P

Packets When data is transmitted through networks it is often split up into small packets rather than being sent as a continuous byte stream.

Packet-switched network A network in which data is transmitted in units called packets. The packets can be routed individually over the best available network connection and reassembled to form a complete message at the destination.

Packet Switching Type of data transfer that occupies a communication link only during the time of actual data transmission. Messages are split into packets and reassembled at the receiving end of the communication link.

PAD (Packet Assembler-Dissembler) The mechanism for disassembling packets at the sending end and assembling them to form the complete message at the receiving end; traditionally used in X.25 networks.

PAL Programmable Array Logic

PAP Password Authentication Protocol

PAR Positive Acknowledgment with Retransmission

Parallel TaskingTM 3Com technology which allows adapters to transmit data to the network before an entire frame has been loaded from the computer into the adapter's buffer, and to transmit data to the computer's main memory before an entire frame has bee received from the network. In effect, a frame can reside on the network, the adapter and in computer memory simultaneously, boosting throughput.

Path Control Layer Layer 3 in the SNA architecture model. This is the SNA layer that routes packets through an internetwork.

PBX Private Branch Exchange

PCI Peripheral Component Interconnect (Bus)

PCM Physical Connection Management (FDDI)

PCMCIA (Portable Computer Memory Card Industry Association) An industry group that has developed a standard for credit-card-size peripherals for portable computers.

PDH Plesi-synchronous Data Hierarchy.A transmission method used on some high-speed data lines.

PDN Public Data Network

PDS Packet-Driver Specification

PDU (Protocol Data Unit) OSI terminology for "packet." A PDU is a data object exchanged by protocol machines within a given layer of the OSI Reference Model containing both Protocol Control Information and user data.

Peer-to-Peer Communications A type of communications and data exchange between peer entities on two or more networks.

PEM Privacy-Enhanced Mail. Relatively new standard for email security using crytographic techniques for authentication and privacy of messages. Defined in RFCs 1421, 1422, 1423 and 1424. RIPEM is an attempt to implement this.

Perimeter Network A small, single-segment network between a firewall and the Internet for services that the organization wants to make publicly accessible to the Internet without exposing the network as a whole.

Personal office internetworking Providing individual remote users access to corporate LAN resources.

Personal Routing system architecture A combination of hardware and software that makes a remote user, connected over a WAN link, appear as a logical local client of the enterprise network.

PEP Packet Exchange Protocol

PHY The physical layer of FDDI; also, a term for FDDI fiber optic cable. In the layer structure, PHY is positioned between the MAC and the PMD.

Physical Control Layer Layer 1 in the SNA architecture model.

Physical Layer First layer of the OSI Reference Model; governs hardware connections and byte-stream encoding for transmission. It is the only layer that involves a physical transfer of information between network nodes.

Physical Media Any physical means for transferring signals between OSI systems. Considered outside the OSI Model, and sometimes referred to as "Layer 0," or the bottom of the OSI Reference Model.

Ping (Packet internet groper) A program used to test reachability of destinations by sending them an ICMP echo request and waiting for a reply. Ping is used as a verb: "Ping the host to see if it is available."

PLL (Phase Locked Loop) Function that ensures accurate signal timing in a token ring network automatically, as opposed to Tank circuits which must be adjusted manually.

PMD (Physical layer Medium-Dependent single mode) The sublayer of the ATM Physical Layer that defines connectors, fiber optic parameters, etc.

PNNI Private Network-Node Interface. Allows multi-vendor switch interoperability for SVC setup. It will eventually allow dynamic ATM networks to be constructed with heterogeneous (multi-vendor) components.

Polling A method of controlling the sequence of transmission by devices on a multipoint line by requiring each device to wait until the controlling processor requests it to transmit.

PoP Point of Presence

POP Post Office Protocol - POP2 and POP3 and standards used on some email servers.

Port 3 meanings:

1 - the physical connection that allows data flow into or out of a computer (eg the serial port of a pc).

2 - part of an address (eg a URL) on a networked computer that specifies a particular service running on that computer. Most services run on standard port numbers in which case the port need not be specified.

3 - the translation of an application from one computer platform to another.

Port Density The number oof ports, physical or logical, per network device.

Postscript A page description language, accepted by many printers.

Postmaster Most sites that can receive email have someone responsible for ensuring the delivery of email, known as the postmaster. If you have a problem with email or can't find a user's address by other means, you can send email to postmaster@host (where host is the site you're trying to contact).

POTS POTS stands for Plain Old Telephone Service. POTS is the existing analog telephone lines.

PPNCG Particle Physics Network Coordinating Group. For more information see PPNCG web page.

PPP (Point-To-Point Protocol) Successor to SLIP; provides router-to-router and host-to-network connections over both synchronous and asynchronous circuits.

Pps Packets Per Second

Presentation Layer OSI layer that determines how application information is represented (encoded) while in transit between two end systems.

Presentation Services Layer Layer 6 of the SNA architecture model.

Primary Rate ISDN (PRI) The Primary Rate Interface consists of 6-30 (23 in the US) B channels and two (one in the US) 64 K bit/second D channel, delivered over a single E1 link (2.08 M bit/second). In the US PRI is delivered over the same physical link as a T1 (1.55 M bit/second) link.

PROM Programmable read-only memory

Protocol converter Device for translating the data transmission code and/or protocol of one network or device to the corresponding code or protocol of another network or device, enabling equipment with different conventions to communicate with one another.

Protocol Islands Network topologies confined to a single leaf network that have no interconnection needs with other leaf networks or the central node.

Protocol Stack Related layers of protocol software that function together to implement a particular communications architecture.

Protocol Translator Network device or software that converts one protocol into another, similar, protocol.

PSDN Packet Switching Data Network

PSTN Public Switched Telephone Network. The traditional analogue telephone network as opposed to ISDN.

PTO Public Telephone Operator. See PTT.

PTT PTT, or Public Telephone and Telegraph, is a generic term for European telephone companies. Most are (currently) state owned and operated. The Deutsche Bundespost is one example of a PTT.

PU Physical Unit

PU2 Physical Unit type 2

PVC (Permanent Virtual Circuit) Generally, a virtual circuit that is permanently established. PVCs save bandwidth associated with circuit establishment and tear down in situations where certain virtual circuits exist all the time.