新的无线局域网标准（一）

With portable computers and wireless LANs, users can enjoy greater productivity while away from their desks, whether they are in conference rooms, public areas or remote offices.

Until recently, however, wireless LANs were too slow for most enterprise applications.based on the IEEE 802.11 standdrd, they ran at 1M to 2M bit/sec.

Now a new high-rate extension to the standard, 802.11b, lets wireless networks support data rates to 11M bit/sec.

Ratified in 1997, the original 802.11 standardunited the wireless industury by defining a low-level protocol architecture that worked with conventional upper-layer enterprise protocol stacks. Also, 802.11 maintained compatibility with the three most popular radio transmission types: direct sequence spread spectrum, frequency-hopping spread spectrum, and infrared.

Essentially, this new architecture added intelligence at the medium access control（MAC）layer 2 and at the physical（PHY）layer 1, fosteing cooperation between the two layers in performing the critical tasks involved with initiating and maintaining wireless communi-cations.

For instance, to ensure reliability of the wireless link, MAC and PHY work together to determine if a clear path exists before they start a transmission.

During transmission, they employ special collision-avoidance and arrival-acknowledgment techniques that are not required in wired ethernet LANs.

In september 1999, the IEEE approved a new designation, known as 802.11.intended to retain the error-correction，security, powermanagement and other advantages of the original, a key ingredient-a thchnique for increasing bandwidth to 11M bit/sec.

Called complementary code keying（CCK）the technique works only in conjunction with the DSSS technology sprcified in the original standard . it does net work with frequency-hopping or infrared transmissions.