Reliance E1 Links with Loop Convertor needs to invert clock at times to work properly with Cisco routers V.35 cards.

We had an E1 from Reliance in North India- Delhi.
This was provided on their MUX located at both the ends. So effectively there was no copper on the path.

Both the end terminals (DTE) were Cisco routers with V.35 interfaces.
At end A the modem used was: Loop 1510-S-11-120-AC
At end B the modem used was: Loop 1510-S-11-120-AC

We were seeing heavy CRC errors on the Link once they were connected to the cisco routers. whereas the BERT run from a loop given at one end and tested on a BERT meter for over 24 hours was error free.

So Reliance, arrogantly at that , insisted that there was no problem at their end and that we need to call the cisco vendor to get the DTE (routers, cables, cards) tested.

however we did all that we could do at the Cisco End, we swapped cables, cards at both ends & still had the same CRC errors problem.

After which we contacted Reliance Mumbai engineer. He suggested we try toggling the CRC & the Clock DIP switches located on the rear of the LOOP modems.

It has been noticed that some cisco gear invert the clock and hence modems need to counter the same.

Thus we toggled the following DIP switch(es) on the LOOP modem(s)
Clock Switch: 7th DIP Switch at End B
CRC Switch: 3rd DIp Switch to set CRC OFF: at End A and End B.

And Voila, the Link is stable now.
Pls find below the o/p of various cisco commands which might help you troubleshoot a similar problem



*******At End A*****
#sh int s0/0/0
Serial0/0/0 is up, line protocol is up
Hardware is GT96K Serial

Cisco IOS Software, 3800 Software (C3845-ENTBASEK9-M), Version 12.4(5a), RELEASE SOFTWARE (fc3)

*******At End B********
#sh int s0/1/1
Serial0/1/1 is up, line protocol is up
Hardware is GT96K Serial

Cisco IOS Software, 2800 Software (C2800NM-ENTBASEK9-M), Version 12.4(5a), RELEASE SOFTWARE (fc3)