sensitive data will not have to wait for access to the network.

Bus Topology A Bus topology is a multipoint. One long cable acts as a backbone to link all the

devices in a network. Nodes are connected to the bus cable by drop lines and

taps. A drop line is a connection running between the device

and the main cable. A tap is a connector that either splices in to the main

cable or punctures the sheathing of a cable to create a contact with the metallic core.

Bus topology Continued… As a signal travels along the backbone,some of its

energy is transformed in to heat.

Therefore it becomes weaker and weaker.

For this reason there is a limit on the number of taps a bus can support and on the distance between those taps.

Advantages & Disadvantages of using bus topology

Advantages: Ease of installation. Less cabling than mesh, star or tree. Only one backbone cable stretches through the entire facility. For n devices to connect only one backbone cable and n drop

lines are needed. Disadvantages: Difficult reconfiguration and fault isolation. It can be difficult to add new devices. Signal reflection problem. Adding new devices may require replacement of the

backbone. A fault in backbone can damage the entire network causing

noise problems in both directions.

Ring Topology In ring topology every device has a dedicated point to

point line configuration only with the two devices on either side of it.

A signal is passed along the ring in one direction from device to device until it reaches its destination.

Each device in the ring incorporates a repeater. Token passing is frequently used in ring topology.

Advantages & disadvantages of using ring topology Advantages: Easy to install and easy to reconfigure. Adding and deleting a device only involves two

connections. Easy fault isolation. If one device does not receive a signal within a specified

period,it can issue an alarm. Disadvantage: Uni-directional traffic. A break in a ring can disable the entire network. This weakness can be solved by a dual ring.

Hybrid TopologiesOften a network combines several topologies

as subnetworks linked together in a large topology.

For example, one department of a buisness may have decided to use a bus topology while another department has a ring.

All can be connected with a central controller in a star toplogy.

Have a useful sunday!