Cell Relay Archive

[Paul Koning <pkoning@lucent.com>]

Santa Yeh wrote:
> 
> Does anyone explain what difference between abr and abr? What benefit
> for each since VBRnrt can assign PCR/SCR vs ABR with PCR/MCR?

I assume you meant "difference between VBR and ABR".

ABR is the only one that has a feedback system.  All the other
service categories are open loop.  In other words, with VBR you 
indicate at the time the VC is created what resources you need,
and those remain reserved for the life of the VC.  Even if there
is a lot of additional capacity available for part of that time, 
you do not have access to it.  (Well, except if you use VBR.3, but
then the additional traffic is sent effectively as best effort
traffic, without delivery guarantees.)

ABR, on the other hand, uses closed loop flow control.  If at
any time the network has additional resources it can let you use,
it will adjust the rate parameters so you can do that.  And it will
maintain low loss probability.

The price you pay is that ABR is extremely complex, sufficiently
so that it has only been analyzed in simulations where a lot of
simplifying assumptions were made.  Also, there is disagreement
on whether the layer 2 flow control in ABR interacts well with 
the layer 4 flow control in TCP.  As a result, I'm not sure that
ABR has seen much real world use.  Certainly it has little or
none in the WAN world, even though immense effort was spent in
the ATM Forum specifically to fit it into that world.  (If you want
LAN only service similar to ABR, you could do so VERY much more
easily using completely different mechanisms, but those were
rejected...)

	paul