Delayed Speech?

[thunderheart]

Dear VOIP Gods,

The users of our system are complaining about what I can best describe as a "delay" in the speech path that they find annoying. I personally don't notice it much but I have often imagined that it was occuring. I can best describe it using an example. When a TDM call is routed over a non-terrestrial trunk or around a fiber loop with abnormally long propagation delay, you will often hear this effect. It takes a small but finite time for the remote party to hear your sentence and respond. You think they didn't hear you or are waiting for you to go on so you end up talking over them. Much like watching a newscaster here in the US interview a reporter live in Iraq.

Since almost all of our calls are local, and the only IP leg is from our phones to the PBX (all "trunks" analog) I'm at a loss to understand where this delay is being introduced. There is no transcoding as the phones use ulaw as do the trunks. The PBX is a fairly hefty server with an AMD Sempron 64 3400+ and a gig of PC3200. There are at most 5 simultaneous calls and I doubt it has ever been more than 3 honestly. The PBX sits on a gigabit managed LAN, all nodes (excluding the phones) have gigabit interfaces.

I know this isn't a PIAF issue but I'm hoping someone will have an idea.

Thanx,

Dallas

 

[steve54301]

It sounds like network latency. And apparently maybe in one particular direction, and not on the return path.

It's the same kind of thing people typically get with satellite based internet.

It interesting because you didn't mention observed echo or other artifacts, but not unheard of.

Network buffering and hardware buffering both contribute to the overall total audio delay.

I'd start troubleshooting by pinging both ways and looking at the return reply times. Is one of them excessive?

Then I'd look at what hardware is in those paths; routers, switches, and anything else in between. Does one path have some sort of transcoding and the other doesn't.

Those are my thoughts.

 

[thunderheart]

Thanks KB

Yeah, it seems obvious that there is some network delay somewhere but again, it can only be between the phones themselves and the PBX. Remember, no VOIP trunking involved. Since the PBX and all the phones are physically plugged into the the same 24-port, gigabit switch which has VERY little traffic, I can't see where the delay would come from. The switch shows really clear stats for all the ports involved (no collisions, bad crc's etc) so I can't blame it on noisy cabling or other phy layer stuff.

Unfortunately, I can't really ping from the phone but pinging to it from the PBX is interesting. The latency will start around 10 milliseconds and slowly fall off over about 20 or so packets to around 1 millisecond, and then instantly spike back to 10 milliseconds. This cycle repeats over and over. Again interesting but 10 ms is really not that high.

I could enable QOS and make SIP traffic the highest priority but I wouldn't think that would be needed in a system with such low utilization.

As for echo, yeah ... there is none but my tdm card has hardware EC. It may be workin' its tail off and I just don't know it.

Dallas