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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 volume of
informitter is now able to send multiple bits of information with
each DSSS code ,enough to make possible the 11M bit /sec in the
original standard .
The 802.11b standard benefits users by delivering wireless
ethernet speeds of 11M bit/sec that can reliably support everyday
business applications,e-mail,internet and server network access.
With support from the new wireless ethernet from the new wireless
ethernet compatibility alliance ,founded by 3com,lucent,nokia and
several other companies in the wireless LAN business,the new
standard will also promise certified interoperability across
multivendor platforms.
Finally, the 802.11b standard serves as a clamoring for a
simplified wireless LAN landscpape.
Vendors can now focus on a single ,high-speed standard , and users
can cut through the clutter of wireless options by fovusing on a
standard that delivers multibvendor interoperability and the
performance to meet their application needs.
有了便携式和无线局域网，用户在离开其办公桌的时候，不管是在会议室、公共区域还是在远处办公室，都能享有更高的生产效率。
然而，时至今日，无线局域网对多数企业应用来说还是太慢。依据IEEE802.11标准，他们运行速度为1兆至2兆位/秒。
现在对此标准的一个新的更高速扩展，能让无线网支持高达11兆位/秒的数据速率。
最初的802.11标准是在1997年提出的，通过定义能与常规的上层企业协议组一起工作的低级协议体系结构，把无线行业团结起来了。802.11也保持了与三种最流行的无线电传输方式（直接顺序扩频、跳频扩频和红外线）的兼容性。
本质上，这种新的体系结构在介质接入控制（MAC）层（第二层）和物理层（第一层）增加了智能，建立这两层之间在执行及开始和保持无线通讯的关键任务时的协作关系。
例如：为确保无线连接的可靠性，MAC层和物理层一起工作，以确定在他们开始传输之前是否有一条清晰的路径。
在传输过程中，他们采用有线以太网不需要的、特殊的避免碰撞和到达应答技术。
1999年9月，IEEE批准了称作802.11B的新名称，作为802.11的高速扩展。此新的802.11B标准在保留原标准的纠错、安全、电源管理和其他优点的情况下，增加了一项关键内容，即把带宽增加到11兆位/秒的技术。
此项技术叫做互补码键控（CCK），他只与原标准中规定的直接顺序扩频（DSSS）技术一起工作，不能用跳频或红外线传输方式工作。
CCK所做的工作是把复杂的数学公式应用于DSSS代码，以允许该代码在每个时钟周期表示更多的信息。现在发射机能在每个DSSS代码中发送多个信息位，足以实现每秒11兆位/秒的数据传输率，而不是原标准的2兆位/秒。
802.11B
标准给出11兆位/秒的无线以太网速度，给用户带来好处，速度能可靠地支持日常的业务应用、电子邮件、因特网和服务器网络的接入。
由3COM、LUCENT、NOKIA和其他几家有无线局域网业务的公司组成的“无线以太网兼容性联盟”支持新的标准，此标准也有望实现跨多厂商与用户团结起来的作用。
现在，厂商能专注在单一的高速标准上，用户也能以通过关注给出厂商互用性和性能。以满足他们应用要求的标准上，解决了从一大堆无线网中作选择的难题。[em09] |
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发表于 2003-9-9 02:13:45
发表于 2003-9-9 04:59:15