Wireless Sensor Networks: Time Synchronization Services

1
Wireless Sensor NetworksTime Synchronization
Services
AND
Jeremy Elson and Deborah Estrin Laboratory for
Embedded Collaborative Systems --
http//lecs.cs.ucla.edu
Goal To reconcile the many needs for
synchronized time in sensor networks...
Data aggregation, multisensor integration
Energy-efficient radio scheduling (TDMA)
Guard band due to clock skew sync error
means receiver cant exactly predict packet
arrival
which have diverse requirements on many axes,
even within an application...
with the different sync methods available,
minimizing wasted resources.
Our Approach
First Results Post-Facto Synchronization

• Use multiple modes - because no singlemethod can
  be optimal on all axes
• Extend existing sync methods (e.g. NTP)
• Develop new methods, and compositions of methods
• Characterize these methods, and the requirements
  of applications, along the axes described above
• Use tiered architectures
• Not a single hardware platform but a range of
  hardware
• Analogy memory hierarchy creates the illusion
  of a system large and cheap per bit as disk, but
  fast as cache
• Goal a palette of tunable time sync methods
• Ideally, the set can always provide service that
  is both necessary and sufficient, to minimize
  resource waste
• Dont spend energy to get better sync than app
  needs

• Basic idea
• A set of receivers is waiting for an event
• Locally timestamp an event when it happens
• Reconcile clocks in the set after the fact
• Avoids wasting energy on unneeded sync
• Also multi-modal train local clocks with NTP
• NTP is good at correcting frequency
• A local pulse is good at correcting phase
• How well does the multi-modal combination work?

Ref Elson and Estrin, Time Synchronization for
Wireless Sensor Networks, Proc. 2001 Intl
Parallel and Distributed Processing Symposium
(IPDPS), Workshop on Parallel and Distributed
Computing Issues in Wireless Networks, April
2001, San Francisco, CA, USA. (Also UCLA CS Tech
Report 200028)
Combination of NTP and the pulse has almost 10x
better error bound than either mode
alone. Instantly (i.e. with little energy)
creates highly precise timebase, albeit localized
and ephemeral
Time Sync Parameter Space (lifetime, scope, max
error, etc.)