STEAM

1
STEAM
Umesh Shankar, Naveen Sastry, David Wagner
NEST Retreat UC Berkeley
Algorithms
Goals

• Parent Switching
• Senders switch parents if their messages are not
  getting through for whatever reason. We dont
  distinguish between connectivity problems,
  interference, or schedule conflicts.
• Nodes use EWMA estimator to decide when to
  switch.
• Channel Switching
• Parents need to switch channels if they are
  overhearing messages that are not destined for
  them.
• But there is a cost to switching children must
  detect that the node has switched channels and
  fail over.
• Compute conflict fraction f, which measures how
  often a node hears messages for another node.
• Compute expected conflict fraction e, which
  estimates what the conflict fraction should be,
  on average.
• Compute cost function g(c), where c of
  children
• Switch channels with probability p (f e) /
  g(c)

• State Exchange
• Want to communicate channel information to
  neighbors using minimal number of sends/receives
  of announcements.
• We only need each node to have one or two backup
  parents (and to have that information be
  relatively recent).
• Idea have each node listen or send with
  exponentially decreasing probability unless new
  information is needed or heard.
• In the steady state, almost no energy used, but
  the probability of hearing useful information is
  still high.
• Variable Listen Rate
• Idea If incoming message rate is low, do not
  need to listen every period.
• Keep estimate of message rate using EWMA
• EWMA of samples 1 if received a message, 0 if
  not
• Keep upper and lower thresholds if estimate goes
  above upper threshold, increase listening
  frequency, and if esimate goes below lower
  threshold, decrease frequency.
• Announce changes, but if decreasing, observe old
  frequency during transition.

• Energy efficient tree-based communication
  scheduling over time and radio frequencies
• Handle dynamic changes
• Node joining or leaving
• Changes in radio connectivity
• Changes in application traffic patterns

Overview

• Largest time unit period
• Receivers choose channels (frequency, time
  offset) on which they listen each period for
  messages.
• Receivers announce their choices on a special
  channel no reservations with senders needed.
• Senders choose some parent and send on
  appropriate channel.
• System converges to a good state senders switch
  parents using EWMA estimator and receivers switch
  channels according to a cost-benefit model.
• Announcement traffic goes to near-zero in the
  steady-state.

Simulation Results
Channel Map (time, frequency)
Current Status
Node 2 dies, Node 100 appears

• Analytic results of performance
• Early stages of implementation

Nodes 3,4 have heard about 100, but dont yet
know that Node 2 has died
Nodes 3,4,5,6 are all sending to Node 100