[AG-TECH] AG Network Requirements Question

Markus Buchhorn Markus.Buchhorn at anu.edu.au
Sun Nov 17 17:56:57 CST 2002

At 10:41 AM 15/11/2002 +0000, Michael Daw wrote:
>I was recently involved in the production of a report on the comparative
>merits of different videoconferencing systems, including Access Grid.

and a very nice report it was too! (the UK e-Science report?)

>In the section on AG, I had the following sentence:
>"The Local Network connection requires 100Mbps connectivity. The Wide Area
>Network requires 10Mbps."
>However, this assertion has since been questioned (not surprisingly, on
>reflection). Why would you need a larger local connection than WAN
>connection? Surely, there's no point in having local connectivity that's
>higher than your WAN? Also, doesn't the size you need for your WAN
>connection depend on what/who else is using it?

Your last point is spot on, of course. It also depends on what your WAN is - in my case I have 3 hops across campus before I hit the outside world, and the next link it takes right there depends upon the destination (commodity, R&E, domestic/international). Which one of these is my WAN link? 

Perhaps a better statement would be what you need to maintain an AG connection between sites. The normal rule of thumb I have seen is around 2Mb/s per site, i.e. with 6 sites in total you'd transmit 2M and receive 10M. So if you typically want to use AG venues with 5 other sites, you need to have 10M clear bandwidth coming your way (and that's sustained, not burst capacity - don't let the ATM/Frame-Relay guys tell you anything else) from wherever all the incoming streams get aggregated on their way to you, i.e. where your multicast choke-point is. This of course scales up with more sites involved, or with higher-bandwidth codecs, etc.

Regarding subnet capacity being higher than WAN, I suspect it's partially due to being able to buy cheap 100M Ethernet switches nowadays, whereas a pure 10M switch is a rarity :-). It is a bit of overkill, if it's much higher than your "WAN" connectivity, since you clearly can't shove that much data off your subnet. However, one benefit I can see is that if the switch is "dumb", i.e. does not do multicast, it will treat multicast traffic as broadcast locally - so every port will see every multicast packet (in- and outbound), so every PC will get an aggregate of the video and audio streams coming down, and also what you are transmitting out. If you're filling your downstream 10M AG-pipe with AG traffic, you can exceed 10M on the LAN links. Plus there's also local management traffic (e.g. if you use VNC locally, and any file-sharing, plus anything else coming from off the WAN). So it's probably covering that off, plus future-proofing for when you get your 100M WAN link :-)

So basically the LAN requirement is what is technically reasonable, cost-effective and won't introduce additional congestion/loss; the WAN requirement is a rule of thumb for a "typical" (if that is meaningful) number of sites in a venue.


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

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