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

Private Telephone Systems

Private telephone systems are independent telephone systems that are owned or leased by a company or individual. Private telephone networks include key telephone systems (KTS), private branch exchange (PBX) and computer telephone integration (CTI).

Key telephone systems provide customers with telephones that have multi-line capability. The key telephone system is composed of multi-line telephones and a key service unit (KSU). Each multi-line telephone has several buttons (switches) to allow the user to select from several telephone lines. The KSU coordinates the activity of each line (e.g., ring or call hold).

The private branch exchange (PBX) is a computer-controlled switch that is an extension of the carrier’s central office switch. These switches are used primarily to switch a large number of “private” lines within an organization onto a smaller number of trunk lines connected to the carrier’s central office switch. Using direct inward dialing (DID), the PBX allows one caller to directly dial another line within the organization without the involvement of the switching equipment at the central office. The PBX can provide services to an organization that include, call pick up groups, call forwarding, conference calling, call detailing and automatic routing.

Computer telephony integration (CTI), allows external computers to automate the call processing activities of the PBX in real-time and consists of a set of application program interfaces that can be used to provide advanced services and complex switching functions. Figure 1.8 shows the different types of private telephone systems. The most simplistic private telephone network is a single telephone attached to a business line. Key systems allow each telephone in a business to answer and originate calls on several business lines. A PBX system allows many extensions within a business to call each other and the PSTN...