Question

I have a voice-chat service which is experiencing variations in the delay between packets. I was wondering what the proper response to this is, and how to compensate for it?
For example, should I adjust my audio buffers in some way?

Thanks

Answer 1

You don’t say if this is an application you are developing yourself or one which you are simply using – you will obviously have more control over the former so that may be important.

Either way, it may be that your network is simply not good enough to support VoIP, in which case you really need to concentrate on improving the network or using a different one.

VoIP typically requires an end to end delay of less than 200ms (milli seconds) before the users perceive an issue.

Jitter is also important – in simple terms it is the variance in end to end packet delay. For example the delay between packet 1 and packet 2 may be 20ms but the delay between packet 2 and packet 3 may be 30 ms. Having a jitter buffer of 40ms would mean your application would wait up to 40ms between packets so would not ‘lose’ any of these packets.

Any packet not received within the jitter buffer window is usually ignored and hence there is a relationship between jitter and the effective packet loss value for your connection. Packet loss typically impacts users perception of voip quality also – different codes have different tolerance – a common target might be that it should be lower than 1%-5%. Packet loss concealment techniques can help if it just an intermittent problem.

Jitter buffers will either be static or dynamic (adaptive) – in either case, the bigger they get the greater the chance they will introduce delay into the call and you get back to the delay issue above. A typical jitter buffer might be between 20 and 50ms, either set statically or adapting automatically based on network conditions.

Good references for further information are:
– http://www.voiptroubleshooter.com/indepth/jittersources.html
– http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a00800945df.shtml

It is also worth trying some of the common internet connection online speed tests available as many will have specific VoIP test that will give you an idea if your local connection is good enough for VoIP (although bear in mind that these tests only indicate the conditions at the exact time you are running your test).