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From: smb@ulysses.UUCP (Steven Bellovin)
Newsgroups: net.dcom
Subject: Re: RJ-41S and RJ-45S
Message-ID: <1086@ulysses.UUCP>
Date: Fri, 18-Jan-85 18:57:33 EST
Article-I.D.: ulysses.1086
Posted: Fri Jan 18 18:57:33 1985
Date-Received: Sat, 19-Jan-85 01:56:20 EST
References: <90@tove.UUCP>
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 39

The purpose of RJ41 and RJ45 jacks is to provide a measurement of the loss to
the central office.  This permits a modem to depend on amplitude of the
received signal.  What happens when the jack is installed is that the actual
loss is measured, and a resistor of an appropriate value is made available
(via pins 7 and 8, as noted) to the modem to permit it to adjust its transmit
level.

If you want to try faking things, buy an ordinary RJ11 cord (at Radio Shack
or wherever) and plug it in to the modem and your wall jack.  The RJ11 cord
(which has provision for 6 pins, though normally only the central 4 are used)
is designed so that the important signals (RING and TIP) are properly connected
when that cord is plugged into an 8-pin jack.  Similarly, your voice phone
that you'll use to make the connection can be connected via an RJ11 plug to
the modem.  (As for how you make the connection:  you dial the number normally,
then press the CONN "button" on the AJ modem and hang up your phone.  No
special phone is needed.)  One thing to watch out for:  if you have two phone
lines in your house, the second pair of wires running through the wall (black
and yellow) may be the other line; this signal will show up on AJ's pins 3&6
(mode indicator and mode indicator common), which will confuse matters...  If
this is the case, disconnect the wires from your jack to the junction box, leaving
only the red and green connected.  But let the black and yellow feed through
the box, of course.  (Btw, the arrangement of having two lines hooked up to one
sack is an increasingly common one, and there are phones and gadgets thawayuse
this configuration.  Radio Shack even has something that looks like a 3-way
T-splitter that will break out line 1, line 2, and line 1+2.  I believe this
arrangement is called an RJ13 jack.)

It's problematic whether or not this arrangement will work with the AJ modem,
though.  It really does need an accurate level setting, we've found.  When
I brought one home, it worked well most of the time; from my supervisor's
house, though, it wouldn't work at all till he played games with the resistor
setting.  (We were using a RJ45 jack attached to a short RJ-11 plug; that
let us have some value in place but still use an ordinary phone line.  We
didn't try using the RJ11 option setting inside the modem.)

		--Steve Bellovin
		AT&T Bell Laboratories

P.S.  These are my opinions, etc., only, not the company's.