Introduction to 802.11

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Introduction to 802.11
2
Wireless LANs 802.11

• Supports both Asynchronous data transfer and time
  bound services
• Asynchronous traffic insensitive to time -
  email, FTP
• Time bound services sensitive to time - voice
  traffic
• Different MAC strategies to support these classes
  of traffic
• Asynchronous traffic is supported through
  Distributed Co-ordination Function (DCF)
• Time bound traffic is supported through Point
  Co-ordination Function (PCF)
• DCF is mandatory, while PCF is optional

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802.11 Terminologies

• BSS Basic service set
• Group of stations under the direct control of a
  single co-ordination function
• All stations in a BSS can directly communicate
  with each other, without any infrastructure
• Geographical area covered by BSS is BS Area
• BSA is similar in concept to a cell in a
  cellular network

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802.11 Terminologies

• Access Point (AP)
• Similar to the base station in cellular network
• Supports range extension by providing network
  connectivity between multiple BSSs
• Multiple BSSs are connected together through a
  Distribution System (DS)
• DS is similar to a backbone network
• BSSs connected by a DS form an Extended Service
  Set (ESS)

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802.11 Terminologies
PORTAL Acts as the gateway to other networks
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(No Transcript)
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802.11 MAC Sub-layer

• Transmission medium can
• Operate only in contention mode, called as
  Contention Period - CP
• Alternate between contention contention free
  modes
• In contention free mode, the medium access is
  controlled by the AP
• Contention Free mode is also known as Contention
  Free Period (CFP)

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802.11 MAC Frames

• Management Frames Used for
• Station association, dissociation, timing and
  synchronization, authentication
• Control Frames Used for
• Handshaking during CP (RTS/CTS)
• ACK frames during CP
• Data Frames Used for
• Sending data during CP and CFP

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802.11 DCF

• Fundamental channel access method in 802.11
• Used by asynchronous data services
• Basis for PCF
• Based on CSMA/CA (Collision Avoidance)
• Collision detection is not used, because a
  station cannot listen to the (air) channel for
  collisions when transmitting

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802.11 DCF Carrier Sensing

• Two kinds of sensing
• Physical carrier sensing
• Takes place at the air interface
• Detects signal strength from other sources
• Virtual Carrier Sensing
• Takes place at the MAC layer (explained shortly)
• Channel is said to be idle only when both the
  sensing mechanisms report idle

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802.11 DCF Basic Operation

• Inter frame space (IFS) Time interval between
  transmission of frames
• Three IFS values are specified
• DCF-IFS (DIFS)
• PCF-IFS (PIFS)
• Short-IFS (SIFS)
• SIFS lt PIFS lt DIFS
• Priority access to the medium is controlled
  through these three IFS intervals

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802.11 DCF Basic Operation

1. A station wanting to transmit, senses the medium
2. If it is idle, waits for DIFS interval, and
   senses again
3. If the medium is free, frame is transmitted
4. If the receiver gets the frame correctly, it
   sends an ACK to the sender after a SIFS interval

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DCF Basic Operation

• If the medium is found to be busy either in step
  1 or 2, back-off procedure is invoked
• What is back-off?
• How is idle detected?
• Clear Channel Assessment
• If the received signal strength exceeds the CCA,
  the channel is declared to be busy
• Note that this does not require locking to the
  carrier, just signal strength
• NAV (Network Allocation Vector) or virtual
  channel sensing

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DCF Virtual Carrier Sensing

• The MAC header has the duration field, which
  indicates for how long the sender wants to hold
  the medium
• Other stations read this field, and adjust a
  parameter called Network Allocation Vector
  (NAV)
• NAV indicates the amount of time that must elapse
  until the channel can be sampled again for idle
  status
• NAV value thus indirectly indicates if the
  channel is free at any given time
• Using NAV value to determine channel status is
  called as Virtual Carrier Sensing

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DCF Data Transmission
16
DCF with RTS/CTS

• Sender cannot detect a collision during
  transmission
• If a large frame is sent, and if collision
  occurs, then lot of channel BW is wasted
• To prevent this waste, Request to Send (RTS) and
  Clear To Send (CTS) control frames are used
• RTS/CTS helps a station to seize the channel for
  a short time so no collisions

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DCF RTS/CTS

• A sender, after contending for the channel, sends
  a RTS frame
• RTS/CTS frames are small (20 bytes), so in case
  of collision, less BW is wasted
• All stations in the BSS, read the RTS frame and
  adjust their NAV accordingly
• After SIFS amount of time, the destination
  responds to the RTS with a CTS frame

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DCF RTS/CTS

• Stations modify their NAV on reading CTS
• This helps to combat the hidden terminal
  problem (what is this?)
• On receiving CTS, sender transmits the data frame
  after an interval of SIFS, which the receiver
  ACKs
• RTS/CTS prevents BW waste, but
• Creates additional delay under light loads
• Causes collisions
• Probability should not be used

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802.11 MAC Frame
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Frame sizes and contents
MAC frames are called MPDU (MAC protocol Data
Unit)

• Frame formats
• RTS (20B)
• Frame control (2B)
• Protocols version (2bits) (802.11 00, so if not
  00, then the following means something different)
• Type (2 bits)- management, control,data, reserved
• subtype (4bits), e.g., controlRTS,
  managementbeacon,
• To DS (DP) (1 bit)
• From DS (1bit)
• More frag (1b)
• This is a Retry (1b)
• Pwr mgt (1b) (1 means the STA will go to power
  save (i.e., periodic sleep) after the frame is
  received)
• More data (1b) if a STA is in power save mode,
  then this is used to signify that more data is
  waiting, so maybe the STA should wake up and get
  the data!)
• Protected frame (1b) (WEP, etc.)
• Order (1b)
• Duration / ID (2B) (but MSB is zero, so 15 bits
  for the duration) in microsec (rounded up) (ID is
  used in power save mode)
• RA receiver address (6B)
• TA transmitter address (6B)
• FCS CRC-32 (4B)
• CTS (14B)

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Physical Protocol Data Unit (802.11b)

• PLCP (phy layer convergence protocol phy with
  MAC frame inside)
• PLCP preamble (144 bits)
• SYNC (128b) to let the physical layer acquire
  sync (transmitter actually begins transmitting
  before this. This period is called ramp up where
  the transmitter goes from 0 zero to full/desired
  power. This typically takes 1us)
• SFD (16b) like a radio ID, IEEE 802.11 has
  SFD-F3A0hex
• PLCP header (48 bits)
• Signal (8b) specifies the modulation used for
  the MAC frame
• Service (8b) reserved
• Length (16b) in micro seconds (not MAC duration,
  So this seems to be ignored)
• CRC (16b)
• PLCP SYNC and header always take 192 us
• MPDU (MAC protocol data unit) or PSDU (PLCP
  service Data unit)

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Short PLCP (optional)

• Preamble 72 bits at 1 Mbps
• PLCP header 48 bits at 2Mbps
• The preamble and header takes 96 us
• MPDU / PSDU at 2, 5.5, Mbps
• Since the PLCP is sent at 2 Mbps, the channel
  must be able to support 2 or more Mbps
• In practice the short header does not work well
  unless the channel is very good.

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802.11 g

• PLCP preamble and header are similar to 802.11b
• PSDU
• Long Training sequence
• 1.6 us guard interval
• Long training symbol (3.2 us)
• Long training symbol (3.2us) (again)
• Total 8 us
• OFDM signal
• Guard interval 0.8 us (This is quite short)
• Signal 3.2 us
• Total 4 us
• Transmitted at 6 Mbps OFDM modulation which is
  the worst performing modulation it has the same
  SNR-BER relationship at 9 and 12 Mbps.
• Data
• 6 us of quiet time (time to decode and error
  correct) to SIFS is 16 us

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802.11 overhead

• Four 192b from PLCP
• At 1Mbps 192 microsec so 768 microsec
• 82B from RTS, CTS, ACK, data
• 656 mic sec at 1Mbps
• 328 micro sec at 2Mbps
• 131 at 5Mbps
• 60 at 11Mbps
• 12 at 54Mbps
• 1 DIFS, 3 SIFS, 4 propagation delays
• 30 microsec 320 micro 42 microsec 98
  microsec
• Total overhead time
• 1552 mic sec at 1Mbps
• 1194 micro sec at 2Mbps
• 997 at 5Mbps
• 925 at 11Mbps
• 878 at 54Mbps
• Data duration
• 40B packet
• 320 mic at 1Mbps gt efficiency data duration over
  all duration 17

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802.11 overhead w/o RTS/CTS

• two 192b from PLCP
• At 1Mbps 192 microsec so 384 microsec
• 14B from RTS, CTS, ACK, data
• 112 mic sec at 1Mbps
• 56 micro sec at 2Mbps
• 22 at 5Mbps
• 10 at 11Mbps
• 2 at 54Mbps
• 1 DIFS, 1 SIFS, 1 propagation delays
• 30 microsec 20 micro 2 microsec 52 microsec
• Total overhead time
• 548 mic sec at 1Mbps
• 492 micro sec at 2Mbps
• 458 at 5Mbps
• 446 at 11Mbps
• 438 at 54Mbps
• Data duration
• 40B packet
• 320 mic at 1Mbps gt efficiency data duration over
  all duration 36

If the packet is smaller than RTSThreshold, then
RTS/CTS are not used
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DCF Fragmentation

• Large data units passed on by higher layers may
  be fragmented
• Fragmentation increases transmission reliability
• If data units are greater than a threshold,
  fragmentation is invoked

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802.11 DCF Basic Operation

• A station wanting to transmit, senses the medium
• If it is idle, waits for DIFS interval, and
  senses again
• If the medium is free, frame is transmitted
• If the receiver gets the frame correctly, it
  sends an ACK to the sender after a SIFS interval
• If the medium is found to be busy either in step
  1 or 2, back-off procedure is invoked

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802.11 DCF Back-Off

• If the medium is found to be busy either in step
  1 or 2, back-off procedure is invoked
• When invoked, station waits until the medium is
  free for a DIFS time, and starts a random timer
• Timer is decremented as long as the medium is
  free
• If the medium becomes busy, the timer is frozen
• Timer is decremented, when medium becomes free
  again for a DIFS time
• When timer reaches zero, the frame is transmitted
• Collisions are detected through a lack of ACK
• If collision occurs, the doubles the size of the
  backoff window and repeats the back-off process

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Back-off (w/o RTS/CTS)
start MAC receives a pkt
CW CWmin (e.g., 7) ShortTryCounter 0
Rand(0,n) Random integer between 0 and N
Channel is busy
Set BOtimer slotTimeRand(0,CW-1)
Wait for idle channel
If channel was not idle but no packet was
decoded, set DIFStimer EDIFS
Channel is idle
Set DIFStimer DIFS
Channel is busy
Wait DIFS
Channel is busy
CW min(1024, CW2)
DIFSTimer 0
backoff
TryCounter lt MaxNumTries
BOTimer 0
transmit
ShortTryCounter
Wait for ACK
No ACK
ACK
done
ShortTryCounter MaxNumTries
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Max number of transmissions

• If an RTS fails to generate a CTS, then the Short
  Retry count is incremented, until is reaches the
  short_retry_limit
• If RTS is received, the ShortRetryCount 0
• If the pkt size is below RTSThreshold (so no RTS
  is transmitted), and fails to generate an ACK,
  then the Short retry count is incremented
• When small data pkt is successful, the short
  retry count is reset to zero
• If pkt size is above RTSThreshold and fails to
  result in an ACK, then the long retry is
  incremented.
• If the pkt is successfully transmitted, then long
  retry count is reset to 0
• If a pkt is dropped due to too many failed
  attempts, the retry counters are set to zero

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Using ACKs to increase data rate

• Envelop analysis
• Suppose that the pkt error prob for 10 Mbps is 0
  and the pkt error prob for 20 Mbps is 0.25.
• The effective data rate at 10 Mbps is 10 Mbps,
  and at 20, is 0.7520 gt 10, so 20 is better, even
  though there will be many losses and
  retransmissions.
• When a transmission fails, it must wait DIFS and
  then back off
• Suppose an empty channel
• DIFS, Data, SIFS, ACK
• If Fails, then wait, 0,7SlotTime and repeat
  the above
• If Fails again, then wait, 0,13SlotTime and
  repeat the above
• What is the effective data rate when 10Mbps is
  used vs 20Mbps?