asterisk users - Dec 2004 - Useful information - UK exchanges and SystemX/SystemY and how ISDN works!

Hi all,

I thought this may be useful information for the Brits reading this list:

I installed a * box (one ZapHFC BRI) for a customer last week and noticed
the following warning every fifty seconds:

  == Primary D-Channel on span 1 down
Dec  6 17:31:19 WARNING[213005]: chan_zap.c:1989 pri_find_dchan: No
D-channels available!  Using Primary on channel anyway 3!
  == Primary D-Channel on span 1 up

Today the machine started having all sorts of problems TEI requests etc. and
wouldn't make/receive calls. A reboot sorted that out, but I decided to
contact BT to see if the D channel going up and down is a problem.

It turns out that this is perfectly normal and the ETSI ISDN specification
doesn't state that the D channel has to be permanently up or down.
Apparently, there are two kinds of exchanges:

System Y exchange (AXE10) - Thorn/Ericsson
Layer 2 (D channel) drops every 50 seconds

System X exchange - Marconi
Layer two stays active permanently

So, if you see this warning, you know what the problem is (or rather that
there isn't a problem!).

Cheers,

Nick.