Unshielded Twisted Pair Cable (UTP)

Unshielded Twisted Pair (UTP) cable is the most common type of cable infrastructure used in Ethernet applications, but also by other networking systems (e.g. telephony).

A UTP cable comprises two to four copper wires. UTP systems such as CAT5/CAT-5e/CAT6/CAT-6a use the eight conductor wires, arranged as 4 pairs (one pair = two copper wires). To reduce common mode interference, each pair is twisted along the cable length. The number of twists per metre for a particular pair also vary along the cable length.

As the name suggests, a UTP cable does not have a foil or braiding shielding to protect against radio interference (as in Shielded Twisted Pair, STP). The pairs of wires are protected by a PVC insulation.

The first widely used Ethernet UTP system was CAT5, which was replaced by CAT5e. For example, enhanced specification of CAT5e also further supports Gigabit Ethernet (1 Gbps) at 100m, with an even higher CAT6 specification supporting 10 Gbps. For 10BT and 100BT Ethernet, data is transmitted using only two pairs of a CAT5 cable, i.e. pairs 1&2 and 3&6, whereas Gigabit Ethernet (1000BT) and higher speeds use all four pairs for data transmission.

This type of cabling usually uses an RJ-45 1 connector and 100 Ohm unshielded twisted pair cabling. Other connections can be used in industrial or other applications (such as the XLR-based RJ-45, EtherCon, used in the Entertainments industry). This cabling is normally used as a structured cable system to connect a workgroup of users. Often an entire outlets are provided to each work area, and the bundle of cables are brought back to a common wiring position for all offices on the same floor of a building (known as a patch panel).

Use with Power over Ethernet (PoE)

When used for Power Over Ethernet (POE), UTP cables can cause heating as power is transferred through the cable.Considering the maximum allowed amounts to a 7V voltage drop, with 350mA output, the loss of 2.45W is observed over the cable length. Cables with a larger conductor diameter have lower resistance, and hence produce less heat. Three essential steps to help keep cable bundles cool to minimise the number of cables per bundle, use lower resistance cabling, and perhaps introduce midspan PoE where existing cable bundling raises issues. It is also generally advised not to use conduit for cable deployments without as the enclosed space will lead to less heat disipation.

See also UTP support in:

Gorry Fairhurst - Date: 31/03/2021