[VoiceOps] Question about Packet Jitter

[Howard Hart]

There's always the RTP/RTCP RFC 3550. See section 6.4.1, interarrival 
jitter subsection for generating RTCP jitter stats. There's even sample 
code to implement this in Appendix A.8

Howard Hart
Ooma Operations

On 10/29/2010 05:15 AM, Jim Dalton wrote:
>> Jitter is an average measure of what's
>> essentially noise (random variations in the arrival times of packets
>> from a mean), and so I'd expect - for two jitter sources in series
>> producing average jitter of j1 and j2 respectively - the overall jitter
>> to be sqrt(j1^2 + j2^2).
>>
>> Intuitively, a packet which is late as a result of the first jitter
>> source might then be further delayed or delivered early by the second -
>> so just summing the average jitter values isn't appropriate.
>>
>> --Dave
>>      
> [JD wrote:] Very good point.  The relationship of overall jitter to be
> sqrt(j1^2 + j2^2) makes sense.  However, I am having trouble working through
> a proof of the math.  Any suggestions of a reference on jitter that might
> address this?
>
> Many thanks for your insight,
>
> Jim D.
>
>    
>> -----Original Message-----
>> From: voiceops-bounces at voiceops.org [mailto:voiceops-
>> bounces at voiceops.org] On Behalf Of David Knell
>> Sent: Friday, October 29, 2010 1:37 AM
>> To: voiceops at voiceops.org
>> Subject: Re: [VoiceOps] Question about Packet Jitter
>>
>> On Thu, 2010-10-28 at 16:26 -0400, Scott Berkman wrote:
>>      
>>> The only portion missing is the jitter through the SBC itself, which
>>>        
>> really
>>      
>>> should be negligible.  Assuming that is 0, your equation below would
>>>        
>> be
>>      
>>> correct.
>>>        
>> I think I'd disagree with this.  Jitter is an average measure of what's
>> essentially noise (random variations in the arrival times of packets
>> from a mean), and so I'd expect - for two jitter sources in series
>> producing average jitter of j1 and j2 respectively - the overall jitter
>> to be sqrt(j1^2 + j2^2).
>>
>> Intuitively, a packet which is late as a result of the first jitter
>> source might then be further delayed or delivered early by the second -
>> so just summing the average jitter values isn't appropriate.
>>
>> --Dave
>>
>>      
>>> -----Original Message-----
>>> From: voiceops-bounces at voiceops.org [mailto:voiceops-
>>>        
>> bounces at voiceops.org]
>>      
>>> On Behalf Of Jim Dalton
>>> Sent: Thursday, October 28, 2010 8:34 AM
>>> To: voiceops at voiceops.org
>>> Subject: [VoiceOps] Question about Packet Jitter
>>>
>>> I have a question about calculating jitter.  Consider the call
>>>        
>> diagram
>>      
>>> below.
>>> A SIP call flows from the source device to the Session Border
>>>        
>> Controller
>>      
>>> (SBC)
>>> to the destination device.  All RTP packets are proxied through the
>>>        
>> SBC.
>>      
>>> +========+                    +=====+
>>>        
>> +=============+
>>      
>>> | Source |  Ingress Call Leg  | SBC |  Egress Call Leg  | Destination
>>>        
>> |
>>      
>>> | Device |------------------->|     |------------------>|   Device
>>>        
>> |
>>      
>>> +========+                    +=====+
>>>        
>> +=============+
>>      
>>>              Jitter Src to SBC
>>>            ------------------->
>>>
>>>                      Jitter Source to Destination
>>>            --------------------------------------------->
>>>
>>> If the packet jitter is known for the Ingress Call Leg from the
>>>        
>> source to
>>      
>>> SBC and for end to end packet flow from the source to the
>>>        
>> destination, is it
>>      
>>> possible to calculate jitter for the Egress Call Leg from the SBC to
>>>        
>> the
>>      
>>> destination device?
>>>
>>> I do not think the following relationship is accurate.
>>> (jitter Source to Destination) less (jitter Src to SBC) = (jitter SBC
>>>        
>> to
>>      
>>> Destination)
>>>
>>> Can anyone provide some guidance on this question?
>>>
>>> Thank you,
>>>
>>> Jim Dalton
>>> VoIP Least Cost Routing, Analysis, Billing
>>> 1.404.526.6053
>>> www.TransNexus.com
>>>
>>>
>>>
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>>>
>>>
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>>> VoiceOps mailing list
>>> VoiceOps at voiceops.org
>>> https://puck.nether.net/mailman/listinfo/voiceops
>>>
>>>        
>> --
>> David Knell, Director, 3C Limited
>> T: +44 20 3298 2000
>> E: dave at 3c.co.uk
>> W: http://www.3c.co.uk
>>
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>> VoiceOps mailing list
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>> https://puck.nether.net/mailman/listinfo/voiceops
>>      
>
>
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