ISDN physical-layer (Layer 1) frame
formats differ depending on whether the frame is outbound (from
terminal to network-the TE frame format) or inbound (from network to
terminal-the NT frame format). Both of the frames are 48 bits long, of
which 36 bits represent data. Actually, the frames are two 24 bit
frames in succession consisting of 2 8-bit B channels, a 2-bit D
channel, and 6 bits of framing information (2*(2*8B+2D+6F) =
32B+4D+12F = 36BD+12F = 48BDF). Both physical-layer frame formats are
shown in the figure. The bits
of an ISDN physical-layer frame are used as follows:
- Framing bit - Provides
synchronization.
- Load balancing bit - Adjusts the
average bit value.
- Echo of previous D channel bits-Used
for contention resolution when several terminals on a passive bus
contend for a channel.
- Activation bit - Activates devices.
- Spare bit - Unassigned.
- B1 channel bits.
- B2 channel bits.
- 8
Added channel bit counts bits.
- D channel bits - Used for user data.
Note that each of the ISDN BRI frames
are sent at a rate of 8000 per second. There are 24 bits in each
frame (2*8B+2D+6F = 24) for a bit rate of 8000*24 = 192Kbps. The effective
rate is 8000*(2*8B+2D) = 8000*18 = 144Kbps.
Multiple ISDN user devices can be
physically attached to one circuit. In this configuration, collisions
can result if two terminals transmit simultaneously. ISDN therefore
provides features to determine link contention. These features are
part of the ISDN D channel, which is described in more detail later in
this chapter.
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