[Info-vax] OT: bit swapping, seamless rate adaptation

[John Wallace]

On Aug 1, 9:33 pm, Neil Rieck <n.ri... at sympatico.ca> wrote:
> This is totally off topic and not related to the original question.
>
> I've been a Bell Canada DSL customer since August 1999. The majority
> of their C.O. equipment is based upon Alcatel technology which can do
> either QAM or DMT. The original Alcatel Speed Touch modem (looks like
> a coffee hot-plate) burned out after 5 years probably because it
> wasn't meant to be on 24/7 (I support folding-at-home so never shut
> down my equipment, ever). Bell replaced the Alcatel modem with a
> SpeedStream 5200.
>
> As most of you may already know, DSL divides the telephone line
> spectrum into a band of 128 virtual modems. Whenever problems are
> encountered by the modem, virtual modems are dropped from the working
> pool which means the connection will slowly degrade over the course of
> days to weeks depending upon your loop length. The only way to get
> them back is to depower/power the modem which forces connection
> renegotiation.
>
> A few months ago (there was snow on the ground) I heard some rumors
> about a router/modem combo from 2wire which could renegotiate/restore
> dropped virtual modems without dropping the connection. Apparently,
> the Alcatel C.O. equipment always had this capability but the el-
> cheapo DSL modems in the field did not. So I visited eBay and
> purchased a 2wire 2701HG-G for $40. This thing works like a charm so
> now I've got a LinkSys WRT300 pluged into the back of the 2701HG-G
> (router plugged into router; it looks like hell but works)
>
> Now here is a really cool surprise. Before installing my 2wire, I
> reset the SpeedStream then I ran line speed tests 5 times but always
> measured 4.7 MB/s. After making the 2wire my primary device, the speed
> had jumped to 5.25 MB/s. YMMV.
>
> NSR

Was your original Alcatel the USB thing frequently referred to over
here as "the frog" (because of its shape, not because of where it came
from)? If so, it was life expired a long time ago, they weren't even
capable of keeping up with the maximum 8Mbit line sync of
originalADSL.

There are defined industry standards for G.DMT (the 'modulation' which
connects your DSL kit with the exchange's DSL kit). The virtual modem
hot swap you describe sounds like 'bit swapping' which should be
present in any correctly impelmented DSL modem.

Modern kit using ADSL2(+) should also support "seamless rate
adaptation" if the telco has chosen to enable it their end, which in
principle allows the line characteristics to be reanalysed and the
whole modem setup to be "retrained", without disrupting user level
traffic.

There are very few different sets of DSL modem hardware internals
around these days, but there is considerable variability in how well
the firmware works. A bit like there being little variability in
mainstream computing hardware these days, but lots of differences in
the software.