Wireless LANs

1
Wireless LANs
2
Wireless LAN

• Basic structure
• Stations plus an access point
• Stations talk to the access point, then to
  outside
• Access point talks to stations
• Stations talk to stations
• Problem
• If more than one station talk at the same time,
  the packets will be corrupted
• Design goal
• A MAC protocol to determine who talks next

3
Wireless communications

• Signal decays according to a power law with the
  distance, at least to the power of -2 with
  distance
• When a station is sending, not all stations can
  hear the medium is busy.

4
Wireless communications

• When a station is sending, he cannot hear other
  stations cannot decide if there is a collision.
  No collision detection in wireless LAN.

5
Wireless communications

• Being able to sense the carrier does not mean
  that you can decode the data
• If received signal having power P means that you
  can decode the data, it may be true that at power
  P/2 you can realize that there is something going
  on

6
Wireless communication

• The received signal can be decoded if the signal
  to noise ratio is larger than a certain
  threshold. Whether there is a collision depends
  on the signal to noise ratio at the receiver.
• You may allow two transmissions at the same time
  without collision.

A
D
C
B
A
D
C
B
A-gtB, D-gtC
A-gtB, C-gtD
7
Wireless communications

• Hidden terminal, A-gtB, C-gtD. C did not hear A.

A
D
C
B

• Exposed terminal. A-gtB, C-gtD. C hears A.

B
A
C
D
8
MAC

• What if we just let everyone talk when he wants
  to?
• There will be collisions.

9
MAC

• When medium is free, what if we say just let
  people who has been waiting to talk?
• More than one station could start at the same
  time.

10
MAC

• How to make sure that a packet is delivered?
• Use ACK.

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12
DCF

• Simplified 802.11 DCF operation for unicast.
  (Automating Cross-Layer Diagnosis of Enterprise
  Wireless Networks, Sigcomm 2007 )