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Real-time
Wireless Sensor Networks
CSC536 Spring 2005
Meng Wan
05/09/2005
Outline
• Background & Motivation
• MAC Protocols in Real-time Sensor
Networks
• Routing Issues in Real-time Sensor
Networks
• Conclusions
Real-time Sensor Networks
Why Need Real-time?
• Since sensor networks operate in real world
environments, in many cases timing
constraints are important
– Implicit time requirements
– e.g., in a monitoring system, when a user enters a
room, the faster the user is recognized the better we
consider the system
– Explicit time requirements
– many systems have explicit real-time requirements related to
the environment
Real-time Sensor Networks
Some Applications
• While tracking an object the object might no longer be in
the vicinity the time the information is received
• An accelerometer might have to be read every 10 ms, or
else there will be a bad estimate of speed and consequently
a high probability of a vehicular crash
Real-time Sensor Networks
Task & Constraints
• Task
– Reliably aggregate and disseminate information within
a time frame that allows the controllers to action
properly, the out-of-date information is not useful
• Constrains
– Small capacities in power, memory, limited CPU
execution speeds, and scarce communication
bandwidth, which bring challenges for real-time
computation and communication in sensor networks
Real-time Sensor Networks
Communications
• Communication layer should effectively
coordinate and control sensors in real-time over an
unreliable network connection
• MAC
– Bandwidth, power consumption, contention, network
connectivity, real-time requirement
• Routing
– Minimum state information and end-to-end signaling,
adaptive routing protocol to avoid unpredictable
congestion, end-to-end timing guarantees
MAC Protocols
• Scheduling-based
• Contention-based
• Collision free
MAC Protocols
Scheduling-Based Protocol
• Algorithm
– Predefines a time slot for each node
– Each node gains at least one slot within a frame, which
is a basic unit of transmission
• TDMA is the main focus of a lot of early work
– Requires global connectivity information
– Not adaptive enough to adjust nodes to dynamic
environments such as topology change
– No consideration on timing constraints
MAC Protocols
Contention-Based Protocol
• Based on carrier sensing mechanisms and employ
additional signaling control
– Typical control packets: request to send (RTS) , clear to send
(CTS)
• CSMA/CD is an example:
– A node that has data to send transmits a short RTS packet
– Nodes within one hop of the sending node hear the RTS and defer
transmission
– The destination responds with a CTS packet, all nodes within one
hop also defer transmission
– The transmitting node assumes that the channel is acquired and
transmits
MAC Protocols
Contention-Based Protocol
• Too costly to send control packets in sensor networks, due
to
– Asymmetric communication
– High message loss
– Interference, etc.
• Distributed and random back off nature does not strictly
guarantee the priority order
– E.g., two higher-priority nodes collide and cause each other to
back off, a third lower-priority node send packets out
MAC Protocols
Collision-Free Real-Time Protocol
• Exploits the data’s periodic nature to
provide guaranteed delays
• Architecture
– cellular network
– router node in the center, with two transceivers,
transmit and receive at the same time using two
different frequency channels
MAC Protocols
Collision-Free Real-Time Protocol
• Intra-cell communication
– Use earliest deadline first (EDF) scheduling
– EDF schedule replicated & updated at each
node
• Inter-cell communication
– Use frequency division multiplexing (FDM)
among adjacent cells, allowing for concurrent
inter-cell communications
MAC Protocols
Collision-Free Real-Time Protocol
MAC Protocols
Collision-Free Real-Time Protocol
• Note
– If combined with a special routing protocol,
end-to-end delay guarantee is the sum of the
bounded delay on each cell along the path
– Six possible directions assigned statically to the
inter-cell frames
MAC Protocols
Analysis
• TDMA
– Fair channel usage
– Maintain too much global connectivity information
– Not adaptive to dynamic environment
So, collisions can not be actually avoided
As a result, real-time requirements not guaranteed
for critical applications
MAC Protocols
Analysis
• Contention-based protocols
– To solve cost issues, some advances in this area largely
reduce chances of collisions and reduce power
consumption, so that it could be useful in some
applications where power consumption is the main
concern
– Hard to provide statistical bound of probability of
priority inversion on real-time requirement
So, it is hard to establish statistical delay guarantees
MAC Protocols
Analysis
• Collision-free protocols
– Save power by eliminating collisions
– Satisfy real-time requirements
– Uses multiple channels, thus higher
requirements on the hardware of the modes
– Assumptions and setup may be not practical
So, more future work needed
MAC Protocols
Summary
• The key challenges in MAC
– Provide predictable delay and/or prioritization
guarantees while minimizing power
consumption
– More consideration on constrains of sensor
networks
Routing
• Requirement
• SPEED – A stateless real-time
communication protocol for real-time
routing in sensor networks
Routing
Requirement
• Maintain minimum global state information
• Geographic routing (location-based) instead
of ID routing (ID-based)
• Minimum end-to-end deadline miss ratio
Routing
SPEED
• Three routing services provided
– Area-multicast
– Area-anycast
– Unicast
SPEED
Basic idea
• Each node records one-hop neighbors’
location & delay
• Feedback-based adaptation algorithms
enforce a per-hop speed in the face of
unpredictable traffic
– Neighborhood Feedback Loop (NFL)
– Backpressure Rerouting
SPEED
Architecture
SPEED
Function
• Four APIs
–
–
–
–
AreaMulticastSend (position, radius, packet)
AreaAnycastSend (position, radius, packet)
UnicastSend(Global_ID, packet)
SpeedReceive()
SPEED
Function
• SNGF: routing module to choose the next hop candidate
• NFL and Backpressure Rerouting: modules to reduce or
divert traffic when congestion occurs, so that SNGF has
available candidates
• The last mile process: when the packet enters the
destination area, provides the three routing services. The
SNGF module controls all the previous packet relays
• Delay estimation: mechanism by which a node determines
congestion
• Beacon exchange: provides geographic location of the
neighbors
SPEED
Evaluation
• Lower deadline miss ratio in face of sudden
congestion
• Reacts to transient congestion in the most
stable manner
SPEED
Evaluation
• SPEED is
– the only one routing protocol especially
designed for sensor networks to satisfy realtime requirements
• SPEED demonstrates
– localized feedback control is a promising
approach for real-time communication in sensor
networks
Conclusions
• Real-time sensor networks become
important for many critical applications
• Real-time requirements mainly lie in
communication layer of the sensor network
• MAC
– Collision-free real-time protocol
• Routing
– SPEED