This work is performed as part of the EYES Project.

1. TiZo-MAC The TIME-ZONE PROTOCOLfor mobile wireless sensor networksbyAntonio G. Ruzzelli Supervisor : Paul Havinga This work is performed as part of the EYES Project.

2. Related WSNs MAC protocol • SMAC : High latency due to data forwarding interruption problems (RTS/CTS) • TRAMA : Good solution but very complex in terms of slot assignment • EMACS : High latency • DMAC : Only unidirectional communication. Moreover it does not support multiple gateways.

3. Targets of TiZo-MAC • Energy efficiency • Reduce the latency • Integrate communication protocol • Suitable for gateway topology network • Include data queries • Routing integrated

4. What is the main IDEA behind this new MAC? Gateway Node Why Time Zones? Nodes with the same color are in the same time zone Nodes within the same subset belong to the same gateway

5. Network topology : • A number of fixed and synchronized gateways; • A large number of mobile sensors (nodes); • Gateways and sensors are wireless; • Network divided in subsets; • Each gateway owns one network subset; • Subset organized in time zones;

6. Data traffic • Subnet flooding by gateway:Gateway msgs are forwarded to all nodes in the subnet 2. Local broadcast by node : Nodes send msgs to all of the direct neighbours. No forwarding is performed. 3. To gateway Transmission by node : Nodes closer to the gateway forward msgs until it reaches the gateway.

7. Time zones : • Number of hops to get the closest gateway • Nodes in the same zone have same hop count • Set up periodically by subnet flooding and local broadcast Nodes features: • Nodes within a time zone own the same slot • Nodes belong to one gateway only • Nodes store the number of hops to the closest gateway

8. Transmission Mechanism • Every slot has a contention period; • Nodes pick up a random time “t” in Contention Period; • Start listen to the channel at “t”; • Channel is Free  Start sending the packet ; • Channel is busy  Turn off the radio until the next scheduled Slot S L O T A B Zone 3 Zone 5 Zone 1 Zone 2 Zone 4 A B

9. 1- Sub-network Set-Up • Useful for initializing and updating the network • Gateways flood the network as follow: Zone 4 Zone 3 Zone 5 2 4 Zone 2 3 4 2 Zone 1 5 1 4 2 1 5 3 4 3 1 2

10. Initialization: • Gateways flood their sub network simultaneously; • Nodes set their hop count number and forward the msg. • Nodes include their hop count number in forwarded messages. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11. Collision reporting • Collision reporting takes place at the end of the slot just before the next contention period; • After transmitting, Tx node listens for a collision report; • In case of collision Rx node broadcasts a collision report; • In case of collision the packet is sent again after a random exponential backoff time. • The collision report is a short burst msg. CP Packet CR

12. Scheduling • Frame is divided in 4 slots; • Nodes in the same zone transmit simultaneously; • Nodes in the next time zone are listening; • Contention period is performed among neighbouring nodes;

13. Possible Schedule tables Upstream fast performing table Downstream fast performing • Packets can be forwarded in 4 zones in the same frame towards the gateway or towards farther zones. Fair fast performing table

14. Implicit Multiple path Performing • Forwarding “Transmission to Gateway” results in multiple copies of the same msg; • Nodes can detect copies of arriving msgs by combination of Source ID and message ID contained in the msg; • Messages arriving at gateway follow multiple paths. • Pro : Greater reliabilty! • Con : Increase overhead! 7 1 6 5 9 3 4 6 2 7 8 6 3 4 5

15. Overhearing • Nodes not getting the slot keep on listening to the beginning of other’s transmitting packet. • Packets have an index containing the ID of included msg; • If sent packet contain one or more same msgs that the node want to send, it can discard the msg. • Pro : Reduce overhead in transmission ! • Con : Small increase of node activity; • Increase complexity.

16. Implementation Nodes with the same colors are in the same zone (same hop Count Number). Beacons are positionated in corners. Simulation result : Nodes :50; Beacons :4; Static Network; Frame length = 1 sec; All Gateways send 1 msg every 32 sec; No gateway transmission; Local broadcast by Localization

17. Implementation has shown: • Most of collisions occur on the border of gateway subsets and around gateways; • TiZo-MAC with static network has given good results in terms of number of collisions without the need of handshake mechanisms among nodes;

18. Summary of TiZo-MAC features • Suitable for mobile sensor networks with a number of fixed gateways; • TDMA based; • No explicit RTS/CTS among nodes; • Implicit handshake mechanism; • Contention period among nodes ; • Collision report in the end of a slot; • Reliability achieved by multi-paths; • High Latency and low bandwidth;

19. Conclusion Future job : • Look at the behaviour with mobile nodes; • Investigate the usage of two different frequencies for “subnet flooding” and “To gateway” transmission • Look at the energy consumption of the MAC • Optimize the overhearing mechanism • An improved routing protocol as an upper layer .

20. Questions are welcome !

21. Using two different frequencies : • Collisions at the border can be eliminated by using 2 frequencies • If a subnet uses f1 for “to gateway” transmission, then adjacent subsets use f2 • If a subnet uses f2 for “subset flooding” then adjacent subnets use f1 Subnet 1 Subnet 2 f2 f1 f1 f2 f2 f2 Subnet 3 f1 f2 f1 f1 f1 f2 f2 f1 f1 = Frequency 1 f2 = Frequency 2