Business Library

An Ethernet LAN can be wired using both conventional copper cabling and optical fibre - or a mixture of the two.

Copper

Coaxial

Such systems wire the devices on the network in a daisy-chain fashion using coaxial cable (familiar to anyone with a TV set). Each device connects to the LAN using a T junction to a single [logical] cable which forms the bus itself.

You may also find coaxial cabling systems for Ethernet LANs referred to as 10base5 ("ten-base-five") or 10base2 ("ten-base-two").

Unshielded Twisted pair (UTP)

UTP is good old fashioned telephone wire and when used in Ethernet is also referred to as 10baseT ("ten base tee").

Each device on the LAN connects in a star configuration to a box called a Hub, as shown in the diagram. The Hub internally forms the Ethernet bus and interconnects all the devices on the LAN segment.

The advantage is cheap cabling in a controlled environment, as the hub can be kept somewhere secure - but this is at the expense of more cabling, the hub, and vulnerability to a single point of failure.

Optical Fibre

The fibre used in an Ethernet LAN is known as 10baseFL ("ten base eff-el") and is configured in a similar way to UTP as described above. The devices connect to a hub - each using two fibres, one for each direction. As before, the hub provides the interconnection between the different devices on the LAN segment.

Fibre has three distinct advantages:

1. Individual connections can be considerably longer without the use of repeaters than when using copper wiring. A repeater is a device which regenerates the original signal, and does nothing else to it, before passing it onto the next length of cable.
2. Fibre does not emit radiation which means it is more secure than copper which can be monitored externally given the appropriate espionage equipment.
3. Fibre is immune to interference from external radiation - which means it is not as susceptible to interference as copper wiring is.

Fibre LANs are considerably more expensive than copper LANs, for example:

• 10baseFL hubs tend to cost quite a bit more than their 10baseT counterparts - whilst providing fewer connections. For example, 3Com hubs from the same range have a capacity of 24 x UTP or 6 x 10baseFL ports.
• 10baseFL adapters, used in the devices which require connection to the LAN, are more expensive than the copper equivalents.
• Optical fibre cables are more expensive than copper ones.

Business Implications?

Ethernet LANs cabled with copper are relatively cheap to set up and appropriate for the vast majority of cases. However, copper does have the following disadvantages which can be overcome by using optical fibres:

• Limited distance
• Emissions which can be picked up externally.
• Susceptibility to interference from other cables and devices.

Fibre is therefore normally used where:

• distance is a problem,
• security is an issue, or
• where the LAN passes through an electrically hostile area such as a car assembly plant.