From: bruce@camb.com (Barton F. Bruce)
Newsgroups: comp.dcom.modems
Subject: Re: 2 vs 4 wire leased lines
Date: 8 Nov 92 07:06:25 EDT
Organization: Cambridge Computer Associates, Inc.

In article <1992Nov6.153712.7645@bert.eecs.uic.edu>, rorem@bert.eecs.uic.edu (Doug Rorem) writes:
>  I was wondering what advantages there are for 4 wire vs 2 wire
> leased lines.  It seems as though more modem manufacturers are
> offering a 2 wire option (if they have leased line capability)
> than 4 wire. Is that because of cost considerations?
> 
>  It is my understanding that all telephone connections between
> telco central offices are 4 wire, so from that standpoint,
> a 4 wire option would seem the best (less echo problems, etc).

DON'T buy analog leased lines unless absolutely necessary.

You want 4 wire if possible, and the telcos will use 4 wire on the long haul
even for 2 wire circuits. The local loop to you may be 4 wire even if they 
give you a 2 wire interface, but sometimes they may use a 2 wire local loop
if it is short. Their option...

It is even possible that they bring 4 wire 56kb DDS to your wall (the long haul
is ALWAYS digital these days) and do the A/D conversion right there rather than
in a D4 channel bank in the CO.

The rub is that 2 wire analog may be the cheapest to get. Long haul 
circuits on an IXC are ALL digital. It is the local loops where the local
telcos really gouge you. 

If they offer 4 wire and it is cheap enough, get it. If 2 wire is a LOT cheaper,
well, you have to decide. In every case see if DDS-II aka GDS or BDS or
simply generic basic non-hubbed 'cheapest-possible' digital is available
and then decide if you need to waste time with slow analog modems.

If BOTH ends of an analog ckt are brought to you digital (its their
option in this case - and would be priced 'analog' rather than 'digital'),
the only product I am aware of that they can use is an Adtran model ADST.
It is a 'Wescom 400' form factor card just like the Tellabs or whatever other
cards they use on Analog circuits - either individually in a single box or
in a wall mounted multi-slot cage. See if it says ADTRAN and is model "ADST"...

If someone were to instead connect a 56kb DSU/CSU before these units,
he might well get a 56kb DDS connection. NOTE WELL that this is ILLEGAL
if the official telco demarc is after the ADST, but you may be the crown
prince of your private island and be able to make use of this info :-)

Using this same info backwards, you can have installed a 56kb DDS line
and then use analog modems on it by buying your own ADSTs. The 'demarc'
is then before the ADST, and yes, ADTRAN has an FCC # for the product's use
by customers on their side of the demarc - so this is legal. There simply 
are NO conditioning issues possible for the 10 feet between the ADST and 
your MODEM. You get the BEST analog circuit conditioning possible 
autonatically!

The ONLY reason someone would have to do such a horrible trick might be
the following: If your modems had a 6 way TDM built in that you needed
to use, and your line was to be converted to DDS when equipment got
upgraded and the TDM function was to be replaced or obviated. Buying 
the ADSTs now to even throw them out later would be less $s than duplicate 
installation for the 2nd type of line.

Of course if you only could use 9.6 or 19.2 due to whimpy older
hardware for now, needed NO TDM functionality, but needed 56kb later, you
order 56kb NOW and use rate adapting CSU/DSUs downspeeding to whatever
you need today. Tommorrow simply dial them up to 56kb.

NEVER ask for simply DDS. They then price using the OLD DDS-I tarrifs.
Make sure you get the newer offerings if available. In NYNEX teritory
it is DDS-II. In California, it is ADN (not available interstate,
but IS instate - even interlata). 
Newsgroups: comp.dcom.modems