[AG-TECH] Feasibility Question

[Markus Buchhorn]

Hi Bronek

At 11:43 AM 18/12/2002 +0000, Bronek Carr wrote:
>Location "S" - Location "F":      2x155Mbps, present utilization 30%; 
>Location "S" - Location "M":     4 Mbps Committed Information Rate  [12 Mbps theoretical], present utilization 50% 
>Location "F" - Location "M":     4 Mbps Committed Information Rate  [12 Mbps theoretical], present utilization 50% 
>Location "S" - Location "L":       4 Mbps Committed Information Rate [6 Mbps theoretical] on a IMA connection [4 ATM lines] and 2 Mbps on a permanent dedicated connection; both the two lines are at present utilized only for small FTP download traffic, but we expect a 50% utilization rate in the future. 

If the 4Mb/s CIR is over ATM, then that is all you will really get with IP. If it is over Frame Relay, you have some hope to achieve the "theoretical" (peak) rate - but it's not guaranteed of course.

>5. whether Gary Refka's figures (see Building an Access Grid Node), namely  256 Kbps per camera/site and 128 Kbps per site, are adequate/required for a high quality communication or not.  If we base our analysis on these figures, 2,256 Kbps should be required/adequate for a four-cameras-per-site video conference 

That's in both directions - you only need 1.25Mb/s in each direction. I presume the above bandwidth figures are duplex (i.e. in each direction)?

256kb/s for video (per camera) is reasonable - it's what you'd typically get with ISDN videoconf at around 300-384kb/s. I wouldn't call it high-quality (CIF resolution, 5-8fps) but it's enough for comfortable use. It also depends on the field-of-view of the cameras - you often don't use the full bandwidth available when shooting a wide-angle view of a room.

4 cameras is quite generous, but really depends on the size of the room. Depending on what you want to do, you could use a video switcher to select from N different sources, but only ever send 2 or 3 video streams?

If you're only doing 2-site events, then most of your links look adequate, but only just based on current usage. If you want to do any 4-site events, you'll be transmitting around 1.1Mb/s, and receiving 3x1.1=3.3Mb/s back - and that is too tight. If you cut back to 3 cameras at each, the per-site traffic comes to about 0.88Mb/s, so in-bound from 3 sites would be 2.7Mb/s which is a bit more reasonable.

You can also drop to a narrower-width audio codec, but it won't save as much as tweaking the camera numbers and bandwidth. You could also have different bandwidths for different cameras - more for speaker-camera, less for audience cameras.

You do have the "benefit" that the AG traffic runs over UDP, which blasts away at bandwidth with no concern for others. email/ftp/web traffic is all TCP based, which backs off in the face of congestion - but it will impact the AG traffic eventually. 

A better approach would be to engineer your traffic policing so that AG traffic is given a higher priority at the choke points (the ends of your narrow links) than other traffic. That way if an AG session starts up (i.e. look for UDP multicast traffic sourced from the AG machines IP addresses) it will get treated as "premium" and other traffic will be forced to drop back. We use that approach with our voice/IP traffic across the country. On the narrow links it means that people email/web/ftp traffic will slow down during AG sessions, but that's usually no big deal. 

I can't immediately see how a mixer would help much beyond what multicast does for you anyway - you only want one copy of each audio/video frame on any link, if there are listeners at a site, and I don't think a mixer can do anything better than that with this topology. You might be able to win something if you can configure the router at S to treat the S-M and S-F-M links like a trunk and load-balance between them, but that will only benefit M, and will hurt F if it is not involved in a session.

Hope this helps?

Cheers,
        Markus


Markus Buchhorn, ANU Internet Futures Project,        | Ph: +61 2 61258810
Markus.Buchhorn at anu.edu.au, mail: Bldg #108 - CS&IT   |Fax: +61 2 61259805
Australian National University, Canberra 0200, Aust.  |Mobile: 0417 281429