ISDN Demonstrator User’s Manual
40-400-00011, Rev. B
40
Packet
A grouping of data, typically from 1 to 512 characters in size which usually represents
one transaction. A packet is always associated with an address header and control
information. The term “Packet” is usually used to refer to a Layer 3 data unit in X.25.
PBX (Private Branch eXchange)
A customer premises telephone switch connecting 20 or more station sets to each
other to the public network and possibly to a private network.
PRI (Primary Rate Interface)
An ISDN User-to-Network Interface. In North America and Japan, it is based on T1
Carrier. In North America, it consists of twenty-four full-duplex channels: Twenty-
three 64 Kb/s B-Channels, and one 64 Kb/s D-Channel ... hence the acronym
“23B+D.” The physical media is two twisted-pairs of wire.
Q.931 (I.451)
Layer 3 protocol for out-of-band message-oriented signaling across user-to-network
interface. Originally specified for the D channel of the ISDN Basic Interface (BRI) and
Primary Rate Interface (PRI).
Q.932
Q.931 signaling protocol extensions to add supplementary services. These include
custom calling features.
RBOC (Regional Bell Operating Company)
Seven operating or holding companies into which the Bell System’s local telephone
companies were assigned as a result of the Modified Final Judgement (MFJ) in 1982.
These companies were assigned the management of local exchange companies
originally owned by AT&T. Distributed regionally, they included Pacific Telesis, U S
West, Southwestern Bell Telephone, Bell South, Bell Atlantic, Ameritech, and
NYNEX.
Ring Back
Also known as audible alerting. The tones the calling party hears while the called
party’s phone is ringing.
RS-232C
Collection of specifications defining electrical and mechanical interfaces between
terminals, computers, and modems.
S Interface
ISDN Interface between station equipment and NT2.
Signaling
Communication between switches, or switches and end points, to set-up, manage,
and tear-down calls. Signaling methods include dial pulse (rotary dial), dual-tone
multifrequency (DTMF) (touchtone), and digital “packet” technology (ISDN, SS7).
SPID
The Service Profile Identifier is used to identify a specific ISDN set when more than
one ISDN set has been attached to the same central office line. In most cases, only
one ISDN set will be assigned to a line, and the SPID is not used by the switch.
