A telephone exchange that serves a particular business or office, as opposed to one that is owned by a common carrier or telephone company and is used by many businesses or the general public. Users of the PBX share a certain number of outside lines for making telephone calls external to the PBX. PBXs have evolved over time, beginning as a manual switchboard or attendant console that was operated by a telephone operator (circuit switching) to the ??????? The IP PBX is the most current PBX. See also “IP PBX.”
PCM (pulse code modulation)
A sampling technique that converts analog signals to digital form. The analog signal is represented by a series of pulses and non-pulses (1 or 0, respectively), and the PCM performs the analog-to-digital conversion of the speech waveform by sampling the signal 8000 times a second, encoding and transmitting the samples in a serial bit stream as 8-bit digital words. Two standards are used for coding the sample level: the Mu-Law standard (North America and Japan) and the A-Law standard (used in most other countries). PCM makes it possible to digitize all forms of analog data, including full-motion video, voices, music, telemetry, and virtual reality.
POTS (Plain Old Telephone Service)
The standard telephone service that most homes use. The POTS network is also called the PSTN. POTS is generally restricted to about 52 Kbps (52,000 bits per second). Non-POTS telephone services, such as ISDN and FDDI, have high-speed, digital communications lines.
Power over Ethernet (PoE)
A technology that allows the LAN switching infrastructure to provide power over a copper Ethernet cable to an endpoint or powered device. With PoE, the electrical current is not carried by power cords.
PRI
A type of ISDN service designed for larger organizations. PRI service is generally transmitted through a T-1 line (or an E1 line in Europe).
Proxy Server
A server (a computer system or an application program) that acts as an intermediary for requests from clients seeking resources from other servers. The VoIP proxy server is used in a DMZ of a company’s secure internal communication network and receives VoIP control messages and VoIP media streams. Using the MAC address and source IP address contained in the control message, the proxy server pushes a policy change to the internal network’s external firewall to open call control protocol ports and Real Time Protocol (RTP) ports only for packets from the source IP address. The VoIP proxy server hides the company’s internal network address and directs incoming VoIP packets to an IP-PBX connected to the company’s internal network.
PSTN (Public Switched Telephone Network)
The network of the world’s public circuit-switched telephone networks. Originally a network of fixed-line analog telephone systems, the PSTN is now almost entirely digital in its core and includes mobile as well as fixed (plain old telephone service, POTS) telephones. The PSTN is largely governed by technical standards created by the ITU-T, and uses E.163/E.164 telephone numbers for addressing.
