Wi Fi

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Wi Fi

  1. 1. WiFi, Bluetooth & Layers
  2. 2. Last week <ul><li>Wifi, Bluetooth: wireless LANs </li></ul><ul><li>Medium Access Control </li></ul><ul><li>Basic example : Aloha </li></ul>
  3. 3. Wifi, Bluetooth, Ethernet Couche 5 Couche 4 Couche 3 Couche 2: Lien Couche 1: Physique <ul><li>Protocol layers 1 et 2 </li></ul><ul><li>Transfer packets over a link </li></ul><ul><li>Standardization body: IEEE </li></ul><ul><li>Standards: 802.11, 803.2, 802.15… </li></ul>
  4. 4. = la norme IEEE 802.11 <ul><li>Norme = règles, techniques, formats communs à respecter </li></ul><ul><li>Protocole = norme de communication entre machines </li></ul><ul><li>IEEE = Institute of Electrical and Electronics Engineering </li></ul><ul><li>IEEE 803.2 </li></ul><ul><li>IEEE 1394 </li></ul><ul><li>IEEE 802.15.1 </li></ul>
  5. 5. IEEE 802.11 standard <ul><li>Communication between terminals and access point </li></ul><ul><li>Direct communication between terminals </li></ul>
  6. 6. Infrastructure mode in urban situation
  7. 7. <ul><li>Emission power 100 mW (1/10 of GSM) </li></ul><ul><li>Bursty packet emissions 2-5-10-..54 Mbits/s </li></ul><ul><li>Range: 100 m outdoor several ten meters </li></ul><ul><li>European ETS 300 328 </li></ul>IEEE 802.11 basic
  8. 8. Frequencies <ul><ul><li>52 MHz bandwidth around 2.4 GHz </li></ul></ul><ul><ul><li>11 channels with partial overlaps </li></ul></ul>
  9. 9. <ul><li>Spread of 11MHz (11 bits Barker sequence) 1 Msymbols/s </li></ul><ul><ul><li>1 Mbps : modulation PSK 1, 1 bit/ symbol (DSSS IEEE 802.11) </li></ul></ul><ul><ul><li>2 Mbps : mod QPSK, 2 bits/ symbol </li></ul></ul><ul><li>Spread of 11 MHz (séquence 8 bits CCK), 1,375 Msymbol/s </li></ul><ul><ul><li>5,5 Mbps : 4 bits/symbole </li></ul></ul><ul><ul><li>11 Mbps : 8 bits/symbole </li></ul></ul>coding IEEE 802.11b <ul><li>Spread spectrum </li></ul>
  10. 10. <ul><li>IEEE 802.11b (1-2-5,5-11 Mbps) </li></ul><ul><ul><li>Bandwidth 2,4 GHz </li></ul></ul><ul><ul><li>Modulation Direct Sequence Spread Spectrum (DSSS) </li></ul></ul><ul><ul><li>No Forward Error Control (FEC) </li></ul></ul><ul><li>IEEE 802.11a (6-54 Mbps) </li></ul><ul><ul><li>Bande 5,2 GHz </li></ul></ul><ul><ul><li>Mod. Orthogonal Frequency Division Multiplexing (OFDM) </li></ul></ul><ul><ul><li>FEC rate ½, 2/3, 3/4 (convolutive code) </li></ul></ul><ul><li>IEEE 802.11g (ERP-OFDM), IEEE 802.11n (MIMO) </li></ul>IEEE 802.11b,a,g,n
  11. 11. Carrier Sense Multiple Access
  12. 12. Basic CSMA: listen before talk <ul><li>node withdraws over signal detection </li></ul>forbidden zone emitter destination packet ack forbidden period DIFS
  13. 13. Hidden nodes collisions avoidance <ul><li>Node withdraws over hidden nodes detection </li></ul>emitter destination packet ack forbidden period RTS CTS
  14. 14. Collision management <ul><li>CSMA/CA Carrier Sense Multiple Access with Collision Avoidance </li></ul>
  15. 15. <ul><li>Random backoff of transmission over forbidden periods </li></ul><ul><ul><li>Evite les collisions répétées </li></ul></ul><ul><ul><li>The node selects a random backoff: a number of mini-slots between 0 and C max -1 (8) </li></ul></ul><ul><ul><li>Mini-slots are not decremented during forbidden periods </li></ul></ul><ul><ul><li>C max double at each collision (lack of CTS or ACK) </li></ul></ul><ul><ul><li>Retry number limited to max_retry (7-16). </li></ul></ul><ul><ul><li>Slot<DIFS (Distributed Inter Frame Space) </li></ul></ul>Forbidden period slot slot slot Forbid. period Example: time for a backoff of 3 slots Retransmissions packet ack Forbidden Period RTS CTS packet ack forbidden period DIFS
  16. 16. <ul><li>Infrastructure mode </li></ul>AP terminal Distribution system BSS ESS AP: Access Point BSS: Basic Set Service ESS: Extended Set Service IBSS: Independent Basic Set Service Terminology IBSS <ul><li>ad hoc mode </li></ul>
  17. 17. <ul><li>IEEE 802.11 packet </li></ul><ul><li>Packet emission </li></ul>preamble MAC header Data part (IP packet) Check sum packet ACK SISF Emitter node Intended Receiver node Formats (packets)
  18. 18. Format (Preamble)
  19. 19. <ul><ul><li>Four addresses in infrastructure mode </li></ul></ul><ul><ul><li>Only two in ad hoc mode </li></ul></ul><ul><ul><li>Control field contains length and mode </li></ul></ul><ul><ul><li>Sequence field for fragmentation </li></ul></ul>Address 1 Address 2 Address 3 Address 4 control sequence Formats (MAC header)
  20. 20. <ul><li>Authentification and encryption (secret key K, symmetric) </li></ul><ul><ul><li>The terminal requires the access point authentification </li></ul></ul><ul><ul><li>The access point sends a challenge of 128 random bits </li></ul></ul><ul><ul><li>The terminal returns the 128 bits xored by K </li></ul></ul><ul><ul><li>The access point confirms authentification </li></ul></ul><ul><ul><li>Default: James Bond overhear the key K via direct comparison between challenge and terminal reply! </li></ul></ul>WEP security <ul><li>Packet encryption (algorithm RC4) </li></ul><ul><ul><li>pseudo random sequence seed=K*IV (Initialisation Vector in packet header) </li></ul></ul><ul><ul><li>Integrity check via an internal check sum </li></ul></ul><ul><ul><li>RC4 is linear (RC4(x  y)=RC4(x)  RC4(y))! </li></ul></ul><ul><li>WEP is very weak and only address unvolontary earsdropping. </li></ul>
  21. 21. <ul><li>WEP improvement with IEEE 802.11i </li></ul><ul><ul><li>Introduction of IEEE 802.1x to manage the secret keys K (Extensible Authentification Protocol- Transport Layer Security, EAP-TLS). </li></ul></ul><ul><ul><li>Authentification made indépendant of encryption </li></ul></ul><ul><ul><li>Introduction of more sophisticated function : (K,IV)  RC4 seed. </li></ul></ul>IEEE 802.11 IEEE 802.1x Authentification agent improved security
  22. 22. = IEEE 802.15.1 <ul><li>Communication between personnal devices </li></ul><ul><li>Architecture piconet master slave: </li></ul><ul><ul><li>7 slaves max per piconet </li></ul></ul><ul><ul><li>Exclusive links slave-master </li></ul></ul><ul><ul><li>Slotted time </li></ul></ul>master slaves piconet
  23. 23. <ul><li>Wide area architecture : scatternet </li></ul><ul><ul><li>Several tiled piconets </li></ul></ul><ul><ul><li>Frequency hopings differ </li></ul></ul><ul><ul><li>certains nodes switch status master-slave </li></ul></ul>IEEE 802.15.1 esclaves
  24. 24. <ul><li>Limited emission power </li></ul><ul><ul><li>Class 1: 100 mW </li></ul></ul><ul><ul><li>class 2: 2,5 mW </li></ul></ul><ul><ul><li>class 3: 1 mW (1/1000 GSM) </li></ul></ul><ul><li>Minimal signal processing </li></ul><ul><ul><li>P eriodic TDMA </li></ul></ul><ul><ul><li>Throughput 1 Mbps max </li></ul></ul><ul><ul><li>Few meters range. </li></ul></ul><ul><li>Profiles </li></ul><ul><ul><li>Standadized applications </li></ul></ul>IEEE 802.15.1
  25. 25. <ul><li>Slotted system managed by the master node over a single frequence </li></ul><ul><li>Adaptative FEC, rate: 1 (no correction), 2/3, 1/3 </li></ul><ul><li>Frequency hopping (1600/sec) </li></ul><ul><ul><li>One hop per slot over 79 channels (2,4 GHz) </li></ul></ul><ul><ul><li>Throughput 1 Mbps, extensions for10 Mbps. </li></ul></ul>IEEE 802.15.1 From master From slave
  26. 26. Bluetooh + WiFi
  27. 27. <ul><li>Format du paquet </li></ul><ul><ul><li>Access Code (AC): synchro, pagination (slot #). Channel AC, Device AC, Inquiry AC. </li></ul></ul><ul><ul><li>Header: address, sequence number, flow control, acquittement </li></ul></ul>Formats
  28. 28. frequency hoping Periodic change of frequencies. Predetermined sequence fixed in standard. Goal: use uncongested frequencies.
  29. 29. <ul><li>Connection establishment </li></ul><ul><ul><li>Inquiry for destination terminal identification (source, destination) </li></ul></ul><ul><ul><li>paging for synchronization of emissions (source, master, destination) </li></ul></ul><ul><ul><li>polling , the master prompts each slave emission. </li></ul></ul><ul><ul><li>Out of connection, the slave can be in wake mode or in sleed mode, otherwise it looses its MAC address. </li></ul></ul>Connection
  30. 30. <ul><li>Authentification (E1 algorithm) </li></ul><ul><ul><li>Secret shared key (link key) (128 bits) </li></ul></ul><ul><li>Encryption (algorithms KG, E0) </li></ul><ul><ul><li>Secret key Kc (deduced from link key par KG) from 8 to 128 bits (negociated) </li></ul></ul><ul><ul><li>Use of slot number in E0 (indicated in paging) </li></ul></ul><ul><ul><li>E1 and E0 differ. </li></ul></ul>Sécurity <ul><li>Default of Bluetooth security </li></ul><ul><ul><li>Keys are too short </li></ul></ul><ul><ul><li>link key and Kc are both function of device PIN (4 bits). </li></ul></ul>
  31. 31. <ul><li>Authentification of B byA </li></ul><ul><ul><li>B sends its address (48 bits) </li></ul></ul><ul><ul><li>A returns rand(A) to B (challenge 128 bits) </li></ul></ul><ul><ul><li>E1(addr B, link key, rand(A))=(SRES,ACO) (32 bits, 96 bits) </li></ul></ul><ul><ul><li>B returns SRES. </li></ul></ul>Authentification
  32. 32. <ul><li>encryption </li></ul><ul><ul><li>Kc depends on link key, ACO and EN_RAND </li></ul></ul><ul><ul><li>The pseudo random word Kstr depends on slot number and the addess of the master </li></ul></ul><ul><ul><li>In packet crypted code=data  Kstr </li></ul></ul>Encryption <ul><li>mode 2 </li></ul><ul><ul><li>Packets are encrypted via individual keys Kc(B)=KG(…,ACO(B)) </li></ul></ul><ul><ul><li>Broadcast packets are not encrypted </li></ul></ul><ul><li>mode 3 </li></ul><ul><ul><li>All packets are encrypted via the key of the master Kc=KG(…,ACO(A)) </li></ul></ul>

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