What is POTS? (Plain Old Telephone System)

POTS is the acronym for Plain Old Telephone System. This is the term used to describe, the old analogue phone system, which is still in use today in many countries. POTS is based on TDM or Time Division Multiplexing which is the technique that is uses to enable a single cable to carry many simultaneous active calls.

Time Division Multiplexing

The PSTN telephone infrastructure consists a central exchange called a CO (Central Office), which connects to remote and urban local exchanges via trunk lines. The local exchanges in turn connect to local household in their area by connections to the local loop. The local loop consists of a network of copper twisted pair cables, which span the area and connect telephone sockets in each household. At the local exchange all the telephone calls made over the local loop are aggregated over a trunk and transported to the CO exchange.

The local exchange aggregates the individual calls by multiplexing them using timeslots over the same communication channel. It does this by assigning each call a 64kbps timeslot and this is known as a Digital Signal 0 (DS0). By sharing the same physical line, and using their own specific timeslot, many active calls are segmented, interleaved and transmitted over the same physical cable. A DS0 is a single call transmitted at 64kbps and is the lowest level of separation, all other Digital Signal levels are aggregates of DS0s, such as the DS1.

Connecting C.O Exchanges with Trunks

In the USA and Japan a DS1, on a T1 line carries 24 DS0 or 24 individual telephone calls simultaneously giving a total bandwidth of 1.5mbs. In Europe and the rest of the world, an E1 line carries 32 individual DS0 though 2 are reserved for signaling giving an available bandwidth of 2mbps. Local exchanges and CO exchange connect using bundles of T/E1 cables to carry the local loop traffic to the central exchange. The T1/E1 lines are called Primary ISDN and are synchronous in that they have their own clock, which allows a peer device to de-multiplex all the separate channels without timing errors.

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