Network Access Lines

Since the early 1900’s, copper telephone lines have been the primary method of connecting customers to the telephone systems or computer networks. Copper wire pairs typically use twisted pairs of copper. The twisting of wires reduces the effects of electrical noise from distorting the desired audio signals. In essence, when the noise is received on one twist, the same noise is received on the other twist. The voltage goes positive on one line while it also goes positive on the other. Basically, the two noise signals are at the same level and they cancel each other (balance). Coax lines use one wire (a shield) to surround the other wire to help contain electrical energy from leaking out.

Telephone lines usually start from the central office’s switching center in the form of bundles of many wire pairs (trunks). These trunks connect the central switching office to distribution cables (cables with a reduced number of wire pairs) that eventually are connected to individual homes or businesses. Trunks may contain thousands of pairs of wires while local distribution cables only contain 25 to 100.

Cables are produced in rolls with a limited length (often 500 feet long). The installation of telephone cables requires several splices points as the large trunk cables connect to the distribution cables that connect to the drop cables to the home.

The switch wires are connected to the local loop lines in a main distribution frame (MDF). This trunk cable is connected to (3) 200 pair distribution cables that supply circuits to nearby neighborhoods. As the cables enter into a neighborhood, they are connected at splice points to smaller distribution cables until a final distribution cable that only holds 25 pairs reaches a telephone pole located near a house. At the telephone pole, usually 2 pairs of wires are tapped to the drop line that enters into the house (to allow up to 2 separate phone lines). These 2 pairs of wire are attached to a network interface device (NID) that protects (isolates) the wiring in the home from the telephone network wiring. Once in the home, twisted pairs of wires are looped from the NID to telephone jacks within the house. This illustration also shows that there is significant potential for different types and sizes of wire and many splice points. This inconsistency can dramatically affect the ability to transfer high-speed digital signals.