MANAGEMENT ISSUES FOR WIRELESS SENSOR NETWORKS, AN OVERVIEW:

1. MANAGEMENT ISSUESFOR WIRELESS SENSOR NETWORKS, AN OVERVIEW: ruzzelli@ucd.ie

2. Examples of WSNs applications environmental data collection: temperature light, humidity, pressure, solar radiation. This network has given biologists new view into the nesting patterns of Storm Petrels. Fire prevention Wind Response Of Golden Gate Bridge

3. Gateway Nodes Cooperation of applications

4. Prototypes of low cost wireless sensors MICA mote sensor prototype, University of Berkeley (US) EYES sensor prototype, University of Twente (NL)

5. How can we make such applications properly working? • Let’s identify some issues: • One application for WSN can include thousands of sensors • Sensors must be cheap • Cheap nodes  Limited memory capabilities •  Limited processing capabilities •  Limited power capabilities •  Maybe not very reliable

6. Some other issues are: • If the radio of a node is always on, the node will be depleted in few days (e.g. Mobile phone)  Node depletion • If a node receives more than one message at the same time, it will not able to understand the content  Packet Collision • Long distance transmission implies very high power consumption : Pc ≈ dⁿ, n≥2.  Limited transmitting power • Identification of useful messages to deliver Efficient type of service

7. Sensor network Design Considerations (1) • Energy efficiency • often difficult recharge or replace batteries: • prolonging the network life expectancy is the MAIN TASK • Scalability • Network may change in size, node density and topology. • Nodes may die over time and new nodes may join later

8. Sensor network Design Considerations (2) • Event detection and periodic measurements • application dependently • Multihop wireless communication • to save energy by relaying messages to close neighbors • Good maintainability • due to hash environment condition • Network self configuration • as network condition change over time

9. How can we address such issues to fulfil the requirements? • Issues are addressed in different layers referred to as network protocols • Adv: Decrease of complexity • Increase of flexibility • Dis: Increase of memory requirements • The whole set of network protocols is known as network architecture (e.g. the ISO/OSI architecture)

10. Internet: The ISO/OSI architecture • Machine/User interaction issues • Language syntax issues • Transmission control e.g. TCP/IP • For routing issues • For medium access control issues • Radio issues Some layers are not suitable for sensor networks!

11. How do protocols for a sensor network differ from traditional ones? • Protocols for ad hoc networks to (e.g. WiFi) • are aimed to obtain: • High bandwidth Utilization • Good fairness • Low latency of packets • High throughput • These are generally the primary concerns in traditional wireless voice and data networks • But in sensor networks they are secondary!

12. Sensing devices Application Data aggregation Sensing coverage Localization Cross layer interaction Routing MAC Physical Antenna Wireless sensor network architectureAn example: • The most suitable network architecture for WSNs is still an open issue • Researchers are evaluating how to best use the cross layer interaction • Each layer has its own task • Any layer try to achieve the task using the smallest amount of energy possible

13. Relevant existing MAC protocol for traditional wireless networks • Bluetooth • Very short computer communication • Used in Personal Area Networks (PAN) e.g. in conference rooms • IEEE 802.11 also known as WiFi • -Short range computer communication • -Used for internet in a Local Area Network (Lan), e.g. the CS department. • Such protocols require nodes to be constantly powered on VERY HIGH ENERGY CONSUMPTION!

14. Wireless Medium Access Control (MAC)protocol fundamentals (1): • The MAC protocol coordinates the times at which nodes access the wireless channel • General Duties: • Packet collision avoidance • Packet collision detection • Efficient channel utilization • Low access delay • Node access fairness

15. node1 TX ? node3 TX node2 ? node1 node2 node3 Wireless Medium Access Control (MAC)protocol fundamentals (2): • Identification of collision TX TX

16. node1 TX ? node3 TX node2 Wireless Medium Access Control (MAC)fundamental mechanisms (3): • General classes of MAC: • Frequency Division Multiple Access (FDMA) • Code Division Multiple Access (CDMA) • Applied on Sensor Networks: • Carrier Sense Multiple Access (CSMA) • Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) • Time Division Multiple Access (TDMA) • TDMA/CSMA hybrid approach

17. New Concepts of Wireless Sensor Networks MACs: • MAC protocols tailored for WSNs have introduced novel concepts: • The node duty Cycle and Wakeup concepts • -To reduce the energy consumption nodes alternate periods of radio activity and inactivity Listening period Sleep period Wakeup period New issues arises a result! Time

18. New issues of Wireless Sensor Networks MACs: The Synchronization issue: -In order to communicate, nodes have to synchronize their internal clock -The low-cost technology of sensors make things worse: -Clock Skew and Offset need to be compensated for periodically Periodically nodes must broadcast their time to neighbours and synchronize clocks together

19. Mechanisms applied in Wireless Sensor Networks MACs: • The CSMA approach (Carrier sense multiple access) • Any node that has a packet to transmit listen to the channel for a period to sense any ongoing transmission in progress • Nothing is sensed Channel free assumed Transmit the packet • Something is sensed Channel busy  Transmission postponed and node switches off the radio node1 TX node3 TX node2

20. Mechanisms applied in Wireless Sensor Networks MACs: The CSMA approach Advantages:Flexibility as nodes works independently Low packet delay Drawbacks: The Hidden Terminal Problem (HTP) The CSMA does not prevent packet collisions if transmitters are not within the same range TX TX ? node1 node2 node3

21. Mechanisms applied in Wireless Sensor Networks MACs: Receiver • The CSMA/CA approach • (CSMA with collision avoidance) • Any node that has a message to transmit applies CSMA then if nothing is sensed: • -The node transmits a Request To Send RTS packet (very short packet) • -The node waits for a Clear To Send CTS Packet • - Once CTS is received, the node is enabled to transmit the message • -The correct message reception is followed by • an acknowledgment ACK Transmitter RTS CTS time Data ACK

22. Mechanisms applied in Wireless Sensor Networks MACs: The CSMA/CA removes the Hidden Terminal Problem -The RTS and CTS contain: message length, transmitter ID, receiver ID. - The CTS notifies the neighbourhood of an imminent transmission -The neighbourhood can switchs off the radio to save energy RTS Transm. Recv. CTS

23. Mechanisms applied in Wireless Sensor Networks MACs: The CSMA/CA approach Advantages:Avoid the Hidden Terminal problem Good Flexibility Disadvantages: High latency of messages Control packet overhead External Terminal Problem ETP Node2 could theoretically transmit to node0 but the CSMA blocks it while node2 transmission is in progress TX node0 node1 node2 node3

24. Node2 Node4 Node1 Node5 Node3 Mechanisms applied in Wireless Sensor Networks MACs: • The TDMA approach • -Time is divided into frames • -Frames are divided into slots • - Each node within the same range owns one slot for transmission … TxN2 TxN1 TxN2 TxN4 TxN5 TxN3 TxN3 TxN1 slot frame

25. Mechanisms applied in Wireless Sensor Networks MACs: • The TDMA approach • Advantages: -Collision free mechanism • -Both HTP and ETP are solved • Disadvantages: • -High packet overhead (all neighbours have to notify their presence before dividing the resource • - Low flexible (e.g. for late joining node)

26. Mechanisms applied in Wireless Sensor Networks MACs: The TDMA/CSMA hybrid approach, the MERLIN protocol as an example

27. MERLIN CSMA/TDMA Hybrid: Transmission Mechanism • Every slot has a contention period to perform CSMA; • Nodes pick up a random time “t” in Contention Period; • Start listen to the channel at “t”; • Channel is Free  Start sending the packet ; • Channel is busy  Turn off the radio until the next scheduled Slot S L O T A B Zone 3 Zone 5 Zone 1 Zone 2 Zone 4 A B

28. Gateway Nodes Conclusion The seminar showed just some of WSNs protocol issues. WSNs are still a World wide work in progress that can be very useful for a great number of applications. By Antonio Ruzzelli