Subject: [Diffserv-interest] DiffServ over ATM

From: John Tillberg <[email-masked]>
Date: Thu, 30 Aug 2001 17:03:50 +0200

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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John Tillberg
Telia Research AB 	Phone:  +46 8-713 82 07
Wireless Solutions	Fax:       +46 8-713 81 49
Vitsandsgatan 9           	Mobile:  +46 702-43 43 90
SE-123 86 Farsta      	Email:  [email-masked]	
Sweden                 	

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