[cisco-voip] Special considerations for IAD or PRInetwork-emulation on Cisco hardware...

[Jason Aarons (US)]

All your questions are around layer1, a show controller or show
service-module would indicate if clocking or timing or other layer1
issues. You have to look at both sides. Your side may be clean but the
other side dirty.

 

Usually a TTC T-Berd with Y-Cable in the middle would help with
analysis. You can even listen in to troubleshoot.  You can rent T-Berd
for a month for less than $250 from Renfro Test Equipment (Google).

 

Other answers inline

 

From: cisco-voip-bounces at puck.nether.net
[mailto:cisco-voip-bounces at puck.nether.net] On Behalf Of Jeff Anderson
Sent: Wednesday, May 06, 2009 5:00 PM
To: cisco-voip at puck.nether.net
Subject: [cisco-voip] Special considerations for IAD or
PRInetwork-emulation on Cisco hardware...

 

I have a couple of customers who are experiencing poor audio using an
IAD to connect their PBX to our h.323 PSTN GW.

 

Basic Layout.

 

Customer PBX PRI > IAD PRI (Network-emulate) converted to VoIP > MPLS
enabled T1 > Core > AS5850 (VoIP converted to TDM) > PSTN.

 

The customer reports that poor audio is experienced inbound into their
PBX. They say their outbound audio is okay. This doesnt happen on every
call but we do have more than one customer reporting the issue.

 

The audio can be described anywhere from echo, cutting out or distorted
audio. The problem has also been reported by users who have 38xx series
gateways so i dont think the problem is IAD specific. The AS5850 is a
shared device used by many different customers, the majority of which
connect using Cisco IP phones and this demographic of customers are not
reporting the problem.

 

I have done packet captures on the AS5850 and there is no packet loss
reported, the packets arrive in sequence and the jitter is very low.

 

In my testing, I have found double-talk to cause the far end audio
(AS5850) to cut-out. All the packets arrive but they contain a payload
value of 7F. Some improvement can be made to this condition by disabling
non-linear progression. 

 

 

 

My questions for the community are related to experiences or lessons
learned from using PRI network emulate.

 

Can there be too many echo cancellers in the call flow? (i.e. Customer
PBX UC500 with Cisco PRI, IAD PRI, AS5850 PRI all with echo cancellers)

 They are usually local link significant. Bad DSP micro code can cause
the issues you are reporting. I've done PCM captures that were good yet
the DSP turned out to be bad.

 

Does the cable length command have any bearing on voice quality?

I haven't seen the cable length command fix quality issues

 

Are there any special commands i should enter when connecting back to
back wth a T1 cable (echo-cancel coverage, nlp, etc)?

Who is clocking? Network vs user side? Clock slips, line code
violations, etc  would create the problems you describe

 

Is a 2 pair sheilded T1 cable necessary for the PRI back to back
connection of less than 20 ft?

I always use shielded T1, who knows when EMF will be present? A 1 ft
cable wrapped around a ballast would be bad..

 

 

Any insight or suggestions would be greatly appreciated. 

 

Thanks,

 

Jeff 

 

 




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