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WiFi, Bluetooth & Layers

WiFi, Bluetooth & Layers

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WiFi, Bluetooth & Layers

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  1. WiFi, Bluetooth & Layers Emmanuel Baccelli

  2. Last week • Medium Access Control • Basic example : Aloha • Wifi, Bluetooth: wireless LANs

  3. Wifi, Bluetooth, Ethernet • Protocol layers 1 et 2 • Transfer packets over a link • Standardization body: IEEE • Standards: 802.11, 803.2, 802.15… Couche 5 Couche 4 Couche 3 Couche 2: Lien Couche 1: Physique

  4. = la norme IEEE 802.11 • Norme = règles, techniques, formats communs à respecter • Protocole = norme de communication entre machines • IEEE = Institute of Electrical and Electronics Engineering • IEEE 803.2 • IEEE 1394 • IEEE 802.15.1

  5. IEEE 802.11 standard • Communication between terminals and access point • Direct communication between terminals

  6. Infrastructure mode in urban situation

  7. IEEE 802.11 basic • Emission power 100 mW (1/10 of GSM) • Bursty packet emissions 2-5-10-..54 Mbits/s • Range: 100 m outdoor several ten meters • European ETS 300328

  8. Frequencies • 52 MHz bandwidth around 2.4 GHz • 11 channels with partial overlaps

  9. coding IEEE 802.11b • Spread spectrum • Spread of 11MHz (11 bits Barker sequence) 1 Msymbols/s • 1 Mbps: modulation PSK 1, 1 bit/ symbol (DSSS IEEE 802.11) • 2 Mbps: mod QPSK, 2 bits/ symbol • Spread of 11 MHz (séquence 8 bits CCK), 1,375 Msymbol/s • 5,5 Mbps: 4 bits/symbole • 11 Mbps: 8 bits/symbole

  10. IEEE 802.11b,a,g,n • IEEE 802.11b (1-2-5,5-11 Mbps) • Bandwidth 2,4 GHz • Modulation Direct Sequence Spread Spectrum (DSSS) • No Forward Error Control (FEC) • IEEE 802.11a (6-54 Mbps) • Bande 5,2 GHz • Mod. Orthogonal Frequency Division Multiplexing (OFDM) • FEC rate ½, 2/3, 3/4 (convolutive code) • IEEE 802.11g (ERP-OFDM), IEEE 802.11n (MIMO)

  11. Carrier SenseMultiple Access

  12. Basic CSMA:listen before talk • node withdraws over signal detection DIFS packet emitter ack destination forbidden period forbidden zone

  13. Hidden nodes collisionsavoidance • Node withdraws over hidden nodes detection RTS packet emitter CTS ack forbidden period destination

  14. Collision management • CSMA/CA Carrier Sense Multiple Access with Collision Avoidance

  15. Retransmissions packet RTS packet DIFS • Random backoff of transmission over forbidden periods • Evite les collisions répétées • The node selects a random backoff: a number of mini-slots between 0 and Cmax-1 (8) • Mini-slots are not decremented during forbidden periods • Cmax double at each collision (lack of CTS or ACK) • Retry number limited to max_retry (7-16). • Slot<DIFS (Distributed Inter Frame Space) ack CTS ack forbidden period Forbidden Period Forbidden period slot slot Forbid. period slot Example: time for a backoff of 3 slots

  16. Terminology Distribution system IBSS • Infrastructure mode ESS AP terminal • ad hoc mode BSS AP: Access Point BSS: Basic Set Service ESS: Extended Set Service IBSS: Independent Basic Set Service

  17. Formats (packets) • IEEE 802.11 packet • Packet emission preamble MAC header Data part (IP packet) Check sum Emitter node packet SISF Intended Receiver node ACK

  18. Format (Preamble)

  19. Formats (MAC header) • Four addresses in infrastructure mode • Only two in ad hoc mode • Control field contains length and mode • Sequence field for fragmentation control Address 1 Address 2 Address 3 sequence Address 4

  20. WEP security • Authentification and encryption (secret key K, symmetric) • The terminal requires the access point authentification • The access point sends a challenge of 128 random bits • The terminal returns the 128 bits xored by K • The access point confirms authentification • Default: James Bond overhear the key K via direct comparison between challenge and terminal reply! • Packet encryption (algorithm RC4) • pseudo random sequence seed=K*IV (Initialisation Vector in packet header) • Integrity check via an internal check sum • RC4 is linear (RC4(xy)=RC4(x)RC4(y))! • WEP is very weak and only address unvolontary earsdropping.

  21. improved security • WEP improvement with IEEE 802.11i • Introduction of IEEE 802.1x to manage the secret keys K (Extensible Authentification Protocol- Transport Layer Security, EAP-TLS). • Authentification made indépendant of encryption • Introduction of more sophisticated function : (K,IV)RC4 seed. Authentification agent IEEE 802.1x IEEE 802.11

  22. = IEEE 802.15.1 • Communication between personnal devices • Architecture piconet master slave: • 7 slaves max per piconet • Exclusive links slave-master • Slotted time master slaves piconet

  23. IEEE 802.15.1 • Wide area architecture : scatternet • Several tiled piconets • Frequency hopings differ • certains nodes switch status master-slave esclaves

  24. IEEE 802.15.1 • Limited emission power • Class 1: 100 mW • class 2: 2,5 mW • class 3: 1 mW (1/1000 GSM) • Minimal signal processing • Periodic TDMA • Throughput 1 Mbps max • Few meters range. • Profiles • Standadized applications

  25. IEEE 802.15.1 • Slotted system managed by the master node over a single frequence • Adaptative FEC, rate: 1 (no correction), 2/3, 1/3 • Frequency hopping (1600/sec) • One hop per slot over 79 channels (2,4 GHz) • Throughput 1 Mbps, extensions for10 Mbps. From master From slave

  26. Bluetooh + WiFi

  27. Formats • Format du paquet • Access Code (AC): synchro, pagination (slot #). Channel AC, Device AC, Inquiry AC. • Header: address, sequence number, flow control, acquittement

  28. frequency hoping Periodic change of frequencies. Predetermined sequence fixed in standard. Goal: use uncongested frequencies.

  29. Connection • Connection establishment • Inquiry for destination terminal identification (source, destination) • paging for synchronization of emissions (source, master, destination) • polling, the master prompts each slave emission. • Out of connection, the slave can be in wake mode or in sleed mode, otherwise it looses its MAC address.

  30. Sécurity • Authentification (E1 algorithm) • Secret shared key (link key) (128 bits) • Encryption (algorithms KG, E0) • Secret key Kc (deduced from link key par KG) from 8 to 128 bits (negociated) • Use of slot number in E0 (indicated in paging) • E1 and E0 differ. • Default of Bluetooth security • Keys are too short • link key and Kc are both function of device PIN (4 bits).

  31. Authentification • Authentification of B byA • B sends its address (48 bits) • A returns rand(A) to B (challenge 128 bits) • E1(addr B, link key, rand(A))=(SRES,ACO) (32 bits, 96 bits) • B returns SRES.

  32. Encryption • mode 2 • Packets are encrypted via individual keys Kc(B)=KG(…,ACO(B)) • Broadcast packets are not encrypted • mode 3 • All packets are encrypted via the key of the master Kc=KG(…,ACO(A)) • encryption • Kc depends on link key, ACO and EN_RAND • The pseudo random word Kstr depends on slot number and the addess of the master • In packet crypted code=dataKstr