Marking Audio stream as EF in Video-Conferencing?

Unanswered Question
Aug 26th, 2010


Traditionally I have marked both the audio and video streams as AF41/CS4 but increasingly I have seen VC vendors marking the audio streams as EF. According to the Telepresence SRND both streams should be marked as CS4 - is there any benefit is classifying the 2 streams differently?


I have this problem too.
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Aaron Harrison Fri, 08/27/2010 - 00:15


I guess the difference would be that if you mark the voice as EF, it's generally going in the priority queue which means it should always get good service.

AF41 is more likely to experience congestion and drops.

Video calls generally involve a G711/G722 voice stream, and a video stream. The two traffic types have different behaviours, so it makes sense for them to be in different classes, especially if you consider that you can generally ignore some video glitches and continue a conversation if the voice stream is arriving happily.

That said, lots of stuff just does mark everything AF41.. I guess it's something to do with keeping them in sync (i.e. sending both types of traffic with the same markings but to me the arguments do seem a little weak. Here's a snip from the old Video SRND 4.1:

The voice component of a call can be classified in one of two ways, depending on the type of call in
progress. A voice-only (or normal) telephone call would have the media classified as CoS 5
(IP Precedence 5 or PHB EF), while the audio channel of a video conference would have the media
classified as CoS 4 (IP Precedence 4 or PHB AF41). All the Cisco IP Video Telephony products adhere
to the Cisco Corporate QoS Baseline standard, which requires that the audio and video channels of a
video call both be marked as CoS 4 (IP Precedence 4 or PHB AF41). The reasons for this
recommendation include, but are not limited to, the following:
• To preserve lip-sync between the audio and video channels
• To provide separate classes for audio-only calls and video calls


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clarke.jason.kit Sun, 08/29/2010 - 18:01

Hi Aaron,

Thanks for your reply - I understand the differences in the streams however if there are glitches in the video picture I feel that in certain situations the quality of the picture woud be just as important as the voice, and therefore the need to priortise the audio stream is negated.

So I am assuming that the parameters that should be set should take into consideration the environment and need of the end-user.

Many thanks


Aaron Harrison Mon, 08/30/2010 - 08:33

Sure.. either way the voice part is pretty easily managed due to it's small and consistent size; if video is important then you need to ensure you have proper network setup to allow for good quality.

You still wouldn't normally put video in a PQ; just having the bandwidth guaranteed for it should be enough.




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