Nortel BCM50 Installation Manual page 6

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While an analog line carries only a single transmission at a time, an ISDN line can carry one or
more voice, data, fax, and video transmissions simultaneously.
An analog modem operating at 14.4 K takes about 4.5 minutes to transfer a 1MB data file and a
28.8K modem takes about half that time. Using one channel of an ISDN line, the transfer time is
reduced to only 1 minute and if two ISDN channels are used, transfer time is just 30 seconds.
When transmitting data, the connect time for an average ISDN call is about three seconds per call,
compared to about 21 seconds for the average analog modem call.
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 two B-channels and one 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 one D-channel (T1 carrier). In Europe, 30B+D consists
of 30 B-channels and one 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.
ISDN overview