WIRELESS LAN B89901048 邱培哲 B89901049 張宏安.

Presentation on theme: "WIRELESS LAN B89901048 邱培哲 B89901049 張宏安."— Presentation transcript:

1 WIRELESS LAN B 邱培哲 B 張宏安

2 This is Wireless LAN … BSS AP

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

4 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)

5 AP，無線網卡，BSS 的關係？ 相當於「基地台」「手機」和「服務範圍」

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

7 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

8 3 Basic Components 無線網路卡 Access Point ( AP ) Basic Service Set ( BSS )

9 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

10 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 .

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

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

13 Ex. Passive Attack to Decrypt Traffic

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

15 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

16 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……

17 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

18 Wireless 會遇到的問題 Hidden Terminal Problem

19 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 )

20 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!!!

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

22 DSSS DSSS ( Direct Sequence Spread Spectrum )

23 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

24 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

25 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

26 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

27 Modulation Technique OFDM

28 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