WinlabMadwifi → MultiMAC

Paper Title:

MultiMAC - An Adaptive MAC Framework for Dynamic Radio Networking

Christian Doerr, Michael Neufeld, Jeff Fifield, Troy Weingart, Douglas C. Sicker, and Dirk Grunwald
University of Colorado, Boulder

Motivation

• Bridging the gap between firmware based MAC and software radios
• Capability to use dynamic MAC plug-in based on scenario.

Primary Achievements

This has been done with Madwifi driver on Atheros chipset.

• Override 802.11 MPDU frame format
  • Necessary to send and receive custom frames
  • In each mode (Master, managed, monitor) card will accept certain types of frames
  • Monitor mode accepts all frames (even with CRC errors)
  • Approach: Use monitor mode to send and receive custom frames (transmit while monitor allowed by chipset)
  • Hardware automatically stamps packets with sequence numbers
  • Use retry = 1 bit to prevent this

• Eliminate automatic ACK and retransmission
  • Again use monitor mode
  • Set multicast bit in dest MAC address using previous packet injection mechanism
  • Need to handle timeouts at sender appropriately

• Eliminate virtual carrier sense (NAV)
  • By operating in monitor mode, NAV is disregarded by default

• Controlling Physical Carrier Sense
  • Tricky! No easy way to do this
  • Introduce desired bias using pre-calibration
    • Card periodically calibrated noise floor
    • Raise the noise floor and bias the calibration
    • Disable periodic calibration after that
    • Cards assume “higher/lower noise” based on calibrated level
  • May be used for immediate send (TDMA based MAC)
  • Does not necessarily mean improved throughput, since Xmission may still interfere (in CSMA based MAC)

• Controlling BAckoff Duration
  • Setting CWMin = CWMax in the driver

• Timer management
  • Atheros chipset has a 64 bit 1-μsec timer, can be read/set by driver
  • Tag packet with delay value before inserting in Tx queue
  • Reset timer to zero
  • Driver spin-waits until the NOW = basetime + delay
  • By fixing CW, the additional defer can also be adjusted for