1 Media Access Control in Wireless Sensor Networks - I

2 The MAC sub layer – Human Analogy I want to talk now Me too In wsgnk fstkgf hgh Determine who goes next on a multi-access channel

3 MAC Protocols: a taxonomy Three broad classes: Channel Partitioning divide channel into smaller “pieces” (time slots, frequency, code) allocate piece to node for exclusive use Random Access channel not divided, allow collisions “recover” from collisions “Taking turns” Nodes take turns, but nodes with more to send can take longer turns

4 Standards Media Access Technology Map 2005

5 Technology Applications

6 Why Zigbee / IEEE ? Standard Bluetooth Version 1.2 Application focusMonitoring & Control Web, , VideoCable Replacement Stack Size (kBytes) <64>1000>250 Battery LifeVery LongShortLong Network Size (#nodes) ~Unlimited (65536) Many7 Range (meters)10~ ~100 Bandwidth (kbps)25011M/ 54 M~1000 Target Cost~$1~$40$10~$15

7 Collision Avoidance Energy Efficiency Scalability Adaptability Channel Utilization Throughput Fairness The key design goals for Sensor Network MAC protocols?

8 What differences between sensor network MAC protocol and Standard MAC protocol? And Why?

9 Taxonomy Asynchronous MAC: B-MAC and X-MAC Senders and receivers have independent schedules Synchronous MAC: S-MAC and Z-MAC Coordinated OO/OFF Schedule between senders and receivers Pros and Cons

10 Two Papers on Asynchronous MAC B-MAC - Versatile Low Power Media Access for Wireless Sensor Networks Joseph Polastre, Jason Hill and David Culler X-MAC: A Short Preamble MAC Protocol for Duty-Cycled Wireless Sensor Networks M. Buettner, G. V. Yee, E. Anderson and R. Han Sensys 2004 Sensys 2006 Feature improvements

11 Collisions/Interference Basic Function of MAC layer Control packet overhead e.g., RTS/CTS, Beaconing Overhearing unnecessary traffic Non-target nodes Long idle time Prepare for receiving packets Synchronization Overhead Preamble and Start of Frame Detection What Causes Energy Waste?

12 B-MAC - Versatile Low Power Media Access for Wireless Sensor Networks Joseph Polastre, Jason Hill, David Culler

13 What is B-MAC? An acronym for Berkeley-MAC Configurable CSMA MAC protocol for WSNs Small core Energy efficient B-MAC = CSMA + LPL + Noise Floor Estimation + Explicit ACK

14 Goals of B-MAC Low power operation Effective collision avoidance Simple and predictable Small code size and RAM usage Tolerable to changing RF/networking conditions Scalable to large numbers of nodes

15 B-MAC Principles Reconfigurable MAC Protocol Flexible Control Feedback to higher protocols Minimal Implementation PHY B-MAC Link/Network Protocols DataControl

16 B-MAC Principles cont …. Clean layering Separation of link-layer from medium access control enables more optimized applications to be built on top. Unscheduled sleep Reduces control overhead (no SYNC) But sender incurs greater overhead to wakeup unsynchronized receiver from sleep. Periodic wakeup Keep wakeup intervals very short CSMA/CA or some other app-specific scheme can be used

17 Low Power Listening Goal: minimize listen cost Sleep/Wake scheduling using Low Power Listening (LPL) Wake up every Check-Interval Sample Channel using CCA If no activity, go back to sleep for Check-Interval Else start receiving packet technique is similar to preamble sampling in Aloha but tailored to different radio characteristics Advantage of this technique is that it works in a completely unsynchronized environment

18 B-MAC Protocol 9 Shift most burden to the sender Sender uses a long preamble before each packet to wake up the receiver. Data transmission can use RTS/CTS or some other strategy. Receive data Rx Tx Long preambleData transmission Carrier sense Check interval

19 Key challenges of B-MAC 9 Check Interval has to be very short to ensure reasonable length preamble Carrier sense duration also has to be very short to ensure receiver does not spend too much energy. Carrier sense has to be very accurate to reduce: latency of transmission energy consumption at sender Receive data Rx Tx Long preambleData transmission Carrier sense Check interval

20 Radio power-up sequence of operations

21 Clear Channel Assessment (CCA) CCA: method used to accurately determine if the channel is clear Requirements: MAC must accurately determine if channel is clear Need to tell what is noise and what is a signal Ambient noise is prone to environmental changes BMAC solution: ‘software automatic gain control’ Noise floor estimation: Signal strength samples taken when channel is assumed to be free such as immediately after transmitting a packet Samples go in a FIFO queue Median added to an EWMA filter Once noise floor is established, a TX requests starts monitoring RSSI from the radio

22 CCA: single-sample thresholding vs. outlier detection Common approach: take single sample, compare to noise floor Large number of false negatives BMAC: search for outliers in RSSI B-MAC searches for outliers in the received signal such that the channel energy is significantly below the noise floor. If an outlier exists during the channel sampling period, B- MAC declares the channel is clear since a valid packet could never have an outlier significantly below the noise floor. If five samples are taken and no outlier is found, the channel is busy

23 CCA Results B-MAC method: Before transmission – take a sample of the channel If the sample is below the current noise floor, channel is clear, send immediately. If five samples are taken, and no outlier found => channel busy, take a random backoff Noise floor updated when channel is known to be clear e.g. just after packet transmission 0=busy, 1=clear Packet arrives between 22 and 54 ms The middle graph shows the output of a thresholding CCA algorithm

24 Short ACK Reliability with minimal overhead ack_code[3] = {0xab, 0xba, 0x83} SR ACK code Data Benefits and Limitation ?

25 Some Evaluation

26 LPL check interval Single-hop data reporting application doing periodic data sampling Sampling rate defines optimal check interval Check interval Too small: energy wasted on idle listening Too large: energy wasted on transmissions (long preambles) Y-axis: Lifetime (years) X-axis: Check Time (ms) it’s better to have larger preambles than to check more often!

27 LPL and neighborhood size More neighbors: more transmissions More time spent receiving packets Less time left to go to sleep To find the best check interval find the expected neighborhood size and move up the y-axis to the lowest line check interval corresponding with this line will yield the maximum lifetime E.g. for a neighborhood size of 20, a check interval of 50 ms is optimal Best result: check interval that gives lowest effective duty cycle Y-axis: Effective duty cycle (%) X-axis: # of neighboring nodes

28 Energy vs. Latency Method: Consider a 10-hop network (shown on top of the figure) Fix throughput to one 100 byte pkt every 10 sec interval B-MAC: choose optimal check interval Results: As latency increases, energy consumed decreases B-MAC is better for lower latencies... Reason: sync packets, probability of multiple schedules --- less time to sleep S-MAC Default Configuration B-MAC Default Configuration

29 Con 2 Con 1 Con 3 B-MAC Pros and Cons 9 Pros Simple to configure the network Easier to tune Has better channel assessment Doesn’t use explicit sync packets Doesn’t use RTS/CTS/ACK if it doesn’t have to Is smaller and less complex Long Delay Overhearing issue Long Preamble

30 Brainstorming How to improve BMAC Performance ?

31 How to Fix Some Issues in B-MAC X-MAC: A Short Preamble MAC Protocol for Duty-Cycled Wireless Sensor Networks M. Buettner, G. V. Yee, E. Anderson, R. Han Sensys 2006

32 = =

33 Take Away Message B-MAC = CSMA + LPL + Noise Floor Estimation + Explicit ACK X-MAC = B-MAC + Early ACK + Encoded preamble (???)