802.11%20continued

1
802.11 continued
2
DCF

• When got a packet to send, sense the channel, if
  idle, send.
• When channel is busy, wait until the channel is
  free. Start to backoff for a random time. If busy
  before reaching zero, freeze bo counter, and
  reactivate when idle again. If channel becomes
  idle, send.
• After receives a packet, send ACK.
• If no ACK received, double the window.

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

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DCF

• Do you want the ACK to have the same priority as
  data packets?
• How do you make sure that ACK has higher
  priority?
• Use time. You have to wait for a certain amount
  time before you can send.
• High priority packets wait shorter.

5
DCF

• The SIFS, DIFS. SIFS is for control packets. DIFS
  is for data packets.
• When a station wants to send, if it is a control
  packet, sense the channel for SIFS, then send. If
  it is a data packet, sense the channel for DIFS,
  then send.

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DCF
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Further improvement

• Further improvement by improving carrier sense
• The problem is other people cannot hear me
  sending, so they will send.
• So, how to make sure that they will know I am
  sending?

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RTS/CTS

• RTS/CTS in the place for carrier sense
• RTS reserves channel for a bit of time, if
  sender hasnt heard other CTSes
• CTS sender replies if it hasnt heard any other
  RTSes
• Both messages include time. Network Allocation
  Vector (NAV)
• If no CTS, exponential backoff
• RTS-CTS-DATA

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RTS/CTS

• 802.11 standardized both CSMA/CA and RTS/CTS
• In practice, most operators disable RTS/CTS
• Very high overhead!
• RTS/CTS packets sent at base rate (often 1Mbit)
• Avoid collisions regardless of transmission rate
• Most deployments are celluar (base stations), not
  ad hoc. Neighboring cells are often configured
  to use non-overlapping channels, so hidden
  terminals on downlink are rare
• Hidden terminal on uplink possible, but if
  clients mostly d/l, then uplink packets are
  small.
• THIS MAY CHANGE. And is likely not true in your
  neighborhood!
• When CS range gtgt reception range, hidden terminal
  less important

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PCF

• The AP acts as the master and sends out beacon
  signals for polling stations and stations can
  sign up for certain amount of bandwidth use
• Co-exists with DCF.
• How to make sure that beacon signals have higher
  priority?
• PIFS