[asterisk-users] cat /proc/zaptel/*

[Jaap Winius]

Quoting Jaap Winius <jwinius at umrk.to>:

Being both impatient and charitable, I'll try answering this myself:

> "ISDN uses LAPD for the D-channel and LAPB for data connections over
> the B-channels. However, LAPB is irrelevant for Asterisk, because when
> the B-channels are used for voice they carry no signaling. This is why
> it is necessary to specify a line code protocol, such as AMI, for the
> B-channels, and a frame type, typically CCS, for the D-channel."
>
> Would that statement be correct?

Basically, although the last line is a little muddled.

> Also, would someone care to elaborate on how the CCS protocol fits into
> this picture, in particular how it relates to LAPD?

LAPD, described in ITU-T recommendations Q.920 and Q.921, is an OSI  
network layer 2 protocol, while CCS (Common Channel Signaling), which  
is described by Q.930 (I.450) and Q.931 (I.451), is layer 3.

Two things that I found confusing here are:

1.) The documentation that explains the Zaptel span configuration  
statement (in /etc/zaptel.conf) describes the D-channel signaling type  
as "framing," which I find misleading. IMO "signaling" would have been  
more accurate.

2.) CCS is a connection control signaling type. The problem is that  
there is more than one CCS type, although my impression is that the  
one used most often for the ISDN D-channel is Q.930/Q.931. The others  
I've heard of are QSIG CCS (Q.931/Q.933) and SS7 (Q.700-series with  
many variants).

Cheers,

Jaap