1 Media Access Control in Wireless Sensor Networks - II

2 What We Have Learned Last Time B-MAC = ? CSMA + LPL + Noise Floor Estimation + Explicit ACK X-MAC = ? B-MAC + Early ACK + Encoded preamble

3 Outline Overview TDMA/CSMA Advantages and disadvantages S-MAC Z-MAC Design concepts, performance evaluation and issues

4 Classification of Multiple Access Protocols Multiple Access Protocols Random AccessControlled Access CSMA TDMA (FDMA, CDMA) ALOHAStatic channel allocation B-MAC X-MAC S-MACZ-MAC

5 CSMA CSMA: listen before transmit: If channel sensed idle: transmit entire pkt If channel sensed busy, defer transmission Persistent CSMA: retry immediately with probability p when channel becomes idle (may cause instability) Non-persistent CSMA: retry after random interval human analogy: don’t interrupt others! I want to talk now Me too

6 TDMA TDMA: Time Division Multiple Access Access to channel in "rounds" Each station gets a fixed length slot (length = pkt Tx time) in each round Unused slots go idle Example: 6-station LAN, 1,3,4 have pkts, slots 2,5,6 idle

7 Medium Access Paradigms Contention Based (CSMA) Random-back off and carrier-sensing Simple, no time synch, and robust to network changes High idle listening and overhearing overheads Solve this by duty cycling TDMA Based (or Schedule based) Nodes within interference range transmit during different times, so collision free Requires time synch and not robust to changes. Low throughput and high latency even during low contention. Low idle listening and overhearing overheads Wake up and listen only during its neighbor transmission

8 TDMA vs. CSMA for Sensor Networks ParameterTDMACSMA Energy for Synchronization BadGood ThroughputGood for multiple sources Good for single source complexityBadGood FairnessGoodBad ?????????????

9 TDMA vs. CSMA for Sensor Networks cont … ParameterTDMACSMA ScalabilityBadGood LatencyBad/GoodGood/Bad Dealing with node failures, new node arrivals BadGood Energy for collision Avoidance GoodBad ?????????????

10 How about combine TDMA and CSMA? S-MAC: for the benefit of Energy Efficiency Z-MAC: for the benefit of throughput

11 Z-MAC: a Hybrid MAC for Wireless Sensor Networks -Injong Rhee, Ajit Warrier, Mahesh Aia and Jeongki Min Medium Access Control with Coordinated Adaptive Sleeping for Wireless Sensor Networks Wei Ye, John Heidemann and Deborah Estrin Sensys 2005 TON 2004

12 S-MAC: Introduction S-MAC stands for Sensor-MAC Key Idea in SMAC: Combine key advantages of scheduled (TDMA) and unscheduled (CSMA) protocols S-MAC = lite Scheduling

13 S-MAC: Energy savings S-MAC tries to reduce wastage of energy from at least 3 sources of energy inefficiency: Nodes periodically sleep to reduce energy consumption in listening to an idle channel Resolve contention by using RTS and CTS Avoid overhearing – S-MAC sets the radio to sleep during transmissions of other nodes

14 Periodic sleeping Problem: Idle listening consumes significant energy Solution: Periodic listen and sleep Turn off radio when sleeping Reduce duty cycle to ~ 10% (120ms on/1.2s off) listen sleep Difference between S-MAC toggling and B-MAC toggling?

15 Deal with global synchronization Node 1 Node 2 sleep listen sleep listen sleep Schedule 2 Schedule 1 Schedules can differ, prefer neighbouring nodes to have same schedule Border nodes may have to maintain more than one schedule.

16 Overhearing Avoidance Problem: Receive packets destined to others Solution: Sleep when neighbors talk Who should sleep? All immediate neighbors of sender and receiver How long to sleep? The duration field in each packet informs other nodes the sleep interval

17 SMAC: Pros and Cons Pros Well-designed, complete protocol that addresses deficiencies of if applied to a sensor network. Schedules sleep and transmit times to enable low-power data transfer with reasonable-latency. Cons SMAC incurs some drawbacks of TDMA schemes Topology maintenance, need for synchronization, additional complexity at border nodes between two schedules… Monolithic system architecture similar to Combines carrier sense, link-layer reliability, RTS/CTS and sleep scheduling into MAC layer.

18 Z-MAC Motivation: Throughput # of Contenders Channel Utilization IDEAL CSMA TDMA

19 Z-MAC: Hybrid Contention Resolution Z-MAC (Zebra MAC) – a Hybrid MAC protocol combines the strengths of both CSMA and TDMA at the same time discounting their weaknesses Z-MAC uses a base TDMA schedule as a hint to schedule the transmissions of the nodes, and it differs from TDMA by allowing non-owners of slots to 'steal' the slot from owners if they are not transmitting MAC Channel Utilization CSMA TDMA Low ContentionHigh Contention HighLow High

20 Z-MAC Features Adaptability to the level of contention in the network Under low contention behaves like CSMA Under high contention behaves like TDMA # of Contenders Channel Utilization IDEAL CSMA TDMA

21 Z-MAC Design Z-MAC has the setup phase in which the following operations are run in sequence: 1.Neighbor discovery 2.Time slot assignment (DRAND) 3.Local frame exchange 4.Time synchronization

22 Neighbor discovery When a node starts up, it runs a neighbor discovery protocol Periodically broadcasts a ping to its one-hop neighbors Ping message contains the current list of its one-hop neighbors Through this message, each node gathers neighbor information Q: How many hops neighbor information is need to avoid interference ? 1 Hop, 2 Hop, More than 2 Hop? What’s the reality?

23 Timeslot Assignment The two-hop neighbor list is used as an input to a time-slot assignment algorithm Current implementation of Z-MAC uses DRAND – a distributed implementation of RAND to assign time slots to every node in the network DRAND ensures no two nodes within a two-hop communication neighborhood are assigned to the same slot. This assignment guarantees that no transmission by a node to any of its one-hop neighbors interferes with any transmission by its two-hop neighbors.

24 DRAND slot assignment C D A F B C D A E B E F Radio Interference Map Input Graph C D A E B F DRAND slot assignment

25 Z-MAC Transmission Control A node can be in one of two modes: Low Contention Level (LCL) or High Contention Level (HCL) Node is in HCL only: when it receives an explicit contention notification (ECN) message from a two-hop neighbor within the last t ECN period. Otherwise, the node is in LCL. A node sends an ECN when it experiences high contention

26 Z-MAC Transmission Control cont… In LCL, any node can compete to transmit in any slot But in HCL, only the owners of the current slot and their one-hop neighbors are allowed to compete for the channel In both modes, owners have higher priority over non- owners. If a slot does not contain an owner or its owner does not have data to send, non-owners can steal the slot. This feature achieves high channel utilization even under low contention as a node can transmit as soon as the channel is available. Z-MAC implements LCL and HCL using the back off, CCA and LPL interfaces of B-MAC

27 Transmission rule (for the owner) Owner takes a random back off within a fixed time period T o When the back off timer expires, it runs CCA and if the channel is clear, transmits the data. If the channel is not clear, then it waits until the channel is not busy and repeats the above process. Busy Owner Accessing Channel Random Back off within T o (Contention Window)

28 Transmission rule (non owner - LCL) Waits for T o and then performs a random back off within a contention window [T o, T no ] When the back off timer expires, it runs CCA and if the channel is clear, then it starts transmission. If the channel is not clear, it waits until the channel is clear, and repeats the above process. Busy Non-owner Accessing Channel ToTo Random Back off within [T o, T no ] (Contention Window)

29 Z-MAC Transmission Control (Continued) AABBBAAAABBB TDMA and Z-MAC under high contention (Two node example) AAAAAA TDMA under no contention (Two node example) AAAAAAAAAAAA Z-MAC under no contention (Two node example)

30 Z-MAC – Performance Evaluation Setup: Single-hop, Two-hop and Multi-hop topology experiments on Mica2 motes. Comparisons with B-MAC (default MAC of Mica2), with different back-off window sizes Metrics: Throughput & energy efficiency

31 Experimental Setup – Single Hop Single-Hop Experiments: Mica2 motes equidistant from one node in the middle. All nodes within one-hop transmission range. Tests repeated 10 times and average/standard deviation errors reported.

32 Z-MAC – Two-Hop Experiments Setup – Two-Hop Dumbbell shaped topology Transmission power varied between low (50) and high (150) to get two-hop situations. Aim – See how Z-MAC works when Hidden Terminal Problem manifests itself Sources Sink

33 Experimental Setup - Test bed 42 Mica2 sensor motes in a Lab. Wall-powered and connected to the Internet via Ethernet ports. Programs uploaded via the Internet, all mote interaction via wireless. Links vary in quality, some have loss rates up to 30-40%.

34 Multi Hop Results – Throughput Why B-MAC is better than Z-MAC in low traffic ? Why B-MAC is worse than Z-MAC in high traffic ? Z-MAC B-MAC MULTI-HOP

35 Multi Hop Results – Energy Efficiency B-MAC Z-MAC HCL MULTI-HOP

36 Summary of Z-MAC Hybrid MAC :Combines strengths of TDMA and CSMA Uses the TDMA schedule created by DRAND as a 'hint' to schedule transmissions The owner of a time-slot always has priority over the non- owners while accessing the medium. Unlike TDMA, non-owners can 'steal' the time-slot when the owners do not have data to send. This enables Z-MAC to switch between CSMA and TDMA depending on the level of contention. Hence, under low contention, Z-MAC acts like CSMA (i.e. high channel utilization and low latency), while under high contention, Z-MAC acts like TDMA (i.e. high channel utilization, fairness and low contention overhead).

37 Discussion Limitation of Z-MAC

38 Take Away Messages Hybrid Synchronous MAC S-MAC = lite Scheduling Z-MAC = CSMA within TDMA slots Asynchronous MAC B-MAC = CSMA + LPL + Noise Floor Estimation + Explicit ACK X-MAC = B-MAC + Early ACK + Encoded preamble

39 MAC Summary Commercial MAC (802.X, Bluetooth) are suitable for wireless LAN with much more powerful devices. Energy is secondary concern compared with throughput. Asynchronous MAC (B/X-MAC) is flexible and works well in low traffic scenario (why is so widely used!) Hybrid synchronous MAC (S/Z-MAC) can achieve better performance in high traffic scenario.