Re: Marker Bit In RTP for Voice Samples for codec like AMR and G729

[Jaap Keuter <jaap.keuter () xs4all nl>]

Hi, 

Check RFC 3551 for "marker bit". 

For Audio it says: 

" For
applications which send either no packets or occasional comfort- noise
packets during silence, the first packet of a talkspurt, that is, the
first packet after a silence period during which packets have not been
transmitted contiguously, SHOULD be distinguished by setting the marker
bit in the RTP data header to one. The marker bit in all other packets
is zero. The beginning of a talkspurt MAY be used to adjust the playout
delay to reflect changing network delays. Applications without silence
suppression MUST set the marker bit to zero." 

For Video it says: 

"
Most of these video encodings also specify that the marker bit of the
RTP header SHOULD be set to one in the last packet of a video frame and
otherwise set to zero. Thus, it is not necessary to wait for a following
packet with a different timestamp to detect that a new frame should be
displayed. " 

And also look at the RTP FAQ [1]: 

" WHAT'S THE MARKER
BIT GOOD FOR?For voice packets, the marker bits indicates the beginning
of a talkspurt. Beginning of talkspurts are good opportunities to adjust
the playout delay at the receiver to compensate for differences between
the sender and receiver clock rates as well as changes in the network
delay jitter. Packets during a talkspurt need to played out
continuously, while listeners generally are not sensitive to slight
variations in the durations of a pause. 

The marker bit is a hint; the
beginning of a talkspurt can also be computed by comparing the
difference in timestamps and sequence numbers between two packets,
assuming the timestamp clock rate is known. 

Packets may arrive out of
order, so that the packet with the marker bit is received after the
second packet in the talkspurt. As long as the playout delay is longer
than this reordering, the receiver can still perform delay adaptation.
If not, it simply has to wait for the next talkspurt. 

" 

Thanks,
Jaap


On 2012-03-21 11:57, NITIN GOYAL wrote: 

> I want to know the
significance of Marker Bit in RTP for Voice packets and is here any RFC
which tell that. 
> I know that the for the Video packets marker bit
means last packet for the same image and hence, its the last packet with
PTS time-stamp corresponding to image but for the Voice Packets for a
codec say AMR-NB or G711 alaw or G729, the Marker Bit is usually false
in each of the RTP packet. 
> So, do the meaning of Marker bit
changes in this case of RTP packets?? 
> Regards 
>
> Nitin




Links:
------
[1] http://www.cs.columbia.edu/~hgs/rtp/faq.html#marker

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