Sensor Networks Pete Perlegos

2 Outline Background Ad-hoc Wireless Networks Smart Dust – TinyOS PicoRadio

3 How are they possible? Moore’s Law: The historical trend…

4 Lets look at the other side Moore’s Law is also pushing a given functionality into a smaller, cheaper, lower-power unit. 486DX DX 2001

5 Other Trends Complete systems on a chip (SoC) Integrated low-power communication RF, optical Integrated low-power transducers power capacitor, solar cell, battery Integrated sensors Detect light, heat, position, movement, chemical presence, etc.

6 Other Trends

7 Outline Background Ad-hoc Wireless Networks Smart Dust – TinyOS PicoRadio

8 Ad-hoc Wireless Networks No base stations or infrastructure required Multi-hop wireless networks Each node can talk with a neighbor Applications Sensor networks Intelligent control applications

9 Ad-hoc Wireless Networks MAC schemes Addressing Routing

10 Geographical Routing Algorithm Geographical network Assumptions: Each node knows its own position and its neighbors’ position Nodes don’t know the global topology Destination address is a geographical position to which the packet is to be delivered

11 A Simple Routing Algorithm Routing Decision: Route to the neighbor which is nearest to the packet destination Source Destination

12 Problem with Simple Routing Source DestinationWall Simple routing does not always work The Geographical routing algorithm is an extension of the simple routing algorithm

13 Route Discovery Packet gets “stuck” when a node does not have a neighbor to which it can forward the packet When a packet is stuck, a Route Discovery is started to destination D A path is found to D Entry [position(D), s(i+1)] is added to the routing table of s(i) Source DestinationWall

14 Routing Tables Routing Table for Station n: (x,y) position Neighbor d (8,6) b Position of n - Position of neighbor a a Routing Algorithm: Packet arrives for position p at node n Node n finds the position to which p is closest and. forwards to the. corresponding neighbor Position of neighbor b Routing Tables:  Routing tables contain some additional entries beside. neighbors

15 Example Pos(A) = (1,1) Pos(B) = (2,2) Pos(C) = (3,1) Links: A ---- B B ---- C A B C Pos(A) --- Pos(B)B --- Pos(A) A Pos(C) C --- Pos(B) B  A gets a packet for Pos(C)  A forwards it to B because pos(B) is closer to pos(C)  B forwards it to C because pos(C) is closer to pos(C) Pos(C)

16 Route Discovery Pos(A) = (1,1) Pos(B) = (2,2) Pos(C) = (3,1) Pos(D) = (2.5,0) Links: A ---- B B ---- C C ---- D B C  A gets a packet for Pos(D)  Packet gets stuck at A because Pos(A) is closest to Pos(D)  Initiate route discovery for D from A  Update the routing tables and forward the packet Pos(D) A D Pos(A) --- Pos(B) B Pos(D)--- Pos(C) C Pos(B) --- Pos(A) A Pos(C) C --- Pos(B) B Pos(D) D B C

17 Desirable Properties of Location Service Spread load evenly over all nodes. Degrade gracefully as nodes fail. Queries for nearby nodes stay local. Per-node storage and communication costs grow slowly as the network size grows

18 Grid Location Service (GLS) n s s s s s s s s s  s is n’s successor in that square. (Successor is the node with “least ID greater than” n ) sibling level-0 squares sibling level-1 squares sibling level-2 squares

19 GLS Updates , 2 11, location table content location update 2

, 2 11, location table content query from 23 for 1 GLS Query

21 Outline Background Ad-hoc Wireless Networks Smart Dust – TinyOS PicoRadio

22 Active Messages - Benefits Event based model : Avoids busy waiting for data to arrive Allows the system overlap communication with computation Lightweight architecture : Balances the need for extensible communication network while maintaining efficiency and agility

23 Active Messages – What are they? Each Active Message contains: the name of a user-level handler to be invoked on a target node upon arrival a data payload to pass in as arguments The handler function serves a dual purpose: extracting the message from the network either integrating the data into the computation or sending a response message

24 Active Messages – What are they? The network is modeled as a pipeline with minimal buffering for messages. This eliminates many of the buffering difficulties faced by communication schemes that use blocking protocols or special send/receive buffers. To prevent network congestion and ensure adequate performance, message handlers must be able to execute quickly and asynchronously.

25 Event Based Programming Event handlers are invoked to deal with hardware events (directly or indirectly). The lowest level components have handlers connected directly to hardware interrupts: external interrupts, timer events, or counter events Events propagate up through the component hierarchy. To perform long-running computation, components request to have tasks executed on their behalf. Once executed by scheduler, tasks run to completion and execute autonomously with other tasks.

26 Event Based Programming Events propagate up through the component hierarchy.

27 Processor Utilization The vast majority of the power consumption occurs in the active state, with very little power used in the idle state The system should embrace the philosophy of getting work done as quickly as possible and going to sleep This is a great benefit of the event based model

28 Outline Background Ad-hoc Wireless Networks Smart Dust – TinyOS PicoRadio

29 PicoRadio The ever-evolving scaling of the semiconductor technology is enabling the co-integration of the interfacing, computation, position location, and communication functions into a single silicon circuit. Benefits of the system-on-a-chip approach: Maximally reduces the size of the sensor node Allows the use of advanced circuit architectures which provide the optimal trade-off between flexibility and energy-efficiency The tight integration of communication and computation functions into a single chip will provide the desired functionality at the lowest possible cost and energy.

30 PicoRadio A range of technologies are still necessary for the realization of ultra-low energy wireless sensor networks: The study of multi-hop networks, and MAC layers that support low (but variable)-rate data transmission, while ensuring low energy-consumption. Chip architectures that enable the implementation of these advanced algorithms. (A heterogeneous combination of programmable, configurable, and fixed components.) Mapping the advanced networking and communication algorithms onto such an architecture is a real design methodology problem. Ensuring and verifying that these distributed and embedded systems will behave correctly is especially hard. An RF front-end that meets the demands of variable bit- rates and energy-efficiency.

31 Protocol Stack Specification of protocol stack derived from: System requirements (top-down) Wireless channel properties (bottom-up)

32 Energy Consumption Multi-hop Networks and Low-Energy Consumption Using several short hops to send a bit is more energy efficient than using one longer hop

33 Energy Consumption Multi-hop Networks and Low-Energy Consumption Energy optimal number of hops as a function of distance

34 Architecture Conceptual PicoNode chip architecture Allows for flexibility Trade-off between flexibility and energy efficiency must be managed

35 Conclusion We seem to be getting close to realizing networks of sensors. Lightweight, event-based communication Shrinking die size (lower power and cost) Advancement of system on a chip development