Medium Access Control in Wireless Sensor Networks

Medium Access Control in Wireless Sensor Networks

• 1. MedIum Access Control In WIreless Sensor Networks MedIum Access Control In WIreless Sensor Networks
• 2. Contents Contents •CSMA/CA recap •S-MAC •B-MAC •X-MAC •SCP •TDMS •Z-Mac •MLA (Mac Layer Architecture)
• 3. CSMA/CA CSMA/CA Carrier sense Collision avoidance via random back-off Virtual Carrier Sense: RTS/CTS
• 4. MAC Challenges MAC Challenges •Traditionally –Fairness –Latency –Throughput •For Sensor Networks –Power efficiency –Scalability
• 5. Energy-Effıcıent MAC Energy-Effıcıent MAC Expected life time of many WSN applications: Months or years Actual lifetime •AA batteries: Max. 2000 mAh •CC2420 radio: 19.7mA when idle but awake (RX mode) •2000mAh / 19.7mA = 101.5 hours = 6 days •Keep radio asleep most of the time •Ideal duty cycle: 0.1% - 1% C. Lu, Washington Univ. Saint Louis
• 6. Types of WSN MAC Types of WSN MAC Scheduled contention: Nodes periodically wake up together, contend for channel, then go back to sleep •S-MAC, T-MAC Channel polling: Nodes independently wake up to sample channel •B-MAC, X-MAC TDMA (Time Division Multiple Access): Nodes maintain a schedule that dictates when to wake up and when they are allowed to transmit •DRAND Hybrid: SCP, Z-MAC, 802.15.4 (contention access period + contention free period)
• 7. S-MAC (Sensor MAC) S-MAC (Sensor MAC) •A node sleeps most of the time •Periodically wake up for short intervals to see if any node is transmitting a packet •Low energy consumption if traffic is light •Accept latency to extend lifetime
• 8. SMAC SMAC •Awake time consists of two parts: SYNC and RTS •A node periodically send SYNC packet to synchronize clocks •CSMA/CA for channel contention C. Lu, Washington Univ. Saint Louis
• 9. S-MAC S-MAC •RTS is section used to transmit data •CSMA/CA followed by RTS/CTS
• 10. S-MAC S-MAC CTS for somebody else  Sleep Sender does one RTS/CTS and then sends data for the rest of the frame •Prefer application performance to node level fairness ACK every data packet •Packet fragmentation for higher reliability
• 11. Pros and Cons of S-MAC Pros and Cons of S-MAC More power conserving than standard CSMA/CA During the listening interval, everyone needs to stay awake unless someone transmits •Waste energy when network traffic is light Time sync overhead RTS/CTS/ACK overhead Complex to implement
• 12. B-MAC (Berkeley MAC) B-MAC (Berkeley MAC) •Clear Channel Assessment (CCA) –Measure the SNR by taking a moving average when there seems to be no traffic •Low Power Listening (LPL) –Periodic preamble sampling: Preamble > Sleep period –No sync between nodes •Hidden terminal and multi-packet mechanisms not provided Sleep t Receive Receiver Sleep t Preamble Sender Message Sleep
• 13. Pros and Cons of B-MAC Pros and Cons of B-MAC No need for everybody to stay awake when there is no traffic •Just wake up for preamble sampling and go back to sleep Better power conservation, latency and throughput than S-MAC Simpler to implement Low duty cycle  longer preamble •Little cost to receiver yet higher cost to sender •Longer delay •More contention
• 14. X-MAC: Overhearıng avoıdance X-MAC: Overhearıng avoıdance Include destination address in short preambles Non-receiver avoids overhearing
• 15. X-MAC: Early ACK X-MAC: Early ACK •Receiver acknowledges preamble  Sender stops sending preamble
• 16. Thoughts on X-MAC Thoughts on X-MAC Better than B-MAC in terms of latency, throughput and power consumption Energy consumption due to overhearing reduced Simple to implement On average the preamble size is reduced by half compared to B-MAC  Still considerable overhead
• 17. SCP-MAC SCP-MAC Scheduled Channel Polling by everybody - Avoid long preambles in - LPL (Low Power Listening) supported by B-MAC Wake up tone - Much shorter than preamble in LPL followed by data
• 18. SCP-MAC SCP-MAC Adaptive channel polling A sends to B, B adds N dynamic high frequency polls If any of them is useful, B adds N polls in the next frame –Otherwise, switch back to the low frequency channel polling All data can travel N hops using N polling periods
• 19. TDMA TDMA - Predictable delay, throughput and duty cycle - Little packet losses due to contention - Scheduling and time sync are difficult - Slots are wasted when a node has nothing to send
• 20. Z-MAC (Zebra MAC) Z-MAC (Zebra MAC) •Runs on top of B-MAC •Rely on CSMA under light load  Switch to TDMA under high contention TDMA Pros - Naturally avoids collisions Cons - Complexity of scheduling - Synchronization needed CSMA Pros - Simple - Scalable Cons - Collisions due to hidden - terminals - RTS/CTS is overhead
• 21. Thoughts on Z-MAC Thoughts on Z-MAC Good idea to combine strengths of CSMA and TDMA Complex Especially hard to implement TDMA part –How to deal with topology changes? MAC protocols supported by TinyOS –CC1100: experimental B-MAC –CC2420: X-MAC
• 22. MLA (MAC Layer Archıtecture) MLA (MAC Layer Archıtecture) Low-level abstractions for radio functionality High-level implementations of common MAC algorithms Implemented in TinyOS 2.0.2 Used to implement platform independent MAC –B-MAC, X-MAC, SCP, TDMA, a variant of Z-MAC K. Klues, G. Hackmann, O. Chipara and C. Lu, A Component Based Architecture for Power-Efficient Media Access Control in Wireless Sensor Networks, ACM Conference on Embedded Networked Sensor Systems (SenSys'07), November 2007.