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CMPE 259 Sensor Networks

Katia Obraczka

Winter 2005

Transport Protocols II

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Announcements

Feedback on project proposals. Project resources.

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Transport protocols (cont’d)

RMST CODA Summary

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RMST

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RMST

Reliable Multi-Segment Transport.

Where to do reliability? MAC. Transport. Application.

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MAC reliability

802.11. RTS/CTS, Data, Ack. Basic stop-and-wait ARQ. No ARQ when in broadcast or multicast

modes.• Random slot selection.

Options: No ARQ. AEQ always. Selective ARQ.

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MAC reliability (cont’d)

Without ARQ: Use broadcast mode. For unicast: address screening at routing layer. +’s: no overhead.

With ARQ: Unicast transmissions. For broad- & multicast, use multiple unicast. Number of retries is configurable.

Selective ARQ: Unicast uses ARQ. Broad- and multicast use no ARQ.

• E.g., route discovery.

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Transport reliability

Strictly e2e. Initiated by sink.

Local recovery. Intermediate nodes trigger repair when loss

is detected. Nodes cache packets.

NACK-based.

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Application-layer reliability

Directed-diffusion based. Sink sends out request (“interest”). When complete data received, sink removes

request.

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Question?

Benefits of lower-layer reliability? Additional overhead?

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RMST overview

Functions: Fragmentation/reassembly. Guaranteed delivery.

Unique identifiers: “No fragments”. Fragment id’s and number of fragments.

Loss detection and repair: Sequence # holes and timers. Loss detection at either sinks or

intermediate nodes. NACKs.

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Preliminary analysis

Demonstrate the benefits of hop-by-hop reliability.

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RMST evaluation

MAC-only reliability. Local recovery.

With and without MAC reliability. End-to-end reliability.

With and without MAC reliability.

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Observations

When there is no transport reliability: MAC reliability critical in lossy links.

Hop-by-hop transport reliability: Adds little to reliable MAC. But, hop-by-hop transport reliability only more

efficient than adding MAC reliability.• MAC ARQ overhead incurred in every packet.

E2E transport reliability: When no MAC reliability is used, simulation does not

terminate: hop-by-hop recovery is critical. If MAC reliability used, hop-by-hop and e2e transport

reliability are equivalent.

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Observations (cont’d)

Experiments with high error rates: Hop-by-hop transport reliability without MAC

reliability. Hop-by-hop transport reliability+Sel. ARQ. E2e transport reliability+ Sel. ARQ.

Hbh transport reliability without ARQ breaks down at high error rates. Routing has hard time establishing routes.

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CODA

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COngestion Detection and Avoidance Importance of congestion control.

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What is CODA ?

Energy efficient congestion control. Three mechanisms are involved:

Congestion detection Open-loop hop-by-hop backpressure. Closed-loop multi-source regulation.

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Congestion detection

Accurate and efficient congestion detection is important Channel loading – sample channel at

appropriate rate to detect congestion.

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Open-loop h-by-h backpressure

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Congestion detected

Upstream nodedecides to propagatebackpressure or not.

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Closed loop multi-source regulation

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1,2,3

ACK

4,5,6 Congestion detected

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Regulate bit is set

ACK

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Congestion detection schemes Buffer occupancy.

Not reliable in CSMA networks. Channel loading.

Good for the immediate neighborhood. Energy considerations.

Report rate. Report rate goes down, congestion. Detection based on report rate needs to

react on longer time scale.

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CODA overview

Combination of backpressure (fast time scale) with closed-loop congestion control.

Backpressure targets “local” congestion, whereas closed-loop regulation targets persistent congestion.

Backpressure is cheaper/simpler since it’s open loop.

Congestion control requires a feedback loop. Uses ACK from sink to self-clock.

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CODA performance metrics

Average Energy Tax = Total packets dropped in network / Total packets received at sink

Average Fidelity Penalty = Difference between average

number of packets delivered at sink using CODA and using ideal congestion scheme.

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Simulation Setup

Random network topologies with network size from 30 to 120 nodes.

2Mbps IEEE 802.11 MAC (RTS/CTS are disabled).

Directed diffusion is used as routing core.

Fixed work load, 6 sources and 3 sinks. Source generate data at different rates. Event packet is 64 bytes and interest

packet is 36 bytes.

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Simulation Results(Case 1: Dense Source , High Rate)

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Simulation Results(Case 2: Sparse Sources, Low Rate)

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Simulation ResultsCase 2: Sparse Source, Low Rate

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Simulation Results(Case 3: Sparse Sources, High Rate)

Network Size (#no of nodes)

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Conclusion

CODA’s energy efficiency. CODA’s ability to handle persistent and

transient congestion.

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Transport protocols: summary

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Pump Slow Fetch Quickly PSFQPump Slow Fetch Quickly PSFQ

For sink-to-source communication (e.g. network reprogramming)

Reliability via retransmissions

Sequence-driven loss detection

C.Y. Wan, A.T. Campbell, and L. Krishnamurthy. PSFQ: A Reliable Transport Protocol for Wireless Sensor Networks. WSNA'02, September 28, 2002, Atlanta, Georgia, USA.

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RMSTRMST

End-to-end or hop-by-hop repair (the latter is generally better)

Suggests that repair could be done at either MAC layer (ARQ retransmissions) or Transport Layer (requests based on fragment numbers etc.)

Timer-driven loss detection and local data caches Fits with the Directed Diffusion API

F. Stann and J. Heidemann. RMST: Reliable Data Transport in Sensor Networks. IEEE SNPA'03.

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ESRTESRT Aim for overall quality of service rather than node-to-node

reliability

Sankarasubramaniam, Y., Akan, O.B., and Akyildiz, I.F., "ESRT: Event-to-Sink Reliable Transport in Wireless Sensor Networks ", In Proc. ACM MobiHoc`03

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CODACODA

Sankarasubramaniam, Y., Akan, O.B., and Akyildiz, I.F., "ESRT: Event-to-Sink Reliable Transport in Wireless Sensor Networks ", In Proc. ACM MobiHoc`03

Receiver based congestion detection Open loop hop-by-hop backpressure Closed-Loop multi-source regulation

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Summarizing Transport IssuesSummarizing Transport Issues Because of harsh conditions and severe

constraints, it may be better to implement reliability in a hop-by-hop rather than end-to-end manner at either the MAC or transport layer

For energy efficiency, it is best to avoid congestion entirely, or have packet losses occur close to the source. Back pressure is a useful technique.

Where possible, scheduled solutions are preferable.

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