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The infrared option never gained market support because

it requires unobstructed line-of-sight and because the avail-

able data rates are limited. The other two schemes use

spread-spectrum approaches, which require a much wider

bandwidth than is actually necessary to support a given data

rate.The idea behind using the wider bandwidth is to min-

imize interference and drastically reduce the error rate.The

FHSS scheme achieves spread spectrum by frequently

jumping from one carrier frequency to another; thus, any

interference or performance degradation at a given fre-

quency affects only a small fraction of the transmission.

The DS-SS scheme increases a signal’s data rate by map-

ping each data bit into a string of bits, with one string used

for binary 1 and another for binary 0.The higher data rate

uses a greater bandwidth. The idea is to spread each bit

The ﬁrst is open-system authentication, in

which two parties simply agree to exchange

identities before sending data. One party

sends a MAC control frame (see the “What Is

the MAC Protocol?” sidebar), which indicates

that this is an open system authentication

exchange. The other party responds with its

own authentication frame, and the process is

complete.

In the second algorithm, shared-key

authentication, the two parties share a secret

key that no other party has, and the key is the

basis for authentication.A handshaking pro-

tocol enables the two sides to verify that each

has that key.

Privacy. With a wireless LAN, eavesdropping

is a major concern because of the ease of cap-

turing a transmission.To assure privacy, IEEE

802.11 provides for the optional use of

encryption by specifying a scheme based on

the Wired Equivalent Privacy (WEP) algo-

rithm. To provide both privacy and data

integrity, the WEP algorithm uses an encryp-

tion scheme based on the RC4 encryption

algorithm. The idea in RC4 is that two com-

municating parties must share a 40-bit key,

which encrypts and decrypts all frames.

Although the 40-bit key provides only a mod-

est level of security, it is enough to protect

against casual eavesdroppers. For much

stronger protections, some 802.11 vendors

offer optional 128-bit encryption.

802.11 protocol layers

Figure 3 shows the standard’s layered pro-

tocol architecture.

The top and middle layers deﬁne a rather

complex set of mechanisms for regulating

access to the LAN and for providing security.

The physical layer speciﬁes the actual trans-

mission details and has been the focus of much work in the

past three years.

The IEEE issued the physical layer for 802.11 in three

stages.The ﬁrst part, issued in 1997, is called simply IEEE

802.11. As Figure 3 shows, it includes the MAC layer and

three physical layer speciﬁcations—all operating at data

rates of 1 and 2 Mbps:

• direct-sequence spread spectrum (DS-SS), operating in

the 2.4-GHz ISM (Industrial, Scientific, and Medical)

band;

• frequency-hopping spread spectrum (FHSS),operating

in the 2.4-GHz ISM band; and

• infrared, operating at a wavelength between 850 and 950

nm.

Data rates of

1 Mbps and 2 Mbps

2.4-GHz

frequency-

hopping

spread

spectrum

2.4-GHz

direct-

sequence

spread

spectrum

Infrared 5-GHz

orthogonal

frequency-

division

multiplexing

Data rates

of 6, 9, 12,

18, 24, 36,

48, 54 Mbps

2.4-GHz

direct-

sequence

spread

spectrum

Data rates

of 5.5 Mbps

and

11 Mbps

Distributed coordination function

(DCF)

Logical link control

Point coordination function

(PCF)

Contention service

Contention-free service

MAC

layer

Physical

layer

IEEE 802.11 IEEE 802.11a IEEE 802.11b

Figure 3. IEEE’s 802.11’s

layered protocol architecture.

The lowest (physical) layer, deﬁnes the frequency band, data rate,

and other details of the actual radio transmission.Above the phys-

ical layer is the medium access control (MAC) layer, which regu-

lates access to the shared radio frequency band so that station

transmissions do not interfere with one another. The MAC layer

has two sublayers. The lower one is the distributed coordination

function, which uses an Ethernet-style contention algorithm that

provides access to all trafﬁc. Ordinary asynchronous trafﬁc uses

this coordination function. The upper MAC sublayer is the point

coordination function, a centralized MAC algorithm that provides

contention-free service by polling stations in turn. Higher-

priority traffic—traffic with greater timing requirements—uses

this coordination function. Finally, the logical link control layer

provides an interface to higher layers and performs basic link-

layer functions such as error control.

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