146 Communication Networks
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defines the physical U interface for basic and primary rate access. The data link layer provides two HDLC-type protocols, one for use with B channels LAP-B and one
for use with D channels LAP-D. The network layer provides the X.25 packet level protocol for packet switching on either the B channel or the D channel and a call
control protocol for D channel signaling. Layers 4-7 are concerned with the end-to- end exchange of user data.
Figure 11.126 ISDN protocol architecture. OSI Layer
ISDN Layer Purpose
higher layers 4+ B Channel End-to-End
User Signaling Higher-level functions related to end-to-end
signaling. Network
3 Circuit-
X.25 Call
Control X.25
Circuit-switching, packet switching, and call control signaling.
Data Link 2
Switched LAP-B
LAP-D Data link control for voicedata channels and
signaling channels B and D. Physical
1 Physical
Physical interface for basic access and primary access T and S interfaces.
Below we will look at each of layers 1-3 separately.
11.2.1. The Physical Layer
The physical user interfaces for basic and primary rate access are, respectively, defined by CCITT recommendations I.430 and I.431. The S and T interfaces both
use four wires to provide full-duplex connections as two separate physical connections, one in either direction. Because of the very short distances involved
between the U interface and the S and T interfaces, this is the most effective approach to full-duplex connection.
The basic access interface at the S or T reference point consists of 2B+D channels which are multiplexed onto a 192 kbps channel using TDM. As mentioned
earlier, this carries an overhead of 48 kbps which accounts for framing and synchronization. Each frame is 48 bits long see Figure 11.127 and consists of: 16
bits from either B channel marked as B1 and B2, 4 bits from the D channel marked as D, and 12 framing and synchronization bits shown in italics. As shown
in Figure 11.127, the structure of frames sent from an NT to a TE is somewhat different from the structure of frames sent from a TE to an NT. The Framing Bit
marks the beginning of a frame. The Balancing Bit is a negative pulse and is intended to balance the DC voltage. These two bits are used for frame
synchronization. An NT echoes the most-recently received D channel bit from a TE using the Echo Bit reasons explained below. The Activation Bit is used by an NT
to activate or deactivate a TE. The Auxiliary Bit and the Auxiliary Complement Bit which is simply the logical negation of the Auxiliary Bit are used for frame
alignment. The Multiframe Bit is used for multiframing.
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147
Figure 11.127 Basic rate frame structures. NT to TE Frame
TE to NT Frame Bit
Description Bit
Description
1 Framing Bit
1 Framing Bit
2 Balancing Bit
2 Balancing Bit
3-10 B1
3-10 B1
11 Echo Bit
11 Balancing Bit
12 D
12 D
13 Activation Bit
13 Balancing Bit
14 Auxiliary Bit
14 Auxiliary Bit
15 Auxiliary Complement Bit
15 Balancing Bit
16-23 B2
16-23 B2
24 Echo Bit
24 Balancing Bit
25 D
25 D
26 Multiframing bit
26 Balancing Bit
27-34 B1
27-34 B1
35 Echo Bit
35 Balancing Bit
36 D
36 D
37 Reserved for future use
37 Balancing Bit
38-45 B2
38-45 B2
46 Echo Bit
46 Balancing Bit
47 D
47 D
48 Balancing Bit
48 Balancing Bit
The primary access interface at the T reference point consists of 23B+D or 30B+D channels which are multiplexed onto a 1.544 or 2.048 mbps using TDM.
The 1.544 mbps interface is used in North America and Japan, and employs a 193- bit frame structure, while the 2.048 interface is used mainly in Europe and employs a
256-bit frame structure see Figure 11.128. The 23B+D frames are organized as multiframes of 24 frames, with the Framing Bit of every fourth frame set to 0 or 1 to
generate the bit pattern 001001 for the multiframe. This is used for frame alignment and synchronization. The 30+D frames, on the other hand, use the first 8 bits as a
Framing Channel. Of these, the first bit is not used and bits 2-8 are set to 0011011 for frame alignment and synchronization.
One notable difference between the basic access and primary access interfaces is that the latter can only be used for point-to-point connections, while the former can
also be used in a multidrop arrangement, where an NT serves a number of TEs via a simple bus. Such an arrangement would allow the TEs to individually use the B
channels and share the D channel. Since the B channels cannot be shared, at most two TEs at any one time can have access to them. D channel frames destined for the
TEs are addressed using LAP-D explained in the next section. To transmit a frame, a TE listens to the echo of a series of 1 bits that it transmits on the D channel in
absence of a signal. These bits can be echoed back by the NT using the Echo Bit. When the TE receives a certain number of successive echoes of its 1 bits the exact
number is dependent on the TE’s predetermined priority level, it concludes that the
