1. ADCA: An Asynchronous Duty Cycle Adjustment MAC Protocol for Wireless Sensor Networks
GLOBECOM 2008
Yu-Chia Chang, Jehn-Ruey Jiang and Jang-Ping Sheu
CSIE Department
National Central University

2. Outline Introduction Related work ADCA MAC protocol
Experiments and simulation results
Conclusions

3. Introduction
Wireless sensor networks is composed of a large number of battery-operated sensor nodes
Energy conservation is one of the most important issues
RF transceiver is the biggest power consumer in a sensor node

4. Introduction (Cont.) Four modes of the radio transceiver: transmitting
receiving
listening
sleeping
The first three modes are also called active or wake modes
The power consumption of the four modes of MICAz mote is 52.5, 59.1, 59.1 and mW

5. Introduction (cont.) Dominant in sensor nets What causes energy waste?
Collisions
Control packet overhead
Overhearing unnecessary traffic
Long idle time
bursty traffic in sensor-net apps
Idle listening consumes 50—100% of the power for receiving (Stemm97, Kasten)
Dominant in sensor nets

6. Introduction (cont.)
Idle listening can be solved by sleep/wake protocols
When the duty cycle (i.e., active period) of the radio is reduced to 1 percent, the power consumption of the sensor node can be reduced by a factor of 50.

7. Introduction (cont.)
Collision can be solved by RTS/CTS (CSMA/CA ) (up to 36% improvement)
Overhearing can be solved by RTS/CTS with NAV (Network Allocation Vector)
However, the overhead of RTS/CTS is relatively large when used in in WSNs since WSN packets are usually very small.
In MICA Mote, the maximum data packet size is 41 bytes and the size of an RTS/CTS packet is 18 bytes.

8. CSMA/CA Contention-based protocols
CSMA — Carrier Sense Multiple Access
Ethernet (CSMA/CD) is not suitable for wireless (collision at receiver cannot be detected at sender)
MACA — Multiple Access w/ Collision Avoidance
RTS/CTS for hidden terminal problem
RTS/CTS/DATA A B C
Hidden terminal: A is hidden from C’s CS

9. CSMA/CA Contention-based protocols (contd.) MACAW — improved over MACA
RTS/CTS/DATA/ACK
Fast error recovery at link layer
IEEE Distributed Coordination Function (DCF)
Largely based on MACAW
Called CSMA/CA

10. Hidden Terminal Problem
Data
Data A B C
A and C want to send data to B
A senses medium idle and sends data
C senses medium idle and sends data
Collision occurs at B

11. Collision Avoidance w/ RTS/CTSB
3.Data C
A and C want to send to B
A sends RTS (Request To Send) to B
B sends CTS (Clear To Send) to A C “overhears” CTS from B
C waits for duration of A’s transmission

12. Virtual Carrier Sense
Timing relationship

13. Introduction (cont.) ADCA MAC protocol for WSNs Goals
Asynchronous sleep/wake protocol
Duty cycle adjustment
Goals
Energy efficiency
Without degrading performance
Without lengthening transmission delay
Without lowering throughput

14. Related work

15. Related work preamble-based slot-based
B-MAC, Wise-MAC, SyncWUF
slot-based
P-MAC, TRAMA, Z-MAC, H-MAC
duty cycle synchronization-based
S-MAC, T-MAC, U-MAC

16. B-MAC Drawbacks Overhearing Bad performance at heavy traffic case
Long transmission latency

17. Wise-MAC Drawbacks Maintain neighbors’ time offset
Transmission latency is increased

18. P-MAC - Pattern MAC
P-MAC [12] divides time axis into frames, each of which consists of two parts: the Pattern Repeat part and the Pattern Exchange part. Both parts contain many slots.
During the Pattern Exchange part, nodes advertise their intended sleep/wake patterns, which represent one slot by one bit (0 for sleeping mode and 1 for active mode) and can be dynamically adjusted based on traffic conditions.
And during the Pattern Repeat part, a node wakes up according to the advertised pattern.
A node also wakes up at a time slot t, if one of its neighbors has advertised to be awake at the time slot t and it has data for sending to the node.
STF = Super Time Frames
PRTF = Pattern Repeat Time Frame
PETF = Pattern Exchange Time Frame = Maximum number of neighbors -> Overhead

19. S-MAC or Sensor-MAC
Medium Access Control with Coordinated, Adaptive Sleeping for Wireless Sensor Networks
[YHE03]

20. S-MAC: Periodic Listen & Sleep
Frame
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

21. Periodic Listen and Sleep
Schedules can differ
Node 1
Node 2
sleep
listen
Prefer neighboring nodes have same schedule
— easy broadcast & low control overhead
Before nodes perform periodic listen and sleep, they need to choose a schedule about when to listen and when to sleep.
This figure shows that even if two nodes have different schedules, they can still talk to each other as long as they know each others’ schedules. For example, if node 1 wants to talk to node 2, it just wait until node 2 is listening. However, we prefer neighboring nodes to have the same schedule, so that it’s easy to do broadcast and the control overhead is low.
But in a large network, we cannot guarantee that all nodes follow the same schedule. For example, in this figure, there are two different schedules on each side. The node on the border will follow both schedules. When it broadcasts a packet, it needs to do it twice, first for nodes on schedule 1 and then for those on schedule 2.
Schedule 2
Schedule 1
Border nodes:
two schedules
broadcast twice

22. 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
Re-broadcast a SYNC packet
Adopting multiple schedules
Receives a schedule after choosing/announcing
Follow both the schedules – suffer more energy loss
A node only re-broadcast SYNC for the first schedule heard

23. S-MAC: Coordinated Sleeping (2)
Neighbor Discovery
chance of failing to discover an existing neighbor
corrupted SYNC packet, collisions, interference
New sensor in the border of two schedules discover 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

24. S-MAC: Coordinated Sleeping (3)
Maintaining Synchronization
Clock drifts – not a major concern (listen time = 0.5s, which is 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

25. S-MAC: Coordinated Sleeping (4)
Adaptive Listening – Low-duty cycle to active mode
* Overhearing nodes – wakeup at the end of the current transmission (duration field in RTS/CTS)
ListenR
ListenON
RTS
Sender
Receiver
CTS
Overhearing nodes (ON)
DATA
ACK
Sleep (based on RTS)
Sleep (based on CTS)
Wakes up even though it is not the correct listen-interval

26. S-MAC Summary
Latency and throughput are problems, but adaptive listening improves it significantly
Energy saving is significant compared to “non-sleeping” protocols

27. S-MAC Information URL: http://www.isi.edu/scadds/
Released S-MAC source code (for TinyOS 0.6.1)
Currently porting to nesC environment (TinyOS 1.0)

28. An Adaptive Energy-Efficient MAC Protocol for Wireless Sensor Networks
T-MAC or Timeout-MAC
An Adaptive Energy-Efficient MAC Protocol for Wireless Sensor Networks
[vL03]

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

30. T-MAC: Preliminaries Adaptive duty cycle:
A node is in sleep mode if no activation event occurs for time TA
Active
Sleep TA

31. 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 (a short time between the end of the RTS packet and the beginning of the CTS packet)
TA = 1.5 * (C+R+T);

32. T-MAC: Asymmetric Communication (1)
Early-Sleeping Problem – in convergecast (A to D)
C – may lose medium to B (RTS) or A (B’s CTS)
C loses to B; D will hear CTS from C;
C loses to A; D will hear nothing, since C is silent; A B C D
contend
RTS
CTS
DATA
ACK
RTS?
active
sleep TA

33. T-MAC: Asymmetric Communication (2)
Future RTS (FRTS)
Let others know that it cannot access the medium; C – sends FRTS – has duration field; receiver of FRTS – schedule timer;
FRTS might affect data; so, DATA postponed until FRTS is over; Prevent others from taking medium, send dummy DS packet; A B C D
contend
RTS
CTS
DATA
ACK
active TA DS
FRTS
TA = C+R+T+CTS_length

34. T-MAC: Asymmetric Communication (3)
Full-Buffer Priority – suitable for unidirectional flows
Buffer – almost full – prefer sending than receiving
Receive RTS, send its own RTS back instead of CTS
Higher chance of transmitting its own message, lesser probability of early-sleeping, limited form of flow control
contend A
contend B
contend C
RTS
active D
RTS
CTS
DATA
ACK TA

35. S-MAC and T-MAC

36. U-MAC Similar to S-MAC Calculate utilization (U) Duty cycle adjustment
U = (Ttx + Trx) / (Ttx + Trx+Tidle)
Duty cycle adjustment
If U> high traffic threshold duty cycle increase
If U> low traffic threshold  duty cycle decrease
Drawbacks
High contention
Preliminary parameter setting (un-adaptable)
Rule of thumb
The experience or knowledge of the experiments

37. ADCA- Initial phase Decide its start time of its sleep/wake schedule
Broadcast its schedule to the neighbors
The initial period last for a certain time to collect neighbors’ schedules

38. ADCA- Adjusting phase The cycle length of all the nodes is the same
The schedule of ADCA is adjustable
Contention period
SYN period
Extended period
Sleeping period

39. The schedule of ADCA

40. ADCA
Listen to the channel for the incoming packets at the contention period
Broadcast the SYN packet at the SYN period
The extended period prolongs the active time immediately
Tune into sleeping period to save energy

41. The adjustment of ADCA The extended period adjustment
The next contention period adjustment

42. The radio status Bad situation
Success reception but destining to other nodes
Failed reception
Channel unstable

43. The adjustment of ADCA
The extended period compensates for the bad receiving situations
The next contention period accommodates to the variable traffics
Time records
Ti: idle listen
Tb: channel busy
Noh: number of overheard packets

44. The length of the extended period
Tbad = time of collision + channel unstable

45. The length of the next contention period
CCP is the length of the current contention period
: negative value (idle time)
Decrease the length
: positive value (busy time)
Increase the length

46. Simulation environments
NS-2 simulator
Two scenarios
All-to-one and end-to-end (n-to-n)
Packet size
Data packet: 50 bytes
Control packet: 10 bytes
Deploy 20, 30 and 40 nodes in 100 x 100 m2 area (average node degree is 4, 6 and 8)
Traffic (CBR)
1, 10, 20, 30, 40, 50 and 60 packets per second
Energy consumption
Tx:Rx:Idle:Sleep = 52.5: 59.1: 59.1: (mW) =17.4: 19.7: 19.7: (mA)

47. Energy consumption (all-to-one)

48. Energy consumption (n-to-n)

49. Transmission delay (all-to-one)

50. Transmission delay (n-to-n)

51. Goodput (all-to-one)

52. Goodput (n-to-n)

53. Implementation
an indoor wireless sensor network testbed which consists of a number of Octopus II sensor nodes
Each Octopus II sensor node is equipped with a MSP430 microcontroller and a CC2420 radio module, which operates at 2.4 GHz and transmits at 250 Kbps.

54. Implementation
And each node is also attached to a USB interface that provides both power supply and a backchannel for programming and data collection.

55. Implementation

57. The average energy consumption for the all-to-one scenario

58. The average energy consumption for the end-to-end scenario

59. The average transmission delay for the all-to-one scenario

60. The average transmission delay for the end-to-end scenario

61. The average packet transmission success rate for the all-to-one scenario

62. The average packet transmission success rate for the end-to-end scenario

63. Conclusion Asynchronous Duty Cycle Adjustment MAC protocol
Two adjustments are presented to compensate for bad situation and to adapt to the current traffic
The asynchronous scheme separates the competitors into different transmission period which reduce the affection of collision and overhearing
Improve utilization, lower energy consumption, lower transmission delay and increase success rate