UBI532-WIRELESS SENSOR NETWORKS

1. UBI532-WIRELESS SENSOR NETWORKS International Computer Institute by Murat Kurt 22.05.2008

2. PaperPresentation

3. 1. Introduction • Enviromental Monitoring = Data Gathering • Example applications: • Precision agriculture • Glacier displacement measurments • Natural habitat monitoring • Microclimatic observations

4. 1. Introduction (2) • Enviromental Monitoring = Data Gathering • Characteristics: • Continuous data gathering • • Unattended operation • • Low data rates • Light traffic load • Low bandwidth requirements • • Battery powered • • Network latency (X) • • Dynamic bandwidth demands (X) • Energy conservation is crucial to prolong network lifetime.

5. 1. Introduction (3) • Energy-Efficient Protocol Design • Communication subsystem is the main energy consumer • – Power down radio as much as possible • Issue is tackled at various layers • – MAC • – Topology control / clustering • – Routing • Orchestration of the whole network stackto achieve duty cycles of ~1‰ Table 1: Mesured current consumptions of the TinyNode platform in different states at 2.5 volt

6. 2. Dozer System • Tree based routing towards data sink • – No energy wastage due to multiple paths • – Current strategy: SPT • • TDMA based link scheduling • – Each node has two independent schedules • – No global time synchronization • • The parent initiates each TDMA round with a beacon • – Enables integration of disconnected nodes • – Children tune in to their parent’s schedule

7. 2. Dozer System (2) • Parent decides on its children data upload times – Each interval is divided into upload slots of equal length – Upon connecting each child gets its own slot – Data transmissions are always ack’ed • No traditional MAC layer – Transmissions happen at exactly predetermined point in time – Collisions are explicitly accepted – Random jitter resolves schedule collisions

8. 2. Dozer System (3) • •Dozer system can be subdivided into four logical components. • – Tree Maintenance Module • – Scheduler Module • – Data Manager Module • – Command Manager Module • Tree Maintenance Module • – Coordinates a node’s integration in the data gathering tree of Dozer and • guarantees constant connectivity • – In case of a network failure it sets the node in an • energy efficient suspend mode until a reintegration • in the tree becomes possible.

9. 2. Dozer System (4) • Tree Maintenance Module - Connection Setup • – Nodesare ranked according to a rating function. Function parameters; • Node'sdistance to the sink • Its load (the number of direct children) • – To minimize tree depth, distance has a higher weight thanload in the • computation • – The node now tries to connectto the highest rated neighbor and the gathered informationabout all other overheard potential parents is stored • Tree Maintenance Module - Connection Recovery • Tree Maintenance Module - Suspend Mode Figure 2: Connection setup -The parent nodesends a beacon (B). Upon beacon reception the childsends a busy tone to activate the contention window. The child then transmits its connection request (C). A handshake (H) serves as an acknowledgment. Shaded areas denote the times a node isactually listening.

10. 2. Dozer System (5) • Scheduler Module • – The energy efficiency of the Dozer system mostly stems from the Scheduler • module • – Communication between a parent and its children is coordinated by a TDMA • protocol • – Dozer only aligns one hop neighbors in the data gathering tree • – The seed value of the randomnumber generator used for calculating the next • random offset is included in each beacon.

11. 2. Dozer System (6) • Data Manager Module • – While anode's data upload times are strictly defined by the Scheduler module, • data injection by the application is always possible. • – The Data Manager module features a message queue buffering injected data • pending for transmission. • – If a message transfer fails to be acknowledged the child immediately stops its • data upload for thisround of the schedule . • – In case of consecutive transmission failures over multiple upload slots the Data • Managerinstructs the Tree Maintenance module to switch to anotherparent. Figure 3: Message reception of a parent with twochildren. Upload slots are determined by parentbeacon (B). All data messages (D) are explicitly ac-knowledged (A).

12. 2. Dozer System (7) • Command Manager Module • – While data flow in Dozer is strictly unidirectional towardsthe sink it is often • desirable to be able to send informationto one or several nodes in the network. • – Dozer establishessuch a lightweight backward channel by making use of the • beacon messages. • – Commands injected at the data sink areincluded in the sink's next beacon • message. • – Every node receiving a beacon containing a command temporarily stores • the command and includes it in its next beacon. • – By repeating this procedure at each level of the tree the command is • disseminated through the whole network. • – Besides addressing a command to all nodes in the network the injection • of commands for individual nodes is also supported. • – Upon reception of a beacon message from the parent theTree Management • component hands the command to theCommand Manager module for further • processing. Themodule checks if this node belongs to the set of intended • recipients of the command.

13. 3. EVALUATION • Dozer’s performance tests are done under different conditions in real-world testbeds • – A set of preliminary measurements on a small-scale network are conducted to • estimate the scalability of the system • – We present results of a deployed indoor networkconsisting of 40 sensor nodes • Platform • – TinyNode 584 platform produced by Shockfish SA • – MSP430mirocontroller with 10 kB of RAM and 48 kB ofprogram • memory • – 512 kB of external flash are also available • – The platform includes a Semtech XE1205 radio transceiver • – Good transmission ranges • – High data ratesof up to 153 kbit/s.

14. 3. Small Scale Experiments • Energy consumption is measured indirectly • All nodes log their radio duty cycles (differences between radio startup and shutdown times) • This information is propogated to the base station • The collected information can be converted to power consumption values using Table 1 • Results can be considered as an upper bound for actual power consumption • To investigate the relation between a node’s power drain, its number of children, and the beacon interval time we conducted a series of experiments on a small network with predefined topology • Figure 4 shows that the duty cycle decreases as the beacon interval grows larger • Figure 4 also shows that costs for additional children do not necessarily have to grow linearly. Uploading data from two or more children in one upload slot saves additional overhead of turning on the radio for each of these children individually.

15. 3. Small Scale Experiments (2) Figure 4: Radio duty cycle of a node depending onits number of children. Measurements were performed with beacon intervals of 15 s (square), 30 s(circle), 1 min (triangle), and 2 min (star), respectively.

16. 3. Office Floor Experiments • Dimensions of the building are approximately 70 x 37 meters resulting in an testbed area of morethan 2500 square meters. • The floor was populated by more than 80 peopleduring office hours. • Constructed a network with heterogeneous density • 40 Nodes • Indoor deployment • > 1 month uptime • Information forming the basis ofthe evaluation in this section were gathered during one weekof operation. • Each node thereby sent approximately 5000data messages to the sink • 30 sec beacon interval • 2 min data sampling interval (4xbeacon interv.) • Node 0’s was chosen to get a deep data • gathering tree and to enforcemulti-hop • communication

17. 3.1 Tree Topology

18. 3.1 Tree Topology (2)

19. 3.1 Tree Topology (3) • Base station (Node 0) has numerous children. Two reason; • The parentrating function promotes connections to the sink since it has zero • tree depth. • The sink was configured to accept more than one child per connection phase. • Hardly anyconnections passed the central core of the building. • We examine the stability of the data gathering tree byinvestigating topologychanges and message loss.Topologychanges are indicated by a node exchanging its parent. • As hoped for, message loss was low, in average 1.2% and at maximum 3.15%. • However, Node 128 is excluded from this analysis. It was only able toconnect to one single other node (Node 112) • Suspend mode • Message lossof approximately 30%

20. 3.1 Tree Topology (4) On average %1.2 1 week of operation Figure 6: Number of successful (black) and failed(grey) connection attempts per node. Per nodepacket loss on the second y-axis.

21. 3.2 Energy Consumption • Energy consumption of the deployed nodes were measured indirectly via their duty cycles. • Figure 7 depicts the average radio activity of each node in the network. • The upward error bar shows the RMS error of all measurments exceeding the average duty cycles. • The overall average duty cycleof all sensing nodes is 1.67‰ with a standard deviation of0.0004. • Applying the values from Table 1 results in a meanenergy consumption of 0.082 mW. • The sink had by far the highest radio uptime of almost 1%. • Ithad to process the data of the whole network. • The extended contention window.

22. 3.2 Energy Consumption (2) On average 1.67‰ Sink radio uptime 1% Figure 7: Average radio duty cycle of each nodeincluding RMS errors. • Mean energy consumption of 0.082 mW • Highest duty cycles; Node 114 = 3.2‰and Node 124 = 2.8‰

23. 3.2 Energy Consumption (3) Node 124radio uptime 2.8‰ Figure 8: Radio duty cycle of Node 124 over a periodof three days. • As can be seen for most of the time the node ran at a dutycycle of 0.7‰

24. 3.2 Energy Consumption (4) • Node 124: • The node is a leafin the data gathering tree. • Threedifferent energyintensive effects can be observed • Exceeding 20% are scans for a full beacon interval • The overhearing phase once everyfour hours results in a temporary duty • cycle of around 1% • The potential parents updates lead to the fringes ofup to 1‰. • Located in a small storage room • It had but a small neighborhood • Consequently, in times ofnormal operation it was able to run at nearly optimal • dutycycle. • In case of connection interruptions the interference affected all its possible • connections resulting in afallback to bootstrap mode.

25. 3.2 Energy Consumption (5) Node 114radio uptime 3.2‰ Figure 9: Radio duty cycle of Node 114 over a periodof three days. • Parents updates and overhearingcan also be spotted in this chart • It acts asa relay for several children and thus cannot reach minimalduty cycles as low as Node 124.

26. 4 CONCLUSION • Environmental monitoring requirements – Long network lifetime – High delivery rateof sampled sensor readings to a central authority. • Dozer, a new data gathering system designedto meet these requirements. • Conclusions – Dozer achieves duty cycles in the magnitude of 1.6‰. – Abandoning collision avoidance was the right thing to do. • Future work – Incorporate clock drift compensation. – Optimize delivery latency of sampled sensor data. – Make use of multiple frequencies to further reduce collisions. • It is our goal to see Dozer running ona genuine large scale sensor network collecting meaningfuldata.