Information Technology Reference
In-Depth Information
The second ISDN interface is the PRI. In North America and Japan, PRIs offer 23 B channels
and 1 D channel (23B+D). In other countries, PRIs offer 30 B channels and 1 D channel
(30B+D). In PRIs, the D channel has 64 kbps of bandwidth. The 23B+D PRIs run over T1
circuits with 1.522 Mbps of total bandwidth, and 30B+D PRIs run over E-1 circuits with
2.048 Mbps of total bandwidth.
More information on ISDN BRI and PRI is in the ITU-T specifications I.430 (BRI) and
I.431 (PRI).
ISDN Layer-1 Frames
ISDN frames are 48 bits in length. There are two formats: one for outbound (toward the ISDN
network) and inbound (from the network). Figure 5-11 shows the frame formats.
ISDN Layer-1 Frame Formats
Figure 5-11
N
T
Fr
a
me sent from n
e
tw
o
rk to t
e
rminal
FLB1
(8 bits)
LDL F L B2
(8 bits)
LDLB1
LDL B2
L
D
L
TE Fr
a
me sent from te
r
mi
n
al
t
o n
e
two
r
k
FLB1
(8 bits)
EDA F F B2
(8 bits)
EDSB1
EDS B2
E
D
S
The bits are as follows:
•
F—Framing bit for synchronization
•
L—Load balancing bit that adjusts the average bit value
•
E—Echo of previous D-bit, for contention resolution
•
A—Activation bit
•
S—Unused
•
D—D channel (16 kbps)
•
B1—First B channel
•
B2—Second B channel
ISDN Layer-2 Protocols
At Layer 2 the ISDN B channels can be encapsulated with X.25, Frame Relay, HDLC, or PPP.
The D channel is encapsulated with LAPD or X.25. These protocols are covered in this section.
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