What should you know about a computer network & WHAT IS DATA TRANSMISSION CHANNELS.

What should you know about a computer network? 

A computer network requires that the computers must somehow be connected with each other. Thus, you require a physical connection between two or more computers. This connection may be through physical wired media or wireless medium. In addition, it will require certain devices that will enable the connection. These concepts are explained in brief in section 1.3 and 1.7 A related question here is: Are the computer in networks connected arbitrarily or there exists some architecture and structure? Section 1.4 and 1.5 provide details of some simple topological structures and network architectures for networks. It also details the classification of networks. Another related point is how the data will be transmitted over these connections. We have provided some information on these points in section 1.2.3. For more details on these topics you should refer to the further readings.

Finally, one of the major issues is how two computers will be able to exchange information over the network. This will require discussion on the term protocols and networking software. Section 1.6 and 1.8 covers some basic concepts of these. For more details, you may refer to further readings.

WHAT IS DATA TRANSMISSION CHANNELS.?

The data transmission has to be done over a transmission channel or media. It can be classified as: a) Guided Channels b) Unguided Channels

Guided Media 

Guided media provide a physical connection between two devices. A signal traveling through guided media is directed and contained within the physical limits of the medium. There are several different Guided media, however we define only the most popular as given below:

 a) Twisted pair cable

 b) Optic Fiber cable

a) Twisted Pair Cable Twisted pair cable is still the most common transmission media. A twisted pair cable consists of two conductors which are normally made of copper. Each conductor has its own plastic insulation typically 1 mm thick. These cables are twisted together. The wires are twisted in a helical form, similar to a DNA molecule. Twisting is done to reduce crosstalk. Twisted Pairs (Figure) are very effective for relatively short distances (a few hundred feet), but can be used for up to a few kilometers. A twisted pair has a bandwidth to distance ratio of about 1 MHz per kilometer. The performance of the twisted pair can be substantially improved by adding a metallic shield around the wires. Shielded wires are much more resistant to thermal noise and crosstalk effects. Twisted pairs are used for long distance connections e.g. telephone lines which are usually organized as larger cable containing numerous twisted pairs.Twisted pair cabling comes in several varieties, two of which are very important: Category 3 and Category 5. Category 5 has more twists per centimeter resulting in less crosstalk and a better quality signal.

                                 

                                                                                                                          Figure . Twisted Pair Cable

 b) Optical Fiber An optical fiber consists of two concentric cylinders: an inner core surrounded by a cladding. Both the core and the cladding are made of transparent plastic or glass material as shown in the Figure 1.4, which transmit signals in the form of light. Optical fiber use reflections to guide light through a channel. The density of the core and cladding must differ sufficiently to reflect the beam of light instead of refracting.

The core is used for guiding a light beam, whereas the cladding (which has a different refractive index) acts as a reflector to prevent the light signal instead of electrons, it does not suffer from the various noise problems associated with electromagnetic signals. The signal is usually generated by a laser or Light Emitting Diode (LED). Optical fibers can provide bandwidth to distance ratios in order of 100s of MHz per kilometer. Like other cables, hundreds of optical fibers are usually housed within one cable. They are being increasingly used as telecommunication carriers for long distance digital trunk lines. Current trends promise that they will replace twisted pair residential loops in the near future.

                                                          
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 Advantages:-

1) Higher Band width – it can support higher band width and hence can transfer data at a higher rate.

2) Less signal attenuation – its transmission distance is greater than the twisted pair and it can run for 50Kms without regeneration.

3) Immunity to electromagnetic interface

4) These cables are much lighter than the copper cables

5) These cables are more immune to tapping than the copper cables.

 Disadvantages:-

1) Installation or maintenance – it needs expertise which is not available everywhere.

2) Unidirectional – Propagation of light is unidirectional and we need two fibers for bidirectional communication.

3) Costly – the cables and interfaces used are relatively expensive.

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