The Internet Encyclopedia (Volume 3)

P1: 211

Zarki WL040/Bidgolio-Vol I WL040-Sample.cls September 15, 2003 8:55 Char Count= 0

WIRELESSINTERNET—ISITHAPPENING? 843

Table 3Features of 802.11b

Advantages Disadvantages

2.4-GHz band is almost Prone to interference

universally available. from other devices

that operate in the

2.4-Ghz band.

the MAC layer, they both use the CSMA/CA protocol. At

the physical layer, 802.11b uses the DSSS radio trans-

mission method and operates in the 2.4-GHz ISM band

(see Table 3), while 802.11a uses orthogonal frequency

division multiplexing (OFDM)—a more recent modula-

tion scheme that is claimed to offer better performance at

higher data rates (Bingham, 1990)—and operates in the

5-GHz UNII (unlicensed national information infrastruc-

ture) band (see Table 4).

802.11b—Offers data rates of up to 11 Mbps (see Table 5).

802.11a—Offers data rates of up to 54 Mbps due to higher

carrier frequency and a more sophisticated encoding

technology (see Table 5).

802.11g.While the 802.11g standard is yet to be finalized

(draft standard was ratified in November 2001), it seeks to

offer current users of 802.11b data rates up to 54 Mbps in

the same 2.4-GHz band. Because 802.11b and 802.11g use

the same frequency band, an 802.11b radio interface will

work with an 802.11g access point albeit at an 802.11b

rate. However, the physical range of 802.11g will be less

than 802.11b, so higher concentration of access points will

be necessary to obtain the full 54-Mbps rate throughout

the service area.

Two mandatory and two optional modes are part of

the draft standard. Use of OFDM (similar to one in use

for 802.11a) is mandatory for data rates greater than

20 Mbps and support for complementary code keying

(CCK) is necessary to ensure backward compatibility with

existing 802.11b radios. The RTS/CTS mechanism de-

scribed previously is used to ensure that both OFDM and

CCK can coexist in the same 2.4-GHz band. An addi-

tional benefit of 802.11g is that since OFDM is already

required for 802.11a, it is possible to build dual-band

Table 4Features of 802.11a

Advantages Disadvantages

Better performance in office Limited number of

environment (multipath channels available

reflection recovery). outside the

Higher data rates and less United States.

congestion in the UNII

band.

Some chip makers promise

proprietary modes that will

deliver up to 72 Mbps.

Table 5Comparison of 802.11b and 802.11a

Technologies

Standard 802.11a 802.11b

Speed, up to 54 Mbps 11 Mbps

Range 300 feet 300 feet

Radio technology OFDM DSSS

MAC protocol CSMA/CA CSMA/CA

Frequency 5 GHz 2.4 GHz

Power management Embedded power

saving protocol

(2.4 and 5 GHz) radios without extra hardware complex-

ity.

The popularity of IEEE 802.11a/b/g makes it a very seri-

ous challenger for 3G. It is being deployed in many public

places to provide high-speed access to mobile users. The

ease of deployment and maintenance of 802.11 is at the

heart of its success. It is cheap (compared to 3G), robust,

and simple to use, three cornerstones of any networking

technology.

Bluetooth and PANs

The concept of personal area networks (PANs) has been in-

troduced to enable wireless communication between de-

vices in direct vicinity of a user. Examples of such devices

are laptop computers, personal digital assistants (PDAs),

cellular phones, printers, and photo cameras. PAN tech-

nology is characterized by low cost, low power consump-

tion, and ad hoc network organization. Wireless PANs are

being standardized in the IEEE 802.15 working group.

The Bluetooth technology is being promoted as an indus-

try standard for PANs, and is forming the basis for IEEE

802.15 standardization.

Bluetooth is a low-power, low-cost wireless technol-

ogy for distances up to 10 m, at data rates up to 1 Mbps

(and higher in newer versions of the standard) (Haartsen,

1998). It operates in the same 2.4-GHz ISM band as the

IEEE 802.11b standard, using fast (1600 hops/s) FHSS.

Bluetooth nodes are organized in so-called piconets, con-

sisting of a master node and up to 7 slave nodes (that

number is now being increased to 25 in the newer version

of the standard). A slave can simultaneously be a slave or

a master in another piconet, with a different frequency

hopping sequence. This allows the construction of larger

networks, called scatternets, where communication be-

tween nodes is carried out using multiple hops across pi-

conets. Within piconets, the Bluetooth MAC uses polling

to regulate access to the radio interface. A slave is only

allowed to transmit a slot of data when polled by the mas-

ter. Transmission of data from master to slave is consid-

ered as an implicit poll. Using this polling scheme, Blue-

tooth can provide both synchronous connection-oriented

(SCO) links, e.g., for support of voice telephony applica-

tions, and asynchronous connectionless (ACL) links, e.g.,

for IP packet transfer.

Bluetooth products are rapidly being introduced into

the market. Many new cellular telephone models are