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- From: bruce@camb.com (Barton F. Bruce)
- Newsgroups: comp.dcom.modems
- Subject: Re: 2 vs 4 wire leased lines
- Message-ID: <1992Nov8.070628.40049@camb.com>
- Date: 8 Nov 92 07:06:25 EDT
- References: <1992Nov6.153712.7645@bert.eecs.uic.edu>
- Organization: Cambridge Computer Associates, Inc.
- Lines: 66
-
- 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).
-