CMPE 259
Sensor Networks
Katia Obraczka
Winter 2005
Transport Protocols II
1-1
Announcements
 Feedback on project proposals.
 Project resources.
1-2
Transport protocols (cont’d)
 RMST
 CODA
 Summary
1-3
RMST
1-4
RMST
 Reliable Multi-Segment Transport.
 Where to do reliability?
 MAC.
 Transport.
 Application.
1-5
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.
1-6
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.
1-7
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?
1-10
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.
1-11
Preliminary analysis
 Demonstrate the benefits of hop-by-hop
reliability.
1-12
RMST evaluation
 MAC-only reliability.
 Local recovery.
 With and without MAC reliability.
 End-to-end reliability.

With and without MAC reliability.
1-13
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.
1-14
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.
1-18
Congestion detection
 Accurate and efficient congestion
detection is important

Channel loading – sample channel at appropriate
rate to detect congestion.
1-19
Open-loop h-by-h backpressure
1
2
3
4
Upstream node
decides to propagate
backpressure or not.
5
Congestion
detected
6
1-20
Closed loop multi-source regulation
1
2
1,2,3
Regulate
bit is set
ACK
4,5,6
Congestio
n detected
7,8
ACK
1-21
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.
1-22
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.
1-24
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.
1-25
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 Results
Case 2: Sparse Source, Low Rate
1-28
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.
1-30
Transport protocols: summary
1-31
Pump 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.1-32
WSNA'02, September 28, 2002, Atlanta, Georgia, USA.
RMST
 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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ESRT
 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 Sensor1-34
Networks ", In Proc. ACM MobiHoc`03
CODA
 Receiver based congestion detection
 Open loop hop-by-hop backpressure
 Closed-Loop multi-source regulation
Sankarasubramaniam, Y., Akan, O.B., and Akyildiz, I.F., "ESRT: Event-to-Sink Reliable Transport in Wireless Sensor1-35
Networks ", In Proc. ACM MobiHoc`03
Summarizing 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.
s
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