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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