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GL-18 GLOSSARY
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Open Systems
Interconnection (OSI)
reference model
A logical model, consisting of seven levels of interfaces, created by the
International Standards Organization (ISO) to show how interconnected
telecommunications systems should interface.
OSI See Open Systems Interconnection reference model.
packet A group of bytes — including a message (the data) and control
information (the header) — transmitted as a discrete unit. In each packet,
the message and control information are arranged in a specified format. In
many systems, the packet is further encapsulated with additional header
and trailer elements to form a frame. See also packet switching.
packet
assembler/disassembler
(PAD)
A functional unit that enables data terminal equipment (DTE) not
equipped for packet switching to access a packet-switched network.
packet switching A data-transmission technique whereby user information is segmented and
routed in discrete data envelopes called packets, each with its own
appended control information for routing, sequencing, and error checking.
Packet switching allows a channel to be occupied only during the
transmission of a packet; on completion of the transmission, the channel is
made available for the transfer of other packets. See also BX.25, packet,
and X.25.
PAD See packet assembler/disassembler.
parallel data transmission An operation whereby a group of data bits is processed or transmitted
simultaneously over essentially parallel paths.
parity A method of checking and, in some cases, correcting the accuracy of bits.
partitioning The capability to impose restrictions on each tenant’s users to limit their
access to only a specified subset of the communications system’s facilities
and services. Partitioning allows a multitenant system to comply with
state or federal regulations and to provide security and cost allocation
among tenants.
pattern For Automatic Alternate Routing — Automatic Route Selection (AAR-
ARS), a series of trunk groups arranged in a preferential order. Also
called routing pattern.
PBX See communications system.