On Sun 17 Mar at 18:27 John Clarke (3:713/730{at}fidonet) wrote to Arthur Marsh:

AM> For the best possible V.34 operation between a site with 
AM> ISDN and a remote site without ISDN, the ISDN equipped 
AM> site should have a 4 wire interface to its V.34 modem 
AM> (either internally as Ascend presumably does, or by 
AM> having a 4 wire interface on the ISDN terminal adapter).

 JC> Why?  I don't see the difference that having the last few 
 JC> inches as 4 wire will make.

Infinite return loss...

AM> Four wire analogue operation may also be easier to 
AM> provide than ISDN for remote sites that do need high 
AM> speed data operation beyond 7 km from the nearest 
AM> exchange/RIM/RCM/...

 JC> Now this I agree with.  ALL long loop services should be 4 
 JC> wire as this allows the insertion of amplifiers without the 
 JC> problem associated with 2 wire amps.  I've always thought it 
 JC> was stretching the friendship to try running bi-directional 
 JC> 2B+D services on a 2 wire circuit although the cost of 
 JC> copper will always make telecom companies try.

 JC> I'd even like to see the situation where loops longer than 
 JC> xKm (or loop loss greater than y db at 2KHz) MUST be 4 wire 
 JC> and have amps fitted at the exchange and user ends to bring 
 JC> the losses back within spec.  While I am pipe dreaming the 
 JC> amps should be equalized to give a flat response at the 
 JC> receiving end.

I've heard of 4 wire analogue to an equalisation amplifier/hybrid at the
customer's premises being implemented in the USA, but not Basic Rate ISDN
through 4 wires.

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