MACAW

1
MACAW

• CS294-1
• Joe Polastre
• September 9, 2003

2
Main Issues

• Whats wrong with CSMA?
• Hidden Terminal Problem
• Whats wrong with MACA?
• Fairness
• Exponential backoff scheme in MACA causes unfair
  channel utilization

3
Assumptions

• Symmetric Links
• Large data transfers
• Near/short range radios
• Single hop networks (pads and bases)
• Ignore capture
• Unlikely in short range radio to have one node
  dominate another with a signal power ratio of
  10db
• Ignore interference
• If radio signal drops like r3, interference
  unlikely with near range low-power radios

4
Design of MACAW

• Backoff
• Sharing (Fairness)
• Multiplicative increase, additive decrease
  (Performance)
• Per-destination backoff (Performance)
• Region backoff copying is a problem
• If CTS fails, was there really a collision?
  Differentiate between RTS and CTS failures
• Multiple Streams
• Treat each stream as a separate queue(co-located
  streams)
• Message Exchange Changes
• ACK
• reliability
• DS
• hidden terminal problem
• RRTS
• notify transmitter you got RTS and channel is now
  clear

5
Whats now fixed by the design?

• Fairness

6
Unsolved
Woo Multihop Tree

• Performing RTS-CTS for multicast in a single (or
  few) rounds of contention
• Multihop tree transfer from children to base
  station

MACAW unsolved terminal problem
7
Results

• MACAW prevents starvation
• Additional control signals kill your performance
• Most prevalent in low density environments

MACA RTS-CTS-DATA-ACK 53.07
MACAW RTS-CTS-DS-DATA-ACK 49.07
8
Results

• What about high density with near range radios

9
How Do Sensor Nets Differ?

• Low-cost short range radios
• Much larger transmission range
• Short bursty traffic and infrequent large data
  transfers
• Multi-hop topologies
• Capture? Interference? Symmetric Links?

10
Symmetric Links??

• How reasonable is the assumption of symmetric
  links in sensor networks?
• Low cost, low power radios dont transmit
  uniformly (Alec Woo did a study of this)
• Delay Tolerant Networking (Fall et al)
• Unidirectional Links in Wireless/Sensor Nets (R.
  Prakash / V. Ramasubramanian)
• Unidirectional Links Prove Costly in Wireless
  Ad-Hoc Networks 1999.
• Thoughts?

11
Near Range Radio

• Wireless nets use short range low cost radios
• Range is much further than assumed in MACAW
  (larger radio range)
• More Interference
• More transmission power ? capture
• Can CSMA be effective for this scenario?

12
Overhead

• What happens in a bursty small data payload
  environment?
• RRTS/RTS/CTS/DS/ACK messages are (almost) as
  large as entire data packets!
• At least on the Mica2 platform
• Same issues with radios similar to the
  Chipcon(large class of radios!) due to
  synchronization
• Lots of state to keep track of

13

• Unclear how well MACAW would perform in this new
  environment
• Questions?