[asterisk-dev] q931 decoding question

[Klaus Darilion]

Hi!

Please take a look at the following incoming SETUP message:

< Protocol Discriminator: Q.931 (8)  len=42
< Call Ref: len= 2 (reference 5566/0x15BE) (Originator)
< Message type: SETUP (5)
< [04 02 88 90]
< Bearer Capability (len= 4) [ Ext: 1  Q.931 Std: 0  Info transfer 
capability: Unrestricted digital information (8)
<                              Ext: 1  Trans mode/rate: 64kbps, 
circuit-mode (16)
<                              Ext: 0  User information layer 1: Unknown 
(24)

lets decode it:

       bearer capability IE
      /
     /  length
    |  /
    |  |   ext=1, coding standard=0 0=ITU, information tranfer 
capability=0 1 0 0 0=Unrestricted digital information
    |  |  /
    |  |  |    ext=1, transfer mode=0 0=circuit mode, information
    |  |  |    transfer rate=0 0 0 0 0
    |  |  |  /
    |  |  |  |
< [04 02 88 90]

this means the bearer capability IE does not include the user 
information layer 1 octet. Nevertheless it gets parsed.

<                     Ext: 0  User information layer 1: Unknown (24)

My questions: Why does it gets parsed? Why is it set to 24? I checked 
libpri source code but could not find the relevant code. Maybe someone 
can point to the corresponding code.

thanks
Klaus