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Wi Fi

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Transcript

  • 1. WiFi, Bluetooth & Layers
  • 2. Last week
    • Wifi, Bluetooth: wireless LANs
    • Medium Access Control
    • Basic example : Aloha
  • 3. Wifi, Bluetooth, Ethernet Couche 5 Couche 4 Couche 3 Couche 2: Lien Couche 1: Physique
    • Protocol layers 1 et 2
    • Transfer packets over a link
    • Standardization body: IEEE
    • Standards: 802.11, 803.2, 802.15…
  • 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.
    • 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 300 328
    IEEE 802.11 basic
  • 8. Frequencies
      • 52 MHz bandwidth around 2.4 GHz
      • 11 channels with partial overlaps
  • 9.
    • 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
    coding IEEE 802.11b
    • Spread spectrum
  • 10.
    • 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)
    IEEE 802.11b,a,g,n
  • 11. Carrier Sense Multiple Access
  • 12. Basic CSMA: listen before talk
    • node withdraws over signal detection
    forbidden zone emitter destination packet ack forbidden period DIFS
  • 13. Hidden nodes collisions avoidance
    • Node withdraws over hidden nodes detection
    emitter destination packet ack forbidden period RTS CTS
  • 14. Collision management
    • CSMA/CA Carrier Sense Multiple Access with Collision Avoidance
  • 15.
    • 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 C max -1 (8)
      • Mini-slots are not decremented during forbidden periods
      • C max double at each collision (lack of CTS or ACK)
      • Retry number limited to max_retry (7-16).
      • Slot<DIFS (Distributed Inter Frame Space)
    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.
    • Infrastructure mode
    AP terminal Distribution system BSS ESS AP: Access Point BSS: Basic Set Service ESS: Extended Set Service IBSS: Independent Basic Set Service Terminology IBSS
    • ad hoc mode
  • 17.
    • IEEE 802.11 packet
    • Packet emission
    preamble MAC header Data part (IP packet) Check sum packet ACK SISF Emitter node Intended Receiver node Formats (packets)
  • 18. Format (Preamble)
  • 19.
      • Four addresses in infrastructure mode
      • Only two in ad hoc mode
      • Control field contains length and mode
      • Sequence field for fragmentation
    Address 1 Address 2 Address 3 Address 4 control sequence Formats (MAC header)
  • 20.
    • 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!
    WEP security
    • 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.
    • 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.
    IEEE 802.11 IEEE 802.1x Authentification agent improved security
  • 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.
    • Wide area architecture : scatternet
      • Several tiled piconets
      • Frequency hopings differ
      • certains nodes switch status master-slave
    IEEE 802.15.1 esclaves
  • 24.
    • Limited emission power
      • Class 1: 100 mW
      • class 2: 2,5 mW
      • class 3: 1 mW (1/1000 GSM)
    • Minimal signal processing
      • P eriodic TDMA
      • Throughput 1 Mbps max
      • Few meters range.
    • Profiles
      • Standadized applications
    IEEE 802.15.1
  • 25.
    • 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.
    IEEE 802.15.1 From master From slave
  • 26. Bluetooh + WiFi
  • 27.
    • Format du paquet
      • Access Code (AC): synchro, pagination (slot #). Channel AC, Device AC, Inquiry AC.
      • Header: address, sequence number, flow control, acquittement
    Formats
  • 28. frequency hoping Periodic change of frequencies. Predetermined sequence fixed in standard. Goal: use uncongested frequencies.
  • 29.
    • 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.
    Connection
  • 30.
    • 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.
    Sécurity
    • Default of Bluetooth security
      • Keys are too short
      • link key and Kc are both function of device PIN (4 bits).
  • 31.
    • 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.
    Authentification
  • 32.
    • 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
    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))