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BSMI Debugging
November 2009 107
Interpreting Information Elements
For Q.931 call control messages, the first Information Element (IE)
starts at byte offset 9 in the hexadecimal string. Each message can
contain several IEs of either fixed (single byte) or variable length.
Figure 8 on page 108 compares the general IE format against the
first IE contained in the example SETUP message, and illustrates
the following points:
A value of 0 in the shaded bit position indicates a variable-length
IE; a value of 1 in that position indicates a single byte IE.
Note: Single byte IEs are commonly used for locking codeset shifts.
Locking shift IEs appear only after all variable-length IEs
within the message. Refer to the Bellcore Technical Reference
TR-TSY-000268 for more information on the structure and use
of single byte IEs and codeset shifts.
The IE identifier value 0x04 indicates a Bearer Capability IE;
refer to
Table 4 on page 108 for possible IE identifier values. IEs
appear in messages in ascending order according to their
identifier number.
The 2-byte length of the IE value indicates that it contains only
the required structures for a Bearer Capability IE.
The IE contents indicate an information transfer capability of
unrestricted digital information (0x88) and a transfer rate/mode
equal to 64
kbps/circuit mode (0x90).
011110117B Information
011011106E Notify
011111017D Status
0111010175 Status Enquiry
Table 3. Q.931 Message Types (Continued)
Message Type Bits Hex Message
IEs
00C10000080101050 4028890 180181 2C0735353532303030
03 278D Xmit 00 60 0 Setup 00 00 0 0018