1 Types of Power-Saving MACs Types of Power-Saving MACs (cont.) - PDF document

What Is Media Access Control?  Media Access Control (MAC) determines who gets access to the shared medium, and when Media Access Control  In wired settings, usually just tries to avoid Greg Hackmann contention  In wireless settings, can also significantly reduce CSE467S Spring 2009 energy consumption 2 Carrier Sense Multiple Access, Collision Avoidance (CSMA/CA) Hidden Terminal Problem CS CS 3 4 Virtual Carrier Sense: RTS/CTS Power-Saving MAC  Many applications’ expected lifetime: months or years RTS Payload  Actual lifetime: AA batteries: 2000 mAh (if you’re lucky)  CC2420 radio: 19.7 mA when idle but awake (RX mode)  2000 mAh / 19.7 mA = 101.5 h ≈ 6 days  CTS  This is a problem!  Solution: keep the radio asleep most of the time NAV Duty cycles on the order of 0.1% – 1%  5 6 1

Types of Power-Saving MACs Types of Power-Saving MACs (cont.)  Scheduled contention: nodes periodically wake up in Different trade-offs for latency, power consumption,  unison, contend for access to channel, then go back runtime overhead, etc. to sleep  Motivation for hybrid protocols: SCP [Ye 2006], Z- MAC [Rhee 2005], Funneling MAC [Ahn 2006] S-MAC [Ye 2002], T-MAC [van Dam 2003]   Channel polling: nodes independently wake up to sample radio channel B-MAC [Polastre 2004], X-MAC [Buettner 2006]   Time division multiple access (TDMA): nodes maintain schedule of when to wake and when they’re allowed to transmit 802.15.4 [IEEE 2003], DRAND [Rhee 2006]  7 8 The Sensor MAC (S-MAC) S-MAC: Boot Phase  Nodes stay asleep most of the time Waking up in 3 s Waking up  Periodically wake for short in 5 s intervals (0.5 s) to see if Waking up anyone’s sending in 4.5 s 3 s  Very low energy consumption when traffic is low 4.5 s 5 s 3 s 4.5 s 5 s 9 10 S-MAC: Sending a Packet S-MAC: Sending a Packet  Time awake divided RTS section used for  SYNC RTS SYNC RTS into two parts: SYNC transmitting data and RTS CSMA/CA again, followed  by RTS/CTS Receiver Receiver  Node periodically CS CS send SYNC packet to Wants to SYNC Wants to SYNC keep clocks in sync CS  CSMA/CA used to contend for access Wants to send data Wants to send data to wireless channel CS CS CS Wants to SYNC & send data Wants to SYNC & send data 11 12 2

S-MAC: Sending a Packet S-MAC: Evaluation  CTS for someone else RTS -> go to sleep CTS ACK ACK Overhearing avoidance  Receiver  Sender does one RTS/ CTS then sends data Sleep for rest of frame  All data packets are RTS DATA DATA ACKed RTS Winner Packet fragmentation =  Sleep higher reliability 13 14 S-MAC in Summary Berkeley MAC (B-MAC)  Power savings over standard 802.11 MAC  Long listening period is expensive Everyone stays awake unless somebody transmits   Time synchronization overhead even when t ≥ t network is idle  RTS/CTS and ACK overhead when sending data  Complex to implement 15 16 B-MAC: Throughput B-MAC: Power Consumption 17 18 3

B-MAC in Summary X-MAC: Room for Improvement Low overhead when network is idle  Header Simple to implement  Payload Better power savings, latency, and throughput than S-MAC  Sender  Lower duty cycle -> longer preambles: Higher average latency  Higher cost to send Recipient  Higher cost to overhear  More contention  Neighbor 19 20 X-MAC: Overhearing Avoidance X-MAC: Preamble ACKing Destination Header Destination Header Payload Payload Sender Sender ACK Recipient Recipient Neighbor Neighbor 21 22 X-MAC: Current Draw X-MAC: Latency 23 24 4

X-MAC in Summary TDMA Better latency, throughput, and power consumption than  Frame B-MAC for unicast traffic  Little energy consumed by overhearing Still simple to implement  No improvements over B-MAC for broadcast traffic  On average, cuts preamble by half -> sending packets is  still expensive Time A B C Sleep Sleep Sync Transmits Transmits Transmits Slot 25 26 SCP: Scheduled Contention + Channel TDMA in Summary Polling  Predictable latency, throughput, and duty cycle  Low packet loss due to contention  Schedules must be recreated when nodes leave/ enter neighborhood t t  Time synchronization overhead  Slots wasted when scheduled node has nothing to send  Often complex to implement 27 28 SCP: Adaptive Channel Polling SCP: Multi-Hop Pipelining Receiver Hop 2 Sender Hop 1  Add N sub-intervals whenever packet received Reduces latency of bursty data  Sender  Continue adding sub-intervals as long as they’re needed, and as long as there’s room Sender “gives up” first regular check after sending multi-  hop traffic Whole network quickly moves into adaptive polling mode  29 30 5

SCP: Energy Consumption SCP: Multi-Hop Latency 31 32 Zebra MAC (Z-MAC): TDMA + SCP in Summary Channel Polling  Channel polling: low overhead when idle Time A B C Sleep Sleep Sync Transmits Transmits Transmits  Scheduled contention: low cost to send  Low latency for multi-hop traffic A  Complex to implement  Experimental radio data required  Overhead due to time synchronization B Reduced by piggybacking on data packets  C 33 34 Z-MAC in Summary So Why Are They Rarely Used?  Reduces waste from unused slots  MAC protocols have additional radio-dependent requirements beyond normal application code Higher throughput and lower latency   Throughput, latency, and duty cycle no worse Turn radio on and off, low latency I/O, carrier sense,  etc. than pure TDMA  Typically implemented by forking radio stacks Hard to implement   Nodes still stay awake if no one transmits Must be maintained as original radio stack changes   Still overhead from time sync  Complex to implement  MAC layers supported by TinyOS today: CC1000: “experimental” B-MAC  CC2420: BoX-MAC (X-MAC-like + pieces of B-MAC)  35 36 6

A Better Approach: MLA References  MAC Layer Architecture (MLA) [Klues 07]  W. Ye, J. Heidemann and D. Estrin, Medium Access Control with Coordinated, Adaptive Sleeping for Wireless Sensor Networks, IEEE/ACM Low-level abstractions of radio functionality  Transactions on Networking, June 2004. High-level implementations of common MAC logic   J. Polastre, J. Hill, and D. Culler, Versatile Low Power Media Access for Wireless Sensor Networks, SenSys, 2004.  I. Rhee, A. Warrier, M. Aia, and J. Min, Z-MAC: a Hybrid MAC for Wireless  Implemented on TinyOS 2.0.2 Sensor Networks, SenSys, 2005.  Used to create 5 platform-independent MAC  M. Buettner, G. Yee, E. Anderson, R. Han, X-MAC: A Short Preamble MAC Protocol for Duty-Cycled Wireless Sensor Networks, SenSys, 2006. layers  W. Ye, F. Silva, and J. Heidemann, Ultra-Low Duty Cycle MAC with B-MAC, X-MAC, SCP, TDMA, variant of Z-MAC Scheduled Channel Polling, SenSys, 2006.   K. Klues, G. Hackmann, O. Chipara and C. Lu, A Component-Based Architecture for Power-Efficient Media Access Control in Wireless Sensor Networks, SenSys 2007. 37 38 7