1. Duty Cycled MAC protocols for Wireless Sensor networks
Alex Koslik
Alex Netes

2. Outline Abstract Introduction Duty cycle
Mac protocols for sensor networks
Synchronic
Asynchronic

3. Abstract

4. The Internet vs Sensor Network
Independent hosts
Collaborative use
End to end flows
Collect , disseminate
Infrastructure
Ad – hoc
Wired (generally)
Wireless
Latency , throughput
Energy
Bandwidth is relatively cheap
Bandwidth is expensive

5. Introduction

6. Physical size

7. Introduction
Smart environments represent the next evolutionary development step in :
Building
Utilities
Industrial
Home
Shipboard
Transportation systems automation

8. Introduction

9. Network Topology
The basic issue in communication networks is the transmission of messages to achieve a prescribed message throughput (Quantity of Service) and Quality of Service (QoS). QoS can be specified in terms of :
message delay
bit error rates
packet loss
economic cost of transmission
transmission power, etc.

10. Network Topology

11. General Purposes of MAC Protocol
MAC protocol is to ensure that the channel can be accessed by multiple users, dealing with the situation of interference.
Has a direct bearing on how reliably and efficiently data can be transmitted
Long battery life

12. Requirement of WSN Adaptive to changing topology
Nodes die out
Mobile nodes
Applicable with limited computing capability

13. MAC Protocol for WSN Energy-efficient in sense of achieved throughput
Robust
As simple as possible

14. Major problems in WSN MAC design
Idle listening
Listening when no traffic is sent
Overhearing
Receiving packets destined to other nodes
Collision
Retransmission
Overhead
Headers for signalling

15. Idle Listening Problem
Listening consumes substantial energy
Synchronized protocols
Nodes awake on a schedule
Asynchronized protocol
Uses low power listening

16. Overhearing Receiving packets that are not destined to the node
Interception, waste of energy in receiving, error responding will cause potential collision

17. Traffic Pattern Local broadcast Nodes to sink report
Schedule exchange/update between neighbours
Omni-directional transmission is desired
Nodes to sink report
Payload and signalling
In favour of directional transmission

18. Duty Cycle

19. Duty cycle
Duty cycling is a widely used mechanism in wireless sensor networks.
Reducing energy consumption due to idle listening.
This mechanism also introduces additional latency in packet delivery.

20. Synchronous MAC --------------------------------------------------
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nodes schedule to wake and sleep
periodic sleep, no idle listening
Example – S-MAC, T-MAC

21. -----Sanka---------------------------------------------
Asynchronous MAC
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Nodes having independent schedule
need some mechanism for carrier sensing
Example – B-MAC, X-MAC

22. Asynchronous vs. Synchronous
Advantages
Use extended preamble
Sender and receiver can have decoupled duty cycles.
No synchronization overhead.
Awake periods are much shorter
Disadvantages
Frame exchange delay even if receiver awakes before preamble ends
Overhearing problem
Preamble latency is expensive for multihop routes
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23. Synchronous MAC protocols
T-MAC
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24. S-MAC Synchronizes sensor clusters
Nodes periodically wake-up to receive synchronization info from its neighbors.
Mitigates need for system wide synchronization.
Nodes can belong to more that one virtual cluster.
Communicate using RTS-CTS
Can use adaptive listening
Neighbor briefly wakes up at the end of overheard RTS, CTS
Reduces one-hop latency

25. S-MAC Periodic listen and sleep Collision avoidance
Coordinated sleeping
Overhearing avoidance

26. S-MAC: Periodic Listen & Sleep
Frame
Duty cycle
(Listen Interval / Frame Length)
Frame schedule
Nodes are free to choose their listen/sleep schedule
Requirement: neighboring nodes synchronize together
Exchange schedules periodically (SYNC packet)
Synchronization period (SP)
Nodes communicate in receivers scheduled listen times
Listen
Sleep C A B D

27. S-MAC: Collision-Avoidance
Collision-Avoidance Strategy ~=
RTS/CTS
Physical carrier sense
Virtual carrier sense: network allocation vector (NAV)
RTS
Sender
Receiver
CTS
Other Sensors
DATA
ACK
NAV (based on RTS)
NAV (based on CTS)
Contend for medium access
defer access

28. S-MAC: Coordinated Sleeping (1)
Frame Schedule Maintenance
Choosing a schedule
Listen to the medium for at least SP
Nothing heard, choose a schedule
Broadcast a SYNC packet (should contend for medium)
Following a schedule
Receives a schedule before choosing/announcing
Follows the schedule
Broadcast a SYNC packet
Adopting multiple schedules
Receives a schedule after choosing/announcing
Can discard the new schedule; or
Follow both the schedules – suffer more energy loss

29. S-MAC: Coordinated Sleeping (2)
Neighbor Discovery
chance of failing to discover an existing neighbor
corrupted SYNC packet, collisions, interference
sensor – border of two schedules; discovers only the first schedule, if schedules do not overlap
Periodically, listen for the complete SP
frequency?
 - if a sensor has no neighbors
S-MAC experimental values:
SP = 10 seconds
Neighbor discovery period = 2 minutes, if at least 1 nbr

30. S-MAC: Coordinated Sleeping (3)
Maintaining Synchronization
Clock drifts – not a major concern (listen time = 0.5s – 105 times longer than typical drift rates)
Need to mitigate long term drifts – schedule updating using SYNC packet (sender ID, its next scheduled sleep time – relative);
Listen is split into 2 parts – for SYNC and RTS/CTS
Once RTS/CTS is established, data sent in sleep interval
Receiver
Listen
Sleep
for SYNC
for RTS
for CTS

31. S-MAC: Coordinated Sleeping (4)
Example Scenarios
Listen
for SYNC
for RTS
for CTS
Sleep
Receiver
Tx SYNC CS
Sender 1
Tx RTS
Got CTS
Send data CS
Sender 2
Tx SYNC
Tx RTS
Got CTS
Send data CS CS
Sender 3

32. S-MAC: Overhearing Avoidance
Who should sleep when a node is transmitting?
All immediate neighbors of both sender and receiver should sleep after hearing a RTS or CTS signal
Use NAV to schedule the sleep timer E C A B D F

33. S-MAC: Efficient Message Passing
Sending a long message?
As a single packet:  cost of re-transmission for message corruption
RTS
FRAG1
FRAG-N
Sender
CTS
ACK
ACK
Receiver
NAV (based on RTS)
Other Sensors
defer access
NAV (based on CTS)

34. S-MAC: Efficient Message Passing
RTS/CTS/ACK – has duration fields in it
If ACK is not received, increase the transmission time, retransmit. ACK will be also be updated.
Difference between & S-MAC
Medium is reserved upfront for the whole transmission in S-MAC

35. Drawbacks of S-MAC
Active (Listen) interval – long enough to handle to highest expected load
If message rate is less – energy is still wasted in idle-listening
S-MAC fixed duty cycle – is NOT OPTIMAL

36. T-MAC Listen for a short time after awake period. Sleeps if IDLE.
Improves on S-MAC by shortening the awake period if IDLE.
For variable payloads, T-MAC uses ~20% of energy used in S-MAC.

37. T-MAC: Adaptive duty cycle:
A node is in active mode until no activation event occurs for time TA
Periodic frame timer event, receive, carrier sense, send-done, knowledge of other transmissions being ended
Communication ~= S-MAC
Frame schedule maintenance ~= S-MAC
Active
Sleep TA

38. T-MAC: Contention Interval
waiting/listening for a random time within a fixed contention interval (unlike exponential back-off in )
assumptions:
load is always high, does not vary

39. T-MAC: Choosing TA
Requirement: a node should not sleep while its neighbors are communicating, potential next receiver
TA > C+R+T
C – contention interval length;
R – RTS packet length;
T – turn-around time, time bet. end of RTS and start of CTS;
TA = 1.5 * (C+R+T);

40. T-MAC: Overhearing Avoidance
~= S-MAC
But implemented as an option in T-MAC
Node – goes to sleep after overhearing RTS/CTS of other nodes communication
miss other RTS/CTS transmissions
disturb the medium while waking up
Overhearing avoidance should not used when maximum throughput is required

41. T-MAC: Solution
Full-Buffer Priority – suitable for unidirectional flows
Buffer – almost full – prefer sending than receiving
Receive RTS, send its own RTS back instead of CTS
contend A
contend B
contend C
RTS
active D
RTS
CTS
DATA
ACK TA

42. S-MAC vs. T-MAC Nodes to Sink Communication

43. Asynchronous MAC protocols
B-MAC
X-MAC

44. B-MAC Uses local schedules
Send preamble that is slightly longer than the sleep period.
Long preamble assures that the neighbor will receive packet.
Suffers from overhearing problem.

45. B-MAC
Transmitting node precedes data packet with preamble slightly longer than sleep period of receiver.
During awake period, node samples medium & if a preamble is detected it remains awake to receive the data.

46. X-MAC Short preamble Target in preamble Strobed preamble
Reduce latency and reduce energy consumption
Target in preamble
Minimize overhearing problem.
Strobed preamble
Reduces latency for the case where destination is awake before preamble completes.
Reduces per-hop latency and energy
Dynamic duty-cycle algorithm

47. X-MAC Strobed preamble Allowing interruption and wake up faster
Short preamble
embedded with address information of the target

48. X-MAC: Benchmarking parameters
Design goals of the X-MAC protocol for duty-cycled WSNs:
energy-efficiency
low loss % of packets
low latency for data
high throughput for data
duty cycles

49. Energy Usage
Energy consumption of the LPL protocol increases as network density increases.
For X-MAC, remains relatively constant.

50. Transmission success rates
channel becomes saturated with increase in collisions.
X-MAC receives approximately 90%
LPL loses more packets as density increases.

51. Duty Cycle w/o contention(star topology)
Senders duty cycle is 7.0% for X-MAC versus 9.3% for LPL, accounting for 32.5% increase in energy lifetime

52. Duty Cycle under contention
X-MAC uses less energy for all sleep periods and generation rates, and is less sensitive to network density.

53. Latency (chain topology of 8 nodes)
X-MAC reduces latency by approximately 50%.

54. Conclusions X-MAC employs a strobed preamble approach
Small pauses between preamble packets permit the target receiver to send an ack
Truncating preamble saves energy at transmitter and receiver and allows for lower latency.
Non-target receivers which overhear the strobed preamble can go back to sleep immediately
X-MAC’s strobed preamble approach outperforms traditional LPL.

55. Bibliography Mahesh Arumugam - MAC Protocols for Sensor Networks
F. L. LEWIS - Wireless Sensor Networks
Muneeb 
Ali
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Brief
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Networks
Tijs van Dam ,Koen Langendoen - An Adaptive Energy- Efficient MAC Protocol for Wireless Sensor Networks
Joseph Polastre , Jason Hill , David Culler –
Versalite Low power media access for wireless sensor networks