1. MAC Protocols for Sensor Networks
Paper Presentation of selected topics in EE206A
Based on the work of Alec Woo, David Culler, Wei Ye, Join Heidemann, and Deborah Estrin
Slides by Dayou Qian and Xiaolong Huang

2. Outline The characteristics of sensor networks
Issues of MAC protocols for sensor networks
Proposed solutions for MAC protocols
An adaptive rate control scheme
S-MAC (Sensor-MAC)
Bibliography

3. The characteristics of sensor networks
Tight constraints
Computation power
Storage
Primary functions
Sample the environment for sensory information
Propagate data back to the infrastructure
Traffic pattern
Little activity in lengthy period
Intensive traffic in short time
Highly correlated traffic
End to end flows are required to be fair
Base station
Sensor
Multihop

4. The characteristics of sensor networks
Networking component stack for sensor networks
Complete TinyOS application component graph

5. Issues in MAC protocols for sensor networks
Energy efficiency
Fairness of end to end flows
Related work
IEEE802.11
High energy consumption when the nodes are in the idle mode
CSMA
To improve the energy consumption by avoiding overhearing among neighboring nodes
TDMA
No contention-introduced overhead and collisions
Not easy to manage the inter-cluster communication and interference
Not easy to dynamically change its frame length and time slot assignment
PAMAS
Power off radio when not actively transmitting and receiving packet

6. An adaptive rate control scheme
The metrics for evaluation of a sensor network MAC protocol
Fair bandwidth allocation from each node to the base station over multihops is desired.
Energy efficiency is desired, including energy spent in listening for the channel and all packets transmissions and forwarding.
Design
Carrier sensing
reduce the time for carrier sensing
Backoff
backoff in application layer is desired other than in MAC layer
Contention
RTS-CTS only during high traffic load, while CSMA during low traffic load
Bidirectional multihop network (parenting structure) helps to reduce ACK packets

7. An adaptive rate control scheme
Rate control mechanism
Monitor route through traffic and original traffic
Application data rate , Probability of transmission , actual data originate rate
Increment by a constant , when application traffic is congested
Monitor:
Multiply by a factor , when application traffic is not congested
Monitor: , because dropping route through traffic is more expensive
Hidden terminal problem solved by parenting
If a node hears its parent transmit at time , assume its grandparent will start at time
The node restrains from transmission from time to

8. An adaptive rate control scheme
Simulation settings
Various CSMA schemes with different settings on: random delay, listening time, backoff scheme
Single cell topology
Small data packets 30 bytes
CSMA versus IEEE
Results on throughput
Randomness improves the throughput performance

9. An adaptive rate control scheme
Results on energy consumption
IEEE is worst in terms of listening energy consumption due to carrier sensing during backoff time
The most efficient schemes are those CSMA with constant listening time and random delay
Results on fairness
Different backoff scheme is insignificant for fairness

10. An adaptive rate control scheme
Results on application phase shift
Better fairness is provided
Conclusion
CSMA mechanism should incorporate random delay, constant listen period with radio powered down during backoff period

11. An adaptive rate control scheme
Multihop simulation settings
Multihop environment with bidirectional link
Results on fairness
Fairness is achieved by ARC

12. An adaptive rate control scheme
Results on energy efficiency
Conclusion
The adaptive rate control scheme together with the new CSMA mechanism provides an effective medium access control without explicit control packets, which is effective in achieving fairness while maintaining good aggregate bandwidth with reasonable energy efficiency

13. S-MAC (Sensor-Networks)
Trade Off
Energy efficiency Node-level fairness & Latency
Basic Scheme: Periodic Listen and Sleep
Choosing schedule
The node randomly choose a time to go to sleep.
The node receives and follows its neighbor’s schedule by setting its schedule to be the same.
If the node receives a different schedule after it select its own schedule, it adopts both schedules.

14. S-MAC (Sensor-Networks)
Maintaining Schedule
To update schedule by sending a SYNC packet periodically

15. S-MAC (Sensor-Networks)
Collision Avoidance
Virtual carrier sense
checking the keeping silent time recorded in the NAV (network allocation vector)
Physical carrier sense
Listening to the channel for possible transmission
RTS/CTS exchange
Overhearing Avoidance
Let interfering nodes go to sleep after they hear an RTS or CTS packet

16. S-MAC (Sensor-Networks)
Message passing
Only one RTS packet and one CTS packet are used
To avoid large control overhead and long delay
ACK would be sent after each data fragment
To avoid fragment loss or error
To Prevent hidden terminal problem
After the neighbor node hears the RTS and CTS, it will go to sleep for the time that is needed to transmit all the fragments (using the duration field)

17. S-MAC (Sensor-Networks)
Energy Saving vs. Increasing latency (Multi-hop Network)
Carrier sense delay
Determined by the contention window size
Backoff delay
Because the node detects another transmission or the collision occurs
Transmission delay
Determined by channel bandwidth, packet length and the coding scheme adopted
Propagation delay
Determined by the distance between the sending and receiving nodes
Processing delay
Depends on the computing power of the node and the efficiency of in-network data processing algorithms
Queuing delay
Depends on the traffic load
Sleep delay
Caused by the nodes periodic sleeping

18. S-MAC (Sensor-Networks)
Testbed
Used Rene Motes as the development platform and testbed
TinyOS
3 working modes: receiving, transmitting and sleep
Topology used in the experiment
3 MAC modules on the mote and TinyOS platform
Simplified IEEE DCF
Message passing with overhearing avoidance
The complete S-MAC

19. S-MAC (Sensor-Networks)
The energy consumption result on the source nodes A and B
When the traffic is heavy (the inter-arrival time<4s), S-MAC achieves energy saving mainly by avoiding overhearing and efficiently transmitting a long message
When the traffic is light, the periodic sleep plays a key role for energy savings

20. S-MAC (Sensor-Networks)
The percentage of time that the source nodes are in the sleep mode
The S-MAC protocol adjusts the sleep time according to the traffic patterns
The Module of message passing with overhearing avoidance spend more and more time in idle listening when the traffic is light

21. S-MAC (Sensor-Networks)
Energy consumption in the intermediate nodes
When the traffic is light, S-MAC still outperforms MAC
Reason1:S-MAC has synchronization overhead of sending and receiving SYNC packets
Reason2:S-MAC introduces more latency and actually uses more time to transmit the same amount of data

22. S-MAC (Sensor-Networks)
Conclusion
This new MAC protocol has very good energy conserving compared with IEEE
This new MAC protocol has the ability to make trade-offs between energy and latency according to the traffic conditions.

23. Bibliography The contents of the slides are based on
Alec Woo and David Culler. A transmission control scheme for medium access in sensor networks. Mobicom 2001
Wei Ye, Join Heidemann, and Deborah Estrin. A energy-efficient MAC protocol for wireless sensor networks. Infocom 2002