[SIPForum-discussion] Jitter measurement.
Badri Ranganathan
badri at arcatech.com
Tue Aug 16 10:30:07 UTC 2011
Hi,
Can anyone tell me how "jitter" is "measured" ?
>From what I could gather from the internet -
<<=========================
Jitter is defined as a statistical variance of the RTP data packet inter-arrival time. In the Real Time Protocol, jitter is measured in timestamp units. For example, if you transmit audio sampled at the usual 8000 Hertz, the unit is 1/8000 of a second.
In RFC1889, I can see this definition under RTCP sender reports -
interarrival jitter: 32 bits
An estimate of the statistical variance of the RTP data packet interarrival time, measured in timestamp units and expressed as an unsigned integer. The interarrival jitter J is defined to be the mean deviation (smoothed absolute value) of the difference D in packet spacing at the receiver compared to the sender for a pair of packets. As shown in the equation below, this is equivalent to the difference in the "relative transit time" for the two packets; the relative transit time is the difference between a packet's RTP timestamp and the receiver's clock at the time of arrival, measured in the same units.
===========================>>
Here it says its measured in timestamp units. Cant understand this explanation much. Why cant it be expressed in milliseconds ?
Thanks,
Badri.
-----Original Message-----
From: discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On Behalf Of Steve Underwood
Sent: 13 August 2011 08:58
To: Rohan Almeida
Cc: discussion at sipforum.org
Subject: Re: [SIPForum-discussion] SIP Faxing
A key question is 15% of what kind of calls fail? If you are talking
about a public FAX server, open to all, you might well see 15% to 20% of
failed calls from wrong numbers, voice calls to FAX numbers, etc. If you
are making test calls into a well controlled server and get 15%
failures, that pretty nasty. You should be getting less than 1%
failures, even if the calls are sending tens of pages each.
Steve
On 08/13/2011 01:14 PM, Rohan Almeida wrote:
> 15% failure is too high. 2-10 % is on an average is acceptable.
> but again the fault might not be only in your system. the interconnect
> devices might also be the cause.
>
> On Tue, Aug 9, 2011 at 9:52 AM, Greg Settle <gsettle at opentext.com
> <mailto:gsettle at opentext.com>> wrote:
>
> What is the topology?
>
> SIP trunk - SBC/gateway - fax server? If yes, who is the SIP trunk
> provider? Do they support T.38 FoIP?
> T1/PRI - gateway - fax server? If yes, which gateway(s) is involved?
> Support for T.38?
>
> Assuming a fax server is involved? If yes, which fax server?
>
> Generally, if T.38 FoIP is supported among all endpoints, and proper
> testing was done to verify configuration among the components, SIP
> faxing failure rates should remain low. 15% failure rate is high, and
> points to weak links in the topology where FoIP standards (T.38)
> are not
> being met. Attached is a document which may assist.
> Greg
>
> -----Original Message-----
> From: discussion-bounces at sipforum.org
> <mailto:discussion-bounces at sipforum.org>
> [mailto:discussion-bounces at sipforum.org
> <mailto:discussion-bounces at sipforum.org>] On Behalf Of Steve Underwood
> Sent: Monday, August 08, 2011 9 <tel:2011%209>:47 AM
> To: discussion at sipforum.org <mailto:discussion at sipforum.org>
> Subject: Re: [SIPForum-discussion] SIP Faxing
>
> On 08/04/2011 09:40 PM, Melissa Parsons wrote:
> >
> > Hello,
> >
> > Can anyone tell me what an acceptable failure rate would be when
> > faxing over SIP? Currently we are averaging around 15%. This
> number is
>
> > high I would expect it to be somewhat higher than traditional faxing
> > but not this high.
> >
> > Thank you,
> >
> > *Melissa Parsons*|Enterprise Systems Engineer
> >
> > *MarineMax, Inc.* ( 727.531.1700 <tel:%28%20727.531.1700> office
> | (727.524-3954 <tel:%28727.524-3954> fax
> >
> > 18167 US Hwy. 19 N. Clearwater, Florida 33764 <tel:33764>
> >
> >
> That's vague. Are you using A-law or u-law? Are you using G.726?
> Are you
>
> using T.38? Pretty much anything else you might use will give 0
> success
> rate, so I assume its one of those. The failure rate you get will
> depend
>
> on many factors. A high packet loss rate is a disaster for FAX. High
> jitter levels for the packet delivery time can be too. T.38
> implementations are quite variable in their behaviour, and not always
> that compatible. Many quirks in the SIP signalling arrangements
> exist in
>
> various implementations, too. At the end of the day you'll get
> somewhere
>
> between 0 and 100% success, depending on all these factors. Between
> servers in large data centres, connected straight on to the internet's
> backbone you might be able to send FAXes all week without an error. In
> really bad tributaries of the internet you might get a near 100%
> failure
>
> rate.
>
> Regards,
> Steve
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