Wireshark · Wireshark-users: Re: [Wireshark-users] Marker Bit In RTP for Voice Samples for codec like AMR and G729

[Jaap Keuter <jaap.keuter@xxxxxxxxx>]

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:

"

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