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GLOSSARY GL-19
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permanent switched
connection (PSC)
On the System 75 and Generic 1, a capability that establishes and
maintains a continuous switched connection between two data endpoints.
PSCs are automatically established when the communications system is
started or restarted, and are maintained until the system becomes inactive.
If a PSC is inadvertently dropped, the system automatically reestablishes
the call. When a trunk is involved in the PSC, one of the data endpoints is
local to the controlling system and the other is on a remote system. Also
called nailed-up connection. See also dedicated switched connection.
permanent virtual circuit
(PVC)
A continuous logical circuit between the processors of two
communications systems that carries distributed communications system
(DCS) messages between the systems over a data communications
interface unit (DCIU) or processor interface (PI) link. More than one PVC
can be established on the same DCIU or PI link. PVCs are needed for
proper operation of the DCS data features.
PDM See processor data module.
port A data- or voice-transmission access point on a device that is used for
communicating with other devices.
premises distribution
system (PDS)
A cabling system that consolidates wiring for a customer’s on-site and
remote telephones and data terminals, and channels transmissions to the
communications system.
PRI See primary rate interface.
primary rate interface
(PRI)
A standard Integrated Services Digital Network (ISDN) frame format that
specifies the protocol used between two or more communications systems.
PRI runs at 1.544 Mbps and, as used in North America, provides 23 64-
kbps B-channels (voice or data) and one 64-kbps D-channel (signaling).
The D-channel is the 24th channel of the interface and contains
multiplexed signaling information for the other 23 channels.
primary high-usage trunk
group
A high-usage trunk group that is administered to provide service to first-
route call traffic only.
private branch exchange See PBX.
processor data module
(PDM)
A device that provides an RS-232C data communications equipment
(DCE) interface for connecting to data terminals, applications processors
(APs), and host computers; and provides a digital communications
protocol (DCP) interface for connection to a communications system. See
also modular processor data module.
protocol A set of conventions governing the format and timing of message
exchanges.