Twisted-pair is a type of copper cabling used for telephone communications and most Ethernet networks. A pair of wires forms a circuit that can transmit data. The pair is twisted to provide protection against crosstalk, which is the noise generated by adjacent pairs of wires in the cable. Pairs of copper wires are encased in color-coded plastic insulation and twisted together. An outer jacket protects the bundles of twisted pairs. A twisted-pair cable is shown in Figure 1.
When electricity flows through a copper wire, a magnetic field is created around the wire. A circuit has two wires. The two wires in that circuit have oppositely charged magnetic fields. When the two wires of the circuit are next to each other, the magnetic fields cancel each other out. This is called the cancellation effect. Without the cancellation effect, network communications become slow due to the interference caused by the magnetic fields.
There are two basic types of twisted-pair cables:
- Unshielded twisted-pair (UTP) - Cable that has two or four pairs of wires. This type of cable relies solely on the cancellation effect produced by the twisted-wire pairs that limits signal degradation caused by electromagnetic interference (EMI) and radio frequency interference (RFI). UTP is the most commonly used cabling in networks. UTP cables have a length up to 330 ft. (100 m.).
- Shielded twisted-pair (STP) - Each pair of wires is wrapped in metallic foil to better shield the wires from noise. Four pairs of wires are then wrapped in an overall metallic braid or foil. STP reduces electrical noise from within the cable. It also reduces EMI and RFI from outside the cable.
Although STP prevents interference better than UTP, STP is more expensive because of extra shielding, and more difficult to install because of the thickness. In addition, the metallic shielding must be grounded at one end. If improperly grounded, the shield acts like an antenna picking up unwanted signals. STP is primarily used outside of North America.
Twisted-pair cables come in several categories (Cat). These categories are based on the number of wires in the cable and the number of twists in those wires.
The size of the network determines the type of network cable that will be used. Most networks today are wired using twisted-pair copper cable. The characteristics of twisted-pair cable are shown in Figure 2.
New or renovated office buildings often have some type of UTP cabling that connects every office to a central point called the Main Distribution Facility (MDF). The distance limitation of UTP cabling used for data is 330 ft. (100 m.). Cable runs in excess of this distance limitation need a switch, repeater, or hub to extend the connection to the MDF.
Cables that are installed inside the walls and ceilings of buildings must be plenum rated. A plenum cable is one that is safe for installation between a dropped ceiling and the structural ceiling of a building where air circulation takes place. Plenum-rated cables are made from a special plastic that retards fire and produces less smoke than other cable types.
NOTE: Cat 3 cables use a 6-pin RJ-11 connector, whereas all other twisted-pair cables use an 8-pin RJ-45 connector, as shown in Figure 3.
There are two different patterns, or wiring schemes, called T568A and T568B. Each wiring scheme defines the pinout, or order of wire connections, on the end of the cable. The two schemes are similar except that two of the four pairs are reversed in the termination order.
On a network installation, one of the two wiring schemes (T568A or T568B) should be chosen and followed. It is important that the same wiring scheme is used for every termination in that project. If working on an existing network, use the wiring scheme that already exists.
Using the T568A and T568B wiring schemes, two types of cables can be created: a straight-through cable and a crossover cable. These two types of cable are found in data installations.
A straight-through cable is the most common cable type. It maps a wire to the same pins on both ends of the cable. In other words, if T568A is on one end of the cable, T568A is also on the other. If T568B is on one end of the cable, T568B is on the other. This means that the order of connections (the pinout) for each color is the exact same on both ends.
Two devices directly connected and using different pins for transmit and receive are known as unlike devices. They require a straight-through cable to exchange data. There are two unlike devices that require a straight-through cable, a switch port to router port and a hub port to PC.
A crossover cable uses both wiring schemes. T568A on one end of the cable and T568B on the other end of the same cable. This means that the order of connection on one end of the cable does not match the order of connections on the other.
Devices that are directly connected and use the same pins for transmit and receive, are known as like devices. They require the use of a crossover cable to exchange data. Like devices that require a crossover cable include:
- Switch port to switch port
- Switch port to hub port
- Hub port to hub port
- Router port to router port
- PC to router port
- PC to PC
If the incorrect cable type is used, the connection between network devices will not function.
Some devices can automatically sense which pins are used for transmit and receive and will adjust their internal connections accordingly.