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1 www.caesar-lab.com

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Wireless Sensor Network ProgrammingPart I : Programming Concept


Agenda

WSN Operating System WSN Programming Model WSN Data Link Layer

Data Link Layer Architecture Medium Access Control (MAC) Protocol

Energy Awareness & Energy Calculation


WSN Operating System (1/2)

Should be compact and small in size since the sensor nodes have very small memory.

Should provide real-time support, since there are real-time applications, especially when actuators are involved.

Should provide efficient resource management mechanisms in order to allocate microprocessor time and limited memory.


WSN Operating System (1/2)

Should support reliable and efficient code distribution since the functionality performed by the sensor nodes may need to be changed after deployment.

Should support power management, which helps to extend the system lifetime and improve its performance.

Should provide a generic programming interface up to sensor middleware or application software.


WSN Programming Paradigms

Sequential Programming Model Process-based Concurrency Event-based Programming


Sequential Programming Model

a simple sequential model would run the risk of missing data while a packet is processed or missing a packet when sensor information is processed

This risk is particularly large if the processing of sensor data or incoming packets takes substantial amounts of time, which can easily be the case. Hence, a simple, sequential programming model is clearly insufficient.


Process-based Model

Equating individual protocol functions or layers with individual processes would entail a high overhead in switching from one process to another

This problem is particularly severe if often tasks have to be executed that are small with respect to the overhead incurred for switching between tasks – which is typically the case in sensor networks.


Event-based Programming Model

The system essentially waits for any event to happen, where an event typically can be the availability of data from a sensor, the arrival of a packet, or the expiration of a timer.

Such an event is then handled by a short sequence of instructions that only stores the fact that this event has occurred and stores the necessary information. The actual processing of this information is not done in these event handler routines, but separately, decoupled from the actual appearance of events

An event handler can interrupt the processing of any normal code, but as it is very simple and short, it can be required to run to completion in all circumstances without noticeably disturbing other code.

Event handlers cannot interrupt each other (as this would in turn require complicated stack handling procedures) but are simply executed one after each other.

The idea is to embrace the reactive nature of a WSN node and integrate it into the design of the operating system.


WSN Data Link Layer Architecture


Medium Access Control Protocol for WSN

The assembly of data into a frame for transmission by appending a header field containing addressing information and a trailer field for error detection

The disassembly of a received frame to extract addressing and error control information to perform address recognition and error detection and recovery

The regulation of access to the shared transmission medium in a way commensurate with the performance requirements of the supported application


MAC Protocols

Fixed-Assignment ProtocolsDemand Assignment ProtocolsRandom Assignment Protocols


MAC Protocols : Fixed-Assignment

Each node is allocated a predetermined fixed amount of the channel resources

Each node uses its allocated resources exclusively without competing with other nodes

Example : frequency-division multiple access (FDMA)time-division multiple access (TDMA), code-division multiple access (CDMA)


MAC Protocols : Demand Assignment

To improve channel utilization by allocating the capacity of the channel to contending nodes in an optimum or near-optimum fashion.

Demand assignment protocols ignore idle nodes and consider only nodes that are ready to transmitUnlike fixed-assignment schemes, where channel capacity

is assigned exclusively to the network nodes in a predetermined fashion regardless of their current communication needs


MAC Protocols : Random Assignment

In fixed-assignment schemes, each communicating node is assigned a frequency band in FDMA

systems or a time slot in TDMA systems. This assignment is static, however, regardless of whether or not the

node has data to transmit. These schemes may therefore be inefficient if the traffic source is

bursty. In the absence of data to be transmitted, the node remains idle, thereby resulting in the allocated bandwidth to be wasted.

Random assignment strategies attempt to address this shortcoming by eliminating preallocation of bandwidth to communicating nodes. CSMA (Carrier Sense Multiple Access) CSMA/CD – CSMA with Collision Detection CSMA/CA – CSMA with Collision Avoidance


MAC Protocol for WSN (1/2)

Balance of requirements For the case of WSNs, the balance of requirements is

different from traditional (wireless) networks.Additional requirements come up, first and foremost, the

need to conserve energy. The importance of energy efficiency for the design of MAC protocols is relatively new and many of the “classical” protocols like CSMA contain no provisions toward this goal.


MAC Protocol for WSN (2/2)Energy Problems on the MAC layer

Collisions : collisions incur useless receive costs at the destination node, useless transmit costs at the source node, and the prospect to expend further energy upon packet retransmission.

Overhearing : Unicast frames have one source and one destination node. However, the wireless medium is a broadcast medium and all the source’s neighbors that are in receive state hear a packet and drop it when it is not destined to them.

Protocol Overhead : Protocol overhead is induced by MAC-related control frames like, for example, RTS and CTS packets or request packets in demand assignment protocols, and furthermore by per-packet overhead like packet headers and trailers.

Idle Listening : A node being in idle state is ready to receive a packet but is not currently receiving anything. This readiness is costly and useless in case of low network loads; for many radio modems, the idle state still consumes significant energy.


Low duty cycle protocols and wakeup concepts

By choosing a small duty cycle, the transceiver is in sleep mode most of the time, avoiding idle listening and conserving energy.

By choosing a small duty cycle, the traffic directed from neighboring nodes to a given node concentrates on a small time window (the listen period) and in heavy load situations significant competition can occur.

Choosing a long sleep period induces a significant per-hop latency, since a prospective transmitter node has to wait an average of half a sleep period before the receiver can accept packets. In the multihop case, the per-hop latencies add up and create significant end-to-end latencies.

Sleep phases should not be too short lest the start-up costs outweigh the benefits.


Energy Awareness

Low Power Microcontroller Low Power Consumption Active Mode Many Low Power Modes Fast wake-up time

Low Power Design Extended Ultra-low Power standby mode Minimum active duty cycle Interrupt driven performance on-demand

www.caesar-lab.com

Current Consumption ProfileA Simple Glass Breakage Detector Using the MSP430 (SLAA351)


Energy Calculation Example : Smart Roadbeds

A Wireless Sensor Network for Smart Roadbedsand Intelligent Transportation Systems

http://www.media.mit.edu/resenv/pubs/theses/AraKnaian-Thesis.pdfใ

http://www.media.mit.edu/resenv/pubs/theses/AraKnaian-Thesis.pdf


Energy Calculation

Energy used by node operations

o To determine number of ns-A used by a task, multiply its current draw in mA by its duration is milliseconds.

o To determine the average current drawn in nA by the system due to a task, multiply the number of ns-A amps used by the task by its frequency is Hz


Node Power Consumption Breakdown

The maximum average node current drain is (11.3mA + 1.5mA + 1.5mA + 2.8mA) = 17.1mA.

The Tadiran TL-4935 lithium thionyl-chloride battery can supply 20mA at 3.6V for 90,000 hours, which is more than 10 years.

Documents

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