[ILUG] eircom hi-speed

[Padraig Brady]

Nice summary Niall, comments below..

Niall O Broin wrote:

> 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.
> 


Very interesting.


 
> 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.


Primary rate ISDN was designed to work over the existing "level 1" links,

T1 in the states/japan which is 1.544Mb/s and E1 elsewhere 2.048Mb/s.

T1 rate is (24*64)+(8 for framing). Hence the user rate for T1 ISDN is
23*64Kb/s = 1.472Mb/s. E1 is 32*64Kb/s, 1 slot is used for signalling
and another for framing. So the ISDN user has 30*64Kb/s = 1.92Mb/s.
Note the D channel is 64KHz in both primary rate cases.

Note also that for both basic rate and primary rate ISDN the D channel 
is *always on*, and only a very small % of it is used for call 
signalling. There was a service in the US that basically did IP across 
this (16Kb/s always on can be seriously useful!), and you could then 
dialup incramentally for 144Kb/s (64 + 64 + 16).


> 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 :-)
> 
> Niall

Padraig.