44
40-400-00001, Rev. A
NT2
(Network Termination 1 and 2)
Customer premises device to fan out a user-to-network (T) interface into multiple T or R
interfaces. Concerned with Layer 1, 2, and 3. Examples are a PBX, Key system, LAN, and
terminal controller.
OSI
(Open System Interconnection Reference Model)
Open Systems Interconnection. The International Standards Organization (ISO) model of
how data communications systems can be interconnected. Communication is partitioned
into seven functional layers. Each layer uses and builds on the services provided by those
below it.
Out-of-Band Signaling
A system that uses a separate communications channel or frequencies outside of voice
band for signaling. Digital examples are the ISDN D channel and common channel signal-
ing.
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 compa-
nies 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.
