I'm in the middle of co-managing the state-wide
deployment of DSL for our telephone company. I will
suggest that not all DSL is created equal. So much
depends on the providers equipment and backbone
structure. There are the Ethernet-based DSLAMS in
which the "hi-speed" side of the DSLAM is IP based and
dependent upon the routers used. Lots of overhead.
Then there are the ATM based systems (typically using
G.Lite modems), which can increase "throughput"
dramatically because of superior caching capabilities
in the backbone side. With the ATM based systems it
isn't uncommon to have a 256Kb connection (128Kb CIR)
deliver an "apparant" throughput in the 1-2Mb range.
Of course, everything is dependant on what level of
"oversubscription" is used by the provider. The RBOCs
typically have a 30x oversubscription, wheras a
certain three-lettered company here in the USA has an
oversubscription rate in the 100x range.
However, backbones are expensive and a 30x
oversubscription rate will result in apparant
throughputs close to if not still in excess of the
speeds you are buying. Even in worse case, though,
the throughput of a basic 128/64 DSL connection will
still make your 56k dialup modem blow chunks in
comparison.
Ken Norton
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