Glossary - P

PABX

Private Automatic Branch eXchange
See PBX.

Packet (network, communications)

The unit of data sent across a network. "Packet" is a generic term used to describe a unit of data at any layer of the OSI protocol stack, but it is most correctly used to describe application layer data units ("application protocol data unit", APDU).
See also datagram, frame.

Packet Switching (network)

Communications technique where a message is broken into packets that include address information and are sent to the destination on the fastest routes available; all packets may not take the same route, so the receiving station reassembles the message in proper order before forwarding the complete message to the address.
See also datagram, X25.

PAD

Packet Assembler-Disassembler (network)
Device used to access packet switching netowrk. It adapts terminals working in character by character mode into devices compatible with the X25 standard.

Padding (general)

A filler used to extend a string, record or field to some prescribed lengh.

PAL

Phase Alternate Line (TV standard)

Palmtop [computer] (computer)

Pocket computer smaller than a laptop computer. Type of palmtop: Personal Digital Assistant (PDA), notebook.
Newsgroup: news:comp.sys.palmtops/.

Parity (communications)

A self-checking code employing binary digits in which the total number of ones is always odd or even.

Payload

Information portion of an ATM cell or a protocol packet.

PBX

Private Branch Exchange (communications)
A circuit switch that relays telephones, terminals or other equipment and provides access to the public telephone system.
A small telephone network for customer premises. Provides local connectivity and switching and connections to the wide area voice network. See also PABX.

PC

Priority Control (ATM)
A congestion control function that uses the CLP bit to perform priority queueing and scheduling actions (i.e. to manage priority and resolve congestion problems).

Personal Computer
Generic term for any of a number of manufacturers' desktop computers. A general-purpose single-user microcomputer designed to be operated by one person at a time.

PCI

Peripheral Component Interconnect (hardware)
Set of Intel standards for a high speed bus system. The PCI bus is related to the 486 and Pentium processors.

PCL

Printer Control Language (hardware)
A document description language used by Hewlett-Packard Laserjet printers, a superset of HP-GL/2.

PCM

Pulse Coded Modulation (communications)
A technique for converting an analog voice signal to digital format. Requires sampling the analog signal 8 thousand times per second and converting the sample to an 8-bit value. Hence, PCM requires a 64 Kbps digital channel to carry one voice connection. See DS-O.

PCMCIA

Personal Computer Memory Card International Association (body, hardware, standard)
An international trade association and the standards they have developed for devicies, such as modems and external hard disk drives, that can be plugged into notebook computers.
The standard defines the format and interface for a credit card six device for use with portable or laptop computers. There are 3 types:

• Type 1: the thinnest at 3.3 mm, is normally used for memory type enhancements.
• Type 2: which is 5 mm thick, is for modem and LAN enhancements.
• Type 3, 10.5 mm thick, is for mass storage I/O.

For unknown reason, around 1995(?) the association decided to rename PCMCIA cards "PC Cards".

PCR

Peak Cell Rate (ATM)
A traffic parameter that characterizes the source and gives the maximum rate at which cells can be transmitted across a virtual circuit, specified in cells per second and defined by the interval between the transmission of the last bit of one cell and the first bit of the next.
Also, a field in the RM cell header specifying the largest value to which the ER field can be set.

PDA

Personal Digital Assistant (computer)
A small hand-held computer used to write notes, track appointments and otherwise keep your life in order. PDAs provide all the functionality of a cheap pad of paper at hundreds of times the cost, and with far less storage capacity. Early PDA's required data to be input using a keypad with keys the size of Chiclets, but more recent models (e.g. the Apple Newton) use a combination of pen-based input and character recognition software to render your data incomprehensible.

PDH

Plesiochronous Digital Hierarchy
digital hierarchy developed 40 years ago by Bell Labs to carry digitized voice over twisted wire more efficiently. A PDH evolved into the North American Digital Hierarchy, which is known more commonly by its signal names DS0 through DS3. The term "T1" is also used colloquially to refer to the DS1 signal and "T3" to refer to the DS3 signal.

PDN

Public Data Network

PDS

Premises Distribution System (network)

PDU

Protocol Data Unit
Term for the combination of header and information field at any given layer in the OSI model. The information field is generally the PDU from the layer above. Also known as message, to describe the primitive passed across different layers and contains header, data (payload and trailer information. The information field is generally the PDU from the layer above. Used in other protocols for the same purpose. See PIU.

Peak Duration (ATM)

A source traffic characteristic that gives the duration of a transmission at the peak cell rate (PCR). It is equivalent to the burst length (in cells).

Peer-to-Peer

Peer-to-Peer Network / Computing (network, computing)
A local area network (LAN) in which there is no central controller or file server. The nodes attached to the LAN each have equal access to network resources, every machine run the same general software (or at least recognize a uniform common protocol) and can each function as both server and client.
Also describes communication between implementations of the same OSI reference model layer in two different network devices. Compare with client/server computing.

PGML

Precision Graphics Markup Language (graphics)

PHY

Physical Layer (network)
The lowest layer in the ISO seven layer model. It concerns electrical and mechanical connections and MAC. It is used by the datalink layer. Example physical layer protocols are CSMA-CD, token ring and bus.

PHY-SAP

Physical Layer Service Access Point
The physical interface at the boundary between the PHY and the ATM layers. See also SAP, ATM-SAP.

PING

Packet INternet Groper (network, Internet)
A programm used to test reachability of destinations by sending an ICMP echo request and waiting for a reply.

PIU

Path Information Unit (network)
IBM term for the SNA layer 3 PDU. See SDLC.

Pixel

PICture ELement (graphics)
The smallest resolvable rectangular area of an image, either on a screen or stored in memory. Each pixel in a monochrome image has its own brightness, from 0 for black to the maximum value (e.g. 255 for an eight bit pixel) for white. In a colour image, each pixel has its own brightness and colour, usually represented as a triple of red, green and blue intensities (RGB).

PL

Physical Layer
See PHY.

PLCP

Physical Layer Convergence Protocol (ATM)
A protocol specified within the TCTC sublayer that defines how cells are formatted within a data stream for a particular transmission facility, such as T1, T3, or OC-n.

Plesiochronous (communications)

Literally means "nearly synchronous". A term describing a communication system where transmitted signals have the same nominal digital rate but are synchronised on different clocks. According to ITU-T standards, corresponding signals are plesiochronous if their significant instants occur at nominally the same rate, with any variation in rate being constrained within specified limits.
Refers also to Plesiochronous Digital Hierarchy.

PLP

Packet Level Protocol (protocol, network level)

PM

Physical Medium
One of the two PHY sublayerst hat provides the bit timing and performs the actual transmission of the bits over the physical medium.

PMD

Physical Medium Dependent
Generic term for that portion of a layer's specifications that is unique to the particular physical media being defined. Generally applies to the lowest portion of the data link layer (layer 2) or to the physical layer (layer 1). See MAC, PHY.
Defines the actual speed at which ATM traffic can be transmitted across a given physical medium. The ATM Forum has approved three Sonet interfaces for UNI: STS-1 at 51.84Mbit/s, STS-3c at 155.52 Mbit/s, and STS-12c at 622.08 Mbit/s, as well as DS-1 (T1) at 1.544 Mbit/s, E1 at 2.048 Mbit/s, E3 at 34.368 Mbit/s, and DS-3 (T3) at 44.73 Mbit/s. The forum also has adopted a number of specifications for LAN environments, including a 100-Mbit/s interface using FDDI encoding, a 155-Mbit/s interface using Category 5 UTP (unshielded twisted-pair), and a 51-Mbit/s interface using Category 3 UTP.

PMP

Point to Multipoint (ATM)

P-NNI

Private Network-to-Network Interface (ATM)
The NNI used in private networks. A routing information protocol that allows different vendors' ATM switches to be integrated in the same network. PNNI automatically and dynamically distributes routing information, enabling any switch to determine a path to any other switch.

POF

Plastic Optical Fiber (hardware)
The interest of this optical fiber is its cost: it is cheaper than silicon optical fibers.

POH

Path Overhead (network)
A 9-octet header in the payload of a SONET or SDH frame that defines the structure and content of the payload.

Point-to-Point (Connection) (network)

A connection with only two endpoints.

PON

Passive Optical Network (network)

POP

Post Office Protocol (network, Internet)
A method to access electronic mail or bulletin board messages that are kept on a (possibly shared) mail server. An other protocol to access electronic mail is IMAP.

Point of Presence (communications)

Port (network, Internet)

A port is a transport layer demultiplexing value. Each application has a unique port number associated with it. For instance, 21 is the port number for FTP.
It is also the name given to a physical network socket, especially on switches, hubs...
See also: Transmission Control Protocol, User Datagram Protocol.

Postscript

Description language for the set of elements (text, pictures...) of a document to be printed. It is independant from output device.

POTS

Plain Old Telephone Service
The name recognized around the world for conventional, basic analog telephone service using twisted-pair copper wire.
POTS takes the lowest 4 kHz(300-3400Hz) of bandwidth, and any service sharing a line with POTS must either use frequencies above POTS or convert POTS to digital and interleave with other data signals.

PPP

Point to Point Protocol (network, Internet, service)
The Point-to-Point Protocol provides a method for transmitting packets over serial point-to-point links. It specifies a standard for encapsulating Internet Protocol data and other network layer (level three on ISO's OSI Model) protocol information over point-to-point links; it also provides ways to test and configure lines and the upper level protocols on the OSI Model. The only requirement is a provision of a duplex circuit either dedicated or switched, that can operate in either an asynchronous or synchronous mode, transparent to the data-linklayer frame.
It's superceding SLIP as the standard protocol for telephone-line computer-to-computer communications. One of its advantages is that it can handle both synchronous and asynchronous communication.
PPP is defined in RFCs 1332, 1333, 1334, 1661, 1662, and 1663. See also RFC1841.

PPT

Packet Payload Type (ATM)

Printer Pass-Through (printing)

Presentation Services Layer

Layer 6 of the SNA architecture model.

PRI

Primary Rate Interface (ISDN)
A type of ISDN connection. In North America and Japan, this consists of 24 channels, usually divided into 23 B channels and 1 D channel, and runs over the same physical interface as {T1}. Elsewhere the PRI has 31 user channels, usually divided into 30 B channels and 1 D channel and is based on the E1 or DS1 (1.544 Mbps) interface.
PRI is typically used for connections such as one between a PBX and a CO (central office, of the telephone company) or IXC (inter exchange carrier, a long distance telephone company).

Private Network

A communications network comprised of dedicated circuits between DTEs and other devices (multiplexers, switches, routers) where bandwidth is dedicated and network management is much simpler. Ssee also PVN, Public Network.

PRM

Protocol Reference Model
Model used to described the organisation and layout of different layers and protocols. For instance PRM and ATM are represented like cubes divided in 3 entities when watched from the top:

• User layer,
• Command layer,
• Management layer.

PRMD

Private Management Domain (messaging)
In X400 messaging system, it is a set of users that owns it own management and routing resources.

PROM

Programmable Read-Only Memory (hardware, memory)

Protocol (standard, network)

A set of formal rules describing how to transmit data, especially across a network. Low level protocols define the electrical and physical standards to be observed, bit- and byte-ordering and the transmission and error detection and correction of the bit stream. High level protocols deal with the data formatting, including the syntax of messages, the terminal to computer dialogue, character sets, sequencing of messages etc.
Many protocols are defined by RFCs or by OSI.

Proxy (networking)

See proxy gateway, proxy server.

Proxy Gateway (networking)

A computer and associated software which will pass on a request for a URL from a World-Wide Web browser such as Mosaic to an outside server and return the results.
This provides clients that are sealed off from the Internet a trusted agent that can access the Internet on their behalf. Once the client is properly configured, its user should not be aware of the proxy gateway. A proxy gateway often runs on a firewall machine. Its main purpose is to act as a barrier to the threat of crackers. It may also be used to hide the IP addresses of the computers inside the firewall from the Internet if they do not use official registered network numbers.
Browsers such as Mosaic and Netscape can be configured to use a different proxy or no proxy for each URL access method (or "scheme") - FTP, Gopher, WAIS, news, and HTTP.

Proxy Server (programming, World-Wide Web)

A process providing a cache of items available on other servers which are presumably slower or more expensive to access. This term is used particularly for a World-Wide Web server which accepts URLs with a special prefix. When it receives a request for such a URL, it strips off the prefix and looks for the resulting URL in its local cache. If found, it returns the document immediately, otherwise it fetches it from the remote server, saves a copy in the cache and returns it to the requester. The cache will usually have an expiry algorithm which flushes documents according to their age, size, and access history. Compare proxy gateway.

PRS

Primary Reference Source (hardware, physical layer)

PS

Protection Switching (network)

PSDN

Packet Switching Data Network (network)

PSDN

Public Switched Network (network)

Packet Switched Network (network)

PSTN

Public Switched Telephone Network (network)

PT

Payload Type
See PTI.

PTI

Payload Type Identifier (ATM)
Three-bit field in the ATM cell header. The first bit indicates which AAL was used to format the data field in payload; the second provides explicit forward congestion indication(EFCI), which alerts the application of possible delays by informing it of congestion behind the cell; the third indicates whether the cell contains data OAM information. The ATM payload type field PTI is defined for all AAL types in ITU Recommendation I.361 as follows:

000	User data cell, congestion not experienced, ATM-user-to-ATM user indication 0
001	User data cell, congestion not experienced, ATM-user-to-ATM user indication 1
010	User data cell, congestion experienced, ATM-user-to-ATM user indication 0
011	User data cell, congestion experienced, ATM-user-to-ATM user indication 1
100	OAM F4 segment associated cell
101	OAM F5 end-to-end associated cell
110	Virtual Circuit Resource Management Cell
111	Reserved

PTT

Postal, Telegraph and Telephony company (or Adminsitration)
Generic term for a provider of these services. One of the many national bodies responsible for providing communications services in a particular country, e.g. British Telecom. Most PTTs outside the USA and UK are (state) monopolies.

Public Network

A communications network where users have shared access to the network resources. Network services are usually provided by common carriers (i.e.telephone companies). See also Private Network.

Pull Media (communications, messaging)

A model of media distribution were the bits of content have to be requested by the user, e.g. normal use of HTTP on the World-Wide Web. Opposite: "push media".

P-UNI

Private User Network Interface
The UNI used between a user and a private network.

Push

1. programming: To put something onto a stack. Opposite: "pop".
2. communications: push media.

Push Media (communications,messaging)

A model of media distribution where items of content are sent to the user (viewer, listener, etc.) in a sequence, and at a rate, determined by a server to which the user has connected. This contrasts with pull media where the user requests each item individually. Push media usually entail some notion of a "channel" which the user selects and which delivers a particular kind of content.
Broadcast television is (for the most part) the prototypical example of push media: you turn on the TV set, select a channel and shows and commercials stream out until you turn the set off.
By contrast, the World-Wide Web is (mostly) the prototypical example of pull media: each "page", each bit of content, comes to the user only if he requests it; put down the keyboard and the mouse, and everything stops. Infogate is a very good example of push system on the Internet.

PVC

Permanent (or Provisioned) Virtual Connection (ATM)
A virtual connection(VPC/VCC) provisioned for indefinite use in an ATM network, established by the network management system (NMS). See also SVC.

Permanent Virtual Circuit
A virtual circuit (X.25), virtual connection (Frame Relay) or virtual channel connection (ATM) established by the network management between an origin and a destination that can be left up permanently(in advance of its need). The circuit can be established by manual or semi-automated methods. Analogous to a leased/dedicated/provisioned real circuit.

PVCC

Permanent Virtual Channel Connection (ATM)

PVPC

Permanent Virtual Path Connection (ATM)

PVN

Private Virtual Network
Public communication resources used by a customer logically organised so that they appear like a private network.

PVP

Permanent Virtual Path

Px64

Worldwide videoconferencing standard for two-way, full-motion video. See JPEG, MPEG.

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