By Antonio Ruzzelli MANAGEMENT ISSUES FOR WIRELESS SENSOR NETWORKS, AN OVERVIEW:

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

By Antonio Ruzzelli Cooperation of applications Gateway Nodes

By Antonio Ruzzelli Prototypes of low cost wireless sensors MICA mote sensor prototype, University of Berkeley (US) EYES sensor prototype, University of Twente (NL)

By Antonio Ruzzelli 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

By Antonio Ruzzelli 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

By Antonio Ruzzelli 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

By Antonio Ruzzelli 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

By Antonio Ruzzelli 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)

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

By Antonio Ruzzelli 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!

By Antonio Ruzzelli Wireless sensor network architecture An example: Antenna Sensing devices Application Data aggregation Routing MAC Physical Sensing coverage Cross layer interaction Localization 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

By Antonio Ruzzelli 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 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!

By Antonio Ruzzelli 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

By Antonio Ruzzelli Wireless Medium Access Control (MAC) protocol fundamentals (2): Identification of collision node1 node2 node3 ? TX node1 node2 node3 ? TX

By Antonio Ruzzelli 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 Wireless Medium Access Control (MAC) fundamental mechanisms (3): node1 node2 node3 ? TX

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

By Antonio Ruzzelli 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 New issues of Wireless Sensor Networks MACs: Periodically nodes must broadcast their time to neighbours and synchronize clocks together

By Antonio Ruzzelli 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 node2 node3 ? TX

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

By Antonio Ruzzelli 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 RTS CTS Data Transmitter Receiver time ACK Mechanisms applied in Wireless Sensor Networks MACs:

By Antonio Ruzzelli 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 Transm. RTS CTS Recv.

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

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

By Antonio Ruzzelli 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) Mechanisms applied in Wireless Sensor Networks MACs:

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

By Antonio Ruzzelli 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 Zone 1 Zone 2 Zone 3 Zone 4 Zone 5 CSMADATA A AB B SLOTSLOT

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