1. Richard LaHusen (USGS)
IEEE PerCom March 11, 2009
TreeMAC : Localized TDMA MAC Protocol for Real-time High-data-rate Sensor Networks
Wen-Zhan Song (WSU)
Renjie Huang (WSU)
Behrooz Shirazi (WSU)
Richard LaHusen (USGS)
*This research is supported by NASA and USGS under grant NNX06AE42G.

2. Outline Introduction Related Works Design and Analysis
Experiments and Evaluations
Conclusion and Future Work

3. Medium Access Control (MAC)
send?
receive?
sleep? u
MAC, controlling access to the channel, plays a key role in determining
channel capacity utilization
throughput and delay
energy consumption and lifetime
congestion
fairness

4. MAC Research Challenges
(a) Hidden station problem (b) Exposed station problem
The MAC research challenges in sensor network mainly come from the facts that:
Radio broadcast nature – interferences are complex
Multi-hop networking – nodes are not in one collision domain
Limited energy supplies – nodes are powered by battery only

5. Existing Sensor Net MAC protocols
The first generation sensor networks are characterized by low data rates (sampling intervals on the order of minutes) and very low duty-cycle operation to conserve power
Such as habit monitoring, microclimate monitoring
MAC protocol: periodic wakeup, sense and sleep to reduce idle listening energy waste:
Examples: S-MAC (INFOCOM 2002) and B-MAC (Sensys 2004)
Great Duck Island, Berkeley Redwood Forest, and James Reserve deployments

6. Existing Sensor Net MAC protocols
Recent sensor network applications involve relatively high data rates (e.g., 102 to 105 Hz sampling rate) and precise timing of the captured signals. High throughput, low delay, and network fairness are as important as energy conservations.
Such as structural health monitoring, medical monitoring, industrial process control, volcano monitoring, land seismic.
TDMA MAC protocol is a natural choice:
Examples: Z-MAC (SenSys 2005), Funneling-MAC (SenSys 2006)
UCLA NetSHM for structural health, Harvard CodeBlue, Volcano, Intel North Sea, Berkeley Golden Gate Bridge

7. Existing Sensor Net MAC protocols Z-MAC
Two nodes in the interference range assigned to different time slots: based on DRAND
Hybrid CSMA+TDMA
Use a base TDMA schedule as a hint to schedule the transmissions of the nodes, and it differs from TDMA by allowing non-owners of slots to 'steal' the slot from owners based on CSMA if they are not transmitting. C D A F B E
Radio Interference Map
Input Graph
DRAND slot assignment 1 3 2

8. Existing Sensor Net MAC protocols Z-MAC
Limitations:
The slot assignment overhead is high
Need to re-run DRAND if topology changes
After DRAND, every node needs to decide frame size based on conventional wisdom – synchronize with rest of the network on Maximum Slot Number (MSN) as the frame size
MSN has to flood through whole network.

9. Existing Sensor Net MAC protocols Funneling-MAC
Limitations: assume sink can reach all nodes in intensity region

10. TreeMAC: key observations
In general, wireless mesh networks or other traditional networks are designed to support mutual communications among nodes.
The traffic pattern is any-to-any, driven by user demands.
In many wireless sensor network applications, sensor nodes monitor fields and generate events which traverse hop-by-hop toward one or more sink nodes.
The major traffic pattern is many-to-one, forming a data collection tree.
Equal channel access is not fair for data collection. 1 5 2 3 4 u v
Controlling access to the channel, generally known as MAC
protocol, plays a key role in determining channel capacity
utilization, network delays and energy consumptions. It also
influences congestion and fairness in channel usage.
The fundamental task of any MAC protocol is to regulate the access
of a number of nodes to a shared medium in such a way that certain
application-dependent performance requirements are satisfied.

11. TreeMAC: key observations
depth k
depth k+1
In a data collection tree, the tx time slots of depth-k nodes shall be no less than the sum of the tx time slots of depth-(k+1) nodes and the slot demand of the depth-k nodes’ own data:
T(k) >= T(k+1) + D(k)
Slot reuse maximization based on graph coloring does not necessarily maximize data collection throughput.
the network may have more data flowing, but the sink does not necessarily get more data – it could be dropped in funnelling region.

12. TreeMAC: key ideas …… cycle 1 2 3 4 5 6 7 8 9 A B C D E F1 2 3 4 5 6 7 8 9 A B C D E F
Time divided to cycles
Cycle divided to N frames (configurable, e.g., 16 in the illustration).
Frame divided to 3 slots:
Slot 0: red Slot 1: green Slot 2: blue
Each node picks slot according to its tree-level only (e.g., level%3)
Each node can only send in picked slot per frame, listen/sleep in the other two
Mitigate vertical 2-hop interference
Each node assign its children’s frames in the beginning of each cycle, such that children’s frames does not overlap/conflict with each other.
Node’s tx/rx slot is determined by the frame-slot pair (F, S).

13. TreeMAC: key ideas Children get parent’s frames subset
sink a b c d o h p j f g i k l n m 1 1 1 1 1 2 2 2 1
Every node get number of slots proportional to its bandwidth demand

14. Conflict-free transceiving and snooping
receiver
sender
snooper u y u y w w v x v x
TreeMAC: conflict-free transceiving and snooping
Other MAC: conflict-free transceiving
We proved: with TreeMAC, given any node, there is at most one active sender in its neighborhood (including itself) at any time.

15. Note on Interference Model
Like other MAC protocols, our theory analysis is based on the widely used protocol interference model:
In this model, a transmission by a node vi is successfully received by a node vj iff vj is not in the transmission range of the source of any other simultaneous transmission.
It does not necessarily provide a comprehensive view of reality, due to aggregate effect of radio interference. But it does provide some good estimations of interference and enables a theoretical performance analysis of number of protocols in the literature.
In this model, a transmission by a node vi is successfully received by a node vj iff vj is not in the transmission range of the source of any other simultaneous transmission. 15

16. Optimal and bufferless schedulingb 1 f 1
We proved:
TreeMAC protocol is a fair and bufferless
packet scheduling protocol.
The throughput of TreeMAC protocol is at least 1/3 of the optimum, and the packet delay of per round is bounded by 3 times of the optimum.

17. Possible deployment strategy
gateway
sink 0
sink 1
sink 2
If there are 3 sinks connected to gateway, then
The 3 different sink trees use 3 different channels
Given a node u in the sub-tree of sink i, the transmittable slot tu = (lu+i) % 3

18. TreeMAC design considerations
System Initialization and Time Synchronization
Initially, nodes run CSMA to discover neighbors and routing path, and perform network time synchronization using FTSP. Sink node initiates the TreeMAC mode by assigning the frames to its children.
Any node who receives schedules from its parent, will switch to TreeMAC mode and assign the frames to its own children as well. During the transition period from CSMA to TreeMAC mode, nodes in CSMA mode have less opportunity to access channel using longer backoff period.
To avoid accumulated synchronization error to new cycle, TreeMAC uses two timers: slot timer and cycle timer. When the cycle timer fires, it stops and restart the slot timer, so that the slot timer gets re-synchronized after each cycle.

19. TreeMAC design considerations
Obtaining Tree Structure Information
TreeMAC protocol does not need separate tree formation and maintenance. It obtains the parent and children information from the routing tables of data collection routing protocol.
Dynamic topology update. A neighbor management module maintains children set information based on upstream packets. Use timeout mechanism to detect children join/leave.
If a node changes parent, we do not adjust frame-slot assignment immediately. Instead, wait until next cycle. The temporary mismatch does not necessarily break the TreeMAC foundation that T(k) >= T(k+1) + D(k)

20. TreeMAC design considerations
Frame-Slot Schedule Assignment and Adjustment
Each node’s bandwidth demand level = R/p, here R is total data rate of its subtree, and p is the link reliability of a node to its parent.
Bandwidth demand information is piggybacked in routing beacon message to reduce overhead.
traffic
demand
#1 # Child …
Parent’s own schedule
round robin
Every node uses round robin method to assign frames to its children based on their proportional bandwidth demand

21. Testbed setup
We conducted the experiments on a sensor network test bed with 24 iMote2 motes
We compared TreeMAC with Funneling-MAC and CSMA (the default MAC in TinyOS), under various network traffic and topologies
Under each network condition, the same experiment is repeated 5 times to get an average
Network throughput, energy efficiency, network fairness, signaling overhead are evaluated

22. Testbed setup

23. Network throughput
We measure the network throughput by calculating how many data packets are successfully delivered to the sink from all nodes in one second.
(a) Network throughput with varying injected data rate [4, 12] pkts/second
(b) Network throughput with different network size [8, 24]

24. Network throughput (c) Network throughput over running time [8, 24]
TreeMAC is robust with respect to dynamic topology and can obtain stable performance in the long run.

25. Energy efficiency
Energy efficiency is defined as the ratio of the number of delivered distinct packets to the gateway over the total number of transmitted packet in the network.
D is the set of packets delivered to the sink;
P is the set of all injected packets;
hops(p) ranges over each hop packet p traverses;
xmits(p, h) denotes the number of transmissions a packet p undergoes at hop h.
TreeMAC would rather drop packet at the source node, than drop packet at the intermediate node.

26. Network fairness
Fairness: how equally every node has delivered data to gateway. We define fairness index as:
ri is the average rate of packets delivered from the ith sensor;
N is the number of sensors in the network.
If the network is more fair, the index is closer to 1.

27. Signalling overhead
TreeMAC overhead: the dissemination of bandwidth demand and frame-slot schedule messages.
Funneling-MAC overhead: beacon packets, path information field, schedule packets, meta-schedule
For comparison, the signaling overhead index is defined as : (total control packet bits) / (total data bits that reach the sink).

28. Conclusion and Future Work
TreeMAC has following theory properties:
Given any node, at any time slot, there is at most one active sender in its neighborhood (including itself).
The packet scheduling is bufferless, which therefore minimizes the probability of network congestion.
The delivery throughput and delay to gateway is at least 1/3 of the optimum assuming reliable links.
Experiments on a 24 node test bed show that TreeMAC protocol significantly improves network throughput and energy efficiency
Future work
Make it compatible with MAC framework
Remove needs of global synchronization.

29. Thank You for attention!
WenZhan Song

30. Existing Sensor Net MAC protocols
Cited from 30

31. Experiment parameters
For comparison, we ported Funneling-MAC from mica2 platform with CC1000 radio to iMote2 platform with CC2420 radio.
The only change made is scaling the power level range from [1, 255] for CC1000 to [1, 31] for CC2420. 31

32. Richard LaHusen (USGS)
IEEE PerCom March 11, 2009
TreeMAC : Localized TDMA MAC Protocol for Real-time High-data-rate Sensor Networks
Wen-Zhan Song (WSU)
Renjie Huang (WSU)
Behrooz Shirazi (WSU)
Richard LaHusen (USGS)
*This research is supported by NASA and USGS under grant NNX06AE42G. 32