Network Topologies

• Refers to the way in which the individual computers, called nodes, of a network are linked together

Geometric representation of the relationship of all the links and nodes to each other

• Two relationships: peer-to-peer, primary-secondary

Mesh Topology

Hi. Bus Topology

iii. Ring Topology

iv. Star Topology

V. Tree Topology

Mesh Topology

• Every device has a dedicated point-to-point link to every other device

• The number of links needed to connect n nodes with mesh topology is n(n-1)/2

Advantages of Mesh Topology

• The use of dedicated links guarantees that each connection can carry its own data load, thus eliminating the traffic problems that can occur when links must be shared by multiple devices.

• Robust: If one link becomes unusable, it does not incapacitate the entire system.

• Privacy or security: When every message travels along a dedicated line, only the intended recipient sees it.

• Point-to-point links make fault identification and fault isolation easy. Traffic can be routed to avoid links with suspected problems.

Disadvantages of Mesh Topology

• Difficult to install

• Difficult to reconfigure

• The hardware required to connect each node (I/O ports and cable) can be expensive.

• The sheer bulk of the wiring can be greater than the available space (in walls, ceilings, or floors) can accommodate.

Bus Topology

• A bus topology has multi point connections.

• 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.

• All nodes will receive a message but only the addressed node will respond

• When a node wants to send a message, it appends the destination address to the message and broadcasts it to the line if it is free

• As message travels, each node checks whether it is addressed to it and if it is addressed to it, the message is picked up and an acknowledgement is sent to the source

Advantages of Bus Topology

• Reduction in the physical line

• Reliability is high because the failure of a computer does not affect the functioning of the network

• Addition of new computers is easy

• Easy to install

• Cost of network is low

• Can be extended by using repeaters

Disadvantages of Bus Topology

• Each node must have a good communication and decision making capability

• If the line fails the entire system breaks down

• Heavy network traffic can slow a bus

• Fault isolation is difficult

• Signal reflection at the taps can cause quality degradation

Ring Topology

• Each device has a dedicated point-to-point connection with only 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. When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along.

Advantages of Ring Topology

• Relatively easy to install and configure

• Link failure can be easily found

• No one node can monopolize the network

• Communication is not dependent on a single node

Disadvantages of Ring Topology

• Communication delay is directly proportional to the number of nodes

• Maximum ring length and number of devices is limited

• Adding or removing nodes disrupts the network

Star Topology

• Each device has a dedicated point-to-point link only to a central controller, usually called a hub or a switch

• Centralized control

Advantages of Star Topology

• Has less line cost

• Each device needs one link and one I/O port to communicate to any number of devices

• Additions, moves, deletions involve only one connection

• Transmission delay between 2 nodes does not increase by adding new nodes

• If a computer/link fails, the remaining portions will not be affected

• Provides a centralized control and problem diagnosis

• Access protocols are simple

Disadvantages of Star Topology

• The system crucially depends on the central hub

• Bottlenecks due to overloaded hub/switch

• More cabling is required in star than in other topologies like bus and ring

• Each device requires its own cable segment

Tree Topology

• Shape of network is an inverted tree with central root branching and sub-branching to the extremities of the network

• It is a variation of star topology

• There is one central hub and many secondary hubs

• Majority of the devices connect to secondary hub and that in turn connects to the central hub\

Central hub is an active hub that contains a repeater( which regenerates the received bit patterns before sending them out)

• Secondary hubs may be active or passive (provides a simple physical connection between the attached devices)

• It is a variation of star topology

• There is one central hub and many secondary hubs

• Majority of the devices connect to secondary hub and that in turn connects to the central hub\

Central hub is an active hub that contains a repeater( which regenerates the received bit patterns before sending them out)

• Secondary hubs may be active or passive (provides a simple physical connection between the attached devices)

Advantages of Tree Topology

• Here, we divide the whole network into segments (star networks), which can be easily managed and maintained.

• Expansion of Network is possible and very easy.

• Allows the network to isolate and prioritize communications from different computers

• If one segment is damaged, other segments are not affected.

Disadvantages of Tree Topology

• Because of its basic structure, tree topology, relies heavily on the main bus cable, if it breaks whole network is crippled.

As more and more nodes and segments are added, the maintenance becomes difficult.

Hybrid Topologies

• A combination of several topologies

Selecting a Topology

The main considerations in selecting a topology are:

• Availability and cost of physical communication lines and line bandwidth

• The capability of a node to route information

• Delay due to routing of information

• Reliability of communication between nodes when there is a breakdown of a line or node

• Strategy of controlling communication between nodes- centralized or distributed

• Size of a network - the no. of nodes and geographical span

• Expandability of the system