# [VoiceOps] Question about Packet Jitter

Jim Dalton Jim.Dalton at TransNexus.com
Fri Oct 29 08:15:07 EDT 2010

> 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

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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> >
>
> --
> 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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