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2009-11-01 14:01:32
2009-11-01 14:01:32

TOPOLOGY

Definition:

The term physical topology refers to the way in which a network is laid out physically. It refers to the configuration of cables, computers, and other peripherals. The topology of a network is the geometric representation of the relationship of all the links and linking devices (usually called nodes) to one another.

There are six types of topologies:

i. Mesh topology.

ii. Star topology.

iii. Ring topology.

iv. Tree topology.

v. Bus topology.

vi. Hybrid topology.

Mesh Topology:

In a 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. Here, if we have n nodes, then we need to connect to n-1 nodes and n(n-1) physical links. However, if each physical link allows communication in both directions (duplex mode), we need n(n-1)/2 links.

Advantages of a Mesh Topology

  • Eliminates traffic problems in links sharing.
  • If one link becomes unusable, it does not incapacitate the entire system. Thus, act as robust.
  • It has privacy and security.
  • Point-to-point link make fault identification and fault isolation easy.

Disadvantages of a Mesh Topology

  • Installation and reconnection are difficult.
  • The hardware required to connect each link (I/O ports and cable) is expensive.
  • It is generally too costly and complex for practical networks.

Star Topology:

In local area networks where the star topology is used, each machine is connected to a central hub. The star topology allows each machine on the network to have a point to point connection to the central hub. All of the traffic which transverses the network passes through the central hub. The hub acts as a signal booster or repeater which in turn allows the signal to travel greater distances.

Advantages of a Star Topology

  • Easy to install and reconfigure.
  • No disruptions to the network when connecting or removing devices.
  • Easy to detect faults and to remove parts.
  • Less expensive.
  • Includes robustness, that is, if one link fails, only that link is affected, other links remain active.

Disadvantages of a Star Topology

  • If the hub fails, the whole system is dead.
  • If the hub, switch, or concentrator fails, nodes attached are disabled.
  • Requires more cable length than a bus topology.
  • More expensive than bus topologies because of the cost of the hubs, etc.

Ring Topology:

In local area networks where the ring topology is used, each computer is connected to the network in a closed loop or ring. The signal passes through each machine or computer connected to the ring in one direction, from device to device, until it reaches its destination. Each machines or computers connected to the ring act as signal boosters or repeaters. When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along.

Advantages of a Ring Topology

· It is relatively easy to install and reconfigure.

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

Disadvantages of a Ring Topology

· Only one machine can transmit on the network at a time.

· The failure of one machine will cause the entire network to fail.

Tree Topology:

The type of network topology in which a central 'root' node (the top level of the hierarchy) is connected to one or more other nodes that are one level lower in the hierarchy (i.e., the second level) with a point-to-point link between each of the second level nodes and the top level central 'root' node, while each of the second level nodes that are connected to the top level central 'root' node will also have one or more other nodes that are one level lower in the hierarchy (i.e., the third level) connected to it, also with a point-to-point link, the top level central 'root' node being the only node that has no other node above it in the hierarchy

Advantages of a Tree Topology

  • Point-to-point wiring for individual segments.
  • Supported by several hardware and software venders.

Disadvantages of a Tree Topology

  • Overall length of each segment is limited by the type of cabling used.
  • If the backbone line breaks, the entire segment goes down.
  • More difficult to configure and wire than other topologies.

Bus Topology:

In local area networks where bus technology is used, each machine is connected to a long, single cable. The cable acts as a backbone to link all the devices in a network. Each computer or server is connected to the single bus cable through drop lines and some kind of connector. A terminator is required at each end of the bus cable to prevent the signal from bouncing back and forth on the bus cable.

Advantages of Bus Topology

  • Easy to connect a computer or peripheral to a linear bus.
  • Requires less cable length than mesh or star topologies.
  • It is cheaper than any other topologies.

Disadvantages of Bus Topology

  • If the network cable breaks, the entire network will be down.
  • 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.
  • Include difficult reconnection and fault isolation.
  • The managing cost of network is too high.
  • Addition of new devices requires modification or replacement of the backbone.

Hybrid Topology:

Hybrid networks use a combination of any two or more topologies in such a way that the resulting network does not exhibit one of the standard topologies (e.g., bus, star, ring, etc.). A hybrid topology is always produced when two different basic network topologies are connected.

Advantages of a Hybrid Topology

· It provides a better result by it.

· It can be designed in many ways for various purposes.

Disadvantages of Hybrid Topology

· It is costly.

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

References:

· Data Communication and Networking By Behrouz A Forouzan.

· Wikipedia and other websites.

Created By- A.H.M Tasbir Farid,

B.Sc in Telecommunication & Electronic Engineering, Hajee Md. Danesh Science & Technology University, Dinajpur, Bangladesh.

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