Carrier Systems : Plain Old Telephone Service (POTS)

Carrier systems are a combination of a transmission medium, specification of signal types and levels along with specific protocol controls (communication rules). The types of carrier systems range from simple audio telephone POTS carriers to high-speed optical carrier (OC-x) transmission systems.

Plain Old Telephone Service (POTS)
Plain old telephone service (POTS) is a transmission system that is used to provide basic telephone service. It is the common term used for residential telephone service.

Between the late 1800’s through the 1990’s, telephone transmission had remained basically the same. Acoustic energy from the customer was converted to electrical signal by a microphone in a handset. This electrical energy was applied through a hybrid electrical device to the telephone line through the speaker in the handset. A telephone hybrid device (often called a “magic” device by telephone personnel) transferred energy from the microphone to into the telephone 2-wire line while extracting most the remote microphone energy and applying it to the speaker. At the same time at the other end of the connection, the same process was occurring.

Figure 1 shows a hybrid telephone transmission system. This diagram shows that microphone energy from user #1 is added to the 2-wire transmission circuit via the hybrid assembly and the microphone energy from user #2 is subtracted from the circuit by the speaker #1. At the same time, the microphone energy from user #2 is added to the 2-wire transmission circuit and speaker #2 subtracts the microphone energy from user #1. This hybrid process allows 2 wires to contain a composite of both microphone signals.


Figure 1: Hybrid Telephone Transmission