Compaq Wireless LAN 100 Bedienungsanleitung PDF herunterladen (Seite 4)

20 IT Pro May ❘ June 2001

NETWORKS

bility is also an imperative because applications evolve

and requirements change.

Recognizing the need to balance these two imperatives,

the IEEE tasked a separate working group to develop a

complete set of speciﬁcations for a wireless LAN, includ-

ing the details of the frequency used,transmission method,

means of controlling access by various devices to the LAN,

and security issues. The proposed specifications, IEEE

802.11, had to be flexible enough to satisfy a range of

requirements and intended applications.

Work began on the original 802.11 in 1990. In 1999, at

roughly the same time, the IEEE issued 802.11a and

802.11b. These three standards differ only in the physical

layer of their architecture.

802.11 architecture

In the architecture’s simplest form, each station

belongs to a single basic service set—meaning that it is typ-

ically within wireless range of only the stations

that also belong to that set. The exception is

when two basic service sets overlap geo-

graphically, making the range narrow enough

for a single station to participate in both sets.

The association between a station and its basic

service set is dynamic. Stations can turn off,

come within range, and go out of range.

Figure 2 shows a more complex form of

the 802.11 architecture—an extended service

set—in which a distribution system connects

two or more basic service sets. Typically, the

distribution system is a wired backbone LAN,

but it can be any communications network.

The extended service set appears as a single

logical LAN to the logical link control level

(described later).The access point is the logic

within a station that provides access to the

distribution system by providing services in

addition to acting as a station.

802.11 services

IEEE 802.11 defines several services that

the wireless LAN must provide if its useful-

ness is to match the functionality inherent in

wired LANs.

Association. Before a station can transmit or

receive frames on a wireless LAN, it must

make its identity and address known.To do so,

it establishes an association with an access

point.The access point can then communicate

this information to other access points, which

makes it easier to route and deliver addressed

frames.The reassociation service makes it pos-

sible for an established association to transfer

from one access point to another,which is what

lets a mobile station move.The disassociation

service makes it possible for either a station or an access

point to notify other access points that an existing associa-

tion is terminated. A station should give this notiﬁcation

before leaving an area or shutting down.

Authentication Security. In wired LANs, stations establish

their identity with other stations through a physical con-

nection. The assumption is that access to that connection

conveys the authority to connect to the LAN. With wire-

less LANs, however, users can establish a connection

merely by having an attached antenna that is properly

tuned. Stations must use an authentication service to estab-

lish their identity with other stations. IEEE 802.11 does

not mandate any particular authentication scheme, which

could be anything from relatively unsecure handshaking to

public-key encryption. It does, however, specify two

authentication algorithms, which vendors can decide to

include in their products. It is then up to the buyer to pay

for the extra capability.

Basic

service set

IEEE 802.x LAN

Portal

Distribution system

Station1

Station3

Station4Station2

Basic

service set

Station5

Station7Station6

Figure 2. IEEE’s 802.11’s

extended service set architecture.

The smallest building block of a wireless LAN is a basic service set,

which consists of stations that execute the same MAC protocol

and compete for access to the same shared wireless medium. A

basic service set may be isolated or, as is the case here, connected

to a backbone distribution system through an access point (AP),

which functions as a bridge and is implemented as part of a station.

A central coordination function housed in the access point con-

trols the MAC protocol or the protocol may be fully distributed.

The basic service set generally corresponds to a cell. The distri-

bution system can be a switch, wired network, or wireless network.

The portal integrates the IEEE 802.11 architecture with a tradi-

tional wired LAN. The portal logic is implemented in a device,

such as a bridge or router, that is part of the wired LAN and

attached to the distribution system.

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