[geeks] Roll your own DSL

[Greg A. Woods]

[ On Tuesday, August 14, 2001 at 19:09:42 (-0400), James Sharp wrote: ]
> Subject: Re: [geeks] Roll your own DSL
>
> When the telco provisions a line for ADSL, SDSL, or HDSL (T1 over copper),
> they try to pick cables in the bundle away from other services to help
> limit cross talk and interference.

Where'd you hear that bit of nonsense?

What they really do is to go out in the field and remove any "bridged
taps" (a bridged tap is an unterminated wire pair that sits in parallel
to the main wire pair going between the CO and the in-service customer
equipment).

Of course they also have to add signal repeaters every 5,000 feet or so
too, which is the real fun and expense.

> They don't take such precautions when provisioning an alarm
> circuit...since they're just supposed to carry DC and/or low frequency
> stuff that won't crosstalk or interfere with other cables in the bundle.

Indeed with a plain alarm circuit (it's called "LDDS" in these parts)
they don't bother removing bridged taps.

That's why the likes of PairGain et al have invented techniques to avoid
the problems caused by bridged taps on the line.

Even with el-cheapo gear you can get over 100 Kbps SDSL on even a 18-20k
foot LDDS circuit.

(and often if you take good care of the installer he'll remove any
bridged taps for you and TDR the line to tell you how long it is!  ;-)

-- 
							Greg A. Woods

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