Low-Power Networking

1
Low-Power Networking

• Tzi-cker Chiueh

2
Goals

• Cutting down the power consumption of wireless
  network interfaces
• Slow down the clock
• Go to sleep if there is nothing to do
• First question is this really important?
• Power-aware routing Maximize the lifetime of an
  (ad hoc) network

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Provisions in 802.11

• Power Saving Mode
• Client notifies AP when it enters PSM (with a
  ListenInterval specification)
• Clients wake up periodically to listen to beacon
• Beacon contains traffic indication map to tell
  which node has packets buffered on AP
• Client explicitly asks AP to forward buffered
  packets

4
Problems with PSM

• TCP unfriendly increase in round-trip delay
  (rounded up to bean period, 100 msec) causes
  throughput to drop
• Periodic wake-up costs power for nothing because
  idle period is longer than 100msec
• Optimizations
• Stay-Awake (0.1sec)
• ListenInterval-Backoff (2x up to 0.9 sec)
• Max-Delay never sleeps more than 20 of the last
  activity interval

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Better Optimizations

• TCP-directed sleep interval determination
• Based on round-trip delay measurements of active
  connections
• Based on the fact that there are no pending
  packets
• Apply to only wireless stations that serve as
  pure Internet clients with NRT traffic

6
More Optimization Ideas

• Apply the idea of I-TCP for client-side power
  optimization
• Explicitly tell each MN when to wake up
• Proxy estimate the round-trip delay of Web page
  response time, including caching effects
• Buffer downstream data and avoid trigger
  congestion control at the remote sender
• Coordinate MNs wakeup time and channel access
• Support prefetching to compensate batching delay
  at the client for upstream traffic

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Streaming Downstream Traffic

• Relatively easy to deal with buffer and batching
• Should work in a way that is independent of media
  streams
• Transcoding if necessary

8
Synergy between QoS and LPN

• QoS is about transmission slot allocation
• Low-power networking is about when interface
  should be up to receive packets
• How to integrate the two?
• Tradeoff in latency increase and power reduction
• What can the infrastructure do more to reduce the
  power consumption at the clients
• High-gain antenna
• Prefetching
• Computation