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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.

Different types of switching techniques are employed to provide communication between two computers. These are : circuit switching, message switching and packet switching.
Circuit Switching
In this technique, first the complete physical connection between two computers is established and then data are transmitted from the source computer to the destination computer. That is, when a computer places a telephone call, the switching equipment within the telephone system seeks out a physical copper path all the way from sender telephone to the receiver’s telephone. The important property of this switching technique is to setup an end-to-end path (connection) between computer before any data can be sent.
Message Switching
In this technique, the source computer sends data or the message to the switching office first, which stores the data in its buffer. It then looks for a free link to another switching office and then sends the data to this office. This process is continued until the data are delivered to the destination computers. Owing to its working principle, it is also known as store and forward. That is, store first (in switching office), forward later, one jump at a time.
Packet Switching
With message switching, there is no limit on block size, in contrast, packet switching places a tight upper limit on block size. A fixed size of packet which can be transmitted across the network is specified. Another point of its difference from message switching is that data packets are stored on the disk in message switching whereas in packet switching, all the packets of fixed size are stored in main memory. This improves the performance as the access time (time taken to access a data packet) is reduced, thus, the throughput (measure of performance) of the network is improved.

Optical fibers consist of thin strands of glass or glass like material which are so constructed that they carry light from a source at one end of the fiber to a detector at the other end. The light sources used are either light emitting diodes (LEDs) or laser diodes (LDs). The data to be transmitted is modulated onto the light beam using frequency modulation techniques. The signals can then be picked up at the receiving end and demodulated. The band width of the medium is potentially very high. For LEDs, this ranges between 20 and 150 mbps and higher rates are possible using LDs.
The major problems with optical fibers are associated with installation. They are quite fragile and may need special care to make them sufficiently robust for an office environment. Connecting either two fibers together or a light source to a fiber is a difficult process.
One of the major advantages of optical fibers over other media is their complete immunity to noise, because the information is travelling on a modulated light beam.
A side effect of this noise immunity is that optical fibers are virtually impossible to tap. In order to incept the signal, the fiber must be cut and a detector inserted.
Despite its shortcomings, optical fiber is an important technology and will be a very attractive transmission indeed.