Information Technology Reference
In-Depth Information
tion, but additional wires within the cable provide flexibility and
aid maintenance. Within an RJ11 cable, the wires may be un
twisted, or they may be organized into pairs and the wires in each
pair twisted together. Outside electrical interference has relatively
little effect on twisted pairs used for circuits.
Cables that connect computers to networks often contain
eight wires, organized as four twisted pairs; plugs for these ca
bles are called Ethernet jacks. Wires within these cables may be
connected to Ethernet jacks in the same way at both ends
(straightthrough cable); alternatively, connections of pairs at
one end may be reversed (a crossed cable). These cables are
called Category 5 or RJ45 cables, and several protocols pre
scribe how communications work over these cables and at what
speed. From slowest to fastest, these protocols are designated
10BaseT, 100BaseT, 1000BaseT (or 802.3ab), and 10GBaseT (or
802.3ae).
Cables running directly from one computer to another pro
vide simple pointtopoint communications, but this use of sep
arate wires for each connection does not scale up well. Instead,
a star network uses pointtopoint communication to connect
each computer in a network to a central hub. An Ethernet net
work allows several computers to connect to the same cable. A
tokenring network arranges the computers into a circle with
each computer connected to the ones just before and after it.
Several computers in an area may constitute a natural group or
segment; and star, Ethernet, or tokenring networks work well
for segments. Bridges allow connections among multiple net
work segments.
Wireless technology utilizes many ideas of Ethernet networks. If
some computers are connected directly to the Internet, these form
access points. Portable computers use radio waves or infrared sig
nals to communicate with a nearby access point. As portable com
puters move, they must interact with new access points as signals
from previous ones become weak. Standards prescribe how a
portable computer becomes associated with one access point or how
it reassociates with another access point during travel. If no com
puters have direct connections to the Internet, portable computers
still can interact with each other (but not with the Internet) through
an adhoc network.
