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.（To be continued）
有了便攜式和無線局域網，用戶在離開其辦公桌的時候，不管是在會議室、公共區域還是在遠處辦公室，都能享有更高的生產效率。
然而，時至今日，無線局域網對多數企業應用來說還是太慢。依據IEEE802.11標準，他們運行速度為1兆至2兆位/秒。
現在對此標準的一個新的更高速擴展，能讓無線網支持高達11兆位/秒的數據速率。
最初的802.11標準是在1997年提出的，通過定義能與常規的上層企業協議組一起工作的低級協議體系結構，把無線行業團結起來了。802.11也保持了與三種最流行的無線電傳輸方式（直接順序擴頻、跳頻擴頻和紅外線）的兼容性。
本質上，這種新的體系結構在介質接入控制（MAC）層（第二層）和物理層（第一層）增加了智能，建立這兩層之間在執行及開始和保持無線通訊的關鍵任務時的協作關系。
例如：為確保無線連接的可靠性，MAC層和物理層一起工作，以確定在他們開始傳輸之前是否有一條清晰的路徑。
在傳輸過程中，他們采用有線以太網不需要的、特殊的避免碰撞和到達應答技術。(末完待續)
文章來源于領測軟件測試網 http://www.kjueaiud.com/
