Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
Community Member

qos issue between a vg224 and a 2800 router - weird jitter on fax calls

Have a vg224 and a 2800 router. vg224 has fax machines and 2800 has PRI to PSTN. Fax calls from the vg224 to the PSTN have very high jitter often (400-1200 ms). However, regardless of jitter value the faxes always succeed. (The jitter is a problem because Callmanager ART reports the jitter with a QoS Alert; we are getting false positives making the QoS Alert ineffective).

Strangely, jitter is only reported on the vg224, not the 2800. There are no routers in the mix, everything is plugged into the same switch on the same subnet, QoS configs have been verified by Cisco. No problem jitter coming from anywhere else on the network.

Here is the output of jitter resulting from a "sh call active voice | inc Jitter":

2800

================

CPRTR01#sh call active voice | inc Jitter

PlayDelayJitter=0

FaxRelayMaxJitterBufDepth = 0 ms

FaxRelayJitterBufOverFlow = 0

vg224

==================

CPRTR03#sh call active voice | inc Jitter

PlayDelayJitter=468

One solution I am thinking about is somehow configuring the vg224 to consider it's leg a fax call the same way the 2800 considers it (you'll notice that the 2800 has a fax relay jitter buffer as well as, I assume, a packetized voice jitter buffer, which may be what accounts for fax success??). Perhaps this is IOS version problem, I don't know.

Configs are attached.

vg224-i6s-mz.124-4.T3.bin

c2800nm-advipservicesk9-mz.124-9.T.bi

4 REPLIES
Community Member

Re: qos issue between a vg224 and a 2800 router - weird jitter o

so no ideas eh? One thing I forgot to mention is that it is an mgcp to h.323 interworking.

Bronze

Re: qos issue between a vg224 and a 2800 router - weird jitter o

First thing I'd do is do a "show controller T1 0/0/0" on the 2800 and look for any clock slips. I don't know if that will cause jitter, but do know that slips will cause problems for faxes.

Community Member

Re: qos issue between a vg224 and a 2800 router - weird jitter o

Thanks for your reply. All other phone calls to this gateway experience no undue jitter. The controllers are clean.

Bronze

Re: qos issue between a vg224 and a 2800 router - weird jitter o

Also, it looks like the 2800 is configured to use G.729 is the preferred codec, but 14,400 bps is the rate. This might be a problem because G.729's rate is only 8,000 bps.

Both sides are set to prefer T.38 fax relay with Cisco as fallback. That should be fine, but fax relay is designed for networks with high latency or packet loss. Fax Passthrough might be more appropriate for your setup.

364
Views
0
Helpful
4
Replies
CreatePlease to create content