ATM Wide-Area Networks (was: sell shell accounts?)
curtis at ans.net
Wed Jul 24 00:53:45 UTC 1996
In message <199607231657.LAA14164 at uh.msc.edu>, Tim Salo writes:
> On a more theoretical note, switches, being circuit-switched, make
> the complicated decisions when the connection is established, (or
> configured for PVCs), and need to make relatively simple decisions
> to switch each cell. This probably scales very well is terms of
> speed, although at some point might have some difficulty in scaling
> to a very large number of simultaneous connections.
> Routers, on the other hand, have to make a bit more complicated
> decisions per packet. This has some limitations in terms of speed
> and number of simultaneous "connections."
Tim et al,
Delete now if you have anything better to do. :-)
Just addressing you theoretical note here for the moment. Routers
scale well to "number of connections if there are a large number of
sources and destinations per routing prefix (ie: good aggregation).
Routers scale O(logN) wrt the number of prefixes used for forwarding
if a radix tree is used or required O(N) storage for hashed lookup
methods (failure of the host based cache - proven avout 2 years ago -
dead horse dept). Switches scale O(N) with the number of connections
established and torn down. Routers are not affected by
So the (theoretic) question is whether the O(logN) swamps the
forwarding lookup in the router model before the O(N) connection setup
overhead kills the switch.
Back to real world considerations. Which scales better depends on
things like average packet size for the router limits and average
connection duration for the switch limits. Good ol' HTTP is a
nightmare for either one. As a result, hybrid approaches start to
look attractive, using routers on the periphery and building fat pipes
through the switches. What you end up with there is all sorts of
traffic on the same fat pipe (or forget about setup scaling) and so
out the window goes "the advantages of ATM QoS".
The state of the art of ISP needs (where the barage of tinygrams and
very short flows is felt full force) is PVC pipes between routers to
offload the routers a little bit. In practice the advantage is very
little (but enough to have MCI doing it). For all the investment made
in custom silicon for ATM it may turn out that a general purpose
processor and a good router design (such as the DEC Alpha used in the
BBN router) will take us to OC12.
Hope its been amusing. I'd say the jury is still out on this one. :-)
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