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Glossary

MTPSR1-202ST

multiplexing to send them over the same transmission facility.

Parity bit: An extra bit attached to each byte of synchronous data used to detect errors in transmis-

sion.

Permanent Virtual Circuit (PVC): A connection between two endpoints dedicated to a single user.

IN ISDN, PVCs are established by network administration and are held for as long as the user

subscribes to the service.

Physical Unit (PU): The component that manages and monitors the resources (such as attached

links and adjacent link stations) associated with a node, as requested by an SSCP via an SSCP-PU

session. An SSCP activates a session with the physical unit in order to indirectly manage, through

the PU, resources of the node such as attached links. This term applies to type 2.0, type 4, and type

5 nodes only.

Point of Presence (POP): The central office’s end points of the long distance carriers.

Point-to-Point Protocol (PPP): A protocol that lets a PC user access TCP/IP (Internet member)

using an ISDN terminal adapter or a high-speed modem over a standard telephone line.

Port: A location for input or output data exchange. Computers, muxes, etc. have ports for various

purposes.

Primary Rate Interface (PRI): Used on ISDN. In North America, and Japan, PRI is one 64 Kbps D

channel and 23 B channels. Elsewhere, it is one D channel and 30 B channels.

Primitive: An abstract representation of interaction across the access points indicating that informa-

tion is being passed between the service user and the service provider. The OSI Reference Model

defines four types of primitives: Request, Indication, Response and Confirm.

Private Branch Exchange (PBX): A telephone exchange located on the customer’s premises. The

PBX provides a circuit switching facility for telephone extension lines within the building, and access

to the public telephone network. See also “Exchange”.

PROM (Programmable Read Only Memory - pronounced “prom”): A permanent memory chip

that can be programmed or filled by the customer after by the manufacturer has set initial values.

Contrast with ROM.

Protocol: 1. A set of semantic and syntactic rules that determines the behavior of functional units in

achieving communication. 2. In Open Systems Interconnection architecture, a set of semantic and

syntactic rules that determine the behavior of entities in the same layer in performing communication

functions. 3. In SNA, the meanings of and the sequencing rules for requests and responses used for

managing the network, transferring data, and synchronizing the states of network components. 4.

Synonymous with line control discipline.

PSTN (Public Switched Telephone Network): A worldwide public voice telephone network that is

used as a telecommunications medium for the transmission of voice, data and other information.

Public Data Network (PDN): A packet-switched network that is available to the public for individual

(“subscriber”) use. Typically, controlled by a government or a national monopoly.

Public Switched Telephone Network (PSTN): The group of circuit-switching voice carriers, which

are commonly used as analog data communications services.

Pulse Code Modulation (PCM): 1. In data communication, variation of a digital signal to represent

information; for example, by means of pulse amplitude modulation (PAM), pulse duration modulation

(PDM), or pulse position modulation (PPM). 2. Transmissions of analog information in digital form

through sampling and encoding the samples with a fixed number of bits.

Pulse dialing: One of two methods of dialing a telephone, usually associated with rotary-dial phones.

Compare with “tone dialing”.

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