Network Topology

Network topology describes the layout or appearance of a network that is, how the computers, cables and other components within a data communication network are interconnected, both physically and logically. The physical topology describes the way in which a network is physically laid out, and the logical topology describes how data actually flow through the network. In data communication network, two or more devices are connected to from a link whereas two or more links from a topology. The topology of a network is the geometric representation of the relationship of all the links connecting the devices.

Types of Network Topology:

1.      Bus Topology: A bus topology is a multipoint data communication circuit that makes it relatively simple to control data flow between and among the computers because this configuration allows all stations to receive every transmission over the network. 

          The bus topology is usually used when a network installation is small, simple or temporary. On a typical bus network, the cable is just one or more wires, with no active electronics to amplify the signal or pass it along from computer to computer. 

         The speed of the bus topology is slow because only one computer can send a message at a time. A computer must wait until the bus is free before it can transmit. The bus topology requires a proper termination at both ends of the cable. Since, the bus is passive topology; the electrical signal from a transmitting computer is free to travel the entire length of the cable. Without termination when the signal reaches the end of the cable, it returns back and travels break up the cable. 

Advantages of Bus Topology

i) The bus topology is easy to understand, install and use for small networks.

ii) The cabling cost is less as the bus topology requires the least amount of cable to connect the computers.

iii) The bus topology is easy to expand by joining two cables with a BNC barrel connector.

iv) In the expansion of bus topology, repeaters can be used to boost the signal and increase the distance.

Drawbacks of Bus Topology

i) Heavy network traffic slows down the bus speed. In bus topology, only one computer can transmit and         others have to wait till their turn comes and there is no co-ordination between computers for reservation of transmitting time slot. 

ii) The BNC connectors used to expansion of the bus attenuates the signal considerably. 

iii) A cable breaks or loses BNC connector causes reflection and brings down the whole network causing all network activity to stop.

2. Ring Topology: In a ring topology, each computer is connected to the next computer, with the last one connected to the first. Rings are used in high-performance networks where large bandwidth is essential, e.g. time attractive features such as video and audio. In other words, a ring topology is a multipoint data communication network where all stations are interconnected is series to form a closed loop or circle. A ring topology is sometimes called a loop. Each station is the loop is joined by point-to-point links to two other stations. 

        The messages flow around the ring in one direction. There is no termination because there is no end to the ring. Some ring networks do token passing. A short message called a token is passed around the ring until a computer wishes to send information to another computer. That computer modifies the token, adds an electronic address and data and sends it around the ring. Each computer is sequence receives the token and the information and passes them to the next computer until either the electronic address matches the address of a computer or the token returns to its origin. The receiving computer returns a message to the originator indicating that message has been received. 

Advantages of Ring Topology

i) No one computer can monopolize the network because every computer is given equal access to the token.

ii) The fair sharing of the network allows the network continue function in a useful, if slower, manner rather than fail once capacity is exceeded as more users are added.

Drawbacks of Ring Topology

i) Failure of one computer on the ring can affect the whole network.

ii) It is difficult to troubleshoot the ring.

iii) Adding or removing the computers disturbs the network activity.

3. Star Topology: In star topology, all the cables run from the computers to a central location where they are all connected by a device called a hub. Stars are used to concentrated networks, where the endpoints are directly reachable from a central location when network expansion is expressed and when the greater reliability of a star topology is required. 

           Each computer on a star network communicates with a central hub that re-sends the message either to all the computers is a broadcast star network or only to the destination computer in a switched star network. The hub is a broadcast star network can be active or passive. An active hub generates the electrical signal and sends it to all the computers connected to it. This type of hub is usually called a multiport repeater. Active hubs require external power supply. A passive hub is a wiring panel or punch down block which acts as a connection point. It does not amplify or regenerate the signal. Passive hubs do not require electrical power supply. Several types of cables can be used to implement a star network. 

Drawbacks of Star Topology

i) If the central hub fails, the whole network fails to operate.

ii) Many star networks require a device at the central point to rebroadcast or switch the network traffic.

iii) The cabling cost is more since cables must be pulled from all computers to the central hub.

4. 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 two devices if connects. A fully connected mesh network therefore has n(n-1)/2 physical channels to link hn devices. To accommodate those links, every device on the network must have n-1 input/output ports. 

Advantages of Mesh Topology

i) The use of dedicated links guarantees that each connection can carry its own data load, thus eliminating traffic problems.

ii) A mesh topology is robust because the failure of single computer does not bring down the entire network.

iii) It provides security and privacy because every message sent travels along a dedicated line.

iv) Point to point links make fault diagnose easy.

Drawbacks of Mesh Topology

i) Since every computer must be connected to every other computer installation and configuration is difficult.

ii) Cabling cost is more.

iii) The hardware required connecting each link input/output and cable is expensive.

5. Tree Topology: A tree topology is the variation of a star. As in a star, nodes in a tree are linked to a central hub that controls the traffic to the network. However, not every computer plugs into the central hub, majority of them are connected to a secondary hub which, in turn, is connected to the central hub. The central hub in the tree is an active hub which contains repeater. The repeater amplifies the signal and increases the distance a signal can travel. The secondary hubs may be active or passive. A passive hub provides a simple physical hub provides a simple physical connection between the attached devices.

Advantages of Tree topology

i) It allows more devices to be attached to a single hub and can therefore increase the distance of a signal can travel between devices.

ii) It allows the network to isolate and priorities communications from different computers.

Drawbacks of Tree Topology

i) If the central hub fails, the system breaks down.

ii) The cabling cost is more.

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