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.