Strange sound in calls. Samples attached

I got called in to take a look at this setup. GXP-2000 phones, sangoma fxo card. Randomly in calls the gxp-2000 user hears this horrible noise. The only time I have ever heard this particular noise is when I had the wrong isdn signaling protocol on a pri.

Anyway, have a listen, tell me what you think. I’m pretty stumped at this point.

It sounds like something in between the GXP-2000 and the Asterisk box… Could this be network collisions?

GXP-2000 phones have a switch inside them that basically let’s them “share” a single network connection with a PC. Are they trying to do this?

I don’t reccomend doing it. UDP and TCP traffic don’t play well together. the fact that it starts and then stops makes it seem like this might be the case, and that the interrupting traffic might be something like an email client checking for new mail. (But in the process, hogging all the bandwidth and messing up the audio path.)

UDP as a real time protocol really needs all the bandwidth to itself. The Asterisk box should have a gigabit network connection, and the talk path for the audio should have it’s own switched port on the network.

Don’t forget to check the bandwidth of the switch itself. If the backbone of the switch can’t handle the RTP traffic, you’re sure to have problems.

Was the recording done on the server ?

If it was you need to keep the in and out files if possible.

Does it happen on all calls ?


They do have the PC hooked up through the phone. I think it is a problem with the phones themselves because we have yet to replicate this using a soft phone.

Yes it was recorded on the server. The crazy sound you hear is only on the ip side it seems. Because as the end of the call you can hear the pstn caller saying “hello”

Take a few of the phones and PCs and give them their own ports on the network switch. See if the problem goes away.

Leave a few alone too. (To serve as a control group.)

I’m willing to bet that the PC traffic is your big problem.

You might even be able to replicate the problem by simply starting a phone call and doing something network intensive with the PC. Like copying a large file from one PC to a server or something like that. Give that a try as well.

If you can cause the problem with PC network traffic, you can solve it easily by giving each object (PC, VOIP phone, etc) it’s own switch port.

Yeah we tried loading down the network doing a large file transfer to the pc that is plugged into the phone. No joy there, it seems to be completely intermittent.

Out of curiosity, they don’t have any old hubs in the network do they?

I’d also check to make sure all of the individual setup objects of the various ports and NIC cards. Is something setup as half duplex, while the other end is full duplex?

Lastly, check the environment… Cell phones and Blackberry devices are notorious interference generators. Are they using those near the GXP-2000?

I think you’re on the right track. It’s very probably something to do with the GXP-2000. But this is sounding less and less like something you can just “make a change” to solve.