Information Technology Reference
In-Depth Information
a jack or socket, so that all wiring of such connectors is the same
everywhere. Figures 8.2a and 8.2b show this ordering of wires for
both the four and sixwire variations. Notice, in particular, that the
sixwire version includes the same connections as the fourwire ver
sion, with two more wires added.
As already noted, computertocomputer connections using RJ
11 jacks and sockets normally utilize only the middle two (red and
green) wires. The other wires are unnecessary—although reliance on
the RJ11 jack allows commonly available wire to be used in com
puter connections. When used, these wires can form a direct connec
tion from one computer to another, which is called
point-to-point
communication
. RJ11 wiring, jacks, and sockets constitute one
common mechanism for establishing this connection.
These wires make connecting computers an easy enough task,
but the resulting communication is relatively slow; only one signal
at a time can be sent along the wires. Transmission of a byte of data,
for example, would require eight signals, and these would have to
be sent one after the other on RJ11 wiring. Such sequential trans
mission of data is called
serial communication
. In serial communi
cation, one wire of the twisted pair is used to send a signal, and the
other wire (sometimes called a ground) functions to complete the
circuit. In order to send multiple signals at the same time, more than
two wires would be used. In order to keep electrical interference to
a minimum, a cable with eight wires (four twisted pairs) might be
used to send four signals concurrently. By organizing the wires into
twisted pairs, electrical interference for each pair is kept low. Such
sending of several signals at once is called
parallel communication
.
This ability to send multiple signals simultaneously can greatly
facilitate communication by increasing speed. Although different
configurations of wire could be used to accomplish parallel commu
nication, a particularly common choice uses eight wires, and is
called
RJ-45
by the Federal Communications Commission. RJ45
wiring, jacks, and sockets are shown in Figure 8.3; this type of con
nector also is commonly called an
Ethernet jack
and socket.
In comparing Figures 8.2 and 8.3, observe that the two jacks and
sockets look similar; however, the RJ45 jack has two more wires
and is somewhat wider than the RJ11. For comparison, Figure 8.4
shows the back of an Apple Macintosh PowerBook G4 laptop com
puter that contains both RJ11 and RJ45 sockets; the RJ45 socket
near the left is wider than the RJ11 socket on the right.
