Wireless Sensor Networks

Transcription

1 Wireless Sensor Networks 1 Ch. Steup / J. Kaiser, IVS-EOS

2 Ubiquitous Sensing 2 Ch. Steup / J. Kaiser, IVS-EOS

3 IEEE 802.x Wireless Communication 3 Ch. Steup / J. Kaiser, IVS-EOS

4 Wireless Technology Comparision Hardware Standard Frequency Range Tx Power Rx Power Idle Power Max. Capacity Qualcomm WSN ac 2,4-2,5 GHz 5,1-5,9 GHz ~100m mw Mbit/s Atmel AT86RF23 3 Nordic nrf24l01+ Nordic nrf ,4-2,5 GHz ~200 m 1mW ~40 mw ~ 0,1 µw 2 Mbit/s 2,4-2,5 GHz ~150m 1mW ~45 mw ~ 2.7µW 2 Mbit/s propriatary ,4-2,5 GHz ~100m 1mW ~48 ma ~ 1,5 µw 1 Mbit/s Wisair WSR a / Wireless USB 3,1-4,8 GHz ~10 m 10nW Mbit/s 4 Ch. Steup / J. Kaiser, IVS-EOS

5 Architecture Applications ZigBee Network Routing Address translation Packet Segmentation Profiles IEEE MAC IEEE /915 MHz PHY IEEE MHz PHY 5 Ch. Steup / J. Kaiser, IVS-EOS

6 Provides two services to physical layer management entity (PLME) PHY data service exchange data packets between MAC and PHY PHY management service interface Clear channel assessment (CCA) 3 methods: Energy above threshold, Carrier sense only, or Carrier sense w/ energy above threshold Energy detection (ED) Used by network layer (channel selection) Link Quality Indication (LQI) Used by higher layers Uses ED and/or SNR estimate Channel Assignment Steven Myers Electrical and Computer Engineering University of Wisconsin Madison 6 Ch. Steup / J. Kaiser, IVS-EOS

7 Star Topology PAN coordinator coordinator Master/slave FFD RFD 7 Ch. Steup / J. Kaiser, IVS-EOS

8 Optional Superframe Structure Beaco n GTS GTS Beaco n inactive (low power) CAP CFP Superframe Duration: SD = abasesuperframeduration*2 SO symbols (0 SO BO 14) Beacon Interval: BI = abasesuperframeduration * 2 BO (0 BO 14) Beacon: sent by PAN coordinator in the first slot of the superframe. Contains network information, frame structure and notification of pending node messages. Contention Access Period (CAP): Communication using slotted CSMA-CA Contention Free Period (CFP): Guaranteed time slots (GTS) given by coordinator (no CSMA) Beacon Order (BO): Describes the interval at which the coordinator shall transmit its beacon frames. Superframe Order (SO): Describes the length of the active portion of the superframe. 8 Ch. Steup / J. Kaiser, IVS-EOS

9 9 Ch. Steup / J. Kaiser, IVS-EOS

10 Arbitration Slotted CSMA-CA: Used in superframe structure Backoff periods are aligned with superframe slot boundaries of PAN coordinator Used in CAP, must locate boundary of the next backoff period to transmit data Un-slotted CSMA-CA: Non beacon enabled network Backoff periods are not synchronized between devices 10 Ch. Steup / J. Kaiser, IVS-EOS

11 Arbitration in Slotted CSMA Slotted CSMA NB=0, = 0, CW=2 = 0 Delay for random(2 BE - 1) unit backoff periods Battery life extension? Y BE = lesser of (2, macminbe) Perform CCA on backoff period boundary N BE = macminbe Channel idle? Y N Locate backoff period boundary CW = 2, NB = NB+1, BE = min(be+1, amaxbe) CW = CW - 1 CW: contention window length NB: number of times the CSMA-CA algorithm was required to backoff BE: backoff exponent N NB> macmaxcsmabackoffs? Y CW = 0? Y N Failure Success 11 Ch. Steup / J. Kaiser, IVS-EOS

12 Problems Hidden Terminal Problem? Multiple Access with Collision Avoidance (MACA) data X data time RTS data CTS CTS blocked 12 Ch. Steup / J. Kaiser, IVS-EOS

13 More problems Exposed Terminal Problem r1 s1 s2 r2 r1 s1 s2 r2 X data time RTS data CTS RTS no CTS! blockiert? s2 sends RTS/CTS to improve throughput 13 Ch. Steup / J. Kaiser, IVS-EOS

14 Big Problem: idle listening Rx active power is sometimes greater than Txactive power, due to the larger number of signal processing circuits that must be active It s more power-efficient to blindly transmit than to blindly receive Energy efficient protocols try to minimize the time of active listening! Approaches: - Scheduling (TDMA) - activation channel (narrow band additional channel) - Preamble - Adaptive schemes 14 Ch. Steup / J. Kaiser, IVS-EOS

15 CSMA S-MAC a priori fixed active and sleep intervals During the activity periods the node must transmit local data + the messages which are relayed in the multi-hop network. Problem with S-MAC: fixed periods 15 Ch. Steup / J. Kaiser, IVS-EOS

16 T-MAC: Time-Out-MAC Determine the activity and sleep periods adaptivly. T-MAC spans a time-out (active) interval of 15 ms. If no event is detected within this interval it enters the sleep state again. An activation event is given by: - Alarm of a periodic timer; - Reception of a message; - Detection of some communication (also collisions are such events); - Termination of the own transmision or of an ack. - The knowledge that a communication by some neighbors has been terminated. (detected by overhearing) All communication is performed in "bursts" at the start of the aktive period. 16 Ch. Steup / J. Kaiser, IVS-EOS

17 Example: Forest Fire Detection - Multi-Hop Communication - Possibly unreliable links - Nodes may fail silently - Needs appropriate routing 17 Ch. Steup / J. Kaiser, IVS-EOS

18 Problem: Embedded Routing in Mesh Networks Goal: Find next Node in vicinity to forward packet to Challenges: - Low energy consumption - Low latency - High throughput - Reliability - Scalability 18 Ch. Steup / J. Kaiser, IVS-EOS

19 Flooding - Extended Broadcast - Very robust - Very high message overhead event A C S G event B D 19 Ch. Steup / J. Kaiser, IVS-EOS

20 Flooding - Extended Broadcast - Very robust - Very high message overhead A event C S event G B event D 20 Ch. Steup / J. Kaiser, IVS-EOS

21 Flooding - Extended Broadcast - Very robust - Very high message overhead A C event S event G B D event 21 Ch. Steup / J. Kaiser, IVS-EOS

22 Optimized Flooding: Gossip - Extended p-persistant Broadcast - Not robust - Adjustable overhead - Statistical coverage event A C S G event B D 22 Ch. Steup / J. Kaiser, IVS-EOS

23 Optimized Flooding: Gossip - Extended p-persistant Broadcast - Not robust - Adjustable overhead - Statistical coverage 0.6>0.5 A event C S event G B D 0.3> Ch. Steup / J. Kaiser, IVS-EOS

24 Optimized Flooding: Gossip - Extended p-persistant Broadcast - Not robust - Adjustable overhead - Statistical coverage 0.3>0.5 A C S G B 0.6>0.5 event D 24 Ch. Steup / J. Kaiser, IVS-EOS

25 Optimized Flooding: Gossip - Extended p-persistant Broadcast - Not robust - Adjustable overhead - Statistical coverage A C S event event G B D 0.8> Ch. Steup / J. Kaiser, IVS-EOS

26 Flooding Pro: - very easy to implement - very limited local storage needed - robust Cons: - heavy overhead or statistical coverage 26 Ch. Steup / J. Kaiser, IVS-EOS

27 Distance Vector Routing (DVR) - Pro-active - Sharing of Routing tables - Broadcast every 30 s S A C G B D From S Via A Via B Via C Via D Via G To A 1 To B 1 To C To D To G 27 Ch. Steup / J. Kaiser, IVS-EOS

28 Distance Vector Routing (DVR) - Pro-active - Sharing of Routing tables - Broadcast every 30 s S A C G B D From S Via A Via B Via C Via D Via G To A 1 2 To B 2 1 To C 2 3 To D 3 2 To G Ch. Steup / J. Kaiser, IVS-EOS

29 Ad-hoc Distance Vector Routing (AODV) - Reactive - Nodes query local nodes - Recursive lookup RReq: S to G A C S G RReq: S to G B D 29 Ch. Steup / J. Kaiser, IVS-EOS

30 Ad-hoc Distance Vector Routing (AODV) RRep: S to G via A: 3 A C S G RRep: S to G via B: 3 B D 30 Ch. Steup / J. Kaiser, IVS-EOS

31 Distance Vector Routing Pro: - easy to implement - robust Cons pro-active: - high message overhead - long propagation latency Cons active: - initial floodding - tradeoff between robustness and overhead 31 Ch. Steup / J. Kaiser, IVS-EOS

32 Hierarchical Routing: Clustered stars - for example, cluster nodes exist between rooms of a hotel and each room has a star network for control. FFD RFD router coordinator 32 Ch. Steup / J. Kaiser, IVS-EOS

33 Multi-Hop vs. Single Hop Less Energy Less Collisions More Reliable From: Holger Karl, Lecture Sensor Networks, Uni Paderborn 33 Ch. Steup / J. Kaiser, IVS-EOS

34 Lessons Learned - Parameters of WSN Technologies - Major Problems: - Limited Memory - Limited Bandwidth - Limited Energy - Routing Approaches in WSN - Ubiquity of Embedded Systems 34 Ch. Steup / J. Kaiser, IVS-EOS