P

packet

In data communications, the basic logical unit of information transferred. A packet consists of a certain number of data bytes, wrapped or encapsulated in headers and/or trailers that contain information about where the packet came from, where it’s going, and so on. The various protocols involved in sending a transmission add their own layers of header information, which the corresponding protocols in receiving devices then interpret.

packetizing

Adding equipment to a network for packetizing voice, and the delays inherent in the data network, makes managing delay a critical factor in integrating voice and data networks.

packet switch

A physical device that makes it possible for a communication channel to share several connections, its functions include finding the most efficient transmission path for packets.

packet switching

A networking technology based on the transmission of data in packets. Dividing a continuous stream of data into small units— packets—enables data from multiple devices on a network to share the same communication channel simultaneously but also requires the use of precise routing information. Contrast with: circuit switching.

PAP

Password Authentication Protocol: In Point-to-Point Protocol (PPP) networks, a method of validating connection requests. The requesting (remote) device must send an authentication request, containing a password and ID, to the local router when attempting to connect. Unlike the more secure CHAP (Challenge Handshake Authentication Protocol), PAP sends the password unencrypted and does not attempt to verify whether the user is authorized to access the requested resource; it merely identifies the remote end. See also: CHAP.

parity checking

A method of error-checking in data transmissions. An extra bit (the parity bit) is added to each character or data word so that the sum of the bits will be either an odd number (in odd parity) or an even number (even parity).

partial mesh

A type of network topology in which some network nodes form a full mesh (where every node has either a physical or a virtual circuit linking it to every other network node), but others are attached to only one or two nodes in the network. A typical use of partial-mesh topology is in peripheral networks linked to a fully meshed backbone. See also: full mesh.

path determination

Path determination means that the router knows a route that leads to the desired destination address.

PCR

Peak Cell Rate: As defined by the ATM Forum, the parameter specifying, in cells per second, the maximum rate at which a source may transmit.

PDN

Public Data Network: Generally for a fee, a PDN offers the public access to computer communication network operated by private concerns or government agencies. Small organizations can take advantage of PDNs, aiding them creating WANs without investing in long-distance equipment and circuitry.

Peers

Other network devices or nodes of similar type.

per-destination load balancing

Load balancing can be configured in two different modes. Per-destination load balancing is load balancing based on the destination; it is on by default and must be turned off to enable the other mode, per-packet load balancing.

per-packet load balancing

Load balancing can be configured in two different modes. Per-packet load balancing is based on the packet. The other mode, per-destination load balancing, is on by default and must be turned off to enable per-packet load balancing.

PGP

Pretty Good Privacy: A popular public-key encryption application offering protected transfer of files and messages.

phantom

Used in a Hot Standby Routing Protocol (HSRP) network to provide an IP default gateway address to hosts.

Physical layer

The lowest layer—Layer 1—in the OSI reference model, it is responsible for converting data packets from the Data Link layer (Layer 2) into electrical signals. Physical-layer protocols and standards define, for example, the type of cable and connectors to be used, including their pin assignments and the encoding scheme for signaling 0 and 1 values. See also: Data Link layer, Network layer, Application layer, Session layer, Presentation layer, and Transport layer.

ping

Packet internet groper: A Unix-based Internet diagnostic tool, consisting of a message sent to test the accessibility of a particular device on the IP network. The acronym (from which the “full name” was formed) reflects the underlying metaphor of submarine sonar. Just as the sonar operator sends out a signal and waits to hear it echo (“ping”) back from a submerged object, the network user can ping another node on the network and wait to see if it responds.

pinhole congestion

Some routing protocols base cost on the number of hops to a particular destination. These routing protocols load balance over unequal bandwidth paths as long as the hop count is equal. Once a slow link becomes saturated, however higher capacity links cannot be filled.

pleisochronous

Nearly synchronous, except that clocking comes from an outside source instead of being embedded within the signal as in synchronous transmissions.

PLP

Packet Level Protocol: Occasionally called X.25 Level 3 or X.25 Protocol, a Network-layer protocol that is part of the X.25 stack.

PNNI

Private Network-Network Interface: An ATM Forum specification for offering topology data used for the calculation of paths through the network, among switches and groups of switches. It is based on well-known link-state routing procedures and allows for automatic configuration in networks whose addressing scheme is determined by the topology.

point-to-multipoint connection

In ATM, a communication path going only one way, connecting a single system at the starting point, called the “root node,” to systems at multiple points of destination, called “leaves.” See also: point-to-point connection.

point-to-point connection

In ATM, a channel of communication that can be directed either one way or two ways between two ATM end-systems. See also: point-to-multipoint connection.

poison reverse updates

These update messages are transmitted by a router in order to overcome large routing loops and offer explicit information when a subnet or network is not accessible (instead of merely suggesting that the network is unreachable by not including it in updates).

polling

The procedure of orderly inquiry, used by a primary network mechanism, to determine if secondary devices have data to transmit. A message is sent to each secondary, granting the secondary the right to transmit.

POP

1. Point Of Presence: The physical location where an interexchange carrier has placed equipment to interconnect with a local exchange carrier.

2. Post Office Protocol: A protocol used by client e-mail applications for recovery of mail from a mail server.

port number

The default identifier for a TCP/IP (Transmission Control Protocol/Internet Protocol) or Internet process.

For example, ftp (File Transport Protocol), HTML (HyperText Markup Language), and Telnet are all available at preassigned unique port numbers so that the computer knows how to respond when it is contacted on a specific port; Web servers use port 80, and SMTP (Simple Mail Transfer Protocol) e-mail is always delivered to port 25. You can override these defaults by specifying different values in a URL, but whether they will work depends on the configuration on the target system.

A total of 65,535 port numbers are available for use with TCP, and the same number are available for UDP (User Datagram Protocol).

PPP

Point-to-Point Protocol: The protocol most commonly used for dial- up Internet access, superseding the earlier SLIP. Its features include address notification, authentication via CHAP or PAP, support for multiple protocols, and link monitoring. PPP has two layers: the Link Control Protocol (LCP) establishes, configures, and tests a link; and then any of various Network Control Protocols (NCPs) transport traffic for a specific protocol suite, such as IPX. See also: CHAP, PAP, and SLIP.

Presentation layer

Layer 6 of the OSI reference model, it defines how data is formatted, presented, encoded, and converted for use by software at the Application layer. See also: Application layer, Data Link layer, Network layer, Physical layer, Session layer, and Transport layer.

PRI

Primary Rate Interface: A type of ISDN connection between a PBX and a long-distance carrier, which is made up of a single 64Kbps D channel in addition to 23 (T1) or 30 (E1) B channels. See also: ISDN.

priority queueing

A routing function in which frames temporarily placed in an interface output queue are assigned priorities based on traits such as packet size or type of interface. Also referred to as PQ.

private addresses

IP addresses that are reserved and blocked on the Internet so they can be used on private networks. See also: IP address.

private branch exchange (PBX)

A private branch exchange (PBX) is essentially a telephone switch used by private enterprises, where public telephone switches are owned and operated by telephone companies.

Private IP v4 addresses

According to RFC1918, IP addresses that should be used in private networks. 10.0.0.0/8, 172.16.0.0 to 172.16.31.255/16 and 192.168.0.0/16

process switching

As a packet arrives on a router to be forwarded, it’s copied to the router’s process buffer, and the router performs a lookup on the Layer 3 address. Using the route table, an exit interface is associated with the destination address. The processor forwards the packet with the added new information to the exit interface, while the router initializes the fast- switching cache. Subsequent packets bound for the same destination address follow the same path as the first packet.

PROM

Programmable read-only memory: ROM that is programmable only once, using special equipment. Compare with: EPROM.

protocol analyzer

A hardware or combined hardware and software product used to analyze the performance data of the network and to find and troubleshoot network problems.

Protocol analyzers vary greatly in complexity. Some use dedicated hardware and can decode as many as 150 protocols; others convert an existing networked PC into a network-specific analyzer.

propagation delay

The time it takes data to traverse a network from its source to its destination.

protocol

In networking, the specification of a set of rules for a particular type of communication. The term is also used to refer to the software that implements a protocol.

protocol data unit (PDU)

The processes at each layer of the OSI model. PDUs at the Transport layer are called segments, PDUs at the Network layer are called packets or datagrams, and PDUs at the Data Link layer are called frames. The Physical layer uses bits.

protocol stack

A collection of related protocols.

Proxy ARP

Proxy ARP means that a particular machine (such as a router) will respond to ARP requests for hosts other than itself. This can be used to make a router disappear from the workstations on a network and eliminate configuration of the workstations.

proxy services

A software package running on a server positioned between an internal network and the Internet. The proxy server filters all outgoing connections so that they appear to be coming from the same machine, in an attempt to conceal the underlying internal network structure from any intruders. By disguising the real structure of the network, the proxy server makes it much more difficult for an intruder to mount a successful attack.

A proxy server will also forward your requests to the Internet, intercept the response, and then forward the response to you at your network node. A system administrator can also regulate the external sites to which users can connect.

PSE

Packet Switch Exchange: The X.25 term for a switch.

PSN

Packet-switched Network: Any network that uses packet-switching technology. Also known as packet-switched data network (PSDN). See also: packet switching.

public switched telephone network (PSTN)

Colloquially referred to as plain old telephone service (POTS). A term that describes the assortment of telephone networks and services available globally.

PVC

Permanent Virtual Circuit: In a frame-relay network, a logical connection, defined in software, that is maintained permanently. Compare with: SVC. See also: virtual circuit.

PVP

Permanent Virtual Path: A virtual path made up of PVCs. See also: PVC.

PVP tunneling

Permanent Virtual Path Tunneling: A technique that links two private ATM networks across a public network using a virtual path, wherein the public network transparently trunks the complete collection of virtual channels in the virtual path between the two private networks.