Presentation Outline Introduction B-MAC Implementation Versatile Low Power Media Access for B-MAC Results Wireless Sensor Networks Critique Comparison Presented By: Eitan Marder-Eppstein 2 Why is B-MAC Needed? B-MAC's Idea of MAC for WSN B-MAC: A carrier sense media access protocol for wireless S-MAC and T-MAC: sensor networks No flexibility – What if network conditions change? Too much code – Limited memory on motes B-MAC Goals: Doesn't scale well – As network size increases, nodes must maintain more Low Power Operation schedules Effective Collision Avoidance Well... there's also WiseMAC: Simple Implementation, Small Code and RAM Size Efficient Channel Utilization Regardless of Data Rate Has no mechanism to reconfigure based on services using the protocol Reconfigurable by Network Protocols Motivated by needs of monitoring applications Tolerant to Changing Network Conditions Highly Scalable 3 4 B-MAC Implementation B-MAC Implementation Carrier Sense Multiple Access with Collision Avoidance Clear Channel Assessment (CCA) Listen for a pre-determined amount of time for network activity Ambient noise changes If the network is idle then send a packet depending on environment If activity is detected then wait for a random amount of time (called backoff B-MAC employs software factor) before re-attempting transmission to estimate the noise floor Search for outliers Backoff counter only decremented when channel is clear significantly below the noise Receiver floor since a valid packet Could Exist could never have one D Backoff Set Sender Hidden Terminal If clear then transmit If busy then backoff A B C F Backoff Set 5 ,4,3 E 2,1,0 5 6 1
B-MAC Implementation B-MAC Implementation Low Power Listening (LPL) B-MAC = Link Protocol When a node awakes it checks for radio activity using CCA Only a small core of media access functionality included with B-MAC If activity is detected, stay awake to receive the packet and then sleep Network services (organization, synchronization, routing) built above B-MAC If no packet (false positive) timeout forces node to sleep What about Hidden Terminal, Fragmentation, etc? Preamble Length No “built in” mechanisms for handling these Preamble must be at least as long as duty cycle for reliable data Instead, B-MAC exposes a set of interfaces to allow services to tune B-MAC reception Protocols built on B-MAC can optimize performance in their environment Wake Up Interface MacControl{ Interface LowPowerListening{ EnableCCA();/DisableCCA(); Set/GetListeningMode(); Sender Receiver Receiver EnableAck();/DisableAck(); Set/GetTransmitMode(); HaltTx(); Set/GetPreambleLength(); A B Set/GetCheckInterval(); } Preamble Data 100ms 100ms Interface MacBackoff{ } Sender Preamble Duty Cycle initial/congestionBackoff(); } 7 8 B-MAC Implementation B-MAC – Adjusting Parameters Link Layer Acknowledgment Calculate Optimal Parameters If enabled, B-MAC transfers an ack code after receiving a unicast packet Specifically, minimize a node's energy consumption to maximize lifetime Solve system of 6 equations to find minimum energy consumption for a Small Code Size given network configuration: ROM and RAM limited Small code size important 9 10 B-MAC – Results B-MAC – Results Shows trade off of more frequently checking the radio to shorten packet transmission time Penalty for idle listening is much more severe than sending packets that are longer than necessary Node lifetime in years based on How duty cycle is affected by LPL check time and network density network density and LPL interval If both neighbourhood size and Best check interval is lowest line at check interval are known, their a given network density intersection gives expected lifetime using optimal parameters 11 12 2
B-MAC – Protocol Comparison Results - Latency/Energy Critique B-MAC to WiseMAC Comparison After talk of WiseMAC as the only serious competitor to B-MAC at the release date of this paper, the authors neglect to test against it. Is scheduling always a bad thing B-MAC does away with synchronization between nodes, but does this always improve performance. Is there a better way to schedule. Energy Floor for LPL The authors of the paper mention that there is an energy floor for LPL. Doesn’t this mean that for a network with high latency constraints B-MAC is not the most energy efficient protocol. In fact, results from the paper confirm that S-MAC uses less energy than B-MAC at high latency. Optimizing for Average Conditions: What if they often change? 13 14 Comparison and Questions B-MAC – Protocol Comparison Results - Throughput SCP Synchronize the entire network in an effective and efficient manner Eliminate the long preamble associated with LPL Adapts well to variable traffic loads Reduce duty cycle from 1-2% for current MAC protocols to 0.1% ? 15 16 B-MAC – Protocol Comparison Results - Fragmentation 17 3
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