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BSMI Debugging
November 2009 105
Figure 6. I Frame Formats
Interpreting the Message Header
The Message header starts at byte offset 5 of the hexadecimal string.
For Q.931 call control messages, this header identifies the D-channel
message and references the call for which the message applies.
Figure 7 compares the general structure for a Message header
against the example SETUP message header, and illustrates the
following points:
The Protocol Discriminator value is 0x08 for all Q.931 call
control messages.
Modules assign 1-byte call reference values for Q.931 messages,
so the call reference length is always 0x01 and the third byte in
the Message header contains the call reference value (0x0001 in
the example).
Example
Byte
n - 2
n - 1
Bits
8765 4321
01111110
01111110
SAPI
TEI
N(s)
N(r)
Message
- - - - - - - - - - - - FCS - - - - - - - - - - - - -
0
1
0
P/F
C/R
General I Frame
Bits
8765 4321
0000000
1100000
0000000
0000000
Example SETUP I Frame
Not Shown in Trace
Not Shown in Trace
See Figure 7
Not Shown in Trace
0
1
0
0
1
2
3
4
5
-
n
-
00
C1
00
00
See Figure 7
Message
Header
00C10000 08010105040288901801812C0735353532303030
03 278D Xmit 00 60 0 Setup 00 00 0 0018