Subject: [AVT] definition of jitter

From: "Guler, Amos" <[email-masked]>
Date: Wed, 1 Aug 2001 20:59:14 +0300

I have a question regarding the definition and meaning of the interarrival
jitter parameter in the RTCP report.
In RFC 1889 it is said that *calculating* the difference in "relative
transit time" (where "relative transit time" is defined as the difference
between a packet's RTP timestamp and the receiver's clock at the time of
arrival) gives the interarrival jitter. However, the interarrival jitter is
*defined* as the (mean deviation of the) difference in packet spacing at the
receiver compared to the sender for a pair of packets.
This way of *calculating* the interarrival jitter is claimed to be
equivalent to the *definition*, but it seems to ignore variable packet
durations. Consider, for example, a clean network that has no jitter in it.
However, the RTP packets contain either 10ms or 30ms of audio. Therefore, if
the first packet had timestamp 0 and its packet duration was 10ms, the next
packet will have timestamp 10ms. If this packet's duration is 30ms, it will
be sent 30ms after the first packet. Therefore if the network is clean, this
packet will also arrive 30ms after the first packet and the jitter was in
fact 0 (the packet spacing at the receiver is 30ms; the spacing at the
sender is also 30 ms, as the second packet was sent 30 ms after the first
one; the difference is 0). The difference in "relative transit time", on the
other hand, will be 20ms: arrival time difference = 30ms; timestamp
difference = 10 ms; difference between the two differences = 20ms.

It seems to me that the correct calculation of interarrival jitter can
remain being based on the difference in "relative transit time", where
"relative transit time" is now re-defined as the difference between the
'sending time' and the receiver's clock at the time of arrival. The 'sending
time' is the packet timestamp PLUS the packet duration.
I will be glad to hear what you think about this matter.

Best regards,
Amos Guler.
Tel/Fax: (+972)-3-645-2779 
Mobile:  (+972)-58-54-2779
[email-masked]

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
http://www.ietf.org/mailman/listinfo/avt

---

From: "Fairlie-Cuninghame, Robert" <[email-masked]>
Date: Wed, 1 Aug 2001 14:21:19 -0700

I think it might also be helpful if these jitter and packet loss
descriptions mention the effect/impact of silence indication schemes (such
as G.729B) on the calculation. (for instance by using the M bit in the
calculcation). The actual calculation may be algorithm specific but the
general idea of what the statistic should be reporting when silence
indiciation is used would be helpful.

Cheers,

Robert.

-----Original Message-----
From: Guler, Amos
To: [email-masked]
Sent: 01/08/01 10:59
Subject: [AVT] definition of jitter

I have a question regarding the definition and meaning of the
interarrival
jitter parameter in the RTCP report.
In RFC 1889 it is said that *calculating* the difference in "relative
transit time" (where "relative transit time" is defined as the
difference
between a packet's RTP timestamp and the receiver's clock at the time of
arrival) gives the interarrival jitter. However, the interarrival jitter
is
*defined* as the (mean deviation of the) difference in packet spacing at
the
receiver compared to the sender for a pair of packets.
This way of *calculating* the interarrival jitter is claimed to be
equivalent to the *definition*, but it seems to ignore variable packet
durations. Consider, for example, a clean network that has no jitter in
it.
However, the RTP packets contain either 10ms or 30ms of audio.
Therefore, if
the first packet had timestamp 0 and its packet duration was 10ms, the
next
packet will have timestamp 10ms. If this packet's duration is 30ms, it
will
be sent 30ms after the first packet. Therefore if the network is clean,
this
packet will also arrive 30ms after the first packet and the jitter was
in
fact 0 (the packet spacing at the receiver is 30ms; the spacing at the
sender is also 30 ms, as the second packet was sent 30 ms after the
first
one; the difference is 0). The difference in "relative transit time", on
the
other hand, will be 20ms: arrival time difference = 30ms; timestamp
difference = 10 ms; difference between the two differences = 20ms.

'sending time' and the receiver's clock at the time of arrival. The
'sending

Best regards,
Amos Guler.