Message from discussion
DACS and second line
From: b...@dsl.co.uk (Brian {Hamilton Kelly})
Subject: Re: DACS and second line
Date: 2000/03/01
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In article <88pu1p$6m...@apple.news.easynet.net>
bj.mor...@ukonline.co.uk "Bazza" writes:
[I see that others have already replied to this, but Bazza seems not to
have read and understood, so I'll try]
> AIUI it's not the distance from the exchange that matters, but the
> number of lines the exchange can provide. If every line is used then
> the option is either to expand the exchange or Dacs lines .... guess
> which is more cost effective from BT's point of view ?
You understand it wrong. To connect a DACS requires two pieces of
equipment: one the DACS up the pole at the subscriber end of the cable
run; the other a similar unit at the exchange end. EACH unit is
connected to two physical lines: two subscribers' premises equipment at
their end, and TWO outlets of line card(s) at the exchange end.
Therefore a DACS solution CAN NOT be deployed if there is no spare line
capacity at the exchange. DACS is *purely* a solution to lack of
capacity in line plant between the exchange's site and some particular
outlying DP site. Capisce?
[BTW Read exchange/concentrator as appropriate throughout.]
Taking up another posting of yours in this thread, there is no way that
there can be any interaction between the two "lines" if both are in use.
DACS works very similarly to BRI ISDN on the cable between the two sets
of converters (i.e. data is carried digitally on a 192kb/s link). There
is no "multiplexing" in the fashion that Bazza appears to believe.
The reason that DACS cannot provide 56kb/s operation is NOT that the
digitization of the voice channels is "crap"; they use the same system as
is used for the digitization at the exchange for conventional exclusive
lines. The operation of 56kb/s modems (which is only attainable on the
downlink from the ISP to the user) is possibly only because the ISP
injects the digital data directly, without at any stage being an analogue
signal, and that there is only one D-to-A conversion (at the line card,
to a POTS line) between the ISP and the user's modem. The ONLY reason
why you cannot get a V.90 modem to work is that there are now TWO D-to-A
conversions involved in the downstream direction; one at the line card in
the exchange, followed by another at the output of the DACS unit near the
subscriber's premises. Notwithstanding this, the bandwidth of the
digital channels is such that one can attain the maximum throughput that
was possible before V.90 came along, namely V.34+.
Bazza's empirical observation of a degradation when using both lines must
be investigated more closely; for a start, just HOW (computer/modem/phone
configurations, etc) was he using both lines when this degradation was
observed?
--
Brian {Hamilton Kelly} b...@dsl.co.uk
"If we could apply the power of electrical repulsion to preserve us from
ever hearing anything more of them [Americans], I should think we had for
once derived a benefit from science." T L Peacock, /Gryll Grange/ 1861
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