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A protocol means the rules that are applicable for a network. Protocol defines standardized formats for data packets, techniques for detecting and correcting errors and so on.
To understand the concept of a communication protocol, let us assume that A and B need to talk to one another. They want to exchange their ideas. But it turns out that, both, A and B are egoists. They start talking again simultaneously, then pause for breath simultaneously, and then start talking again. Now imagine the confusion and chaos. To avoid it, they must follow a set of rules while talking. For instance, say first A must talk, then he/she must give B a chance to put forward his/her ideas, and so on. This common set of rules would be known zz communication protocol for A and B.
Thus for effective use of a network it must follow a standardized protocol. There are various protocols that are used in various types of networks. For example, IBM LAN software, DEC net (Digital’s family of communication protocols), TCP/IP (Transmission Control Protocol/Internet Protocol) etc.

1. Short cable length and simple wiring layout. Because there is a single common data path connecting all nodes, the linear topology allows a very short cable length to be used. This decreases the installation cost, and also leads to a simple, easy to maintain wiring layout.
2.  Resilient Architecture. The LINEAR architecture has an inherent simplicity that makes it very reliable from a hardware point of view. There is a single cable through which all the data propogates and to which all nodes are connected.
3. Easy to extend. Additional nodes can be connected to an existing bus network at any point along its length. More extensive additions can be achieved by adding extra segments connected by a type of signal amplifier known as repeater.
Disadvantages of the Linear Topology
1. Fault diagnosis is difficult. Although simplicity of the bus topology means that there is very little to go wrong, fault detection is not a simple matter. Control of the network is not centralized in any particular node. This means that detection of a fault may have to be performed from many points in the network.
2.  Fault isolation is difficult. In the star topology, a defective node can easily be isolated from the network by removing its connection at the center. If a node is faulty on the bus, it must be rectified at the point where the node is connected to the network.
3.  Repeater configuration. When BUS type network has its backbone extended using repeaters1, reconfiguration may be necessary.
4.  Nodes must be intelligent. Each node on the network is directly connected to the central bus. This means that some way of deciding who can use the network at any given time must be performed in each node.

The next task is to distinguish between LANs and WANs. LANs are different in the following important respects.
>• The distance between the nodes is limited. There is an upper limit of approx. 10 km, and a lower limit of 1 m.
>* While WAN usually operate at speeds of less than 1 mbps (one mega bits per second), LANs normally operate at between 1 and 10 mbps. Using optical fiber technology, it is possible to achieve speeds of the order of hundreds of mbps.
>¦ Because of the short distances involved, the error rates in LANs are much lower than in WANs. LANs error rate is 1000 times lower than in WANs so are normal.
>¦ The distance limitations involved in LANs normally mean that the entire network is under the ownership and control of a single organization. This is in sharp contrast to WANs, where the network is normally operated by the countries post and telecommunications authorities rather than by its users.
It can be seen from the above, the LANs, differ from other types of network in that the area they cover is limited. This means they can operate at high speeds and with very low error rates

1. Ease of service. The star topology has a number of concentration points (where connections are joined). These provide easy access for service or reconfiguration of the network.
2. One device per connection. Connection points in any network are inherently prone to failure. In the star topology, failure of a single connection typically involves disconnecting one node from an otherwise fully functional network.
3. Centralized control/problem diagnosis. The fact that the central node is connected directly to every other node in the network means that faults are easily detected and isolated. It is a simple matter to disconnect failing nodes from the system.
4. Simple access protocols. Any given connection in a star network involves only the central node. In this situation, contention for who has control of the medium for the transmission purposes is easily solved. Thus in a star network, access protocols are very simple.
Disadvantages of the Star Topology
1. Long cable length. Because each node is directly connected to the center, the star topology necessitates a large quantity of cable. Whilst the cost of cable is often small, congestion in cable ducts and maintenance and installation problems can increase cost considerably.
2. Difficult to expand. The addition of a new node to a star network involves a connection all the way to the central node.
3. Central node dependency. If the central node in a star network fails, the entire network is rendered inoperable. This introduces heavy reliability and redundancy constraints on this node.
The star topology has found extensive application in areas where intelligence in the network is concentrated at the central node.

There are many ways of transmitting digital information through a medium. Making the choice between one technique and another is normally a question of comparison between performance, in terms of the speed and accuracy of transmission and cost.
There are two major obstacles to successful transmission : attenuation and noise. Noise can rise from a variety of sources in the environment and serves to distort the signal.
Attenuation is a measure of how much the strength of the signal is reduced in passing through the medium. It is proportional to the distance travelled.
For a particular medium, there will be a range of frequencies that can be transmitted through it.
In determining how much information can be sent through the cable, the most important aspect’ ; to consider is the width of this frequency range. This is known as the bandwidth of the medium.
Here there is a choice of baseband that carries a digital signal, or broadband that carries a radio frequency (RF) signal on the cable.
In baseband modulation, interfaces are relatively inexpensive as they require no special devices for generating the digital. Only one channel is available over the cable for communications and hence the signal can be transmitted at a single frequency at a time.