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MultiModem ZPX User Guide 41

Packet: 1. In data communication, a sequence of binary digits,

including data and control signals, that is transmitted and

switched as a composite whole. The data, control signals and,

possibly, error control information are arranged in a specific

format. 2. Synonymous with data frame. 3. In TCP/IP, the unit of

data passed across the interface between the Internet layer and

the link layer. A packet includes an IP header and data. A packet

can be a complete IP datagram or a fragment of an IP diagram. 4.

In X.25, a data transmission information unit. A group of data and

control characters, transferred as a unit, determined by the

process of transmission. Commonly used data field lengths in

packets are 128 or 256 bytes. 5. The field structure and format

defined in the CCITT X.25 recommendation.

Packet Assembler/Dissembler (PAD): Used by devicesto

communicate overX.25 networksbybuilding orstripping X.25

information on or from a packet.

Packet Data: The information format (“packetized”) used for

packet-mode calls.

Packet Mode: Refers to the switching of chunks of information

for different users using statistical 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 transmission.

Permanent Virtual Circuit (PVC): A connection between two

endpoints dedicated to a single user. In ISDN, PVCs are establised

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.

Pipe: A logical abstraction representing the association between

an endpoint on a device and software on the host. A pipe has

several attributes; for example, a pipe may transfer data as

streams (Stream Pipe) or messages (Message Pipe).

Plug and Play (PnP): A technology for configuring I/O devices to

use non-conflicting resources in a host. Resources managed by

Plug and Play include I/O address ranges, memory address

ranges, IRQs, and DMA channels.

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 memeber) using an ISDN terminal

adapter or a high-speed modem over a standard phone line.

Polling: Asking multiple devices, one at a time, if they have any

data to transmit.

Port: Point of access to or from a system or circuit. For Universal

Serial Bus, the point where a Universal Serial Bus device is

attached.

Primary Rate Interface (PRI): Used on ISDN. In North America,

and Japan, PRI is one 64Kbps D channel and 23 B channels.

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

Primitive: An abstract representation of interaction across the

accesspoints indicating that information is being passed between

the serviceuser and the service provider. The OSI Reference Model

defines four typesof primitives: Request,Indication, Responseand

Confirm.

Private Branch Exchange (PBX): A phone exchange located on the

customer’s premises.The PBX provides a circuit switching facility

forphone extension lineswithin thebuilding, and access to the

public phone 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,

themeanings of and thesequencingrulesfor 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”.

Quantizing: The process of analog-to-digital conversion by

assigning a range, from the contiguous analog values, to a

discrete number.

Random Access Memory (RAM): A computer’s primary

workspace. All data must be stored in RAM (even for a short

while), before software can use the processor to manipulate the

data. Before a PC can do anything useful it must move programs

from disk to RAM. When you turn it off, all information in RAM is

lost.

Rate Enforcement: The concept in frame relay where frames

sent faster than the CIR are to be carried only if the bandwidth is

available, otherwise they are to be discarded. (The frame relay

network assumes that anything exceeding the CIR is of low

priority.) Rate enforcement makes sure that the network will not

get so congested that it isn’t able to meet the agreed on CIR.

Recognized Private Operating Agency (RPOA): A corporation,

Glossary

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