Compaq Wireless LAN 100 Manual de usuario descargar pdf (Pagina 2)

18 IT Pro May ❘ June 2001

NETWORKS

Single and multiple cells

Figure 1 shows the single-cell conﬁguration—a simple

wireless LAN strategy typical of many environments. It is

so named because all the wireless end systems are within

range of a single control module. Another common con-

ﬁguration is a multiple-cell wireless LAN, in which a wired

LAN connects multiple control modules. Each control

module supports wireless end systems within its trans-

mission range.An infrared LAN, for example,limits trans-

mission to a single room, so each room in an ofﬁce building

would need one cell.

Nomadic access

In this conﬁguration, the wireless LAN links a LAN hub

and a mobile data terminal equipped with an antenna,

such as a laptop or notepad computer.Thus, for example,

an employee returning from a trip can transfer data from

a personal portable computer to an ofﬁce server.Nomadic

access is also useful in an extended environment such as

a campus or a business operating from a cluster of build-

ings. In both cases, users can move around with their

portable computers and access the servers on

a wired LAN from various locations.

Ad hoc network

This network is set up temporarily to

meet some immediate need. It has no cen-

tralized server.Thus, in meetings, a group of

employees, each with a laptop or palmtop

computer, can link their computers in a net-

work that lasts just as long as the meeting.

WIRELESS LAN REQUIREMENTS

As these configurations show, wireless

LANs must meet requirements typical of any

LAN, including high capacity, ability to cover

short distances, full connectivity among

attached stations, and broadcast capability.

They must also meet requirements speciﬁc to

their intended environment.

• Throughput. The medium access control

(MAC) protocol should use the wireless

medium as efﬁciently as possible to maxi-

mize capacity. The “What is the MAC

Protocol?” sidebar describes this protocol

in more detail.

• Number of nodes. Wireless LANs may

need to support hundreds of nodes across

multiple cells.

• Connection to backbone LAN. Most

applications require interconnection with

stations on a wired backbone LAN.Wire-

less LANs easily satisfy this requirement

by using control modules that connect to

both types of LANs.Applications may also require accom-

modating mobile users and ad hoc wireless networks.

• Service area. A typical coverage area for a wireless LAN

has a diameter of 100 to 300 meters.

• Battery life. Mobile workers use battery-powered work-

stations that must have a long battery life when used

with wireless adapters. Thus, the wireless LAN’s MAC

protocol typically should not require mobile nodes to

monitor access points constantly or engage in frequent

handshakes with a base station. Typical wireless LAN

implementations have features to reduce power con-

sumption when the network is not being used, such as

sleep mode.

• Transmission robustness and security. If not properly

designed, a wireless LAN may be prone to interference,

making it easy for intruders to eavesdrop. A properly

designed wireless LAN permits reliable transmission,

even in a noisy environment, and provides some level of

security from eavesdropping.

• Collocated network operation. As wireless LANs

become more popular, multiple wireless LANs are

100-Mbps

Ethernet switch

Bridge or

router

10-Mbps

Ethernet switch

Server

Ethernet

Figure 1. Single-cell wireless

LAN conﬁguration.

A backbone wired LAN, such as Ethernet, supports servers, work-

stations, and one or more bridges or routers to link with other

networks. A control module (CM) acts as an interface to a wire-

less LAN. The module includes either bridge or router function-

ality to link the wireless LAN to the backbone and some sort of

access control logic, such as a polling or token-passing scheme, to

regulate access from the end systems. Some of the end systems

are stand-alone devices, such as a workstation or a server. Hubs

or other user modules (UMs) that control several stations off a

wired LAN may also be part of the conﬁguration.

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