[cisco-voip] CMR reports large jitter on gateway side only

[Andre Beck]

Hi,

while debugging certain voice quality issues in a network based on 7925s,
Unified WLAN (WiSMs and 1142s), CUCM 7.1(3) and 2811 MGCP PRI gateways
to a local PBX, we activated CDR and CMR recording. After collecting data
for some days, we see approx. 30% of calls that CAR considers Poor or just
Acceptable QoS. Looking closer, there's something dubious about these stats:

* They are only matching the jitter trigger, by having jitter way beyond
  100ms. Other triggers (Packet Loss, Delay) don't seem to match.

* Jitter is *always* bad on the gateway side, never on the phone side.
  Incoming calls have the poor QoS on the originating side, outgoing
  calls have it on the destination. I would expect some asymmetry, but
  not that much.

* Now for the strangest detail: Tandem calls (coming into the gatway
  on the PRI and leaving it on the same way, due to a call forward
  no answer or busy) which obviously never even hit the LAN are
  tracked with high jitter, too! We are seeing jitter values up to 800ms
  in one of the CMRs in these cases, even though the gateway is talking
  to itself on the loopback interface (if it does so at all, dunno exactly
  how tromboned calls are handled exactly, though there is no QSIG path
  replacement in action).

Now the question is, when it does report obvious nonsense like in the
tandem call case, are the other CMRs involving actual RTP streams over
the (W)LAN at all reliable?

Anyone having an explanation for the absurd gateway loopback jitter?

TIA,
Andre.
-- 
                    Cool .signatures are so 90s...

-> Andre Beck    +++ ABP-RIPE +++      IBH IT-Service GmbH, Dresden <-