[asterisk-dev] proper way to signal user information layer 1 (ISDN)

[Matthew Fredrickson]

Klaus Darilion wrote:
> Hi!
> 
> In case of digital ISDN calls I want to signal the ISDN user information 
> layer 1 values between the dialplan (and maybe some apps) and the ISDN 
> channel driver.
> 
> I see two ways of doing this:
> 
> 1. an ast_variable (a global one with _ prefix so that it gets copied 
> automatically) which is read/written by the ISDN channel drivers and the 
> apps.
> 
> 2. Adding ast_channel.userinformationlayer1. The channel driver and apps 
> can access the value directly and from dialplan this can be done by 
> enhancing func_channel.c (http://bugs.digium.com/view.php?id=10217)
> 
> This value exists only in ISDN, thus I'm not sure if a new member to 
> ast_call is the proper way - but ISDN transfer capability is similar and 
> this is already added to ast_chan.

User information layer 1 is like a codec field in ISDN land.
[From Q.931, here are the different options for this field]

0 0 0 0 1 ITU-T standardized rate adaption V.110, I.460 and X.30. This 
implies the presence of
octet 5a and optionally octets 5b, 5c and 5d as defined below
0 0 0 1 0 Recommendation G.711 [10] μ-law
0 0 0 1 1 Recommendation G.711 A-law
0 0 1 0 0 Recommendation G.721 [11] 32 kbit/s ADPCM and Recommendation 
I.460
0 0 1 0 1 Recommendations H.221 and H.242
0 0 1 1 0 Recommendations H.223 [92] and H.245 [93]
0 0 1 1 1 Non-ITU-T standardized rate adaption. This implies the 
presence of octet 5a and,
optionally, octets 5b, 5c and 5d. The use of this codepoint indicates 
that the user rate
specified in octet 5a is defined by the user. Additionally, octets 5b, 
5c and 5d, if
present, are defined in accordance with the user specified rate adaption
0 1 0 0 0 ITU-T standardized rate adaption V.120 [9]. This implies the 
presence of octets 5a and
5b as defined below, and optionally octets 5c and 5d
0 1 0 0 1 ITU-T standardized rate adaption X.31 [14] HDLC flag stuffing

I think that probably making chan_zap so that it is more codec aware 
would be the right way to adjust this field.

-- 
Matthew Fredrickson
Software/Firmware Engineer
Digium, Inc.