The little plastic tips on the ends of phone cords and cables are
modular plugs. They fit into modular jacks.
Despite their male name, jacks are female. Plugs are
male. If you don't understand this, the next time you're naked,
look in the mirror.
The word "modular" refers to a phone construction format introduced by
AT&T in the1970s, that allowed installers to assemble phones at customer
locations by selecting specific components that plugged together, instead
of needing hard wiring. The modular connector design was
also applied to the jacks that phones plugged into, to get dialtone.
plugs are made in three sizes:
- The smallest plug, known
as 4-position, 4-wire, is used for handset cords. A position
is a groove molded into the plastic that could contain a little
bit of gold-plated wire to make contact with wires inside the jack.
- The middle-size plug is
the most common. It has six positions, and either two, four, or six
wires. It is used for most line cords that connect phones, modems and
other devices to phone jacks.
- The largest plug, with
eight positions and eight wires, is usually used for LANs
(Local Area Networks) and sometimes for four-line phones. It is often
called an RJ45, but that designation is inaccurate for LAN connections.
plugs and jacks are also used on some ATT/Lucent phone systems.
If you are going to
re-use jacks previously installed for a Merlin or other ATT / Lucent /
Avaya phone system that uses the "T568B" wiring scheme, you will either
have to re-arrange the wires inside the jack, or connect the circuit
that would normally go on the white/orange wire pair, to the white/green
At least 99% of all phone jacks used with
one-line phones are really two-line jacks. In the same actual jack, if two
wires are connected, it's a one-line jack. If four wires are connected,
it's a two-liner.
The FCC and phone companies have codes to
identify how jacks are wired. A single-line jack designed for mounting a
wall phone is an RJ-11W. A single-line jack for a table or desk phone is
an RJ-11C. Two-line jacks are RJ-14W and RJ-14C. Three-line jacks are
RJ-25. If you know that much, you know more than many phone company
employees. Four-line jacks are RJ-61. If you know that, you know more than
all phone company employees. "RJ" stands for Registered Jack. "W"
stands for Wall. Apparently only one person knew what the "C" stood for,
and he died without telling anyone.
||If you look at the
springy wires inside a common phone jack, the two inner ones are used
for line #1, and the two outer ones are used for line #2. The flat
cords that commonly connect phones to phone jacks follow this same
Cords used for three-line phones have two
additional conductors for the third line, outside the second pair, for a
total of six wires (three pairs). If you look at a cross-section of a
six-conductor phone cord, the line circuits could be considered to look
like this: 321123. If you plug a single-line phone into a two-line jack,
it will work on line #1. If you plug a two-line phone into a three-line
jack, it will work on lines #1 and line #2.
Four-line cords have two more wires (total
of eight wires, in four pairs). Four-line non-system phones can use two
two-line cords, or one four-line cord.
used in modern electronic or digital
phone systems usually use cords with four or six internal wires,
regardless of the number of lines the phone can handle.
Phone jacks are made in
three general formats:
The surface jack, is a cube, roughly 2
inches by two inches by 3/4 of an inch, and mounts on the surface of the
wall. It is usually installed after a wall is constructed, often with wire
stapled to the baseboard. They are available in a variety
of colors, with connections for two, three or four pairs.
flush jack is flat, like an electrical outlet, and is usually
installed in new construction, with wire concealed inside the wall.
Flush jacks are available in a variety of colors, for two, three or
four pairs of wire, and in both standard and "Decora" styles (at
wall jack, usually mounted about five feet above the
floor, has two metal mounting studs, to support a wall phone. They are
available with plastic or stainless steel front plates, with two or three
pairs of wire.
Both surface and flush jacks are available
in multi-port versions, with up to six ports in a
single-gang plate or12 ports in double-gang
plate. You can use a mixture of jacks and other connectors, for phone,
fax, LAN, modem, answerer, TV, etc. Multi-port assemblies are available in
both standard and Decora styles, in a variety of colors.
jacks and mounting plates are available for installation in
the floor, outdoors, in modular furniture, etc.