Ten Years of Wireless Sensor Network MACs: Aspects of time, frequency, and energy

1. Ten Years of Wireless Sensor Network MACs:Aspects of time, frequency, and energy Anish Arora Wenjie Zeng Jing Li

2. What is Medium Access Control (MAC) • MAC provides two major functions • Addressing • Channel access – the focus of this tutorial • Requirements for multiple access protocols • Reliability – high delivery probability • Throughput capacity – high delivery rate • Latency – timely delivery • Fairness – fair chance to deliver for everyone • Energy Efficiency – low cost per delivery • Critical for battery-powered sensor platforms

3. Under the Hood – The Physical Layer Model • When is a reception successful? • Physical Model: when Signal to Interference Ratio (SINR) is greater than some threshold β • Protocol Model: when the other senders are far away • Terminals A and B send, C “captures” B • Capacity is underestimated under protocol model - Received signal strength from node i N – Background noise - Distance between node i and j Δ - Some positive value A B C

4. Challenges for Wireless Sensor MACs • Low radio sensitivity + high attenuation • Wired Medium: Interference Range = Carrier Sensing Range = Communication Range • Wireless Medium: Interference Range ≠ Carrier Sensing Range ≠ Communication Range • B sends to A, C wants to send to D • C has to wait, CS signals a medium in use • Since A is outside the radio range of C waiting is not necessary • C is “exposed” to B • A sends to B, C cannot receive A • C wants to send to B, C senses a “free” medium (CS fails) • Collision at B, A cannot receive the collision (CD fails) • A is “hidden” for C D A B C Hidden Terminal Problem Exposed Terminal Problem A B C

5. Challenges for Wireless Sensor MACs (cont.) • Global information sharing is difficult • Time synchronization, time/frequency schedules • The network is mobile in nature • The wireless medium is unreliable - link mobility • Node mobility • Robustness of MACs matters • Energy constraint • High bandwidth, low latency -> high energy consumption • Reliability -> retransmissions -> high energy consumption

6. Contention Based Medium Access • Carrier Sense Multiple Access (CSMA) • Transmit when you feel like transmitting • Problem: Receiver must be awake as well • Minimize collision – backoff scheme is critical • Initial backoff – delay before putting the bits into the air • Avoid collision in the first place • Congestion backoff – delay after the medium is detected busy • Avoid repeated collisions • Retransmission needed when backoff fails • Backoff scheme may make multiple attempts in itself

7. Contention Based Medium Access (cont.) • A cookbook for a backoff scheme • A backoff probability distribution p(n): the probability to tx in slot n • Uniform backoff probability (CW is the contention window): p(i) = p(j), for any slot i and slot j • Non-uniform backoff probability • The minimum and maximum backoff window • Example 1: Binary Exponential Backoff (BEB) • Uniform backoff probability • Starts with some minimum backoff window • Backoff window doubles upon each failure until a max value is reached • Example 2: Sift • Exponential backoff probability • Fixed backoff window • More suitable for WSNs: receiver’s radio consumes energy throughout the backoff period

8. A Classic Contention MAC: Slotted Aloha • We assume that the stations are perfectly synchronous • In each time slot each station transmits with probability p • In Slotted Aloha, a station can transmit successfully with probability at least 1/e, or about 36% of the time

9. Unslotted (Pure) Aloha • Unslotted Aloha: simpler, no (potentially costly!) synchronization • However, collision probability increases. Why? • There is a factor-2-handicap of Unslotted vs. Slotted

10. Aloha Robustness • Round robin has a problem when a new station joins. In contrast, Aloha is quite robust • Example: If the actual number of stations is twice as high as expected,there is still a successful transmission with probability 30%. If it is onlyhalf, 27% of the slots are used successfully. So nodesjust need a good estimateof the number of nodes intheir neighborhood

11. Scheduling Based Medium Access • Schedule transmissions across • Time: Time Division Multiple Access (TDMA) • Use a “pre-computed” schedule to transmit messages • Distributed, adaptive solutions are difficult • Frequency: Frequency Division Multiple Access (FDMA) • Choose different frequencies for interfering neighbors • Permanent (radio broadcast), slow hopping (GSM), fast hopping • Space: Space Division Multiple Access (SDMA) • Segment space into sectors, use directed antennas • Reuse frequencies in different cells • Code: Code Division Multiple Access (CDMA) • People talk simultaneously in different languages

12. Design Space for MAC Protocols • Share a common sense of time? • Synchronous vs. Asynchronous It’s about slot 10 at R, and he’s supposed to be awake I’d better made sure that R is awake when I talk to him S R S R Asynchronous Synchronous

13. Design Space for MAC Protocols (cont.) • Who controls the coordination? • Sender-centric vs. Receiver-centric S:I’m sending R: z.. S:I’m sending R: zz.. S:I’m sending R: zzz.. S:I’m sending R:roger that S:I’m waiting. R: z.. S:I’m waiting.. R: zz.. S:I’m waiting… R: zzz.. S:finally… R:send now S R S R Sender-centric and asynchronous Receiver-centric and asynchronous

14. Sender-Centric and Asynchronous MACs:B-MAC and X-MAC • Low Power Listening (LPL) • Introduced by B-MAC • Periodically wake up shortly to detect incoming traffic • Senders transmit preambles as long as the sleep interval before sending the actual data to ensure the receiver is listening for the packet • X-MAC improves upon B-MAC • Embed node ID into preambles • Avoid overhearing • Send only the 802.15.4 header to reduce receiver detection time • Packetized preambles • Receiver acknowledges preamble packets destined to itself • Allow early preamble termination

15. Sender-Centric and Asynchronous MACs:BoX-MAC • Improved upon B-MAC and X-MAC • Key observation • In X-MAC, receiver detection time is coupled with length of preamble packets • Decouple by detecting presence of energy in the channel • Use data packet as preambles • Save energy in handshaking

16. Sender-Centric and Synchronous MACs:S-MAC • All nodes choose and broadcast awake schedules • Synchronized sleep/wake within cluster • Boarder nodes • Low channel utilization b/c of control overhead and introduced cluster contention • Use RTS/CTS to resolve contention • Allow senders go back to sleep if another sender wins the medium Schedule 1+2 Schedule 2 Schedule 1 increased latency

17. Sender-Centric and Synchronous MACs:SCP-MAC • Network wide synchronized channel polling • Short tones at polling time to signal pending traffic • Geared towards ultra-low duty cycle • Short polling instead of longer listen period • Adapt listen interval dependent on the current network load SCP-MAC S-MAC

18. Receiver-Centric and Asynchronous MAC:RI-MAC • Flip the LPL operations • Senders listen for receiver preambles if they have pending data • Allow short preambles from the receiver • Sender long preamble becomes long listening • Better channel utilization

19. Receiver-Centric and Synchronous MAC:O-MAC • Neighborhood synchronization vs. network synchronization • Larger memory footprint vs. longer convergence time • Each node wakes up at pseudo-random slots • Compute neighbor’s schedule using its broadcasted seed • Approximate TDMA at low cost • Ideally, the wakeup frequency should minimize contention per wakeup • Slots need not be aligned

20. A Brief Summary of Exemplar MACs

21. Applications Define the “Best” MAC • Synchronous MACs are more efficient if control overhead can be mitigated, e.g. in static networks • Synchronization can be complicated in mobile network • Node mobility due to movement • Link mobility due to changing link quality • Asynchronous MACs are more robust • Over a wide range of traffic loads and network sizes, receiver-centric and synchronous MACs are most energy efficient

22. Challenges of Multi-channel Protocols • Coordination of communicators • Compared with single channel protocols, needs agreement on both channel and time • Restricted ratio interfaces • Most hardware platforms for WSN do not support multiple interfaces • Channel switch time is not negligible • Restricted energy supply • Control overhead has to be reduced for energy efficiency

23. Main Categories of Multi-channel MACs • Static scheme • Different channels are chosen for 2-hop neighbors to avoid potential interferences • Dynamic scheme • Negotiate which channel to use per packet • Load balancing scheme • Splitting traffic loads evenly over available channels • Probabilistic scheme • Probabilistically allocates a fraction of nodes into the next channel when the current channel becomes overloaded

24. Static Channel Assignment • MMSN (Infocom’06): • Frequency assignment • every node is assigned a channel for data reception such that most of two-hop neighbors do not communicate on the same channel. • Toggle snooping • When a node has a unicast packet to send, the node snoops on two frequencies alternatingly: fself and fdest • Cons: • High overhead of distance-2 coloring & toggle snooping • Regardless of real traffic conditions

25. Dynamic Channel Assignment • TMMAC (ICC’07) • A TDMA-based multi-channel MAC protocol • Consists of a 2-dimensional ATIM window wherein each node negotiates time slot and channel it will use before transmission • ATIM window is updated and checked at each node • Cons: • Message overhead of explicit negotiation • Energy overhead of continuous listening for channel updates

26. Load Balancing Channel Assignment • Y-MAC (IPSN’08) • Extended from Crankshaft protocol, TDMA-based • Every receiver wakes up at its non-overlap slot within each frame on home channel • If more packets need to be received, the receiver hops to the next channel for reception • Cons: • per slot channel switching • Instantaneous channel condition is not considered

27. Probabilistic Channel Assignment • A practical multi-channel MAC (IPSN’08) • Starts in home channel • Feedback control scheme for channel expansion & shrinking • Switch probability is a function of success transmission rate & sender/receiver role • Periodic channel status updates • Cons: • Channel update & control overhead • Instantaneous channel condition is not considered

28. Throughput Capacity of Wireless Networks • Definition: for each source-destination pair, the average number of bits delivered successfully from a source to a destination • The problem: how does throughput capacity change with an increasing number of nodes in an area of fixed size

29. Network Model • Network of n nodes i.i.d in a unit area region • An intuitive observation – the worst-case throughput capacity λ(n) is • This occurs when the n nodes form a clique network • Static network, independent and random destination • Each node can transmit at W bps • Packets sent from node to node in multihop fashion • Transmissions are slotted • Several nodes may transmit simultaneously • Use the protocol model

30. Results for Always On Network • The throughput capacity λ(n) is • The result carry over to the physical model • Dividing W into multiple channels does not change result • Implications: • Network aimed at smaller scale of users are better • When nodes communicate only nearby nodes at a distance of less than O(1/sqrt(n)) away, a constant bit rate is feasible per source-destination pair • E.g. smart home networks

31. Results for Duty Cycled Network • If every node runs at a duty cycle ψ, the throughput capacity λ(n) is • Throughput capacity scales better than linearly in ψ (note that 0≤ ψ ≤ 1) • Throughput capacity scales better than the reciprocal of

32. Energy Efficiency of Wireless Networks • Definition: the ratio from the energy expense in receiving unique and useful data packets to the total energy expense • To optimize energy efficiency • Maximize throughput capacity • Minimize total energy consumption • For full-duty-cycled networks, maximizing energy efficiency is equivalent to maximizing throughput capacity

33. Results for Energy Efficiency in MAC • Deriving the exact form of energy efficiency as a function of network size n turns out to be difficult • Given a network size, optimal efficiency is obtained by searching through different duty cycles and traffic loads • Numerical and experimental results are obtained

34. References • B-MAC: Versatile Low Power Media Access for Wireless Sensor Networks, Polastre, J. and Hill, J. and Culler, D., SenSys 04’ • X-MAC: X-MAC: A short preamble mac protocol for duty-cycled wireless sensor networks, Buettner et.al., SenSys 06’ • BoX-MAC:BoX-MACs: Exploiting physical and link layer boundaries in low-power networking, Moss, D. and Levis, P., Technical Report SING-08-00, Stanford University, 08’ • S-MAC: An Energy-Efficient MAC Protocol for Wireless Sensor Networks, Ye, W. and Heidemann, J. and Estrin, D., Infocom 02’ • SCP-MAC: Ultra-low duty cycle MAC with scheduled channel polling, Ye, W. and Silva, F. and Heidemann, J., SenSys 06’ • RI-MAC: RI-MAC: A receiver-initiated asynchronous duty cycle MAC protocol for dynamic traffic loads in wireless sensor networks, Sun, Y. and Gurewitz, O. and Johnson, D.B., SenSys 08’ • O-MAC: O-MAC: A Receiver Centric Power Management Protocol, Hui Cao, Kenneth W. Parker, Anish Arora, ICNP 06’

35. Place Holder Slide The previous page is the last slide. This page can be removed.

36. Low Power Listening (B-MAC) • Nodes wake up for a short period and check for channel activity. • Return to sleep if no activity detected. • If a sender wants to transmit a message, it sends a long preamble to make sure that the receiver is listening for the packet. • preamble has the size of a sleep interval • Very robust • No synchronization required • Instant recovery after channel disruption preamble data listen channel sniff

37. overhearing problem Low Power Listening (B-MAC) • Problem: All nodes in the vicinity of a sender wake-up and wait for the packet. • Solution 1: Send wake-up packets instead of preamble, wake-up packets tell when data is starting so that receiver can go back to sleep as soon as it received one wake-up packet. • Solution 2: Just send data several times such that receiver can tune in at any time and get tail of data first, then head. • Communication costs are mostly paid by the sender. • The preamble length can be much longer than the actual data length. • Idea: Learn wake-up schedules from neighboring nodes. • Start sending preamble just before intended receiver wakes up. • WiseMAC • encode wake-up pattern in ACK message

38. Sensor MAC (S-MAC) • Coarse-grained TDMA-like sleep/awake cycles. • All nodes choose and announce awake schedules. • synchronize to awake schedules of neighboring nodes. • Uses RTS/CTS to resolve contention during listen intervals. • And allows interfering nodes to go to sleep during data exchange. increased latency frame sleep sleep listen listen time

39. Sensor MAC (S-MAC) • Problem: Nodes may have to follow multiple schedules to avoid network partition. Schedule 1+2 Schedule 2 Schedule 1 • A fixed sleep/awake ratio is not always optimal. • Variable load in the network. • Idea: Adapt listen interval dependent on the current network load. • T-MAC

40. Hybrid Protocols • Protocols may use information from upper layers to further improve their performance. • Information about neighborhood • Routing policies • Minimize costly overhearing of neighboring nodes • Inform them to change their channel sniff patterns • Use randomization to resolve schedule collisions optimization for WiseMAC like in Dozer schedule collision

41. Motivation – Hidden Terminal Problem • A sends to B, C cannot receive A • C wants to send to B, C senses a “free” medium (CS fails) • Collision at B, A cannot receive the collision (CD fails) • A is “hidden” for C Interference Range ≠ Carrier Sensing Range A B C

42. Motivation – Exposed Terminal Problem • B sends to A, C wants to send to D • C has to wait, CS signals a medium in use • Since A is outside the radio range of C waiting is not necessary • C is “exposed” to B D A B C

43. Adaptive Slotted Aloha • Idea: Change the access probability with the number of stations • How can we estimate the current number of stations in the system? • Assume that stations can distinguish whether 0, 1, or more than 1 stations transmit in a time slot. • Idea: • If you see that nobody transmits, increase p. • If you see that more than one transmits, decrease p. • Model: • Number of stations that want to transmit: n. • Estimate of n: • Transmission probability: p = 1/ • Arrival rate (new stations that want to transmit): λ (with λ < 1/e).

44. Adaptive Slotted Aloha Q&A Q: What if we do not know , or  is changing? A: Use  = 1/e, and the algorithm still works. Q: How do newly arriving stations know ? A: We send with each transmission; new stations do not send before successfully receiving the first transmission. Q: What if stations are not synchronized? A: Aloha (non-slotted) is twice as bad. Q: Can stations really listen to all time slots (save energy by turning off)? Can stations really distinguish between 0, 1, and ¸2 sender? A: Maybe. One can use systems that only rely on acknowledgements.

45. Backoff Protocols • Backoff protocols rely on acknowledgements only. • Binary exponential backoff • If a packet has collided k times, we set p = 2-k • Or alternatively: wait from random number of slots in [1..2k] • It has been shown that binary exponential backoff is not stable for any arrival rate λ > 0 (if there are infinitely many potential stations) • Interestingly when there are only finite stations, binary exponential backoff becomes unstable with λ > 0.568; Polynomial backoff however, remains stable for any λ < 1.