An Energy-Efficient and Low-Latency Routing for Wireless Sensor Networks

1. An Energy-Efficient and Low-Latency Routing for Wireless Sensor Networks Antonio G. Ruzzelli, Richard Tynan, G.M.P. O’Hare. Adaptive Information Cluster project (AIC) Smart Media Institute (SMI) Department of Computer Science University College Dublin Ireland. http://www.adaptiveinformation.ie/home.asp

2. Overview of Wireless Sensor Networks • Large number of tiny sensors (nodes) distributed in an area network; • Sensor nodes: • have sensing devices attached; • are self-organizing; • are usually battery operated and of low cost hence power limited  • multi-hop communication to save energy;

3. Objectives Experimental evaluation of two essential scheduling regimes within the MERLIN* protocol to be injected dynamically in the network. ↓ An increase of network adaptivity to save energy by choosing the appropriate scheduling with respect to the application requirements. * Mac Energy-efficient, Routing and Localization Integrated, (see reference in the paper)

4. Motivation for MERLIN? • Separated MAC and Routing layers for WSNs cause an extremely high latency (e.g. SMAC and DSR  tens of seconds delay for packets of nodes in hop 10 or more) • MERLIN integrates MAC+Routing features into the same simple architecture; • No usage of handshake mechanisms; • Latency is considerably reduced while ensuring a very low energy consumption

5. What is the main IDEA behind the MERLIN protocol? (European EYES project, NL) 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 --------------------------------- Nodes within the same zone wake up and go inot sleep simultaneously

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. Transmission Mechanism (I) S L O T CP DATA A B Zone 3 Zone 5 Zone 1 Zone 2 Zone 4 A CR B • Nodes in the same zone share the medium; • Nodes in the same zone use a contention period (CP) for CSMA; • Nodes in the neighbouring zones are listening; • Nodes 2 hops away are sleeping; • A collision report period (CR) is provided at the end of the slot. • The CR message is a short burst tone. Note: MERLIN, not addressing the receiving neighbouring node, can cause multiple copy of the same msg sent increase overhead

8. Packets are organised in multiple msgs of the same data traffic type; Packets start with an index containing the ID of included msgs; Nodes, which lose the contention, keep on listening to the beginning of the transmitted packet then go into sleep; Nodes discard from their queue the msgs already fowarded. Transmission Mechanism (II) Channel contention P a c k e t messages Msg-index • Pro : Reduce overhead in transmission! • Con : Small increase of node activity; • Increase complexity. Discard msgs already forwarded from their queue Listen to the packet index

9. Transmission Mechanism (III) • Mitigating the Hidden terminal problem (HTP) [WiseMac]: • The CSMA can be Adjusted by the SNR; • Interference range of a signal can be set greater than the range of correct signal reception; • MERLIN sets a certain received power threshold for a correct reception of a packet; • E.g. A threshold of 15 dB SNR for a correct reception results in a maximum of 50m transmitting range and over 100m interference range. CP C Transmitting range A B Interfering range Zone N+1 A B Con: throughput reduction Zone N

10. Scheduling tables: V-schedule vs. X- Schedule • Frame is divided in 8 slots; • Nodes in the same zone transmit simultaneously • The X scheduling is obtained by super positioning 2 V-sched one of which upside-down • Nodes go into sleep immediately after the transmission

11. Simulation and result Nodes with the same colors are in the same zone (same hop Count Number). Number slot /frame = 4 Contention period = 30ms DataRate = 115200 bits/sec DataSize = 16+8 Bytes (data + 3 bytes preamble + starting code) Eyes node

12. X-scheduling vs V-scheduling 300 250 200 Network Lifetime (days) 150 100 50 0 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Frametime (sec) 1 Gateway 100 Nodes rand. Distributed. 800*500 area network Min signal strength(12 m) 50 msg/min sent by 5 rand. nodes Static network X Scheduling V-Scheduling V and X scheduling Network lifetime. • The network lifetime depends linearly on the frame length; • The usage of V-sched results in a longer network lifetime than X-sched; • The network is considered to fail when 30% of nodes are depleted. • Lifetime calculated for a linear depletion of 2 AA batteries.

13. X and V scheduling setup time • V-sched shows double network setup time with respect to X-sched; • X and V scheduling can be setup in less than 10 seconds for 250 nodes network density.

14. End-to-end packet delay X-scheduling V-scheduling • The controlled multiple path mechanism may cause a lower delay for nodes farther from the gateway than other; • A periodic and discontinuous increase of latency at the intersection of data traffic flows due to: • X-sched: Packet Collisions hence retransmission; • V-sched: Periodical stop of nodes activity that go into sleep.

15. Average end-to-end packet delay • X-sched presents a more accentuated linear behaviour than the V-sched; • V-sched shows a considerable increase of packet delay than X-sched; • X-sched has a greater throughput than V-sched. Which scheduling to adopt should be based on both the application requirements and network conditions.

16. Conclusion and future work • The absence of handshake mechanisms like RTS/CTS by means of MERLIN can considerably reduce the end-to-end packet delay; • The X scheduling should be used for applications in which some energy can be traded off for a decrease of latency of messages and for applications in which latency is a tighter constraint; • V-scheduling is more suitable for low data traffic applications where the need of saving energy is of paramount importance. • Future work: As a result of the same nature of X and V scheduling together with a fast setup time, MERLIN is suitable for a dynamic scheduling switching in accordance with change of network conditions, e.g. by means of migrating agents to be injected.

17. Thank you for your kind attention

18. 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