Topology Topology is physical or logical arrangement of computer. The

physical topology of a network refers to the configuration of cables, computers, and other peripherals.

Physical topology should not be confused with logical topology which is the method used to pass information between workstations.

It is the geometric representation of the relationship of all the links and linking devices i.e. nodes to each other. Depending on the need of an organization different arrangements of computers are available.

Types of Topologies

Mesh TopologyStar TopologyTree TopologyBus TopologyRing Topology

Mesh Topology

In mesh topology each node is connected to every other node by direct links. For n nodes , there would be n(n-1)/2 physical links.

To accommodate that many links, every device on the network must have n-1 input/output ports.

Every device has dedicated point to point link to every other device.

Disadvantage

• The use of dedicated links will increase the number of input/output ports.

• Installation and reconfiguration are difficult in this topology.

• The hardware required to connect each other can be prohibitively expensive.

• The cable length is much more in this topology comparing to other.

Advantages• The use of dedicated links guaranteed that each connection can carry its

own data.

• The Mesh topology does not have traffic congestion problems.

• Point to point links make fault identification and fault isolation easy in this topology.

• It is also robust in nature. Of one link is down, all the rest of the network can still continue.

• When every message sent travels along dedicated line, only intended recipient sees it. Physical boundaries prevent other users from gaining access to messages, so we can say that there is data security.

Star Topology

• As we know that mesh topology have major drawback of cabling that has been overcome here in star topology with a special network device that is HUB.

• In star topology, each device has a dedicated point to point link hub. The devices are not directly linked to each other.

• A star topology does not allow direct traffic between devices. There devices can only communicate via the hub.

Advantages• It is less expensive than mesh topology.

• Each device needs only one link and one input port to connect it to any number of other.

• It is easy to install and reconfigure.

• Less cabling is required.

• Moves and deletion involves only connection between the device and the hub.

• Robustness i.e. if one link fails, only that link is affected and so helps in easy fault identification and fault isolation.

• Hub is used to monitor link problems and by pass defective links.

Disadvantages

• Two terminals/devices can’t interact directly i.e. they have to go via central controller hub. This leads to no privacy in the network.

• The network’s dependence on the main computer is more i.e. if the central computer (HUB) breaks down the entire network stops functioning.

Tree Topology

• The tree topology is a variation of a star. The tree topology requires the computer to be linked in a hierarchical way.

• Nodes in a tree are linked to a central hub that controls the traffic to the network.

• Not every device plugs directly into the central hub. the majority of devices connect to a secondary hub that in turn is connected to the central hub.

• A central hub in the tree is an active hub. An active hub contains a repeater.

Tree Topology

• Repeater is a hardware device that regenerates the received bit patterns before sending them out.

• The secondary hubs may be active or passive hubs.

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

• The packets carrying the addresses of the destination nodes should have the complete address i.e. all the nodes above in the hierarchy must also have to be mentioned.

Advantages

• Point to point wiring for individual segments.

• Supported by several hardware and software vendors.

Disadvantages

• Overall length of each segment is limited by the type of cabling used.

• Id the backbone line breaks, the entire segment goes down.

• More difficult to configure and wire than other topologies.

Bus Topology• A linear bus topology consists of a main run of cable with a

terminator at each end.

• All nodes are connected to the linear cable.

• Ethernet and Local Talk Networks use a linear bus topology.

• This topology is a multipoint configuration. One long cable acts as backbone to link all the devices in the network. All nodes are connected to bus that runs through the network.

Bus Topology• Drop line is a connection running between the device an the

main cable.

• A tap is a connector that either splices into the main cable.

• As a signal travels along the backbone, some of its energy is transformed into heat.

• Signal becomes weaker and weaker when farther it has to travel.

• There is a limit on the number of taps a bus can support depending on the distance between those taps.

Advantages

• Easy to install. Easy to connect a computer or peripheral to linear bus. Requires less cable length.

• Backbone cable can be laid along the most efficient path, then connected to the nodes by drop line of various length.

• Only the backbone cable stretches through the entire facility. Each drop line has to reach only as far as the nearest point on the backbone.

Disadvantages

• Entire network shuts down if there is a break in the main cable.

• Terminators are required at both ends of the backbone cable.

• Difficult to identify the problem if the entire network shuts down.

• Not meant to be used as a stand alone solution in a large building.

Ring Topology

• In a ring topology, each device has dedicated point to point line configuration only with two devices on either side or 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.

• To connect n devices in a ring topology we need n cable links.

Advantages

• A ring is relatively easy to install and reconfigure, each device is linked only to its immediate neighbours.

• To add or delete a device requires moving only two connections. The only constraints are media and traffic considerations.

• Fault isolation is simplifies. In a ring a signal is circulating at all times. If one device does not receive a signal within a specified period. It can issue an alarm. The alarm alerts the network operator of the problem and its locations.

Disadvantages

• Supports unidirectional traffic.

• In a simple ring, a break in the ring can disable the entire network. This weakness can be solved by using dual ring.