[asterisk-dev] Switching unrestricted digital calls

[Pavel Troller]

Hi!
  I was trying to google answers to my questions, but I didn't find anything
relevant, so I'm asking here.
  I'm developing an application, which needs to establish ISDN 64kbps
Unrestricted Digital call. These are coming from PRI ports (which are connected
to a legacy PBX) and they are going to either the same or another PRI port.
  There are no problems with signalling, but with the media. The terminal
devices (ISDN TAs) are having problems establishing and maintaining V.120 
connection.
  To find the problem, I used a BRI tester (connected instead of the ISDN TA)
and called to a single internal extension, which does just Echo(). A BERT test
was then started on the tester. And this is the most interesting thing there.
Depending on the BERT pattern selected, various results were obtained. Complex
patterns, like 2^15-1 or similar, showed permanent error rate about 4E-4. The
errors were distributed smoothly in time, every second appeared about 30
errors. Simple patterns, like 1:1, 1:3 or hex pattern 55 AA, were transmitted
with no errors during 4 consecutive hours.
  Of course the PRI E1 stream was perfectly synchronized. BERT towards a
loopback in the PBX into which the tester is connected is also error free
with any BERT pattern. Dahdi channels are configured with echo cancellation
disabled.
  Any hints ?

  With regards, Pavel Troller