[time-nuts] Frequency from telco

Michael Sokolov msokolov at ivan.Harhan.ORG
Sun Sep 24 19:22:53 EDT 2006


Hal Murray <hmurray at suespammers.org> wrote:

> Does anybody know how DSL works?  If I poke around with a scope in my DSL 
> modem/router, will I find a clock locked to the telco's master clock?

I doubt it.  DSL lines come from DSLAMs, not from "real" telco equipment
like switches and synchronous digital hierarchy (SDH) equipment where
those precise synchronised clocks live.  DSLAMs switch and multiplex at
the packet or ATM cell level and thus have no need for syntonisation of
the actual DSL carrier frequencies.  Although I've never poked
inside a DSLAM, only customer premises equipment (CPE), I would expect
that the clock for the per-line DSL transceivers comes from an ordinary
commercial grade crystal oscillator on the line card.

There are some DSLAMs on eBay oftentimes and I don't even see where one
would connect to a telco master clock.

> How many different versions of DSL are there?

My personal interest focuses on SDSL, but even that is already a
mouthful of flavors:

http://ifctfvax.Harhan.ORG/OpenSDSL/

But unfortunately a lot of the original DSL diversity has died out and
most of the world has concentrated on the completely boring and
uninteresting (to me at least) ADSL/DMT.

MS



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