Network Topology





The pattern of combination of computers for transferring data is called network topology. It is a physical layout of the network’s computers, terminals, and links. These are Star, Ring, Bus, Mesh and Hybrid topology.

 

 

(a) Star Topology:

 

All the nodes are connected to server or master computer. The master computer controls all the local work stations or computers (nodes). The failure of nodes is detected easily. Host computer controls communication between any two computers. Local computers are not attached directly to each other. In this topology, several devices are connected to central devices.

Advantage of Star Topology:

  • Star topology has minimal line cost because only n-1 lines are required for connecting ‘n’ nodes.
  • Transmission delays between two nodes do not increase by adding new nodes to the network because any two nodes may be connected via two lines only.
  • If any of the local computers fail, the remaining portion of network is unaffected.

 

(b) Ring Topology:

 

A networking topology, in which one node is connected with other, and so on, to make ring like pattern, is called ring topology. Each node is connected to repeater, all repeaters are connected to nearby repeater, in this way, a pattern is formed which look like as ring.

Ring topology

Ring topology

  • Local repeaters are responsible for data transmission from node to cable and cable to node. Data is transmitted in the form of frames. Each frame has control part (address) and data part. Frame transmitted by one node, circulates unidirectional through cable around network and at destination node, frame is copied and again circulated to return back to source node where it is removed.
  • No host or master computer (server) is used to control data communication.
  • Failure of cables or node disrupts the topology.
  • Not very popular, due to complicated software and error prone.
  • Communication delay is the directly proportional to number of node (n) and links.

 

(c) Bus or Linear Topology:

 

It is a common cable called trunk or data highway also. All the nodes (local Computers) are connected to it. Its each end has a terminator to stop ill flow of data out the bus.

Bus Topology

Bus Topology


(i) Frames: Frame is a block of data transmitted from node. It has two parts- control part and data part. Frames travel across the full path of bus in both directions, where its address is matched, accessed to that node.

(ii) In case, if any node is failed, there is no hazardous effect to other nodes. If bus is failed, whole network is failed.

 

 

(d) Tree Topology:

 

It is a modification of bus topology. Coaxial cable is often used to connect computers in a bus topology. The cable, in most cases, is not one length, but many short stands that use T-connectors to join the ends. T-connectors allow the cable to branch off in a third direction to enable a new computer to be connected to the network.

Tree Topology

Tree Topology


Special hardware has to be used to terminate both ends of the coaxial cable such that a signal traveling to the end of the bus would come back as a repeat data transmission.

Since a bus topology network uses a minimum amount of wire and minimum special hardware, it is inexpensive and relatively easy to install. One can simply connect a cable and T-connector from one computer to the next and eventually terminate the cable at both ends.

The number of computers attached to the bus is limited, as the signal loses strength when it travels along the cable. If more computers have to be added to the network, a repeater must be used to strengthen the signal at fixed locations along the bus.

The problem with bus topology is that if the cable breaks at any point, the computers on each side will lose its termination. The loss of termination causes the signals to reflect and corrupt data on the bus. Moreover, a bad network card may produce noisy signals on the bus, which can cause the entire network to function improperly.

 

(e) Mesh Topology:

 

In this topology, all nodes are connected to each other. So, it is sometimes called completely connected networks. In this topology, more cables are used and linking cost depends on the number of nodes (n).

Mesh Topology

Mesh Topology

 

Advantage of Mesh Topology:

  • This type of network is very reliable, as any line breakdown will affect only communication between the connected computers.
  • Each node of the network need not have individual routing capacity.
  • Communication is very fast between any two nodes.

 

(f) Hybrid Topology:

 

The hybrid topology is the mixture of all the topology. In this topology, ring, star and bus topology are combined to obtain better result. Some standard hybrid topologies are described below:

  • Star Bus: The star bus is a combination of the bus and star topologies. In a star-bus topology, several star topology networks are linked together with linear bus trunks. If one computer goes down, it will not affect the rest of the network. The other computers can continue to communicate. If a hub goes down, all computers on that hub are unable to communicate. If a hub is linked to other hubs, those connections will be broken as well.

 

  • Star Ring:  The start ring (sometimes called a star-wired ring) appears similar to the star bus. Both the star ring and the star bus are centered in a hub that contains the actual ring or bus. Linear-bus trucks connect the hubs in a star bus, while the hubs in a star ring are connected in a star pattern by the main hub.

 

Advantages and Disadvantages of Topologies:

 

Topology Advantages Disadvantages

Bus

(i) Use of cable in economical.

(ii) Media is inexpensive and easy to work with.

(iii) System is simple and reliable.

(i) Network can slow down in heavy traffic.

(ii) Problems are difficult to isolate.

(iii) Cable break can affect many users.

Ring

(i) System provides equal across for all computers.

(ii) Performance is even despite many users.

(i) Failure of one computer can impact the rest of the network.

(ii) Problems are hard to isolate.

(iii) Network reconfiguration disrupts operation.

Star

(i) Modifying system and adding new computers is easy.

(ii) Centralized monitoring and management are possible.

(iii) Failure of one computer does not affect the rest of the network.

If the centralized point fails, the network fails.

Mesh

System provides increased redundancy and reliability as well as case of troubleshooting.

System is expensive to install because it uses a lot of cabling.



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