Subject: re:[Diffserv-interest] DiffServ over ATM

From: "Jeff Norman" <[email-masked]>
Date: Thu, 30 Aug 2001 11:52:12 -0400 (EDT)

John,
  I know others may disagree or agree with me, but IMHO, queuing on
multiple queues based on IP QoS/CoS/DiffServ and then queuing on
another set of multiple queues based on ATM service categories is
somewhat redundant.

Typically it's one or the other, or if both do exist then the ATM queues
should be applying back-pressure onto the IP queues such that packets
can get differentiated and conditioned by the IP queues as well.

It looks like to me in your situation, the back-pressure isn't happening.

Best Regards,
--Jeff

In message "[Diffserv-interest] DiffServ over ATM", John Tillberg writes:

>Hello DiffServers!
>I'm currently doing some simulations (using Opnet) of IP over ATM where I
>use WFQ to differentiate between two separate traffic classes on a network
>with low bandwidth links (2 Mbps). It appears that the WFQ mechanism doesn't
>have any significant impact on the performance of the network. No matter how
>I tune the weights, or even if I overload some of the links, I get almost
>zero delay for both of my queues. However I do get delay on the ATM layer
>(and I am aware of that I have to map my IP QoS classes onto proper ATM QoS
>classes). I presume this is because the forwarding speed on the IP layer is
>much higher than the bandwidth of the network. Hence every (conforming)
>incoming packet will receive service on the IP layer (i.e. sent down to the
>lower layers) right away even if there is a state of congestion on the ATM
>layer. Have I misunderstood the matter? If not: Is this the same case for
>DiffServ? Does the effect of the IP-packet scheduler get  "disabled" in a
>similar manner when you run DiffServ over ATM? 
>Yours Sincerely
>
>John Tillberg 
>
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From: Brian E Carpenter <[email-masked]>
Date: Thu, 30 Aug 2001 13:32:46 -0500

I agree with Jeff. The intent of diffserv is make it unnecessary to mess 
around with parallel ATM VCs and the like - diffserv shares a single pipe
among multiple traffic classes. If each diffserv queue has its own VC,
diffserv is probably a no-op. Also, it's no surprise that IP forwarding is 
not a bottleneck on such slow links.

Now you know why some of us think that running ATM over wireless is
really, really pointless.

  Brian