1. PW-MAC: An Energy-Efficient Predictive-Wakeup MAC Protocol for Wireless Sensor Networks
Lei Tang∗ Yanjun Sun† Omer Gurewitz‡ David B. Johnson∗
∗Department of Computer Science, Rice University, Houston, TX, USA
†Systems and Applications R&D Center, Texas Instruments, Dallas, TX, USA
‡Department of Communication Systems Engineering, Ben Gurion University, Israel
INFOCOM 2011

2. Motivation
(receiver-initiated) as the beacon is substantially shorter than a preamble, wireless bandwidth usage
Reduce the duty cycle for recieivers and senders, whereas RI-MAC only reduce the duty cycle only at receivers
Energy-efficiently resolving collisions and retransmitting lost packets

3. Preamble vs beacon Sender initiated (X-MAC)
Receiver initiated (RI-MAC)

4. Reduce duty cycle
RI-MAC
PW-MAC

5. Energy-efficiently resolving collisions
Pseudo-random wakeup schedule generater
To enable a sender to accurately predict the wakeup times of a receiver(done in wise mac)
To avoid generating same numbers (waking receivers up at the same time)

6. Energy-efficiently retransmitting lost packets
Detecting wireless collision -> switching to sleeping state -> choosing when to wakeup and retransmit the packet

7. On-demand prediction error correction
A node S requests an update of the prediction state of another node R when it detects that the prediction error is larger than the sender wakeup advance time.

8. evaluation Sender wakeup advance time
the prediction error caused by hardware and operating system latency leads to the sender missing some wakeups of the receiver

9. evaluation Conflicting wakeup schedule
Once two receivers wake up at the same time, they will continue waking up at the same time in the following cycles due to the fixed wakeup interval of WiseMAC.

10. evaluation Hidden terminal
WiseMAC is unable to detect the packet collisions of the two hidden senders and does not have an efficient mechanism to handle packet retransmissions

11. evaluation
Multihop network

12. Delay-Bounded MAC with Minimal Idle Listening for Sensor Networks
Yang Peng, Zi Li, Daji Qiao and Wensheng Zhang
Iowa State University, Ames, IA 50011
INFOCOM 2011

13. Motivation
RI-MAC uses shorter and less frequent beacons which consume less bandwidth
A sender needs to remain awake after a data packet arrives, till the receiver wakes up to receive the packet, potentially wastes a lot of time on idle listening.
A receiver sends out beacons at a fixed time interval on average and does not adapt to changes of traffic pattern.

14. Reduce idle listening delay bounded data delivery
only requires the sender to wake up at prescheduled rendezvous times to communicate with the receiver
the receiver wakes up at the scheduled beacon time

15. relative delivery delay bound
Delay between arrival time of packet and when packet is transmitted
The ratio of the data delivery to the average data arrival interval
if data packets arrive every 100 seconds and the delivery delay of a data packet is 10 seconds, the relative delay is 10%.
Conserve energy
if a 10% relative delay bound is acceptable, when the data arrival interval increases from 10 to 100 seconds, the number of beacons sent by the receiver and hence the energy consumed by the receiver can be reduced by an order of magnitude.

16. delay-bounded data delivery services
if a data packet arrives before the next scheduled listen time, the radio won’t be turned on till the scheduled listen time

17. adjusts to the varying traffic condition
Dynamic duty cycle
sensor nodes adjust their duty cycles dynamically to the varying traffic condition
This shows that CyMAC nodes are able to adjust quickly to the varying traffic condition and operate in ultra low duty cycles when the traffic is light.

18. Effects of time asynchrony
clock drift
Difeerent clocks counting time at slightly different rates
Extra delay has been added to the packet delivery delay

19. Evaluation Line topology
Varying flow length & end-to-end relative delay bound
When the flow length is large, RI-MAC cannot provide the desired delay bound even with a higher duty cycle than CyMAC.

20. Evaluation Line topology
Varying end-to-end relative delay bound & the average packet generation interval
Change data intervals

21. Evaluation Star topology
the relative delay in RI-MAC is not affected much by the number of source nodes but by average packet generation interval .

22. Evaluation Mesh topology with multiple flows
CyMAC has lower duty cycle than RIMAC
Achieve the desired delay bound