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Introduction to the ISDN Interface Overview
Revised April 2000 DBS-EX23-530 1-3
Overview
Description of the ISDN Interface
The ISDN Interface is a Primary Rate Interface (PRI) digital trunk card that
provides 23 voice channels (“B” channels) and a control channel (“D”
channel) over a four-wire circuit.
ISDN lines can be leased from local exchange carriers and long-distance
carriers.
Note: The current version of the ISDN supports voice communications only.
Data can be transmitted only if it reaches the ISDN in analog form. Examples of
analog data that can be transmitted over the ISDN include fax transmissions or
PC files that have been converted into analog form using a modem.
Framing Options
The ISDN Interface takes an 8-bit sample from each of the 23 voice channels.
These samples are grouped into 23 bearer time slots, and 1 data channel time
slot.
Since each of the 24 time slots contains 8 bits, the number of sampling bits in
each frame equals 192 (24 x 8 = 192). In addition, a framing bit is added to
the end of each frame to mark where the frame ends. The addition of the
framing bit results in a 193-bit frame.
The ISDN transmits these 193-bit frames at a rate of 8000 per second. The
total number of frames (193) multiplied by the frame speed (8000 per second)
results in a total transmission rate of 1.544 Mbps.
ESF Framing Formats
The extended superframe (ESF) format transmits the sampling frames in
groups of 24 with 23B (bearer) channels at 64 Kbps and 1D (Data) channel at
64 Kbps. With the addition of 2.0 Kbps error checking CRC, 2.0 Kbps
framing, and 4.0 Kbps datalink facility, the total capacity is 1.544 Mbps.