Subject: [AVT] re: use of marker bit in cts rate mode

From: "Paxton Smith" <[email-masked]>
Date: Sat, 4 Aug 2001 19:54:55 -0400

Hi,

This is my first post to the group; I hope this isn't too obvious.
My question concerns the use of the marker bit during a continuous (cts)
rate session between two clients (no dtx).  Say that each client is equipped
with a VAD, such that talkpurts can be identified before transmission. Is it
customary to set the marker bit at the beginning of a talkspurt?  Shouldn't
hurt,
should it?  (Although one may ask why not use DTX if you've got a VAD
onboard
already).  I already looked at RAT for an example, and noticed the marker
bit
is not used when silence suppression is off.

Thanks in advance,

Paxton

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Paxton Smith                                   [email-masked]
Telecommunications & Signal Processing Lab     +1 514 398-2896
Dept. Electrical & Computer Engineering
McGill University
3480 University Street
Montreal, Quebec, Canada H3A 2A7
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_______________________________________________
Audio/Video Transport Working Group
[email-masked]
http://www.ietf.org/mailman/listinfo/avt

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From: Colin Perkins <[email-masked]>
Date: Sun, 05 Aug 2001 20:07:51 +0100

Hi,

--> "Paxton Smith" writes:
>This is my first post to the group; I hope this isn't too obvious.  My
>question concerns the use of the marker bit during a continuous (cts) rate
>session between two clients (no dtx).  Say that each client is equipped
>with a VAD, such that talkpurts can be identified before transmission. Is
>it customary to set the marker bit at the beginning of a talkspurt?

We had a similar discussion with respect to EVRC recently, and came to the
conclusion that the M bit should not be set during continuous speech (even
if talkspurts can be detected). The -11 revision of the profile has revised
text which is intended to clarify this.

In general setting the M bit won't hurt, expect that it will reduce the
efficiency of RTP header compression (which was the concern of the EVRC
folks).

>Shouldn't hurt, should it?  (Although one may ask why not use DTX if
>you've got a VAD onboard already).  I already looked at RAT for an
>example, and noticed the marker bit is not used when silence suppression
>is off.

Rat can't tell the start of talkspurts if silence suppression is switched
off.

Colin

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From: Stephen Casner <[email-masked]>
Date: Sun, 5 Aug 2001 13:28:15 -0700 (PDT)

On Sat, 4 Aug 2001, Paxton Smith wrote:

> customary to set the marker bit at the beginning of a talkspurt?

The purpose of the M bit is to allow the receiver to know that packets
were intentionally not sent (because of silence).  This indicates to
the receiver that it would be a good time to adjust the playout delay
to accommodate clock skew.  If using DTX with no comfort noise
packets, then a silence gap that's long enough for the playout buffer
to drain is an equivalent indication.  The value of the M bit comes
when the silence is short and there may be uncertainty about whether
missing packets are lost or intentionally not set.

For the scenario you ask about, the presence of the comfort noise
packets is already an indication to the receiver that time can be
elastic and the indication comes at the beginning of the silence, so
there is no point in setting the M bit and it is in conflict with the
intended meaning (that packets were intentionally not sent).

							-- Steve