Legacy and Less Common Cabling Systems - LAN Wiring - page 464

Cabling technology to support computer networking has had a very interesting his-
tory. Along the way, quite a number of alternatives have been developed to meet the
needs of a rapidly changing networking world. Eventually, one particular technol-
ogy, twisted-pair Ethernet, became dominant and displaced all of the others. The
only current competition to Ethernet is Token-Ring, and the two are covered in the
main portions of this topic. However, to be a complete reference to LAN wiring,
many of the older technologies, including coaxial Ethernet, will be covered in this
Appendix.
You may still encounter some of these technologies in older installations, and
it is useful to have this information for reference if you ever need to repair an older
network, or to just shake your head in fascination at how far we have come from
the old days.
Ethernet Coax
Ethernet topology is the “granddaddy” of LAN systems. It was the first widely
accepted, nonproprietary, standardized multiple-access network. Intended for the
routine connection of computers and workstations, it allowed many devices to share
a high-speed communications line without regard to the customary hierarchy of per-
manent computer-to-terminal connections.
Ethernet was originally developed by the Xerox Corporation at its Palo Alto
Research Center. In the late 1970s, Xerox Corporation, Digital Equipment
Corporation (DEC), and Intel Corporation agreed to jointly promote Ethernet as an
open standard for computing. This standard eventually became the IEEE 802.3
standard, administered by the Institute of Electrical and Electronics Engineers. IEEE
802.3 has been revised and refined several times and now includes coax, twisted
pair, and fiber optics. 1 Modern Ethernet, as embodied in IEEE 802.3, actually dif-
fers in some ways from the original Xerox/DEC/Intel standard (sometimes called
DIX). This older standard has been revised and lives on as Ethernet Version 2.
However, most installations have shifted to the IEEE standard and we will refer to
that simply as “Ethernet” in accordance with common usage, unless there is a sig-
nificant difference.
Ethernet uses a method of signaling called Carrier Sense Multiple Access with
Collision-Detect (CSMA/CD). At the signaling rate of 10 Mbps, it uses a special
method, called Manchester coding , to preserve a null DC level on the cable; this
allows collisions to be detected when they occur. Each device listens to the network
1 Wireless multiple-access connection methods are standardized in IEEE 802.11, which is covered in
Chap. 13.