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New Member

Rcvr Jitter, meaning of...

I am troubleshooting a "quality" issue with a 7960 phone, and I was looking at the web page that is presented when you point the browser at the IP address of the phone, and click on 'Stream 1' under 'Streaming Statistics'.

Can anyone tell me what the numbers in the "Rcvr Jitter" statement represent, and what level (number) is considered unacceptable.

The cco web documents on the subject say that the numbers in the "Rcvr Jitter" statement represent the "Maximum jitter of stream", but there are two numbers, so that doesn't make much sense.

I could assume that the first number represents average jitter, and the second number max jitter, observed since the receiving voice stream was opened, but it would be good to know if this is what the developers programmed.

What I am looking for is a rough estimate for when the jitter is going to start affecting voice quality (equivalent to a mean opnion score of 3 or so) based on the numbers given in the "Rcvr Jitter" statement.

Thanks,

Greg

  • Other Collaboration Voice and Video Subjects
5 REPLIES
Cisco Employee

Re: Rcvr Jitter, meaning of...

It's very difficult to put an absolute value on this since it depends on many factors. Mainly how full the de-jitter buffer gets. Jitter will not affect voice unless it gets so bad that it overruns the de-jitter buffer on the phone. As a general rule I would say that anything above 80-90ms or so would start to cause concern if you see it consistantly.

So what happens is that the de-jitter buffer will hold a certain amount of the voip packets as they come in, when the congestion that is causing the jitter happens no new packets enter the buffer, if this delay is long enough the buffer will run out of packets to feed to the DSP, you wil hear this. Then when the congestion is clear the buffer will get hammered by all the packets that were delayed. If enough packets are delayed, they will fill the jitter buffer up and it will start overflowing, you loose the packets that overflow the buffer and this will also be audible.

Anonymous
N/A

Re: Rcvr Jitter, meaning of...

The "Rcvr Jitter" is the difference or delta between what was sent from the far end and what was recieved by your phone.

Jitter is bad, you don't want large numbers of Jitter in any direction.

A quick useful way of seeing how much jitter (if any) is affecting a call is to press the round 'i' button on the phone twice while on a call.

This will display the jitter stats and other info for the current call. If you have increasing jitter values, you need to get it sorted.

The first place to look is the duplex setting on the switch. A duplex mismatch between the phone and switch will usually cause lots of jitter.

Next, check the QOS used for the WAN. This is a design issue and can't be answered simply in a forum reply.

The MOS (Mean Opinion Score) is dependant on the codec used, it won't be dependant on the jitter value since this is a variable. g.711 provides the highest MOS value.

New Member

Re: Rcvr Jitter, meaning of...

Thanks, I do understand what jitter is, and what the possible causes are, but I was specifically wondering what the numbers on the web page represent.

Anyone know?

Cisco Employee

Re: Rcvr Jitter, meaning of...

Well the format is this: (avg jitter, mx jitter ).

The values are in milliseconds.

The definition of how the numbers are calculated:

This is the packet arrival variance that typically looks like a Gaussian distribution. Jitter is usually given in terms of standard deviation. For instance "packets arrive at their theoretical arrival times, +/- a standard deviation of 5 ms"

Cisco Employee

Re: Rcvr Jitter, meaning of...

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