lesson 4

Network Cabling -

The Physical Media

Three Families of Cable

1. Coaxial:

Thinnet

Thicknet

2. Twisted pair :

UTP (unshielded twisted pair)

STP (shielded twisted pair)

3. Fiber-optic cable

Coaxial Cable

Coaxial means ‘having the same axis’, or the same center.

The metal braid which surrounds the inner copper core protects the signal from interference and crosstalk. This makes coaxial cable a good choice in places that have a lot of electrical noise - such a near machinery.

Coaxial cable also attenuates the signal less than twisted pair, so is a good choice for longer distances, and when low error rates are needed.

There are two common types of coaxial cable:

Thinnet

About as thick as a pencil.

A segment of Thinnet cable can be up to 185 meters in length.

Thinnet is often used for bus topology networks.

There are a whole family of Thinnet cable types called RG-58.

Thinnet is quite flexible and easy to install.

Thicknet

Thicknet cable is about as thick as a finger.

A segment of Thicknet cable can be up to 500 meters in length.

Often used as the backbone in a star bus topology network.

Because it is thick it does not bend easily so is difficult to install.

Both types of coaxial cable use BNC connection hardware. See pp. 80

Twisted Pair Cables

Unshielded Twisted Pair

This is the most commonly used cable in computer networks. It is used in almost all hub-based networks to connect stations to hubs.

The name is misleading as the most commonly used types of UTP cable actually contain 4 twisted pairs (i.e. 8 conductors).

The maximum length for a segment of UTP is 100 meters.

There is a standard for the quality of UTP cable, from Category 1 (OK for telephones, but not LANs) up to Category 5 (good for data transmission up to 100 Mbps).

Shielded Twisted Pair

This is like UTP, but has metal foil wrapped around each pair, and a copper braid around the whole cable. This makes STP less effected by electrical interference. It can also transmit at higher rates and over longer distances than UTP. It is only used in special circumstances, such as in areas of high electrical noise.

UTP and STP cables are connected using RJ-45 plugs and sockets.

Fiber-Optic Cable

Coaxial and UTP/STP cables transmit data using electricity. Fiber-optic is different - it codes the data as pulses of light.

It is impossible to ‘tap’ into the data in a fiber-optic cable, so it is good in a high-security environment.

Transmissions speeds are faster than any other cable.

Cable segments can be kilometers in length.

Fiber-optic cables are completely unaffected by electrical interference.

Fiber-optic cable and the associated hardware are more expensive than copper cables, and require special expertise to install.

Plenum Grade Cables

The spaces above the ceiling or beneath the floor of a modern office block is called a plenum. Network cables often go through the plenum. Fire regulations in many countries, require the use of plenum-grade cables in this situation. The plastic used in plenum grade cable will not give off poisonous gasses if burnt in a fire.

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