System information

Troubleshooting ISDN Connections 17-375
Layer 1
ISDN Primary Rate Interface (PRI) service offers 23 B channels and one D channel in North
America and Japan, yielding a total bit rate of 1.544 Mbps (the PRI D channel runs at 64 kbps). ISDN
PRI in Europe, Australia, and other parts of the world provides 30 B plus one 64-kbps D channel and
a total interface rate of 2.048 Mbps. The PRI physical layer specification is ITU-T I.431.
Layer 1
ISDN physical layer (Layer 1) frame formats differ depending on whether the frame is outbound
(from terminal to network) or inbound (from network to terminal). Both physical layer interfaces are
shown in Figure 17-2.
Figure 17-2 ISDN Physical-Layer Frame Formats
The frames are 48 bits long, of which 36 bits represent data. The bits of an ISDN physical layer
frame are used as follows:
• F—Provides synchronization.
• L—Adjusts the average bit value.
• E—Used for contention resolution when several terminals on a passive bus contend for a channel.
• A—Activates devices.
• S—Unassigned.
• B1, B2, and D—For user data.
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. When an NT receives a D bit from the TE, it echoes back the bit in the next E-bit
position. The TE expects the next E bit to be the same as its last transmitted D bit.
Terminals cannot transmit into the D channel unless they first detect a specific number of ones
(indicating “no signal”) corresponding to a preestablished priority. If the TE detects a bit in the echo
(E) channel that is different from its D bits, it must stop transmitting immediately. This simple
F L B1 L D L F L B2 L D L B1 L D L B2
11 8 11111 8 111 8 111 8
…
NT frame (network to terminal)
Field length,
in bits
F
L B1 E D A F F B2 E D S B1 E D S B2
11 8 11111 8 111 8 111 8
…
TE frame (terminal to network)
A = Activation bit
B1 = B1 channel bits
B2 = B2 channel bits
D = D channel (4 bits x 4000 frames/sec = 16 kbps)
E = Echo of previous D bit
F = Framing bit
L = Load balancing
S = Spare bit
Field length,
in bits