Network Topologies:
The physical arrangement of computers in a network is called as topology. The path taken by data from sender to receiver computer mainly depends upon the topology of the network. Every computer is linked to the network but the way how it is linked is different in different topologies.
The choice of topology depends upon the following factors:
1. Cost
2. Availability of physical communication line.
3. Reliability of the entire system.
4. Number of cables required.
5. Expandablility of the system.
6. Transmission delays.
7. Maximum distance.
8. Maximum number of nodes.
In LAN, various Topologies exist :
6.5.1 STAR Topology
Star Topology is forerunner of all other topologies. Here, every PC on the network is connected to a central controller, called Hub, as shown in fig 6.5.
It is topology upon which our telephone network is built. Every communication from every user's station goes through the central controller before reaching it destination This network is cheapest of all. To connect five systems we just need five cables But failure chances are more if hub fails, whole network will go down. The star topology is very easy to install and reconfigure. The stations can be added or removed without affecting the working of other nodes on the network. Here, only one connection is needed to add a single node to the network.
Merits
1. Cheaper media of Networking.
2. Less number of Cables required.
3. Transmission delays between two nodes do not increase by adding new nodes to the network.
4. If any node fail, the remaining portion of the network is unaffected.
5. Easy to add new nodes.
Demerits
1. The hub offers limited number of connections.
2. If hub fails, the entire network fails.
3. The cabling cost is more since cables must be pulled from all computers to the central hub.
4. Many star networks require a device at the central point to rebroadcost or switch the network traffic.
6.5.2 Ring Topology
Fig. 6.6 shows the circular a ring arrangement of a computer network.
FIGURE 6.6
Ring Topology avoids dependance on Central Computer. Each node on ring can send a message to other terminal with a unique-address.
A node receives data from one of its adjacent node. The only decision a node has to take is whether the data is for its use or not. If it is addressed to it utilises it. Otherwise it passes it on to the next node.
Addition of a new station involves breaking the ring and adding an Ring-Interface Unit. (RIU)
Every Data Unit in ring contains source & destination address.
Merits
1. It is a True Distributed Data processing system.
2. More reliable than STAR because communication is not dependent on Hub
3. In ring topology, a shorter cable length is needed as compared to star topolog
4. Ring topology delivers fast and efficient performance.
5. It is suitable to setup high speed network using optical fibers.
Demerits
1. Addition of New Node increases the communication delays.
2. Complicated control software.
3. In the ring topology, the failure of a node causes failure in the network.
4. As the single channel is shared by various nodes, it is difficult to diagnose the fault.
5. It is difficult to add, remove or reconfigure the nodes in the network.
6.5.3 BUS Topology
In bus topology, a single cable acts as a backbone of the network and all the nodes or computers are attached to it by using T connectors as shown in figure 6.7. Each computer on the network has a direct connection with the backbone cable.
When one computer sends a signal up the cable; all the computers on the network receive the information, but the one with the address that matches the one encoded in the message accepts the information while all the others reject the message.
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 the ends of the cable. Therefore it has special devices called "terminators" which absorb the signal on both ends.
As with ring structure, Bus Network has also a unique address. If geographic coverage needs to be expanded, repeators which interconnect two buses are required.
In Bus topology, least length of physical Transmission medium is used. Coverage can be increased by extending the bus through use of repeators.
Image
The Bus Topology, is one of most popular topology. One reason for popularity is that, its wiring requirements are comparitively simple and growth is obtained easily & quickly. With most bus-structured networks, workstations can be added without distributing the rest of network.
Merits
1. Less number of cable required.
2 Easy to expand.
3 Nodes can be added without distributing the rest of network.
4. Easy to understand and install.
Demerits
1. If the communication line fails, the entire system breaks down.
2. It is very difficult to repair the fault in this topology.
3. Heavy network traffic slows down the bus speed. In bus topology only one computer can transmit and other have to wait till their turn comes and there is no co-ordination between computers for reservation of transmitting time slot.
6.5.4 Fully Associated Topology
In this topology, all nodes are directly connected to each other as shown in figure 6.8. Thus, it is a completely connected network with a seperate physical link
for connecting each node to the other node. The control is distributed with each Computer deciding its communication priorities. In this mode cost is high because more wires are needed to connect nodes but chances of failure are very less.
Merits
1. Very Reliable.
2 Each node of network need not have individual routing capacity.
3. Communication is very fast between two nodes.
4. It is robust because the failure of any one computer does not bring down the entire network.
5. It provides security and privacy because each message sent travels along a dedicated line.
Demerits
1. Most Expensive networks
2. Adding more nodes will increase cost many folds.
6.5.5 Mesh Topology
In mesh topology, each node is connected to more than one nodes of the system. In this way, there exist multiple paths between two nodes of the network. In case of failure of one path, the other one can be used.
A fully connected mesh topology is same as fully associated topology. Advantages and Disadvantages are same as fully associated topology.
6.5.6 Multi Drop Topology
The draw backs of fully interconnected network were removed in this topology by using a single cable around all nodes as shown in figure 6.9. In this topology all nodes are connected via a single link. i.e. Single transmission medium is shared by all nodes. Whenever a message is transmitted, sender and destination addresses are attached as tag and message floats on a single available line. Receiver recieves it and send back acknowledgement message which again floats on same line. There may be many messages floating at a time, that is why name multidrop Configuration. It is also called broadcasting network for LANs.
The topology is preferred by local area networks where high speed communication channel is required and computers are confined to a small area. As compared to fully connected network the number of physical lines is highly reduced to one line and thus the cost. This is reliable network. Failure of any node does not affect other nodes and the network. Addition of new nodes is very simple as compared to fully connected network. But failure of communication line fails the entire network
Merits
1. Reduction in total physical Lines.
2. Reliability is high.
3. Addition of new nodes is easy.
4. Failure of any node does not affect the network functioning for other computer
Demerits
1. If line fails, entire system breaks down.
2. Each node must have good & quick decision making capability.
6.5.7 Hybrid Topology
An hybrid topology is simply a combination of two or more then two topologies. The exact shape configuration of the network depends on the needs and the over all organisational structure of the company involved. In some cases, the hybrid topology. may have components of star, ring and completely connected networks topologies as shown in figure 6.10.
6.5.8 Tree Topology
In this topology all nodes are connected in a hierarchical fashion and requires information to flow through the branches as shown in fig. 6.11.
By the term "hierarchical flow", we mean that the data travels level by level. It starts from one level and travels upto one level down and then to subsequent levels.
FIGURE 6.11
In the tree topology, the server or the host computer is connected at the top. To the server, the most important terminals are attached at next level and clients are attached to these terminals. Data can flow from top to bottom and bottom to top in level by level manner. Tree topology is an extension to star topology.
Merits
1. The controlling and control software are very simple.
2 A new node at lowest levels can be added very easily.
Demerits
1. If top most computer fails it may lead to failure of entire network.
2. If a new node is to be added at higher levels it is very difficult.
3. If the levels increase it may lead to a complex network.
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