[ILUG] eircom hi-speed
Niall O Broin
niall at linux.ie
Sat Nov 3 12:58:51 GMT 2001
On Sat, Nov 03, 2001 at 12:15:09PM +1100, Justin Mason wrote:
> > Out of curiosity, can ADSL be done over an IDSN line ?
> > Kate, who only has idsn.
> Nope -- as far as I know, the ISDN signalling totally gets in the way of
> the DSL signalling. If you have a "dry copper pair", ie. a wire with *no*
> signalling, you could DIY it -- but they're impossible to get in Ireland.
POTS (plain old telephone service) provides one analog voice channel within
the frequency range of 300HZ - 3.4KHz. This limited bandwidth is what keeps
analog modem speeds relatively low, and is also what can make it quite
difficult to distinguish between spoken s and f on a telephone line.
ISDN (integrated services digital network) provides 144 Kbits divided into
two 64Kb data channels (B) (which can be used for voice, fax, data, low-rate
video etc. etc.) and a 16Kb signalling channel (D). This is basic rate ISDN,
often referred to as 2B+D.
ADSL (asymmetric digital subscriber line) provides one analog voice channel
within the frequency range of 300HZ - 3.4KHz AND a digital channel which
uses higher frequencies. The digital channel is available at various data
rates (higher download than upload, hence asymmetric) depending on numerous
factors - line quality, telecom policy etc.
ADSL over ISDN provides the 2B+D of an ISDN line AND the ADSL digital
channel. To my knowledge, this is available in parts of Spain and is the de
facto means of getting ADSL in Germany - in fact, you have to fight quite
hard with Deutsche Telekom to get them to give you "plain" ADSL.
All of the above technologies are provided to the consumer over a copper
pair. Also available is primary rate ISDN which is a ~2 Mbit link giving 30
(or is it 31 ?) data channels & a signalling channel hence it's referred to
as 30B+D but for that you'll normally get a special four wire circuit. When
I think about it, with the increase in copper capacity provided by xDSL
signalling, I don't suppose there's any reason why 30B+D couldn't be
provided over an ADSL channel, but I've no idea if it's been done.
Isn't it wonderful how many nice comms technologies are available to people
who don't live in third world countries. Ah well - be patient. 20 years ago
you could wait 6 or 7 years to get a telephone installed if you lived in th
wrong place. In 20 years time I'm sure ADSL will be widely available here.
Of course by then, the first world will have fibre to the consumer :-)
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