[VoiceOps] Question about Packet Jitter

[David Knell]

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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-- 
David Knell, Director, 3C Limited
T: +44 20 3298 2000
E: dave at 3c.co.uk
W: http://www.3c.co.uk