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–Reliably aggregate and disseminate information within a time frame that allows the controllers to action properly, the out-of-date information is not useful Constrains –Small capacities in power, memory, limited CPU execution speeds, and scarce communication bandwidth, which bring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

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 real- time 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