Before a detailed analysis is undertaken of the principal transmission media it should be established that different media have varying properties, which consequently result in them being used in particular situations.

Co-axial Cable

Co-axial cable - sometimes referred to as "co-ax" - is based on a central copper core encased in a plastic sheath, which is surrounded by a copper mesh all encased in a plastic coating. The signal is carried on the central core with the outer conductor or mesh forming a screen to outside electrical noise. The most common example of co-ax is television aerial cable.

Originally this form of cable was the most common form of LAN cable because:

• It has a high capacity
• It has a resistance to interference

However, it had two main disadvantages:

• Its thickness limited its ability to be run through small cable ducts and around tight angles
• The cost of co-ax is relatively high in comparison to more traditional forms of data cabling.

While co-ax is still widely used, most of the networks which specified this cable type are now able to operate on other types such as twisted pair or optical fibre. Co-ax has two main forms:

• Thick - This form of cabling, often known as "yellow cable", was the original co-ax cable. Its capacity in terms of speed and distance are great, but the cost of the cabling is high and its thickness prohibitive in tight cabling ducts.
• Thin - This form of cabling was introduced to reduce the cost of cabling networks. This was mainly associated with Ethernet and became known as "Cheapernet". Its main sacrifice over "thick" cable is the distance that a single branch can run. However, the cable is much cheaper and thinner, and therefore overcomes some of the disadvantages of the original cable.

Twisted Pair

Twisted pair cabling is the most common form of cabling. The pairs are twisted to reduce the interference between adjacent pairs in the cable. Usually a series of pairs are encased in a single sheath and colour coded to reduce the numbers of physical cables which need to be pulled through the ducting. There are three main types:

• Unscreened - The main advantages of this type of cable are its low cost, easy handling and the fact that cables already laid for other devices can often be re-used to implement a LAN. However, the main drawbacks are its relatively high error rate and the short distances which can be run without signal regeneration.
• Screened - This form of cabling has been introduced to reduce the number of errors due to outside interference. The wire is encased in a metallic braid, somewhat similar to co-axial cable. Although this reduces errors it also raises costs.
• Uniform - Recent innovation has produced this very high specification twisted pair where each wire in the pair is twisted uniformly during manufacture. This eliminates most cross wire interference and the whole cable is also screened from outside interference. Although this provides run distances and capacity similar to co-ax the cable is extremely expensive.

Optical Fibre

This form of cable is made up of one or more continuous strands of glass. Each is surrounded by cladding and then reinforcing material to protect the fibre, and the whole cable is clad in a sheath.

However, rather than using electrical pulses, light is used to transmit information, which is read at the receiving end and converted into electrical pulses for the processing device. The advantages of optical fibre are:

• It can carry a much higher quantity of data and is immune to normal interference, hence errors are negligible.
• The signals do not weaken to any great degree and therefore much longer distances can be run before signals have to be regenerated.
• Fibre offers the advantage of security as it is almost impossible to tap in to it without being detected.
• The cable is light in weight and small in size relative to the capacity it can handle.

However, it possess the following disadvantages:

• The cost of the cable is high - due to the quality of glass necessary and its fragility, the cost of production is high.
• Termination of fibre cables is a skilled task requiring care and thus installation costs will also be high.
• It is difficult to manufacture T-junctions for optical fibre.

On the other hand although the cable is undoubtedly expensive at present, as demand and use of the cable increases the cost will inevitably fall.