1. Presenter: Abhishek Gupta Dept. of Electrical and Computer Engineering
ECE/MAE 7750: Distributed Control Systems FISP: Focused Independent Study and Presentation
TMAC - An Adaptive Energy-Efficient MAC Protocol for Wireless Sensor Networks
Presenter: Abhishek Gupta
Dept. of Electrical and Computer Engineering
Utah State University
Date: February 23, 2005

2. Outline Overview Issues related to Wireless Sensor networks
Current approaches and their shortcomings
T-Mac protocol design
Performance Analysis
Conclusion

3. Introduction to Ad-hoc Wireless Sensor Networks
An Ad Hoc wireless sensor network is a network of sensor devices that are deployed in an ad hoc fashion and coordinate for sensing a physical phenomenon.
Each wireless sensing node typically includes
A Sensor
A processor
A radio
A battery

4. Introduction MAC protocols
MAC stands for Medium Access Control.
The main purpose of MAC protocol is to ensure that no two nodes interfere with each others transmission.
Essential attributes of MAC are
Collision Avoidance
Energy Efficiency
Scalability and adaptivity
Channel Utilization
Latency
Throughput
Fairness
Primary
Secondary
Secondary

5. Energy Efficiency in Wireless Sensor Networks
Energy efficiency is the primary concern in a wireless sensor networks.
Causes of energy waste
Collisions
Takes place at the receiver
Increases Latency
Overhearing
Happens when the nodes pick up data destined to other nodes
Idle Listening
Listening to traffic that is not sent

6. MAC Protocols Existing Solution
Traditional schedule based protocols are not appropriate for sensor networks.
TDMA
Disadvantage: Very difficult to maintain synchronization.
IEEE with power save mode
Disadvantage: Not designed for multi-hop networks
SMAC protocol with fixed duty cycle

7. Existing Solutions SMAC protocol
SMAC stands for Sensor Medium Access Control (protocol).
Time is divided into fairly large frames.
The ratio of the listen time to Sleep time is called as the duty cycle.

8. Existing Solutions SMAC protocol
During the Sleep time, node turns off its radio.
During the active time, nodes can communicate with each other.
Drawback of SMAC approach
Difficult to optimally tune it under static traffic condition.
Energy waste due to idle listening in active state when the load is low and traffic is dynamic
Increases the latency of the network

9. TMAC Evolution (TMAC – Timeout Medium Access Control)
Adaptive Rate Control
ARC
IEEE 802.3
IEEE
CSMA/CD
CSMA/CA
SMAC
TMAC
Carrier Sense Multiple Access with Collision Detection
Carrier Sense Multiple Access with Collision Avoidance
Fixed duty cycle
Adaptive duty cycle
DMAC/
MMAC
Directional Antennas

10. TMAC design Basic Communication Scheme
Nodes can communicate in active time and turn off radio at sleep time
TMAC uses RTS- CTS-ACK scheme

11. TMAC design Basic Communication Scheme
The active period can be dynamically ended if no activation event is detected.
Activation events include
Firing of periodic frame timer
Reception of any data
Sensing of any communication on the radio
The knowledge that the data exchange of the neighbors have ended
This scheme moves all the communication to a burst at the beginning of the frame

12. TMAC Design Clustering and Synchronization
Nodes exchange their schedule by periodically broadcasting SYNC packet
Nodes take following 2 steps to choose their schedule
Listen for SYNC packets for a fixed amount of time
Case 1: No SYNC packets are received
Case 2: SYNC packet is received.
Case 3: Multiple SYNC packets are received.
Schedule 2
Schedule 1
Border nodes with 2 schedule broadcast twice
Broadcast the chosen schedule by sending out SYNC packet.

13. TMAC Design Additional Features of TMAC: Fixed Contention interval
In traditional protocols, back off scheme is used
Back-Off scheme reduces the probability of collisions when the load is high
TMAC transmits its queued message in burst at the start of the frame.
TMAC uses fixed contention interval
Contention time is always used, even if no collision has occurred yet.

14. TMAC Design Additional Features of TMAC: RTS retries
When a node sends a RTS but does not receive CTS, there are 3 possible reasons
The receiving node has not heard RTS
The receiver is prohibited from replying
The receiver node is sleeping
TMAC retries by sending additional RTS before giving up on the node

15. TMAC Design Overhearing Avoidance
SMAC have introduced overhearing avoidance by putting nodes to sleep after hearing RTS\CTS packets destined for other nodes
Main Advantage: Energy is saved
Disadvantage: Higher Collision overhead.
TMAC has kept overhearing avoidance feature of SMAC as an optional feature.

16. TMAC Design Early Sleeping Problem
This problem is seen in unidirectional traffic, like in node to sink communication.
Early sleeping problem reduces the maximum throughput of the network.

17. TMAC Design Early Sleeping Problem Proposed Solution: Future-Request-To-Send (FRTS)
The idea is to let other node know that we still have a message for it, but are ourselves prohibited from using the medium.
The FRTS packet contains the length of time for which communication is blocked.
The node that receives the FRTS packet knows that it will be the future target of an RTS packet and must be awake by that time.

18. TMAC Design Early Sleeping Problem Proposed Solution: Full Buffer Priority
When node’s transmit/routing buffer are almost full, it may prefer sending to receiving.
Advantages:
Probability of early sleeping problem is reduced.
The scheme introduces limited flow control in the network.

19. TMAC Design Simulation set up and parameters
For protocol design simulations and implementations, EYES wireless sensing nodes were used.
EYES nodes have TI MSP430F149 processor, 60Kb flash memory and the processor runs on variable clock rate
The capabilities and power consumption of EYES nodes are quite similar to other prototype sensor nodes, for example, popular Berkeley motes
3 Protocols were used and compared.
Carrier Sense Multiple Access
SMAC with varying active time from 75 ms to 915 ms
TMAC protocol with frame length of 610 ms and TA of 15 ms

20. TMAC Design Performance: Homogenous Local Unicast
Homogenous unicast is the best case for SMAC since the load is constant

21. TMAC Design Performance: Event based local unicast
x axis- message frequency
y axis- average energy consumption
Figure shows an event based unicast scenario

22. TMAC Design Conclusion
TMAC offers a novel way of dynamically adapting the duty cycle and thus obtaining optimal performance.
TMAC offers significant energy efficiency over always listening MAC protocols.

23. References:
Medium Access Control With Coordinated Adaptive Sleeping for Wireless Sensor Networks, by Wei Ye, John Heidemann, and Deborah Estrin (IEEE/ACM TRANSACTIONS ON NETWORKING, VOL. 12, NO. 3, JUNE 2004 )
An Adaptive Energy Efficient MAC Protocol for Wireless Sensor Networks by Tijs van Dam, Koen Langendoen (SenSys’03, November 5–7, 2003, Los Angeles, California, USA.)
Presentation by Wei Ye on MAC Layer Design for Wireless Sensor Networks
Presentation by Ranjith Udayshankar on Medium Access Control With Coordinated Adaptive Sleeping for Wireless Sensor Networks