Friday, November 23, 2012

Notes - Wireless LANs

The following are notes from Computer Networks written by Tanenbaum 5th edition.
  • wireless LAN standard 802.11
The 802.11 Architecture and Protocol Stack
  • connect clients via AP(Access Points)
  • several access points can be connected together in a distribution system
  • ad hoc network also possible, computers can directly send frames to each other
  • structure is as follows
  • transmission techniques of frequency hopping and infrared are now defunct
  • spread spectrum is now known as 802.11b
  • OFDM is 802.11a
  • multiple antenna techniques now caled 802.11n
The 802.11 Physical Layer
  • transmission techniques make it possible to send a MAC frame over the air from one station to another
  • short range radios in 2.4GHz or 5 GHz ISM frequency bands
  • rate adaptation
    • if signal is weak low rate can be used, if clear highest rate can be used
  • 802.11b
    • spread spectrum similar to CDMA but there is only one spreading code
    • satisfy FCC requirement
    • Barker sequence
      • autocorrelation is low
      • send at rate of 1 Mbps Barker sequence used with BPSK modulation to send 1 bit per 11 chips, 11 Mchips /second
      • send at 2Mbps used with QPSK modulation to send 2 bits per 11 chips
      • CCK(Complementary Code keying)
  • 802.11a
    • up to 54Mbps in 5 GHz band
    • uses OFDM(Orthogonal Frequency Division Multiplexing)
    • sent over 52 subcarriers
  • 802.11g
    • copies 802.11a operates in 2.4GHz ISM band
  • 802.11n
    • throughput of 100Mbps
    • doubled channels to 40 MHz
    • signal streams uses 4 antennas to transmit 4 streams at the same time
    • separated using MIMO(Multiple Input Multiple Output) communication techniques
The 802.11 MAC Sublayer Protocol
  • radios are half duplex
  • avoids collision using CSMA/CA (CSMA with collision avoidance)
  • station using channel sensing, exponential backoff 0-15 in the case of OFDM physical layer
  • DCF(Distributed Coordination Function)
    • starting backoffs early
    • acks are used to infer collisions
    • no central control
  • PCF(Point Coordination Function)
    • AP controls activity in cell like base station
    • in practice not used
  • transmission ranges of different stations may be different, exposed/hidden terminal problem
  • NAV(Network Allocation Vector)
    • each station keep record of when channel is in use
    • how long a frame will take to complete
  • Channel sensing
  • RTS is right to send CTS is clear to send
    • has issues, does not help short frames 
    • can slow down operation
  • Wireless networks are noisy and unreliable
    • strategies
    • lower the transmission rates use more robust modulation
    • if too many frames lost can lower the rate again
    • probability of receiving an n bit frame entirely correctly is (1-p)n
    • very difficult to receive full frame for long frame, ethernet frame has less than 30% success
  • fragmentation to split packets into smaller sizes to reduce error
  • power saving beacon frames
    • frames advertises the presence of AP every 100 msec
    • clients set a power management bit to tell it that they are entering power save mode
    • will buffer traffic to power save mode nodes
  • APSD (Automatic Power Save Delivery)
    • sends buffer frames to clients just after client sends frames to AP
    • works well for VoIP wireless phones
  • QoS suffers with wireless to prevent this we have to make VoIP services higher priority
  • intervals after an ack in CSMA/CA
    • SIFS(Short InterFrame Spacing)
      • control frame or next fragment sent here
    • AIFS(Arbitration InterFrame Spacing)
      • there is a high priority and low priority frame here
    • DIFS(DCF InterFrame Spacing)
      • regular frame
    • EIFS(Extended InterFrame Spacing)
      • bad frame recovery done
  • TXOP or transmission opportunity
    • original CSMA/CA mechanisms let stations send one at a time
    • slows down fast senders to speed of slow senders
    • rate anomaly
The 802.11 Frame Structure
  • 802.11 standard defines 3 classes of frames
    • data
    • control
    • management
    • variety of fields used within the MAC sublayer
  • Frame control made up of 11 subfields
    • version set to 00 though allows future versions to be created
    • type data control or management
    • subtype RTS or CTS
    • To DS From DS is whether the frame is incoming or going to APs
    • More fragments mean more fragments will follow
    • Retry is retransmission
    • Power management means to go into power save mode
    • More data means more frames incoming
    • Protected frame means encryption has occurred
    • Order tells that the frames will come in order
  • Duration how long frame and ack will occupy channel
  • Addresses to source and destination, 3rd can be a final destination and second is a relay point
  • sequence numbers frames so duplicates can be detected
  • data stores data, and check sequence is a 32 bit CRC
Services
  • Association service used by mobile stations to connect themselves to APs
    • Reassociation is to change preferred AP
    • disassociation to break AP
  • Authenticate to choose security
    • recommend scheme WPA2(WiFi Protected Access 2)
      • outdated scheme is WEP(Wired Equivalent Privacy)
      • based on AES(Advanced Encryption Standard)
  • distribution service determines routing
  • integration service handles translation for frame to be sent outside a LAN
  • data delivery service
  • QOS traffic scheduling
    • traffic with different priorities
  • Transmit power control service to meat power limitations, dynamic frequency selection to avoid transmitting on reserved frequencies

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