Twisted-pair cable, shown in Figure 18.6, consists of two insulated strands of copper wire twisted around each other to form a pair. One or more twisted pairs are used in a twisted-pair cable. The purpose of twisting the wires is to eliminate electrical interference from other wires and outside sources such as motors. By twisting the wires, any electrical noise from the adjacent pair will be canceled. The more twists per linear foot, the greater the effect.
Twisted-pair wiring comes in two types: shielded (STP) and unshielded (UTP). STP has a foil or wire braid wrapped around the individual wires of the pairs; UTP does not. The STP cable uses a woven-copper braided jacket, which is a higher-quality, more protective jacket than UTP.
Figure 18.6 Twisted pair cable
Of the two types, UTP is the most common. UTP cables can be further divided into five categories:
Category 1: Traditional telephone cable. Carries voice but not data.
Category 2: Certified UTP for data transmission of up to 4 Mbps (megabits per second). It has four twisted pairs.
Category 3: Certified UTP for data transmission of up to 10 Mbps. It has four twisted pairs.
Category 4: Certified UTP for data transmissions up to 16 Mbps. It has four twisted pairs.
Category 5: Certified for data transmissions up to 100 Mbps. It has four twisted pairs of copper wire.
Twisted-pair cable has several advantages over other types of cable (coaxial and fiber-optic)-it is readily available, easy to install, and inexpensive. Among its disadvantages are its sensitivity to EMI (electromagnetic interference) and susceptibility to eavesdropping; it does not support communication at distances of greater than 100 feet; and it requires the addition of a hub (a multiple network connection point) if it is to be used with more than two computers.
Coaxial cable (see Figure 18.7) is made of two conductors that share the same axis; the center is a copper wire that is insulated by a plastic coating and then wrapped with an outer conductor (usually a wire braid). This outer conductor around the insulation serves as electrical shielding for the signal being carried by the inner conductor. Outside the outer conductor is a tough insulating plastic tube that provides physical and electrical protection. At one time, coaxial cable was the most widely used network cabling. However, with improvements and the lower cost of twisted-pair cables, it has lost its popularity.
Figure 18.7 Coaxial cable
Coaxial cable is found in two types: thin (ThinNet) and thick (ThickNet). Of the two, ThinNet is the easiest to use. It is about one-quarter of an inch in diameter, making it flexible and easy to work with (it is similar to the material commonly used for cable TV). ThinNet can carry a signal about 605 feet (185 meters) before the signal strength begins to suffer. ThickNet, on the other hand, is about three-eighths of an inch in diameter. This makes it a better conductor-it can carry a signal about 1,640 feet (500 meters) before signal strength begins to suffer. The disadvantage of ThickNet over ThinNet is that it is more difficult to work with. The ThickNet version is also known as standard Ethernet cable.
When compared to twisted-pair, coaxial cable is the better choice even though it costs more. It is a standard technology that resists rough treatment and EMI. Although more resistant, it is still susceptible to EMI and eavesdropping.
Use coaxial cable if you need:
A medium that can transmit voice, video, and data.
To transmit data longer distances than less-expensive cabling.
A familiar technology that offers reasonable data security.
A Mixed-Cable System
Many networks use both twisted-pair and coaxial cable. Twisted-pair cable is used on a per-floor basis to run wires to individual workstations. Coaxial cable is used to wire multiple floors together. Coaxial cable should also be considered for a small network because you can purchase prefabricated cables (with end connectors installed) in various lengths.
Fiber-optic cable (see Figure 18.8) is made of light-conducting glass or plastic fibers. It can carry data signals in the form of modulated pulses of light. The plastic-core cables are easier to install, but do not carry signals as far as glass-core cables. Multiple fiber cores can be bundled in the center of the protective tubing.
Figure 18.8 Fiber-optic cable
When both material and installation costs are taken into account, fiber-optic cable can prove to be no more expensive than twisted-pair or coaxial cable. Fiber has some advantages over copper wire; it is immune to EMI and detection outside the cable and provides a reliable and secure transmission media. It also supports very high bandwidths (the amount of information the cable can carry), so it can handle thousands of times more data than twisted-pair or coaxial cable.
Cable lengths can run from .25 to 2.0 kilometers depending on the fiber-optic cable and network. If you need to network multiple buildings, this should be the cable of choice. Fiber-optic cable systems require the use of fiber-compatible NICs.