Simple Telecom : Basic Concepts

Most telecommunication customers are served by copper cable (twisted pair or coax) terminated by the local telephone company in a telephone network interface box, called a network termination (NT). The NT is normally located on the side of the building. The network termination isolates the network from the wiring inside the building. From the NT, the “inside wiring” extends the telephone cable to all internal wall and floor jacks.

To reduce the number of copper pairs, telephone systems use a hybrid transmission system to allow both transmission and reception on a single pair of copper wires. By combining both transmit and receive audio signals using a special hybrid combiner, only one-pair of wires is required to operate a standard home telephone. These two lines are routinely referred to as “tip and ring.” This single pair of wires also provides dial tone, dialing pulses or tones, ringing (high voltage signal), and a talk path.

Most of the information that is transferred in voice conversation occurs at frequencies below 3,300 cycles per second (Hertz or Hz) and above 300 Hz. This allows telephone systems to restrict the audio frequency range for voice grade circuits from 300Hz to 3300Hz. Using a restricted frequency range reduces the transmission line and system switching performance requirements. The limiting of the audio frequency range is accomplished through the use of devices known as band-pass filters. Band-pass filters strongly attenuate signal frequencies above and below specific frequencies.

It is possible to send digital information through the hybrid network through the use of a modulator/demodulator (MoDem). The MoDem converts digital signals into analog tones that can be transmitted on standard telephone lines.

Telephone transmission lines can be divided into access lines (local loops) and interconnection lines (trunks). Often referred to as 1FB’s or B1’s, local loops refer to all two-wire voice grade connections between a residence or place of business and the telephone company’s serving end office (e.g., where the dial tone originates). Interconnection trunks refer to high capacity groups of circuits connecting switching sites such as end offices or other switching centers.