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Lesson 2
Selecting Data-Link Layer Protocols
1-17
Fiber-optic cables are also much less susceptible to attenuation than copper
cables.
Attenuation is the tendency of a signal to weaken as it travels along a
medium. This
signal weakening is one of the main reasons for the 100-meter UTP-cable
length limit.
All fiber-optic cables can exceed UTP cables in length, but there are
several different
types of fiber-optic cable, each with different length limitations. The
multimode fiber-
optic cable typically used on LANs can span distances ranging from 400 to
2,000
meters, while single-mode fiber can support cable runs as long as 100
kilometers.
In performance and flexibility, fiber-optic cable is superior to UTP in
almost every way.
Fiber-optic cable is inherently more secure than copper cables because
unauthorized
people cannot easily tap into the cable and intercept its signals as they
can with cop-
per. The drawback of the medium is its additional cost and the special
skills required
to install and maintain it. Virtually all the tools used to install and test
fiber-optic cable
are different from those for copper-based cables. Installers must glue the
connectors
onto a fiber-optic cable—typically using a heat-cured glue and a small
oven—while
UTP connectors are crimped onto the cable. Also, because of the way light
pulses are
transferred through the cable, fiber-optic installers must be careful not to
bend the
cable too sharply.
The raw materials for a fiber-optic installation are far more expensive than
their UTP
counterparts. There are also far fewer fiber-optic installation contractors
than UTP
experts, and their skills come at a premium. In addition, many network
administrators
who have the ability to make minor repairs on UTP cables would be lost when
faced
with a malfunctioning fiber-optic connection.
Planning Generally speaking, network designs call for fiber-optic cable only
when there are
specific reasons for it, such as the need for extra-long cable runs or an
environment with a lot
of interference.
Wireless Networking Although wireless network technologies of various types
have
been around for many years, only recently have they become a viable medium
for the
average network installation. The older wireless LAN technologies were
notoriously
slow and unreliable, but a new standard published by the Institute of
Electrical and
Electronic Engineers (IEEE), called 802.11b, has boosted wireless LAN
transmission
speeds to 11 Mbps (faster than standard cabled Ethernet) and greatly
increased their
reliability.
IEEE 802.11b networks can function using two different topologies. The ad
hoc topol−
ogy, illustrated in Figure 1-4, consists of two or more computers equipped
with wire-
less network-interface adapters that can communicate with each other
interchangeably.
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