Information Technology Reference
In-Depth Information
As shown in Figures 8.2a and 8.2b, a standard RJ11 jack or
socket may have six different wires, although many telephone cables
contain only four wires. Historically, these wires were straight or
untwisted, although modern cables may be either untwisted or
twisted.
Although telephone wiring usually has four or six wires, only
the middle two (the red and green wires) are usually used for basic
telephone or computer connections. A fourwire version includes
white and blue wires (or sometimes black and yellow wires), often
used in telephone operations for multiple telephones, for signaling,
or as spare wires. The sixwire version contains red, green, white,
blue, black, and yellow wires.
So, why include extra wires if they don't do anything? The an
swer involves longterm flexibility and maintenance. Consider, for
example, the telephone wires for my house. (You could check if
your house or apartment is similar.) Because I am a residential cus
tomer who pays for a single telephone line, I need only one pair of
wires (the red and green pair). However, in the future, the telephone
company knows that I might want a second line or other special ser
vices. Because installing new wires is time consuming, labor inten
sive, and expensive, the telephone company has installed a fourwire
cable to my house, and it maintains a connection box on the side of
my house with connections for all four wires. Only two wires are
used now, but the telephone company can provide additional ser
vices whenever I request them.
A similar consideration applies to the wires that run into my
house from the telephone company's connection box. The wires
themselves are small and inexpensive, but installing wires through
the walls takes considerable time and effort. Because labor, not wire,
is the primary cost of installation, most telephone wiring within a
home involves fourwire cable. I may use only two wires for my ba
sic service, but the others are available if I want more service. Also,
as a backup, if I want just two wires but either the red or green wire
breaks somewhere in a wall, having another wire in place allows an
easy substitution. Altogether, the additional cost in going from two
small telephone wires to four or six wires is low, but the cost of in
stalling new lines is high. The use of the largerthanneeded cable al
lows longterm flexibility and aids maintenance at a low cost.
Returning to the RJ11 jacks and sockets, note that the RJ11
standard specifies which color wires are connected to which parts of
