【文章內(nèi)容簡介】 way from their desks ,whether they are in conference rooms, public areas or remote offices.BRUntil recently , however,wireless LANs were too slow for most enterprise on the IEEE standdrd,they ran at 1M to 2M bit/sec. Now a new high rate extension to the standard , lets wireless works support data rates to 11M bit/sec. Ratified in 1997 ,the original standardunited the wireless industury by defining a lowlevel protocol architecture that worked with conventional upperlayer enterprise protocol stacks . also, maintained patibility with the three most popular radio transmission types: direct sequence spread spectrum,frequencyhopping 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 munications. For instance ,to ensure reliability of the wireless link ,MAC and PHY work togetherto determine if a clear path exists before they start a transmission. During transmission, they employ special collision avoidance and arrivalacknowledgment techniques that are not required in wired ether LANs. in september 1999,the IEEE approved a new designation, known as to retain the errorcorrection,security,powermanagement and other advantages of the original , a key ingredienta thchnique for increasing bandwidth to 11M bit/sec. Called plementary code keying (CCK) the technique works only in conjunction with the DSSS technology sprcified in the original standard . it does work with frequencyhopping 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 standard benefits users by delivering wireless ether speeds of 11M bit/sec that can reliably support everyday business applications,,inter and server work access. With support from the new wireless ether from the new wireless ether patibility alliance ,founded by 3,lucent,nokia and several other panies in the wireless LAN business,the new standard will also promise certified interoperability across multivendor platforms. Finally, the standard serves as a clamoring for a simplified wireless LAN landscpape. Vendors can now focus on a single ,highspeed 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. Data Communication Systems There are five basic types of data munication system: Offline data transmission is simply the use of a telephone or similar link to transmit data without involving a puter system .The equipment used at both ends of such a link is not part of a puter, or at least does not immediately make the data available for puter process, that is, the data when sent and/or received are 39。offline39。. This type of data munication is relatively cheap and simple. Remote batch is the term used for the way in which data munication technology is used geographically to separate the input and /or output of data from the puter on which they are processed in batch mode. Online data collection is the method of using munications technology to provide input data to a puter as such input arisesthe data are then stored in the puter (say on a magic disk) and processed either at predetermined intervals or as required. Enquiryresponse systems provide ,as the term suggests ,the facility for a user to extract information from a puter .The enquiry facility is passive ,that is ,does not modify the information stored .The interrogation may be simple ,for example ,39。RETRIEVE THE RECORD FOR EMPLOYEE NUMBER 123439。 or plex .Such systems may use terminals producing hard copy and /or visual displays. Realtime systems are those in which information is made available to and processed by a puter system in a dynamic manner so that either the puter may cause action to be taken to influence events as they occur (for example as in a process control application) or human operators may be influenced by the accurate and uptodate information stored in the puter, for example as in reservation systems. How TCP/IP Works Over the past few years ,one of the most written－ about w