Transmission Media
Copper Wires
Ignoring the ever-increasing popularity of wireless networks, most networks use copper wires as the transmission medium, i.e. the mechanism by which data is transmitted through the network.
Cabling typically uses copper wire because it has the benefits of being both cheap and highly conductive. Since it is conductive and data is transmitted as electrical signals, data can be sent over long distances. Indeed, so prevalent is copper that network cables are frequently referred to simply as copper.
The Problem of Interference
Anyone who's studied physics at school - even if only to GCSE level - will know that when current travels down a wire, an electromagnetic field is created in the area around the wire. In turn, this EM field can effect the current flowing in a near-by wire. This form of interference can seriously impair the network's ability to transmit data over distance. This is especially true when wires sit side-by-side.
In order to minimise the problem of interference, two common strategies are used:
- Twisted pair (TP) cables:
First off, it's important to realise that electrical communication requires a complete circuit. With TP wiring, the copper wires are coated with plastic insulation layers and then, pairs of wires are twisted around each other. One wire of the pair is for current to flow in the outbound direction, while the other wire of the pair provides the return path, often referred to as 'ground'.
By twisting the wires, the electromagnetic properties of the wires are altered, reducing the amount of EM radiation emitted from the wires while simultaneously reducing the susceptibility of the wires to external EM sources.
- Coaxial cables:
A single copper wire is coated with plastic insulation (as with TP) and then this, in turn, is surrounded by a metal shield. The metal shield acts as a barrier against EM interference. Also, this shield provides the ground.
It should be noted that the pair of insulated copper wires used in TP cabling can also be shielded by a metal 'tube', just like in coaxial cables. In this case, the TP cables are known as shielded twisted pair, or STP. With no metal shield, the TP cables are also known as unshielded twisted pair (UTP).
Optical Fibres
There are times when copper just won't do it. For example, due to the electromagnetic interference induced by and experienced by copper wires, there is a limit to the distance that data can be sent using copper. Furthermore, while copper is highly conductive, there is still a limit to the amount of data which can be transmitted and the time taken to send data is short but finite.
To overcome these obstacles, 'cabled' networks extending over large distances and / or networks that must send very large quantities of data in short time use fibre optics. Optical fibres are tiny glass fibres coated in a plastic sleeve to provide flexibility. Signals are transmitted from one end in the form of light pulses using an LED or laser.
Light does not cause EM interference and thus fibre optics can be used over much greater distances. Furthermore, light can be used to send greater volumes of data more quickly. In addition to the advantages already mentioned, fibres - unlike copper cables - do not need to complete an electrical circuit. Instead, fibres can simply be implemented in an end-to-end manner.
While optical fibres are ideal for high throughput networks - e.g. Gigabit Ethernet - they are still very expensive to implement and maintain.
Radio Waves
Wireless networks are all the rage these days. The main advantage is obvious: no cables are needed. Since data is transmitted through the using radio waves, wireless networks are often referred to as radio frequency, or RF networks. (Actually, the frequencies used sit in the region of the EM spectrum where radio waves end and microwaves begin. Note that radio and microwave are really just descriptive terms used to describe electromagnetic waves of a particular wavelength / frequency range.)
What's next
Next stop: a look at LANs and WANs.
| Just Too Good Last updated: June, 2006 (DJL)
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