Matteo Petracca Scuola Superiore Sant Anna, Pisa

Transcription

1 Wireless stack and protection techniques Matteo Petracca Scuola Superiore Sant Anna, Pisa Basic Computing Theory and Practice in WSNs Scuola Superiore Sant Anna, Pisa June 21th 2010

2 Outline Introduction to WSNs Platforms and applications IEEE standard Physical Layer Medium Access Control Layer Data protection techniques for wireless channels Wireless communications issues The Binary Symmetric Channel Automatic Repeat request techniques Forward Error Correction techniques 06/21/2010 Matteo Petracca 2

3 Introduction to WSNs 06/21/2010 Matteo Petracca 3

4 Wireless Sensor Networks WSNs applications: Military monitoring (vehicular traffic, enemy movements) Environmental observations (pollution,temperature,etc...) Building monitoring (intelligent house) Health care (biomedical applications) 06/21/2010 Matteo Petracca 4

5 Famous Platforms for WSNs Telos Developed by the University of Californa, Berkeley MSP430F1611 (8MHz) CC2420 (2.4GHz, 250kbps) 8 Mbit of external memory Mica and Mica2 Developed by the University of Californa, Berkeley ATMEGA128L (8MHz) TR1000 (Mica, 916MHz, 40kbps) CC1000 (Mica2. 916MHz, 38.4kbps) 4 Mbit of external memory TinyNode Developed by the EPFL, Lausanne MSP430F1611 (8MHz) XE1205 (869MHz, 76.8kbps) 4 Mbit of external memory 06/21/2010 Matteo Petracca 5

6 From WSNs to WMSNs A new generation of devices: Stargate board with a medium resolution camera. CITRIC mote with camera and microphone. Multimedia applications over WSNs: Environmental video surveillance Advanced health care monitoring Opportunistic safety applications 06/21/2010 Matteo Petracca 6

7 Sant Anna s solution for WMSNs Motherboard: PIC32 (40MHz) MRF24J40MB (2.4GHz) Ethernet controller RS232 Daughterboard: PIC32 (40MHz) CMOS camera (640x480) FRAM 250KB Ligth sensor 06/21/2010 Matteo Petracca 7

8 IEEE standard 06/21/2010 Matteo Petracca 8

9 Communication stack (ISO/OSI Model) The O.S.I. model (O.S.I. - Open System Interconnection) is a way of sub-dividing a communication system into smaller parts (called layers). Each layer is delegated to specific functionalities Each layer implements: Protocols to manage the communication with the corrisponding layer in other nodes Services provided to adjacent layer through service interfaces 06/21/2010 Matteo Petracca 9

10 Communication stack (ISO/OSI Model) Services and applications Data formatting Services for opening, closing and managing sessions End-to-end communication Routing, Logical addressing LLC, MAC, Physical addressing Bit encoding, Modulation, Physical channel, Transceiver control 06/21/2010 Matteo Petracca 10

11 Communication stack (ISO/OSI Model) Logical link Physical link 06/21/2010 Matteo Petracca 11

12 Communication Overhead Data TRA- Header Data NET- Header TRA- Header Data DL- Header NET- Header TRA- Header Data PHY- Header DL- Header NET- Header TRA- Header Data 06/21/2010 Matteo Petracca 12

13 Collapsed Model for WSNs Multi-Hop Communications Single-Hop Communications 06/21/2010 Matteo Petracca 13

14 IEEE generalities IEEE is a standard which specifies the Physical layer (PHY Layer) and the Media Access Control (MAC layer) for low-rate wireless personal area networks (LR-WPANs) Standard history: 2003: First version of the standard PHY and MAC specifications Over-the-air data rates 250 kbps, 40kbps, 20 kbps 2006: Revision of the standard Additional PHY support Over-the-air data rates 250 kbps, 100kbps, 40kbps, 20 kbps 06/21/2010 Matteo Petracca 14

15 Wireless Personal Area Network (1) A Wireless Personal Area Network (WPAN) is a wireless network used for communications among devices in proximity of individual s body (short range). From the book: Low-Rate Wireless Personal Area Networks 06/21/2010 Matteo Petracca 15

16 Wireless Personal Area Network (2) A Wireless Personal Area Network (WPAN) is a wireless network used for communications among devices in proximity of individual s body (short range). From the book: Low-Rate Wireless Personal Area Networks 06/21/2010 Matteo Petracca 16

17 IEEE PHY: Rates and freq. 4 possible data rates: 27 channels in 3 frequency bands: 868 MHz band, 1 channel (Only Europe) 915 MHz band, 10 channels (Only America) 2.4 GHz band, 26 channels (Worldwide) 06/21/2010 Matteo Petracca 17

18 IEEE PHY: Rates and freq. 4 possible data rates: 27 channels in 3 frequency bands: 868 MHz band, 1 channel (Only Europe) 915 MHz band, 10 channels (Only America) 2.4 GHz band, 26 channels (Worldwide) 06/21/2010 Matteo Petracca 18

19 IEEE PHY: 2.4GHz modulation O-QPSK modulation: DSSS applied before modulation A total of 16 waveforms 62.5 Ksymbols/s * 4 bit per symbol = 250 kbit/s Gray labeling adopted for minimizing the Bit Error Rate 06/21/2010 Matteo Petracca 19

20 IEEE PHY: 2.4GHz modulation The Direct Sequence Spread Spectrum concept: Input data fx modulation PN code PN From the book: Low-Rate Wireless Personal Area Networks 06/21/2010 Matteo Petracca 20

21 IEEE PHY: PPDU The PHY Protocol Data Unit: 7 preamble bytes for syncronization 120 bytes as max size of PHY PDU /21/2010 Matteo Petracca 21

22 IEEE MAC: Network topologies 06/21/2010 Matteo Petracca 22

23 IEEE MAC: Superframe The Superframe structure: Two periods: Active period (tx data) Inactive period (sleep) CAP period + GTS period = 16 slots (Max GTS number = 7) The GTS allocation is decided by the PAN coordinator In the CAP period the Slotted CSMA/CA is performed 06/21/2010 Matteo Petracca 23

24 IEEE MAC: CSMA/CA The Carrier Sense Multiple Access with Collision Avoidance mechanism: The sender listen the channel for a defined amount of time, if the channel is free the node sends the data If the channel is busy the node waits for a backoff period BackoffPer iod = random BE ( 2 1) 06/21/2010 Matteo Petracca 24

25 IEEE MAC: CSMA/CA 06/21/2010 Matteo Petracca 25

26 IEEE MAC: Hidden node The Hidden node problem: B to Hub transmission 1. The node B wants to send a packet to the Hub 2. A starts to send a packet to the Hub 3. B cannot listen A, so it starts to send its own packet 4. Collision!!! No solutions in IEEE (short range networks) 06/21/2010 Matteo Petracca 26

27 IEEE MAC: Star topology Star network Data transfer: Beacon-Enabled mode 1. The node waits for the beacon 2. If the node has a GTS allocated it waits for the appropriate slot 3. If the node has not a GTS it performs an access in the CAP by means the Slotted CSMA/CA mechanism Non-Beacon-Enabled mode 1. The node sends data adopting the unslotted CSMA/CA mechanism 06/21/2010 Matteo Petracca 27

28 IEEE MAC: P2P topology Peer-to-Peer Data transfer: Non-Beacon-Enabled mode 1. The node sends data adopting the unslotted CSMA/CA mechanism 06/21/2010 Matteo Petracca 28

29 IEEE MAC: MPDU The MAC Protocol Data Unit: 3 bytes for Frame Control and Sequence Number Max 20 bytes for the addressing 2 bytes for the Frame Check Sequence 95 bytes for Data Payload without Ausiliary Security Header 06/21/2010 Matteo Petracca 29

30 Zigbee ZigBee is a specification for a suite of high level communication protocols using small, low-power digital radios based on the IEEE standard for Wireless Personal Area Network(WPANs). 06/21/2010 Matteo Petracca 30

31 Data protection techniques for wireless channels 06/21/2010 Matteo Petracca 31

32 The wireless channel Main wireless channel issues: Reflection When a waveform propagating in medium A hits the boundary to another medium B and the boundary layer between them is smooth, one part of the waveform is reflected back into medium A, another one is transmitted into medium B, and the rest is absorbed. Diffraction By Huygen s principle, all points on a wavefront can be considered as sources of a new wavefront. If a waveform hits a sharp edge, it can by this token be propagated into a shadowed region. 06/21/2010 Matteo Petracca 32

33 The wireless channel Main wireless channel issues: Scattering When a waveform hits a rough surface, it can be reflected multiple times and diffused into many directions Doppler fading When a transmitter and receiver move relative to each other, the waveforms experience a shift in frequency, according to the Doppler effect. Too much of a shift can cause the receiver to sample signals at wrong. Attenuation and other factors f v = c s, r 1 f0 P RX = A λ 4πD 2 G RX G TX χ 2 P TX 06/21/2010 Matteo Petracca 33

34 The Binary Simmetric Channel p p p 1-p 0 1 Model definition: P( Rx = 0 Tx = 0 ) = 1 p P( Rx = 0 Tx = 1 ) = p P( Rx = 1 Tx = 0 ) = p P( Rx = 1 Tx = 1 ) = 1 p BSC is a memoryless channel BER=p 06/21/2010 Matteo Petracca 34

35 PLR without protection techniques PLR in case of BSC channel model: BER = p n = number of bits P (Re ceivepacket) = (1 p) n = Probability to have no errors P ( DiscardPacket ) ) P( DiscardPacket) n 1 n = 1 (1 p = Probability to have at least one error 06/21/2010 Matteo Petracca 35

36 Forward Error Correction techniques Main principles of FEC techniques: Redundancy informations (overhead) are added to the original data packet in order to recover bit error The PLR strictly depends on the adopted redundancy technique Request Sensor node Data + redundancy Coordinator 06/21/2010 Matteo Petracca 36

37 Automatic Repeat request techniques Main principles of ARQ techniques: If a packet is corrupted (discarded) or is lost a new transmission is performed p = probability to loose a single packet k = max number of retransmissions k PLR = p k PLR = 0 Request Sensor node Data Coordinator 06/21/2010 Matteo Petracca 37

38 Example Data packet without protection: Header Data FCS n PLR = 1 (1 p) = 1 (1 p) Data packet with protection: 456 Header FCS Data Data Data P H +F P DATA P H + F P DATA = 1 (1 p) PER = (1 p) (1 (1 p 3 3p = probabilit y to discard packet due to H + F errors = probability to discard packet due to Data errors = 0 PLR = 1 (1 p) 216 (1 p)) 240 ) 06/21/2010 Matteo Petracca 38

39 Example PLR comparison: 06/21/2010 Matteo Petracca 39

40 PER: Example 06/21/2010 Matteo Petracca 40

41 thank you! 06/21/2010 Matteo Petracca 41