Title: Wireless Sensor Networks: Random, Ephemeral Identifiers
1Wireless Sensor NetworksRandom, Ephemeral
Identifiers
AND
Jeremy Elson and Deborah Estrin Laboratory for
Embedded Collaborative Systems --
http//lecs.cs.ucla.edu
WSNs create new opportunities...
...while presenting new challenges.
- New technologies have reduced the cost, size, and
power ofmicro-sensors, micro-actuators (MEMS),
and wireless interfaces. - Systems can
- Sense phenomena at close range
- Be embedded in the environment - outdoors,in
buildings, vehicles, equipment, people? - Potential revolutions in
- Environmental monitoring
- Disaster relief scenarios
- Condition-based maintenance
- Fantastic Voyage?
- Energy constraints imposed by untethered and
unattended systems. - Scaling challenges due to very large numbers of
sensors. - Level of dynamics
- Environmental obstacles, weather, terrain,
changes in RF propagation... - System ad hoc deployment, node failures,
self-configuration, re-tasking - Many assumptions in traditional distributed
systems are violated - Passively listening for messages from others is
very expensive - Transmission should be avoided even if the
channel is idle - Localized algorithms - do computation if it
avoids transmission - In-network processing (e.g., summarization), not
simply routing - Self-configuration - system should be adaptive,
robust to dynamics - Synchronization - ad-hoc creation of local time
and space coordinates
In parallel with others advancing the state of
the art in hardware, our lab studies techniques
for scalable, energy-conservative, robust
coordinationhow do you get all those sensors to
do something useful?
Unique Identifiers A staple of distributed
systems, but do they apply here?
Necessary but Too Expensive
Is there another way?
- Sensor nets have many uses for unique
identifiers(packet fragmentation, reinforcement,
compression codebooks...) - But, its critical to maximize usefulness of
every bit transmitted and received each one
brings the net closer to its death (Pottie) - Low data rates mean no space to amortize the cost
of a large, guaranteed unique ID -- e.g.,
statically assigned during manufacture, ala
Ethernet - High dynamics mean the cost of a protocol to
assign small locally unique IDs by resolving
conflicts cant be amortized over lifetime of the
network
- Can we use a series of small, random, ephemeral
identifiers instead? - In case of an identifier collision do nothing!
Avoid persistent collisions by picking a new ID
for every transaction - Marginal cost of losses from ID collisions will
likely be small compared to losses from many
other sources (packet loss, node dynamics, etc.) - Sensor nets must be robust to dynamics anyway -
so, perhaps this small additional source of error
does not cost us anything
Example Packet Fragmentation
- Address-free fragmentation using random
identifiers - Sender picks a random number for each new
packet,allowing receivers demultiplex
concurrently arriving packets - Identifier collisions result in lost packets
(e.g. failed checksum) - Scheme scales beautifully identifier space grows
withtransaction density, not network size, node
density, etc. - Leverages spatial and temporal locality (see
diagram at right) - Requires no configuration
- Random IDs will be IDsif total net size is large and transaction
density small - Can promote good design, making dynamics the norm
A model of the performance of random identifiers,
validated by experiments
- Definition of efficiency
- Total bits transmitted Address or Identifier
Bits Data Bits - For static allocation Bits received Data Bits
Only - For AFF allocation Bits received Data Size
P(No-Collision) - P(N-C) (1 - 1/IDspace) (trans-density)
IDspace 2address-bits
- Implementation on our SCADDS testbed
- Low-power Radiometrix RPC radio (27 byte
frames) - PC104 stacks (just like a PC)
- Test of address-free fragmentation
- Give each node a unique ID, report loss due to ID
collisions - Try a listening heuristic to avoid collisions
3
(address bits are not considered useful)