Network topologies
March 31, 2008
Physical and Logical Topologies:
A network topology is the shape or structure of a network. Physical topology describes the actual appearance or layout of the network. Logical topology describes the flow of data through the network. There are various types of network topologies:
a) Star
Each node is connected to a central device using a point to point link. Central devices are usually known as hubs or concentrators. The hub receives signals from a node and repeats the signal to other nodes.
When more than one hub is required, they are inter-linked using a single cable known as a backbone. The performance of the network is maintained by using a backbone cable that is capable of higher data speeds than the rest of the network e.g. using fiber optic cable to link hubs, while UTP cable is used to link workstations.
Cascaded hubs are also commonly used. The hubs may be linked together using twisted pair crossover cables or in some cases, a button on the hub is used to effect the crossover at a particular point.
Advantages:
- They are easy to configure and reconfigure
- They are easy to troubleshoot because all data goes to a central point.
- Easy to manage and monitor the network.
- Fault in the media, network card or node (device) are automatically isolated.
Disadvantages:
- A hub failure results in failure of all entire section of the network. It provides for a single point of failure.
- Its time consuming to install because there is a separate segment per route.
- It requires more cable relative to other topologies.
b) BUS:
This is a linear topology with all nodes attached directly to the main cable (backbone). The bus is terminated at both ends of the cable to absorb the signal from the cable.
Advantages:
- It is easy to install and use established standards.
- They require little cable compared with other topologies
Disadvantages:
- They are difficult to reconfigure
- They are difficult to troubleshoot
- All devices are affected by media faults
c) RING
Computers are connected in a circle. The ring comprises a series of point to point links between each device. Computers are either attached directly to the ring or indirectly via an intermediary device such as Multi-station Access Unit (MSAU).
Twisted pair cable connects the computer to the MAU. Signals pass from device to device in a single direction. The common implementations of a ring topology are: IBM’s token ring and Fiber Distributed Data Interface (FDDI) or Copper Distributed Data Interface(CDDI).
Advantages:
- Ring networks use a relatively small amount of cable.
- They are simple to install.
- Two ring systems (using dual counter-rotating rings) can be used to provide fault tolerance.
Disadvantages:
- Difficult to reconfigure
- Troubleshooting can be difficult
- Media or device failure can affect all devices.
d) Hybrid
This is combining two different topologies within the same network to benefit from the advantage of both topologies. There can be star bus or star ring.
e) Mesh:
Mesh is commonly used in WAN. It is often found in public networks like the Internet. It require every device has a point to point connection to every other device on the network.
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