PABX (private automatic branch exchange)
Another term for a PBX.
A group of binary digits, including data and call control signals, that is switched as a composite whole. The data, call control signals, and error control information are arranged in a specific format. Also called block, frame, cell, or datagram.
A problem that occurs when there is congestion at the packet switches or routers. It can considerably degrade real-time applications.
A measure of the ratio of the total packet bits occupied by control information to the number of bits of data, usually expressed as a percent.
A data network that uses licensed bandwidth and which is specifically built for two-way data, not for voice communications.
A network consisting of a series of interconnected switches that route individual packets of data over one of several redundant routes. Packet-switched networks include X.25, Frame Relay, IP, and ATM.
A method of transmitting messages through a communication network, in which long messages are subdivided into short packets. Each packet contains the data and a destination address and is passed from source to destination through intermediate nodes. At each node, the packet is received, stored briefly, and then passed on to the next node. The packets are then reassembled into the original message at the receiving end.
PAD (packet assembler/disassembler)
A protocol conversion device that accepts characters in a serial data stream and converts them into packets to send across a packet-switched network (e.g., X.25 network).
PAL (Phase Alternate Line)
The color television broadcasting system developed in West Germany and the United Kingdom that uses 625 picture lines and a 50Hz field frequency.
PAM (pulse amplitude modulation)
A form of modulation in which the amplitude of the pulse carrier is varied in accordance with successive samples of the modulating signal.
PAN (personal area network)
A network that surrounds an individual and provides networking between badge-based computers and other input/output devices.
PAP (Password Authentication Protocol)
A protocol that uses a two-way handshake for the peer to establish its identity upon link establishment. The peer repeatedly sends the password to the authenticator until verification is acknowledged or the connection is terminated.
The simultaneous transmission of all the bids making up a character or byte, either over separate channels, or on different carrier frequencies on the same channel.
The state of being even-numbered or odd-numbered. A parity bit is a binary digit appended to a group of binary digits to make the sum of the digits either all odd (odd parity) or all even (even parity).
A procedure in which the addition of noninformation bits are added to data to make the number of ones in a grouping of bits either always even or always odd. This procedure allows detection of bit groupings that contain single errors. It can be applied to characters, blocks, or any specific bit grouping. Also called VRC.
A passive wiring device that facilitates manual patching of end users onto ports on one or more network hubs.
(1) In a network, any route between any two nodes. (2) The route traversed by the information exchanged between two attaching devices in a network.
Pbps (petabits per second)
PBX (private branch exchange)
A telephone switch located on a customer’s premises that primarily establishes voice-grade circuits between individual users (extensions) and the switched telephone network. Typically, the PBX also provides switching within a customer’s premises and usually offers numerous other enhanced features, such as least-cost routing and call-detail recording. Also called a PABX.
PCM (Pulse Code Modulation)
A scheme used to convert an analog voice signal into a digital bitstream for transmission. Digital transmission technique that involves sampling of an analog information signal at regular time intervals and coding, the measured amplitude into a series of binary values, which are transmitted by modulation of a pulsed, or intermittent, carrier. A common method of speech digitizing by using 8-bit code words, or samples, and a sampling rate of 8,000 times per second.
PCS (Personal Communication Services)
A digital service that operates in the 1.8GHz to 2GHz band and uses both microcell and picocell architectures.
PDC (Personal Digital Cellular)
Also known as Japanese Digital Cellular (JDC), a 2G standard for digital wireless communications deployed widely in Japan.
PDH (Plesiochronous Digital Hierarchy)
The first generation of digital hierarchy, defining the available digital transmission rates and number of channels. It is used by telecommunications operators and implemented according to three standards: T-carrier in North America, E-carrier in ITU-T countries, and J-carrier in Japan. PDH is defined by the ITU-T in its G.703 standard.
PDN (public data network)
A packet-switching network (e.g., X.25, Frame Relay, Internet, IP backbones) that is designed to provide low error-rate data transmission.
PDU (protocol data unit)
The OSI term for a packet.
The interaction by which computers and other network devices communicate with each other as equals and on their own initiative (as opposed to a host/terminal environment).
An arrangement in which operators agree to exchange with one another the same amount of traffic over high-speed lines between their routers so that users on one network can reach addresses on the other.
A major factor on which the total productivity of a system depends. Performance is largely determined by a combination of several other factors: throughput, latencies, response time, and availability.
An environment in which computers are taken out of stand-alone boxes to which we are tied and put into ordinary things, in everyday objects around us. Also called ubiquitous computing.
P-frame (predicted frame)
In MPEG terminology, a frame that is only reconstructed from preceding reference frames. It can also be a reference frame, as it serves to reconstruct other frames in some instances.
PGP (Pretty Good Privacy)
A technique for encrypting messages. PGP is one of the most common ways to protect messages on the Internet because it is effective, easy to use, and free. PGP is based on the public-key method, which uses two keys: a public key that you disseminate to anyone from whom you want to receive a message and a private key that you use to decrypt messages that you receive.
A third voice circuit that is superimposed on two 2-wire voice circuits.
The angle of a wave form at a given moment.
A small, flat antenna that is steered electronically. It provides great agility and fast tracking, as well as the ability to form multiple antenna beams simultaneously. The beam is electrically pointed by adjusting the phases of the individual transmitters. This allows for very fast and precise steering of the communications beam, which is very important for high-bandwidth communication because the data rate is inversely proportional to the angular offset.
A random distortion of signal lengths caused by the rapid fluctuation of the frequency of the transmitted signal. Phase jitter interferes with interpretation of information by changing the timing.
PHS (Personal Handyphone System)
A Japanese standard for 2G PCS wireless networks.
The address of the physical communications device in a system.
The definition of the number of pins in the connector, the number of wires in the cable, and what signal is being carried over which of the pins and over which of the wires, to ensure that the information is being viewed compatibly.
Layer 1 of the OSI model. Defines the electrical, optical, mechanical, and procedural characteristics of the interface.
ping (packet Internet groper)
The name of a program used with TCP/IP internets to test reachability of destinations by sending them an ICMP echo request and waiting for a reply.
pixel (picture element)
In computer graphics, the smallest element of a display space that can be independently assigned color and intensity.
PKE (public key encryption)
A message authentication mechanism that is part of most Web browsers.
PKI (public key infrastructure)
A process that secures e-business applications such as private e-mail, purchase orders, and workflow automation. It uses digital certificates and digital signatures to authenticate and encrypt messages and a certificate authority to handle the verification process.
The physical equipment of a telephone network that provides communications services.
A device that converts computer output into drawings on paper or displays the output on display-type terminals instead of printing a listing.
PLP (Packet Layer Protocol)
A standard in the network layer of X.25.
PM (phase modulation)
A way of modifying a sine wave signal to make it carry information. The sine wave, or carrier, has its phase changed in accordance with the information to be transmitted.
PNAP (private network access point)
A private point of access to the Internet, also called a peering point, that bypasses public NAPs.
A circuit that connects two points directly, where there are generally no intermediate processing nodes, although there could be switching facilities. Synonymous with two-point and always on.
A system in which you can associate information about individual users, groups, organizational units, and entire organizations, as well as events (such as the beginning of the accounting department’s month-end closing), with various network services or classes of service.
A host-system?controlled method for determining whether each of the stations on a communication line has data to send.
PON (passive optical network)
A network in which one access line is shared among multiple buildings. Optical splitters and couplers are used at each fiber connection in the network.
POP (point of presence)
The physical access location into a network.
A point of access into a communications switch, a computer, a network, or other electronic device; the physical or electrical interface through which one gains access; the interface between a process and a communications or transmission facility.
A number in the range 1 to 65,535 that identifies a port. The port number does not represent a physical port, such as the serial port to which a modem or a mouse might be attached; instead, it is more like a regional memory address.
The maximum signaling rate on a digital access line.
POTS (plain old telephone service)
The standard analog telephone service that most homes use.
PPP (Point-to-Point Protocol)
A successor to SLIP, this protocol provides router-to-router and host-to-network connections over synchronous and asynchronous circuits.
PPTP (Point-to-Point Tunneling Protocol)
A Layer 2 protocol that can work in a non-IP enterprise environment, which is one of its strengths for customers who use multiple protocols rather than using only IP. PPTP provides low packet overhead and good compression, but its weaknesses are on the security front.
Layer 6 in the OSI model that provides services to the application layer, enabling it to interpret the data exchanged, as well as to structure data messages to be transmitted in a specific display and control format.
The routine checking of components to keep the system functioning.
PRI (Primary Rate Interface)
A bundle of ISDN circuits, primarily a PBX interface. The United States and Japan use 23B+D, and the ITU uses 30B+D. Also called Primary Rate Access (PRA).
On a point-to-point communication line, the station that gains control of the line first. On a multipoint line, the station controlling communications.
The decryption (reception) or encryption (signature) component of an asymmetric key set.
The channel equipment furnished to a customer as a unit for exclusive use, generally with no access to or from the public switched telephone network. Also called leased line.
A network based on leased lines or other facilities that provide telecommunication services, within an organization or within a closed user group, as a complement or as a substitute to the public network.
A set of rules that govern network communications. Low-level protocols define transmission rates, data encoding schemes, physical interfaces, network addressing schemes, and the method by which nods contend for the chance to transmit data over the network. High-level protocols define functions such as printing and file sharing.
Information in a packet that the protocol needs in order to do its work.
protocol stack (or protocol suite)
A collection of protocols that computers use to exchange information.
A packet-switched VPN that runs across the service provider’s backbone, generally using Frame Relay or ATM.
Proxy ARP (Proxy Address Resolution Protocol)
The technique in which one machine, usually a router, answers ARP requests intended for another by supplying its own physical address. By pretending to be another machine, the router accepts responsibility for routing packets to it. The purpose of proxy ARP is to allow a site to use a single IP network address with multiple physical networks.
A server that provides firewall functionality, acting as an intermediary for user requests, establishing a connection to the requested resource either at the application layer or at the session or transport layer.
PSK (phase-shift keying)
A modulation technique for transmitting digital information to analog whereby that information is conveyed as varying phases of a carrier signal.
PSTN (public switched telephone network)
The complete traditional public telephone system, including telephones, local and interexchange trunks, transport equipment, and exchanges.
PTO (public telecommunications operator)
An incumbent carrier in places other than the United States.
PT&T (postal, telegraph, and telephone) organization
Usually a governmental department that acts as its nation’s common carrier.
A device that is used by algorithms that encrypt and decrypt using asymmetric yet mathematically linked keys. Each security module is assigned a pair of keys: The encryption key is “public” and does not require distribution by secure means. The decryption or “private” key cannot be discovered through knowledge of the public key or its underlying algorithm. Public key algorithms can apply to one or more of the following: key distribution, encryption, authentication, or digital signature.
A momentary, sharp alteration in the current or voltage produced in a circuit to operate a switch or relay which can be detected by a logic circuit; a sharp rise and fall of finite duration.
A common termination point in the wiring closet for wires going out to the individual offices and wall sockets.
A program that updates news, weather, or other selected information on a computer user’s desktop interface through periodic and generally unobtrusive transmission over the World Wide Web.
PVC (permanent virtual circuit)
A defined path that provides essentially a dedicated private line between users in a packet switching network. The network is aware of a fixed association between two stations, permanent logical channel numbers are assigned exclusively to the permanent circuit, and devices do not require permission to transmit to each other.
PWM (pulse width modulation)
The process of encoding information based on variations of the duration of carrier pulses. Also known as pulse duration modulation (PDM).
The ITU-T H.320 standard for interoperability in video conferencing over ISDN.