Types Of Isdn Service; Isdn Layers - Nortel Enterprise Edge 2.0 Programming Manual

372 Appendix B: ISDN Overview

Types of ISDN service

Two types of ISDN services (lines) are available: Basic Rate Interface (BRI) and
Primary Rate Interface (PRI). Each line is made up of separate channels known as
B and D channels which transmit information simultaneously.
• BRI is known as "2B+D" because it consists of
2 B-channels and 1 D-channel.
• PRI is known as "23B+D"(in North America) or as "30B+D" (in Europe). In
North America, "23B+D" consists of 23 B-channels and 1 D-channel (T1
carrier). In Europe, "30B+D" consists of 30 B-channels and 1 D-channel (E1
carrier).
B channels
B channels are the bearer channel and are used to carry voice or data information
and have speeds of 64 kbps. Since each ISDN link (BRI or PRI) has more than one
B-channel, a user can perform more than one transmission at the same time using a
single ISDN link.
D channels
The standard signaling protocol is transmitted over a dedicated data channel called
the D-channel. The D-channel carries call setup and feature activation information
to the destination and has speeds of 16 kbps (BRI) and 64 kbps PRI. Data
information consists of control and signal information and for BRI only,
packet-switched data such as credit card verification.

ISDN layers

ISDN layers refer to the standards established to guide the manufacturers of ISDN
equipment and are based on the OSI (Open Systems Interconnection) model. The
layers include both physical connections, such as wiring, and logical connections,
which are programmed in computer software.
When equipment is designed to the ISDN standard for one of the layers, it works
with equipment for the layers above and below it. There are three layers at work in
ISDN for Enterprise Edge. To support ISDN service, all three layers must be
working properly.
• Layer 1: A physical connection that supports fundamental signaling passed
between the ISDN network (your service provider) and the Enterprise Edge
system.When the LED on a BRI S/T Media Bay Module configured as BRI is
lit, your layer 1 is functioning.
• Layer 2: A logical connection between the central office or the far end and the
Enterprise Edge system.Enterprise Edge has one or two of these connections for
each BRI link, and one for each PRI link. Without Layer 2, call processing is
not possible and there is no dial tone.
• Layer 3: Also a logical connection between the ISDN network (your service
provider) and the Enterprise Edge system. For BRI lines, layer 3 is where call
Enterprise Edge 2.0 Programming Operations Guide P0911588 Issue 01