How are computers connected? - The Tao of Computing - page 216

third pair for voice communications and the fourth pair for supply
ing electrical power.)
The next major step in communication speeds over these ca
bles came in 1995 with the standard for 100BaseT . This ap
proach still uses just two of the four pairs of wires, but provides a
mechanism for transmitting 100,000 bits of information per sec
ond. The next standard using RJ45 cable for highspeed commu
nication was ratified in 1999. This standard is called 1000BaseT
or 802.3ab (following the standards' group at the Institute of
Electrical and Electronics Engineers [IEEE]) and utilizes all four
pairs of wires to communicate at the rate of 1 million bits per sec
ond. The newest standard, called 10GBaseT or 802.3ae , was
adopted on June 17, 2002, and runs at 10 million bits per second,
although it can transmit only over short distances using an en
hanced RJ45 cable; this most recent standard assumes that
most communication at such high speeds will utilize more sophis
ticated cables and communication processes than are possible
with RJ45.
Although RJ45 cable, jacks, and sockets describe essential ele
ments of the wires and connectors, the wires can be connected to
the jacks in several ways; this gives rise to a few different versions
of RJ45 cables. In many modern applications, RJ45 cables con
nect a computer to a special device, such as a modem or hub, and
wires at both ends of the cable connect in identical ways to the
jacks; that is, pin 1 of the jack at one end of the cable is connected
to pin 1 of the jack at the other end, pin 2 of each jack is con
nected, and so on. This is called a straight-through cable ,
and such a cable is used most often in connecting computers to
networks.
When connecting two computers directly with a cable (pointto
point connection), however, a slightly different approach is needed.
In particular, we noted that 10BaseT and 100BaseT standards in
volve one pair of wires for sending data and another for receiving
data. Of course, when two computers communicate, the sending
wires for one machine must correspond to the receiving wires for
the other, and this requires connections for some jack pins to be re
versed. When RJ45 cables are connected to their jacks by reversing
the wire connections at one end, the resulting cablejack combina
tion is called a crossed cable .