Ethernet Cabling Methods

Most Ethernet cabling systems use unshielded twisted pair (UTP) or coaxial cable (thin or thick) to carry LAN packets. Twisted pair is just what its name implies--four wires twisted in two pairs. Twisted pair is used in a star topology for 10Base-T Ethernet networks. UTP runs from each node on a network segment to a hub, which may then connect with other parts of the network through a backbone or stacking hub arrangement.

Twisting the wires reduces electrical interference. Shielding refers to the amount of insulation around the wire and, thus, its noise immunity. Underwriters Laboratories certifies UTP cable as category 1 (voice grade; also called silver satin), category 3 (data grade), or category 5 (high-speed data grade). A 10Base-T network requires category 3, while category 5 is being used with new 100-Mbps Ethernet systems.

The STP (shielded twisted pair) used in some Token Ring networks looks somewhat like the wire used to carry house current (110 V) throughout your home. STP actually carries a relatively low voltage signal, however, and the heavy insulation is for noise reduction, not safety.

Though more expensive than twisted pair, coaxial cable has more advantages. Thick coaxial cable lets you run greater distances and attach more nodes, because it's less prone to signal interference and attenuation. (For these reasons, coaxial cable is also used for cable TV hookups.) Thin coaxial cable lets you daisy chain. Sections of Ethernet networks using coaxial cable employ a linear bus topology.

Standard Ethernet coaxial, or thicknet, cable has a greater degree of noise immunity and is more difficult to damage, but it requires a combination of vampire tap (a piercing connector) and a drop cable to connect to a LAN. The newer thinnet cable is somewhat thinner than thicknet. Although thinnet doesn't carry signals over as long a distance as thick cable, thin Ethernet cable uses a simple BNC connector (a bayonet-locking connector for thin coaxial cables), costs less, and has become a standard for small- to medium-size Ethernet LANs.

In an Ethernet network, the number of connections (or taps) and their intervening distances can be limiting factors for a particular cable type. For thicknet, you can use repeaters to regenerate the signal every 500 meters or so. Without repeaters in a long network, standing waves (i.e., additive signal reflections) distort the signal and cause errors. Detection of collisions (i.e., two network adapters trying to transmit at the same time) depends partly on timing; only five 500-meter segments and four repeaters can be placed in series before the signal propagation delay becomes longer than the maximum time period allowed for detection of a collision. Without this limit, the workstations farthest from the sender would be unable to determine whether a collision had occurred.