Why does wireless lan use CSMA/CA?

[watermelonyu]

Ethernet 的 access method 是採用 CSMA/CD, 為什麼 wireless lan 卻改採 CSMA/CA? 從 collision detection 變成用 collision avoidance...why?

[watermelonyu]

reference from :

http://cweb.msi.com.tw/program/service/terminology/terminology_view.php?SERIALS=137



CSMA/CA 為Carrier Sense Multiple Access with Collision Avoidance的簡寫，譯為「載波感測多重存取碰撞避免協定」，為IEEE 802.11標準所定義。

CSMA/CA是無線網路媒體存取的方式，與乙太網路的CSMA/CD協定相似，差別在於乙太網路是利用碰撞偵測的方式，但由於碰撞偵測無線訊號有所困難，因此CSMA/CA改採取避免碰撞（Collision Avoidance）的方式。避免碰撞發生的方法是以偵測工作頻帶中的電磁波能量來發現頻道是否空閒，若發現空閒也不立刻送出資料訊框，而是自行產生一隨機亂數來當作延遲的時間，當在延遲期間偵測到頻道忙碌時，則此隨機時間會被凍結，直到再度確認頻道空閒時再啟動隨機時間，一旦延遲時間一到才將訊框送出，如此一來資料的碰撞機率便大幅降低。

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[watermelonyu]

Hi Yoda,



CSMA/CA & CSMA/CD 何者效能較好?

[watermelonyu]

Hi Eric:



CSMA/CD be used in wired local area network. CSMA/CA be used in wireless local area network.



There are in diffenet environment, so for the performance comparison, looks need to do more study.



For the performance study of CSMA/CA,

in terms of physical layer implementation, those are InfraRed (IR), Direct Spread Spectrum

Sequence (DSSS), and Frequency Hopping Spread Spectrum (FHSS).



Different behavior for each implementation, due basically to the physical parameters specified for all three implementation.



Using ideal propagation delay conditions, DSSS physical implementation shows the best throughput.



This alos show in today's market, most of 802.11 wireless LAN is using DSSS as physical layer.



There is a paper in the following URL and as attachment,

http://www.eunice-forum.org/eunice99/027.pdf



which may show you a light.



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