Outline Medium Access Control With Introduction to MAC Coordinated - PDF document

Outline Medium Access Control With  Introduction to MAC Coordinated Adaptive Sleeping  S-MAC Overview for Wireless Sensor Networks  S-MAC Evaluation  Critique  Comparison to MACAW Presented by: Arik Brooks Washington University Department of Computer Science and Engineering MAC (Medium Access Control) General MAC Requirements  All shared-medium networks need an  Ensure reliable communication effective MAC protocol  Ensure fair access to shared physical  Controls access to the shared medium transmission medium for all contending  Ensures no two nodes interfere with each streams other’s transmissions  Minimize delay for sending/receiving messages A B C  Maximize bandwidth utilization A sends B receives C sends a message ??? a message Typical MAC protocols Typical MAC protocols  Contention-based  Contention-based  CSMA (Carrier Sense Multiple Access)  Pros – Avoids collisions well, no synchronization  MACA/MACAW (Virtual Sensing RTS/CTS)  Cons – Control overhead, idle listening  IEEE 802.11 (Physical & Virtual Sensing)  Schedule-based  PAMAS  Pros – Conserves energy  Schedule-based  Cons – Doesn’t scale well, synchronization difficult  TDMA (Time-Division Multiple Access)  Others  CDMA (Code-Division Multiple Access) 1

MAC Requirements for S-MAC: Design Wireless Sensor Networks  Energy Efficiency  Uses elements of contention-based and schedule-based MAC protocols  Scalability/Adaptability to network size, density, and topology  Tries to reduce energy waste from all major sources  Less emphasis on fairness, latency,  Collisions, Overhearing, Control packet overhead, throughput, and bandwidth utilization Idle Listening  Trades performance for energy efficiency  Increases per-hop delay  Reduces fairness Sources of Energy Waste S-MAC: Low Duty-Cycle Operation  Collisions  Low duty-cycle operation  Re-transmitting packets takes a lot of energy  Reduces idle-listening by putting nodes to sleep most of the time  Overhearing  However, increases latency, which can  Receiving packets destined to other nodes accumulate on each hop  Control packet overhead  Sending/Receiving control packets  Idle Listening Listen Sleep Listen Sleep L…  An idle radio consumes almost as much power as during reception S-MAC: Coordinated Sleeping S-MAC: Listen Schedule  Uses Coordinated Sleeping to reduce control  Listen broken into slots for SYNC, RTS, CTS overhead and latency  Nodes adopt their neighbors sleep schedule  If conflicting schedules, nodes can adopt both or Receiver Listen just remember the second for transmit SYNC RTS CTS Rx Data/Sleep  Only initiate communication during awake time, during which neighbors should also be awake Transmitter  Periodically broadcast SYNC message to Tx SYNC Tx RTS Rx CTS Tx Data … maintain sync CS CS  Periodic neighbor discovery (listens to whole sync period) 2

S-MAC: Collision Avoidance S-MAC: Message Passing  Sending Long Messages is costly in terms of  Collision and Overhearing Avoidance energy and latency  Uses physical AND virtual carrier sense  Re-transmission of a long message is costly  RTS/CTS/Data/Ack  Transmitting many small fragments requires extra  All nodes that hear a RTS or CTS sleep for the overhead specified TX time to avoid overhearing/collision  Message Passing used to minimize costs of  Inspired by PAMAS sending a long message  Use a single RTS/CTS for all fragments, reducing Sender Receiver control packet overhead A B C D E F  Re-transmission limited to corrupted fragments S-MAC: Example w/o Adaptive S-MAC: Adaptive Listening Listening  Uses Adaptive Listening to minimize delay A B C D  Nodes that hear RTS/CTS wake up and Schedule Schedule listen for a short time immediately after the Wake Wake previous transmission should have ended A Tx Data CS  Not as good as it sounds because RTS/CTS Added RTS CTS ACK B Rx Data packets exchanged during adaptive wakeup CS Delay period are not during the next packets wake CTS RTS CTS C time D CTS S-MAC: Example with Adaptive S-MAC: Energy Performance Listening  Two-hop network A B C D  Savings due to avoiding overhearing and Schedule Adaptive Schedule Wake Wake Wake efficiently transmitting Adaptive long messages A Tx Data Wake CS B RTS CTS ACK Added Source 1 Sink 2 Rx Tx Data CS A E Delay C CTS RTS CTS ACK C Rx CS CS B D D CTS RTS RTS CTS Source 2 Sink 1 Not Seen Not Seen 3

S-MAC: Energy Performance S-MAC: Latency Performance … … 1 2 3 10 11 1 2 3 10 11 Source Source Sink Sink Lowest Traffic Load Highest Traffic Load S-MAC: Throughput Performance S-MAC: Energy-Time Cost Performance … … 1 2 3 10 11 1 2 3 10 11 Source Source Sink Sink Highest Traffic Load Another Traffic Load Key Ideas Critique/Questions  Combines advantages of contention-based  The authors dismiss fairness as an important part of (good collision avoidance) and schedule- the MAC protocol because of an unsubstantiated based (energy efficiency) assumption about the nature of wireless sensor network applications  Low-duty cycle with coordinated sleeping  How does S-MAC compare to other MAC protocols  RTS/CTS/Data/Ack when awake designed for energy efficiency (TDMA, etc…)?  Overhearing avoidance based on info in  Do schedules ever combine or get reformed to RTS/CTS packets synchronize more groups of nodes? This doesn’t  Adaptive sleeping used to reduce latency seem to meet the requirement of being flexible to compared to other schedule-based protocols network changes 4

Comparison to MACAW Where to learn more  MACAW  http://www.isi.edu/scadds/projects/smac/  Designed to improve fairness, throughput, latency, and  General info and links related to S-MAC reliability  http://www.isi.edu/ilense/software/smac/  RTS-CTS-DS-DATA-ACK  Download source code  Dynamic backoff algorithm (lower if contention level is low)  S-MAC  FAQ  Designed to improve energy efficiency  http://www.isi.edu/~weiye/pub/commstack.pdf  Adds coordinated sleep for energy conservation  Detailed description of the modified comm stack  Backoff algorithm is random sleep used to implement S-MAC on TinyOS  Uses fragmentation to reduce control overhead 5