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From: roy@alanine.phri.nyu.edu (Roy Smith)
Newsgroups: comp.dcom.telecom
Subject: Re: Device Given to me With "Data Lines"
Message-ID: <16373@accuvax.nwu.edu>
Date: 24 Jan 91 18:38:53 GMT
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X-Telecom-Digest: Volume 11, Issue 63, Message 4 of 6


> Any time we order a "Data Line" for our computers here, we are given a
> device to put inline between the modem and the phone jack [...] It also
> has a switch labeled FLL and PROG.  There is a label on the box that
> reads exactly as follows: 

> For use as USOC RJ-
> 41S-M, 42S-M, 43S-M,
> RTC=41S & 36X (AA-97A & 635A)

	Personally, I think it's a crock, but anyway, here's what it
all means.  FLL is Fixed Loss Loop and PROG is for Programmed Loss
Loop.  What all that means is that you have a loop of copper wire
running from the central office (CO) to your jack.  Depending on the
length of the loop (i.e. the distance from your building to the CO)
the resistance, and hence the loop loss, will vary.  In the programmed
mode, inside the little box is a resistor which is used to compensate
for the loop loss; the box should have come with a little package of
fixed resistors; the tech who installed the box was supposed to have
measured the loop loss and installed the proper resistor based on the
results of his measurement.

	USOC is just an acronym which stands for Universal Service
Ordering Code.  It's telephone-speak for "part number", more or less.
All of the RJ-4X-Y jacks are variations on an 8-pin data jack.
According to the documentation I have, "Data configurations use jacks
which incorporate components to limit signal power levels of data
equipment.  Data equipment with a maximum signal power output of -9dBm
are not limited to data configurations."; I interpret that last part
as "any modem you buy today can just be plugged into a plain old
ordinary RJ-11 voice jack".

	I have the wiring diagrams for both the RJ-41S and RJ-41M
jacks in front of me right now and can't find any differences; both
have tip and ring on pins 4/5 (prog mode) and also on pins 1/2 under
control of a switch and through what's called a "Pad" (FLL mode), and
the programming resistor on pins 7/8; the resistor has no internal
connections; I guess it's up to whatever is plugged into the jack to
put it in series with some part of the modem circuitry if it wants.
The note on the 41M says it's for use in multiple installations, but I
don't see how it's any different from the 41S.  A RJ-36X is a fancy
jack with shorting bars, apparantly used for putting multiple
series-connected modems on a single line; if you unplug the plug, the
shorting bars just pass the signal through the jack, but somehow I'd
be surprised if that's what you really have.

	The bottom line is that if you are using any sort of standard
off-the-shelf dialup modem (212A, V.22bis, PEP, HST, etc, etc, etc)
you don't need the fancy RJ-4X jack; just have them put in a plain old
RJ-11 voice jack and that's it.  Even better, don't even tell the
installer that it's a data line, that usually just gets them confused.
Come to think of it, even our four-wire LAD circuits that we run 128 kbps
over using special leased-line modems are terminated in plain old
RJ-11's (although, they probably have some other RJ code in that
case).


Roy Smith, Public Health Research Institute
455 First Avenue, New York, NY 10016
roy@alanine.phri.nyu.edu -OR- {att,cmcl2,rutgers,hombre}!phri!roy