IEEE 802.11 WLAN

1
IEEE 802.11 WLAN
2
IEEE 802.11 Protocol Stack
For centralized contention-free channel access
For distributed contention-based channel access
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Possible Network Topologies
BSS mode
ESS mode
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802.11 Channels, association

• 802.11b 2.4GHz-2.485GHz spectrum divided into 11
  channels at different frequencies
• AP admin chooses frequency for AP
• interference possible channel can be same as
  that chosen by neighboring AP!
• host must associate with an AP
• scans channels, listening for beacon frames
  containing APs name (SSID) and MAC address
• selects AP to associate with
• may perform authentication
• will typically run DHCP to get IP address in APs
  subnet

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Distributed Coordination Function (DCF)
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Binary Exponential Backoff

• Backoff Counter is randomly selected from
  0,CW,where CW is contention window
• For each unsuccessful frame transmission, CW
  doubles (from CWmin to CWmax)
• CW ? 2 (CW1)-1
• If successful transmission,
• CW ? CWmin
• Reduces the collision probability

7
Virtual Carrier Sensing
A
B
C
D
E
RTS
RTS
CTS
CTS
DATA
DATA
ACK
ACK

• Any node hearing RTS or CTS sets up their NAV
  (network allocation vector) until end of ACK.
• NAV set -gt node silent (act as if carrier busy).

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802.11 Timeline

• If carrier busy (physical or virtual), schedule
  transmission after a random backoff when carrier
  is free.
• Average backoff interval is doubled for each
  failed attempt.

DATA
RTS
Transmitter
SIFS
SIFS
DIFS
ACK
CTS
Receiver
SIFS
DIFS
Nodes that hear transmitter
NAV (RTS)
t
NAV (CTS)
Nodes that hear receiver
Another transfer
Random backoff
Defer access
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RTS/CTS Mechanism (Optional)

• RTC/CTS solves HTP
• But, non-negligible overhead
• If frame size gt RTSthreshhold,
• RTS-CTS-DATA-ACK
• Otherwise,
• DATA-ACK

802.11b 802.11b
tslot 20usec
SIFS 10usec
PIFS SIFS tslot
DIFS SIFS 2tslot
EIFS gt DIFS
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Collisions are not completely avoidedin IEEE
802.11 !!

• H does not sense any signal during Ds DATA tx
• H may transmit
• Collision in Es reception

11
Energy Conservation Powercontrol

• Power control has two potential benefit
• Reduced interference increased spatial reuse
• Energy saving
• If C reduces transmit power, it can still
  communicate with D
• Reduces energy consumption at node C
• Allows B to receive As transmission (spatial
  reuse)

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802.11 frame addressing
Address 4 used only in ad hoc mode
Address 1 MAC address of wireless host or AP to
receive this frame
Address 3 MAC address of router interface to
which AP is attached
Address 2 MAC address of wireless host or AP
transmitting this frame
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802.11 frame addressing
H1
R1
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802.11 frame more
frame seq (for reliable ARQ)
duration of reserved transmission time (RTS/CTS)
frame type (RTS, CTS, ACK, data)
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The Other IEEE 802.11 Efforts

• 802.11e
• Provides QoS support by differentiating traffic
  streams
• Applicable to 802.11 PHY a, b, and g
• 802.11h
• Supplementary to MAC layer so as to comply with
  European regulations for 5 GHz WLAN
• 802.11i
• Security enhancement
• 802.11n
• Enhancement for higher throughput (gt 100 Mbps )
• Decrease overhead within 802.11 protocol
• Packet preamble, CW, ACK, IFS parameters
• 802.11r
• Speed up handoff between APs (Fast
  BSS-Transition)
• Important for VoWLAN
• 802.11s
• Support mesh networks