1. Sensor Networks Katia Obraczka Winter 2005 MAC II
CMPE 259
Sensor Networks
Katia Obraczka
Winter 2005
MAC II

2. Announcements
Nacho will b talking about data aggregation on Mon., Feb. 28th.
Exam: Wed., March 2nd.
Project status updates.
1 paragraph (plain text) describing progress so far and what remains to be done, plan, etc.
Project submission also includes project report!

3. Today
Finish MAC.
Start “Storage, querying, and aggregation”.

4. FLAMA (cont’d from last class)

5. TRAMA Limitations Complex election algorithm and data structure.
Overhead due to explicit schedule propagation.
Higher queuing delay.
Flow-aware, energy-efficient framework.

6. Flow-aware Medium Access
Avoid explicit schedule propagation.
Take advantage of application.
Simple election algorithm to suit systems with low memory and processing power (e.g., 4KB ROM in Motes).
Incorporate time-synchronization, flow discovery, and neighbor discovery during random-access period.

7. Flow Information
Flow information characterizes application-specific traffic patterns.
Flows can be unicast, multicast or broadcast.
Characterized by source, destination, duration and rate.

8. Example: Data Gathering ApplicationFc Fd D Fb B A
Sink Fe E

9. Flow Discovery Mechanism
Combined with neighbor discovery during random-access period.
Adapted based on the application.
For data gathering application, flow discovery is essentially establishing the data forwarding tree.

10. Example
Sink initiates neighbor discovery, flow discovery, and time synchronization.
Broadcasts periodic SYNC packets.
Potential children reply with SYNC_REQ.
Source reinforces with another SYNC packet.
Once associated with a parent, nodes start sending periodic SYNC broadcasts. B
SYNC A C S
SYNC_REQ
Sink

11. Election Process B wc=1 A C S wa=3 wc=1 Sink ws=0
Weighted election to incorporate traffic adaptivity.
Nodes are assigned weights based on their incoming and outgoing flows.
Highest priority 2-hop node is elected as the transmitter.
A node listens if any of its children has the highest 1-hop priority.
Can switch to sleep mode if no transmission is started. B
wc=1 A C S
wa=3
wc=1
Sink ws=0

12. Simulation Results (16 nodes, 500x500 area, CC1000 radio, grid topology, edge sink)
Delivery Ratio
Queueing Delay

13. FLAMA Summary
Simple algorithm that can be implemented on a sensor platform.
Significant performance improvement by application awareness.

14. Geographic Random Forwarding [Zorzi e al.]

15. Approach: busy tones Nodes assumed to have 2 radios.
One used for data and another for signaling.
Receiver uses busy tone to prevent collisions.
Transmissions are broadcast.
RTS/CTS.
Any node can receive to RTS.
CTSs subject to contention.

16. Protocol operation: transmitter
Node has packet to send:
Listens to both frequencies for T.
If either frequency busy, backoff.
Otherwise, broadcast RTS with location of source and destination.
Listens for CTS.
If CTS received, sends data.
If no CTS received after N trials, abort.
If undistinguished signal, COLLISION.

17. Transmitter (cont’d) After packet transmitted, wait for ACK.
If ACK received, done and sleep.
Otherwise, retransmit.

18. Receiver Nodes wake up and listen. If no activity, go back to sleep.
Otherwise, go to receive state and activate busy tone.
When receiving RTS, node determines its priority as relay.
Based on location info.
First CTS slot, neighbors nearest to destination send CTS.
Nodes listen for data.
If data, only “designated” node receives; all others go to sleep.
“Designated” node is node that sent CTS (specified in data packet header).

19. Receiver (cont’d)
If CONTINUE, i.e., no node in closest region replied with CTS, nodes in 2nd. closest region will “contend”,…
If ABORT, transmitter reached maximum number of CTS slots.
If COLLISION, more than one CTS was received.
Colliding nodes execute collision resolution.

20. Evaluation Analytical. Numerical results. Compared with STEM. Metrics:
Idle nodes sleep.
Dual radio.
Metrics:
Energy.
Latency.
Overhead?

21. Results
From paper…