While the ISDN protocols define the transfer of information, layer 2 and 3 functions are handled
with LAPD. Understanding the information contained in this frame will help you understand
Q.921 and Q.931 debug outputs. Remember that LAPD is the framing protocol used for D channel
data and that the D channel is used to build connections to an ISDN link.
An LAPD frame has six parts to it: Flag, Address, Control, Information, CRC, and a final
Flag. Figure 26.4 shows the LAPD frame and the fields within the frame.
The following information describes the fields within the LAPD frame:
Flag This one-octet field starts and ends the frame with a value of 7E (0111 1110). The LAPD Flag
and Control fields are identical to those of HDLC.
FIGURE 2 6 . 4 Link Access Procedure, Data frame
Flag Address Control Information CRC Flag
Address This field is two octets long and contains some important information. This field identifies
the TE using this link and has four parts: service access point identifier (SAPI), command/
response (C/R), address extension 0 (AE0), and terminal endpoint identifier (TEI).
Service access point identifier (SAPI) This field is six bits long. Table 26.4 shows the SAPI
values that can be used in an LAPD frame.
Command/response (C/R) This field is one bit long. This bit identifies the frame as either a
command or a response. The user side always sends commands with this bit set to 0 and responds
with it set to 1. The network side is the exact opposite, sending a command with this bit set to 1,
or a 0 if it is responding.
Address extension 0 (AE0 and AE1) These are one bit long. Their value indicates whether the
associated octet is the last in the Address field. Setting the value to 1 in the last bit of an address
octet (the AE field) indicates to the receiving device that this is the last octet in the Address field.
Terminal endpoint identifier (TEI) These values uniquely identify each TE on an ISDN S/T bus.
A TEI can be either dynamically or statically assigned. Table 26.5 lists the values for this field.
TABLE 2 6 . 4 SAPI Values
SAPI Description
0 Call control procedures
1 Packet mode using Q.931 call procedures
16 Packet mode communications procedures
32–47 Reserved for national use
63 Management procedures
Others Reserved for future use
TABLE 2 6 . 5 Terminal Endpoint Identifier (TEI) Values
TEI Description
0–63 Fixed TEI assignments
64–126 Dynamically assigned (assigned by the switch)
127 Broadcast to all devices
Control This field has 11 available values, each one listed in Table 26.6, along with its application.
You will see one of three types of information here: Information Transfer, Supervisory,
or Unnumbered.