2.3 Twisted Pair

2.3 Twisted Pair

Twisted pair cable is the most common type of cable used in computer networks. It is reliable, flexible and cost effective. The twisting of the pairs of copper cable is one method used to protect the cable’s signals from interference and crosstalk. This type of protection is known as cancellation.

There are two main categories of twisted pair cable:

  • Unshielded Twisted Pair (UTP) Cable
  • Shielded Twisted Pair (STP) Cable

Unshielded Twisted Pair (UTP) Cable

A common implementation of UTP is 100BaseT, which allows for a maximum cable length segment of 100 metres. There are different categories of UTP cable and one differentiation factor is the number of twists per foot of cable. The more twists the higher quality the cable, i.e. there are less twists per foot in Cat3 compared to Cat5 UTP cable.

Cat5 UTP cable is a popular choice for network cable because it meets the European standard for allowing data transfer speeds of 100Mbps.

100BaseT stands for:

  • Data Transmission Rate of 100Mbps, i.e. 100
  • Uses baseband transmission, i.e. Base
  • The media is twisted pair, i.e. the T.

Shielded Twisted Pair (STP) Cable

STP is similar to UTP but with each pair covered by an additional copper braid jacket or foil wrapping. This shielding helps protect the signals on the cables from external interference.

STP is more expensive than UTP but has the benefit of being able to support higher transmission rates over longer distances.

STP is used in IBM token ring networks.

Twisted Pair Cable Components

Like coaxial cable, twisted pair has connectors which are used to connect to the NIC and other network devices. The connectors used look similar to RJ11 telephone jacks, however, there are differences: the RJ11 is smaller and has only has four wires whereas the UTP RJ45 connectors are larger and have eight wires.

TIA/EIA Standards

The organisation that sets the standards for structured cabling is the Electronic Industries Association and Telecommunications Industries Association (EIA/TIA). There are different standards for the various situations that cabling can be used for ranging from telephone systems to computer networks. There are currently six implemented categories of UTP.

Category Description
Category 1 Traditional UTP telephone cable that can carry voice but not data transmissions. Most telephone cable prior to 1983 was Category 1 cable
Category 2 This category certifies UTP cable for data transmissions up to 4 megabits per second (Mbps). It consists of four twisted pairs of copper wire
Category 3 This category certifies UTP cable for data transmissions up to 16 Mbps. It consists of four twisted pairs of copper wire with three twists per foot
Category 4 This category certifies UTP cable for data transmissions up to 20 Mbps. It consists of four twisted pairs of copper wire
Category 5 This category certifies UTP cable for data transmissions up to 100 Mbps. It consists of four twisted pairs of copper wire.
Cat 5e is an enhanced higher quality version that allows Gigabit transmission.
Category 6 This category certifies UTP cable for transmissions up to 1,000 Mbps, i.e. Gigabit Ethernet. It consists of four twisted pairs of copper wire with all pairs being used.

Other UTP Standards

100BaseTX is a popular choice of UTP cable. With 100BaseTX only two of the four pairs are used, yet this still allows for data transfer rates of 100Mbps. The specified cable run length for 100BaseTX is 100 metres. The TIA/EIA recommendation regarding this is:

  • Workstation to wall plate – 3 metres maximum
  • Horizontal run from wall plate to wiring closet – 90 metres maximum
  • Patch cables to network devices – 6 metres maximum

100BaseTX is also known as Fast Ethernet.

Another UTP standard that is gaining in popularity and use is 1000BaseT. This is also referred to as Gigabit Ethernet and utilises Cat 5e and Cat 6 cable. With these implementations all four pairs of wires are used with each transmitting at 250Mbps.

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