1. An Adaptive Energy-Efficient MAC Protocol for Wireless Sensor Network
Tijs van Dam, Koen Langendoen
SenSys’03
Ku Dara
Network & Security LAB at KAIST

2. Contents Introduction S- MAC drawbacks T- MAC Experiments Conclusions

3. Introduction Traditional MAC Protocols
Design to maximize packet throughput, minimize latency and provide fairness
Protocol design for wireless sensor networks
Focuses on minimizing energy consumption
What was the most wasted energy in traditional MAC protocol?
Idle listening
A node does not know when it will be the receiver of a message from one of its neighbors
It must keep its radio in receive mode at all times
Ex) sensor application : 1/sec, messages fairly short, transmit(5ms) ,
receive(5ms), 990ms on listening while nothing happens(99%)
T-MAC

4. S-MAC Idle listening problem solution S-MAC in sensor network
Duty cycle is involved, each node sleep periodically
S-MAC in sensor network
Single-frequency contention-based protocol
Time is divided into –fairly large-frame (frame: 1sec)
Every frame has two parts : active part (200ms) /sleep part (800ms)
duty cycle = listen interval / frame length (20%)
All messages are packed into the active part
Tradeoff
Energy efficiency ↑, throughput ↓ , latency↑
T-MAC

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

6. T-MAC Preliminaries(1)
Basic idea
To utilize an active and a sleep cycle, similar to S-MAC
To introduce an adaptive duty cycle by dynamically ending the active part
An active period ends when no activation event has occurred for a time TA
Activation event
The reception of any data on the radio (RTS, CTS, DATA, ACK)
The sensing of communication on the radio (overhearing)
Difference in the duty cycle
S-MAC - fixed duty cycle
T-MAC – Dynamic duty cycle
T-MAC

7. T-MAC Preliminaries(2)
Normal MAC protocols: messages are spread out over the whole time frame
S-MAC: active time is fixed
T-MAC: the active time is dynamically adjusted (i.e., be shorten) by timing out on hearing nothing during some time period (TA)
T-MAC

8. T-MAC : RTS Operation (1)
Contention Interval : Fixed contention interval
In contention-based protocols, like IEEE
a back-off scheme is used: contention interval increases when traffic is higher
Reduce the probability of collision when load is high
In the T-MAC protocol
Every node transmits its queued messages in a burst at the start of the frame
In burst, the traffic is mostly high
Waiting and listening for random time within a Fixed contention interval
Tuned for maximum load.
T-MAC

9. T-MAC : RTS Operation (2)
RTS Retries
No CTS reply for RTS?
The receiving node has not heard the RTS due to collision
The receiving node is prohibited from replying due to an overheard RTS or CTS
Receiving node is asleep
Solutions:
Retransmit RTS if no answer
If there is still no reply after two retries, it should give up and go to sleep
T-MAC

10. T-MAC : Choosing TA Determining TA TA > C+R+T
The interval TA must be long enough to receive at least the start of the CTS packet
TA > C+R+T
C – contention interval length:
R – RTS packet length:
T – turn around time, time between RTS end & CTS start:
Larger TA increases the energy used
In experiments, used TA = 1.5 x (C + R + T) A B C
contend
RTS
CTS
DATA
ACK TA
T-MAC

11. 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
Although overhearing avoidance saves energy, it must not be used when maximum throughput is required
T-MAC

12. T-MAC: Asymmetric Communication (1)
Early-Sleeping Problem – unidirectional (A to D)
Node goes to sleep when a neighbor still has messages for it A B C D
contend
RTS
CTS
DATA
ACK
RTS?
active
sleep TA
T-MAC

13. T-MAC: Asymmetric Communication (2)
Future request-to-send (FRTS)
Let others know that we still have a message for it, but cannot access the medium;
C sends FRTS to future target of an RTS packet
FRTS has duration field
FRTS might affect data; so, DATA postponed until FRTS is over; To prevent others from taking medium, A send DS(Data Send) packet; A B C D
contend
RTS
CTS
DATA
ACK
active TA DS
FRTS
TA > C+R+T+CTS_length
T-MAC

14. T-MAC: Asymmetric Communication (3)
Taking priority on full buffers
When a node’s transmit/routing buffers are almost full, it may prefer sending than receiving
Receive RTS, send its own RTS to others instead of CTS
Advantage in a node–to-sink communication pattern A B C D
contend
RTS
CTS
DATA
ACK
active TA
T-MAC

15. Experiments
S-MAC Vs. T-MAC
T-MAC

16. Simulation setup and parameters
Simulator: OMNeT++
Built a network of 100 nodes in a 10 by 10 grid (8 neighbor)
Energy consumption
S-MAC protocol
A frame length of one second, and with several lengths of the active time, varying from 75 ms to 915 ms.
T-MAC protocol
Always used a frame length of 610ms and an interval TA with a length of 15 ms
Can optionally be deployed with overhearing avoidance, full-buffer priority, and FRTS
T-MAC

17. Homogeneous local unicast
Nodes send packets to one of their neighbors at random
T-MAC: Used overhearing avoidance, but no FRTS or full-buffer priority mechanisms
T-MAC

18. Nodes-to-sink communication
Nodes send messages to a single sink node :Send message to corner node
Shortest path routing, no data aggregation
T-MAC: Used overhearing avoidance, FRTS & full-buffer priority mechanisms
T-MAC

19. Early-sleeping Problem
Nodes send messages to a single sink node: Send message to corner node
Shortest path routing, no data aggregation
T-MAC:
FRTS
Vs. Priority
Vs. FRTS + Priority
Vs. No measures
T-MAC

20. Conclusions And Future Work
T-MAC dynamically adapts a listen/sleep duty cycle
Early sleeping problem
Proposed FRTS & full-buffer priority
Trade-off : throughput vs. energy efficiency
Future work
Experiment with mobile network
Apply virtual clustering in the S-MAC
T-MAC