WIRELESS LAN B89901048 邱培哲 B89901049 張宏安

This is Wireless LAN … BSS AP

Access Point (AP) Access Point (AP) provides a MAC bridging function from your wired network to your wireless network.

Basic Service Set The Basic Service Set is a term used to describe the collection of Stations which may communicate together within WLAN (Wireless Local Area Network)

AP，無線網卡，BSS 的關係？ 相當於「基地台」「手機」和「服務範圍」

Why Wireless LAN ? Mobility Anywhere , Anytime Convenience Easy to build up a WLAN Freedom

Then ? What’s next ? Applications : PDA Notebook 家電(冷氣,冰箱,電視,影碟機,微波爐) 印表機 掃描器

3 Basic Components 無線網路卡 Access Point ( AP ) Basic Service Set ( BSS )

Network Security … Something to defend from the attacks WEP：Wired Equivalent Protection (Privacy) MAC：無線網卡的 Serial Number IV：Initialization Vector IC：Integrity Check

How Does WEP Work ? WEP relies on a secret key that’s shared between the mobile station and an AP . The secret key is used to encrypt the packets before they are transmitted ,and an integrity check is used to ensure that packets are not modified in transmit . We do not know how to establish the shared key .

Ex. Active Attack from Both Ends 我是大野狼 ？？？ 我是大野狼

Ex. Active Attack to Inject Traffic 我是大野狼 我是小紅帽

Ex. Passive Attack to Decrypt Traffic

How To Protect Yourself ? 啟動WEP加密 慎選AP位置 使用最新的WPA和EAP協定

Protocol & Technique Issues Multiple Access Problem CSMA/CD CSMA/CA 對抗Jamming & Interference 展頻技術(Spread Spectrum) Bit Rate, Bandwidth & Channel OFDM

Why Need Multiple Access Protocol? 讓每個User在傳輸時都有很高的Data Rate 如果兩個人一起傳,收的人會收不到……

CSMA/CD Carrier Sense Multiple Access / with Collision Detection 先觀察有沒有人在傳 Collision Detection 如果發現撞到了,大家就先休息,不傳 Exponential Blocking 如果Collision發生越多,就等久一點再傳

Wireless 會遇到的問題 Hidden Terminal Problem

Modulation Technique 展頻技術(Spread Spectrum) 需要展頻的理由 展頻的方法 Free Licensed Channel Jamming & Interference 展頻的方法 DSSS ( Direct Sequence Spread Spectrum ) FHSS ( Frequency Hopping Spread Spectrum )

CSMA/CA Carrier Sense Multiple Access / with Collision Avoidance RTS (Request To Send) 向AP提出要傳輸的申請 CTS (Clear To Send) AP說你可以傳了!!!

Jamming & Interference 其他電器 有人表你 Solution : 展頻技術 DSSS FHSS

DSSS DSSS ( Direct Sequence Spread Spectrum )

DSSS DSSS ( Direct Sequence Spread Spectrum ) PN sequence 類似一個加密的動作 將一個Symbol切成很多個小的chips 使得某一頻段的Jamming影響變小 10.4dB processing gain 802.11b所用的技術

Modulation Technique FHSS Frequency Hopping Spread Spectrum 傳輸資料的兩端同時約好轉換傳輸頻率 利用類似產生PN sequence的方法決定下一個moment該用什麼頻段 約2.5Hops/sec 藍芽所用的展頻技術

兩種展頻技術的比較 DSSS 運用技巧讓Jamming變小 FHSS 藉由轉移傳輸頻率,躲避Jamming

Bit Rate, Bandwidth & Channel 傳輸頻寬大,Equalizer很難作 Possible Solution Channel Partition 把資料分成很多個sub channel 去傳

Modulation Technique OFDM

Final … 更高的安全性 更快的傳輸速率 更低的power 更廣泛的應用 更大的服務範圍