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Lecture 8


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Lecture 8

  1. 1. MAC, Physical Layer, Energy Consumpion and IEEE 802.15.4 Lecture 8 September 28, 2004 EENG 460a / CPSC 436 / ENAS 960 Networked Embedded Systems & Sensor Networks Andreas Savvides [email_address] Office: AKW 212 Tel 432-1275 Course Website
  2. 2. Announcements <ul><li>Appointment schedule for projects </li></ul><ul><li>Student presenter for Oct 12 – Diffusion routing </li></ul><ul><li>Project proposal </li></ul><ul><ul><li>1 page description of your project (including references) </li></ul></ul><ul><ul><li>Should include: </li></ul></ul><ul><ul><ul><li>What is the problem your solving and what is the new feature that you are adding to the problem </li></ul></ul></ul><ul><ul><ul><ul><li>Narrow down the problem you will be working on, be very precise with what you are going to do </li></ul></ul></ul></ul><ul><ul><ul><li>Give an initial list of paper references on which your paper will be based </li></ul></ul></ul><ul><ul><ul><li>A list of resources that you will need for the project (any additional HW, SW and sensors) </li></ul></ul></ul><ul><ul><li>Do not exceed 1-page!!!! </li></ul></ul><ul><ul><li>Email to [email_address] </li></ul></ul><ul><ul><ul><li>Filename: name1_and_name2_proposal </li></ul></ul></ul><ul><ul><ul><li>Email Subject: EENG460 Project Proposal </li></ul></ul></ul>
  3. 3. Frequency Bands and Data Rates <ul><li>In 2.4GHz band 62.5 ksymbols/second </li></ul><ul><li>1 symbol is 4 bits </li></ul><ul><li>1 symbol is encoded into a 32-bit pseudorandom sequence the chip </li></ul><ul><li>chip rate = 62.5 x 32 = 2000 kchips/s </li></ul><ul><li>Raw data rate = Symbol rate * chips per symbol </li></ul><ul><li>= 62.5 * 4 = 250kb/s </li></ul><ul><li>In 868/915 MHz bands </li></ul><ul><li>1 bit symbol (0 or 1) is represented by a 15-chip sequence </li></ul>
  4. 4. Physical Layer Transmission Process Binary Data from PPDU Bit to Symbol Conversion O-QPSK Modulator Symbol to Chip Conversion RF Signal
  5. 5. Radio Characteristics <ul><li>Power output </li></ul><ul><ul><li>The standard does not specify a power output limit. </li></ul></ul><ul><ul><li>Devices should be able to transmit -3dBm </li></ul></ul><ul><ul><ul><li>In US 1Watt limit in Europe 10mW for 2.4GHz band </li></ul></ul></ul><ul><li>Receiver should be able to decode a packet with receive power of </li></ul><ul><ul><li>-85dBm in 2.4GHz and -92dBm in the lower frequency bands </li></ul></ul><ul><li>What does that mean in terms of range? </li></ul>
  6. 6. Going from Watts to dBm -10dBm=.1mW -3dBm=.5mW 0dBm=1mW +3dBm=2mW +4dBm=2.5mW +6dBm = 4mW +7dBm=5mW +10dBm=10mW +20dBm=100mW
  7. 7. Friss Free Space Propagation Model Same formula in dB path loss form (with Gain constants filled in): How much is the range for a 0dBm transmitter 2.4 GHz band transmitter and pathloss of 92dBm?
  8. 8. Friss Free Space Propagation Model Same formula in dB path loss form: How much is the range for a 0dBm transmitter 2.4 GHz band transmitter and pathloss of 92dBm? <ul><li>Highly idealized model. It assumes: </li></ul><ul><li>Free space, Isotropic antennas </li></ul><ul><li>Perfect power match & no interference </li></ul><ul><li>Represent the theoretical max transmission range </li></ul>
  9. 9. Propagation Mechanisms in Space with Objects <ul><li>Reflection </li></ul><ul><ul><li>Radio wave impinges on an object >> λ (30 cm @1 GHz) </li></ul></ul><ul><ul><li>Earth surface, walls, buildings, atmospheric layers </li></ul></ul><ul><li>Diffraction </li></ul><ul><ul><li>Radio path is obstructed by an impenetrable surface with sharp irregularities (edges) </li></ul></ul><ul><ul><li>Secondary waves “bend” arounf the obstacle </li></ul></ul><ul><ul><li>Explains how RF energy can travel without LOS </li></ul></ul><ul><li>Scattering </li></ul><ul><ul><li>When medium has large number of objects < λ (30cm @1 GHz) </li></ul></ul><ul><ul><li>Similar principles as diffraction, energy reradiated in many directions </li></ul></ul><ul><ul><li>Rough surfaces, small objects (e.g foliage, lamp posts, street signs) </li></ul></ul><ul><li>Other: Fading and multipath </li></ul>
  10. 10. A more realistic model: Log-Normal Shadowing Model <ul><li>Model typically derived from measurements </li></ul><ul><li>Statistically describes random shadowing effects </li></ul><ul><ul><li>values of n and σ are computed from measured data using linear regression </li></ul></ul><ul><li>Log normal model found to be valid in indoor environments!!! </li></ul>
  11. 11. Transmit Power Levels in Chipcon CC2420 = 1mW = 43.5mW Radio supply voltage= 2.5V And Power = I*V
  12. 12. Budgeting Battery Power <ul><li>Assuming power drain is the same for Transmitting and Receiving = 43.5mW </li></ul><ul><li>We need to power the device from a 750mAh battery for 1 year </li></ul><ul><li>What is the duty cycle we need to operate at? </li></ul>
  13. 13. Budgeting Battery Power <ul><li>Assuming power drain is the same for Transmitting and Receiving = 43.5mW </li></ul><ul><li>We need to power the device from a 750mAh battery for 1 year </li></ul><ul><li>What is the duty cycle we need to operate at? </li></ul><ul><li>1 year has 365 x 24 = 8760 hours </li></ul><ul><li>The average current drain from the battery should be </li></ul><ul><li>Average power drain </li></ul>
  14. 14. Computing Duty Cycle
  15. 15. Energy Implication <ul><li>Active transceiver power consumption more related to symbol rate rather than raw data rate </li></ul><ul><li>To minimize power consumption: </li></ul><ul><ul><li>Minimize T on - maximize data rate </li></ul></ul><ul><ul><li>Also minimize I on by minimizing symbol rate </li></ul></ul><ul><li>Conclusion: Multilevel or M-ary signalling should be employed in the physical layer of sensor networks </li></ul><ul><ul><li>i.e need to send more than 1-bit per symbol </li></ul></ul>
  16. 16. Radio Energy Model: the Deeper Story…. <ul><li>Wireless communication subsystem consists of three components with substantially different characteristics </li></ul><ul><li>Their relative importance depends on the transmission range of the radio </li></ul>Tx: Sender Rx: Receiver Channel Incoming information Outgoing information Transmit electronics Receive electronics Power amplifier
  17. 17. Radio Energy Cost for Transmitting 1-bit of Information in a Packet <ul><li>The choice of modulation scheme is important for energy vs. fidelity and energy tradeoff </li></ul>
  18. 19. Examples <ul><li>The RF energy increases with transmission range </li></ul><ul><li>The electronics energy for transmit and receive are typically comparable </li></ul>nJ/bit nJ/bit nJ/bit GSM Nokia C021 Wireless LAN Medusa Sensor Node (UCLA) ~ 1 km ~ 50 m ~ 10 m
  19. 20. Power Breakdowns and Trends Analog electronics 240 mW Digital electronics 170 mW Power amplifier 600 mW (~11% efficiency) Intersil PRISM II (Nokia C021 wireless LAN) Radiated power 63 mW (18 dBm) <ul><li>Trends: </li></ul><ul><ul><li>Move functionality from the analog to the digital electronics </li></ul></ul><ul><ul><li>Digital electronics benefit most from technology improvements </li></ul></ul><ul><li>Borderline between ‘long’ and ‘short’-range moves towards shorter transmit distances </li></ul>
  20. 21. What is wrong with this model? <ul><li>Does not include many parameters </li></ul><ul><ul><li>DC-DC converter inefficiencies </li></ul></ul><ul><ul><li>Overhead for transitioning from on to standby modes </li></ul></ul><ul><ul><li>Different power consumptions for receiver and transmitter </li></ul></ul><ul><ul><li>Battery discharge properties </li></ul></ul><ul><ul><li>Does not include the processor power and any additional peripherals </li></ul></ul>
  21. 22. Where does the Power Go? Power Supply Battery DC-DC Converter Peripherals Disk Display Communication Radio Modem RF Transceiver Processing Programmable  Ps & DSPs (apps, protocols etc.) Memory ASICs
  22. 23. DC-DC Converter Inefficiency Current drawn from the battery Current delivered to the node
  23. 24. Battery Capacity <ul><li>Current in “C” rating: load current normalized to battery’s capacity </li></ul><ul><ul><ul><li>e.g. a discharge current of 1C for a capacity of 500 mA-hrs is 500 mA </li></ul></ul></ul>from [Powers95]
  24. 25. Microprocessor Power Consumption CMOS Circuits (Used in most microprocessors) Dynamic Component Digital circuit switching inside the processor Static Component Bias and leakage currents O(1mW) Static Dynamic
  25. 26. Power Consumption in Digital CMOS Circuits - current constantly drawn from the power supply - determined by fabrication technology <ul><li>short circuit current due to the DC path between the </li></ul><ul><li>supply rails during output transitions </li></ul>- load capacitance at the output node - clock frequency - power supply voltage
  26. 27. DVS on Low Power Processor <ul><li>Maximum gain when voltage is lowered BUT lower voltage increases circuit delay </li></ul>CMOS transistor threshold voltage Transistor gain factor Dynamic Power Component Number of gates Load capacitance of gate k Propagation delay
  27. 28. Now Back to IEEE 802.15.4 MAC <ul><li>MAC supports 2 topology setups: star and peer-to-peer </li></ul><ul><li>Star topology supports beacon and no-beacon structure </li></ul><ul><ul><li>All communication done through PAN coordinator </li></ul></ul>
  28. 29. Star: Optional Beacon Structure Beacon packet transmitted by PAN Coordinator to help Synchronization of network devices. It includes: Network identifier, beacon periodicity and superframe structure Generic Superframe Structure GTS: Guaranteed time Slots assigned by PAN coordinator
  29. 30. Star Network: Communicating with a Coordinator
  30. 31. Star Network: Communicating from a Coordinator Beacon packet indicates that there is data pending for a network device Device sends request on a data slot Network device has to ask coordinator if there is data pending. If there is no data pending the Coordinator will respond with a zero Length data packet
  31. 32. Peer-to-Peer Data Transfer <ul><li>Peer-to-peer data transfer governed by the network layer – not specified by the standard </li></ul><ul><li>Four types of frames the standard can use </li></ul><ul><ul><li>Beacon frame – only needed by a coordinator </li></ul></ul><ul><ul><li>Data frame – used for all data transfers </li></ul></ul><ul><ul><li>ACK frame – confirm successful frame reception </li></ul></ul><ul><ul><li>A MAC Command Frame – MAC peer entity controltransfers </li></ul></ul>
  32. 33. Beacon Frame
  33. 34. ACK & Data Frames ACK Frame Data Frame
  34. 35. MAC Command Frame
  35. 36. Wrap-up Low Power MAC <ul><li>You now have enough information to do a more detailed power consumption analysis for IEEE 802.15.4 </li></ul><ul><li>Need to factor in different packet structures header and MAC overheads </li></ul><ul><li>What are the issues related with low power MAC protocols? </li></ul><ul><li>Design of low power schemes for peer-to-peer networking… </li></ul>
  36. 37. Concept of Primitives <ul><li>Request: To initiate a service </li></ul><ul><li>Indication: Indicate an N-layer event that is significant to the used </li></ul><ul><li>Response: to complete a procedure previously invoked by an indication primitive </li></ul><ul><li>Confirm: conveys the results of one or more associated previous service requests </li></ul>
  37. 38. Next Lecture <ul><li>Time Synchronization </li></ul><ul><li>Read the paper </li></ul><ul><li>[Elson02] Fine-Grained Network Time Synchronization using Reference Broadcasts , Jeremy Elson, Lewis Girod and Deborah Estrin, Proceedings of the Fifth Symposium on Operating Systems Design and Implementation (OSDI 2002), Boston, MA. December 2002. UCLA Technical Report 020008.  </li></ul>