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.
What CCK does is apply sophisticated mathematical formulas to the DSSS codes, a permitting the code to represent a greater vol |
