WIDE meeting, Eindhoven, April 2-3 2009, Mikael Johansson [email protected] Mikael Johansson KTH ·...

WIDE meeting, Eindhoven, April 2-3 2009, Mikael Johansson [email protected]

Mikael Johansson KTH · Stockholm · Sweden

Control-relevant models oflatency and loss in WSNs


WP2 update

T2.1 Towards ultra-reliable short-range wireless sensor networks (KTH, ESenza)

Roles: Isolation of reliability bottlenecks of industrial wireless standards (ESENZA), development of suggestions for improvements (KTH)

Measure of success: reliability bottlenecks of wireless communication standards isolated, and means for alleviating these suggested

T2.2. Dimensioning and design of heterogeneous wide-area sensor networks (KTH,TUE,UNISI,ESENZA)

Roles: Development of methodologies for design and dimensioning of heterogeneous wide-area sensor networks(ESENZA,KTH, UNISI) and energy-aware sensor placement technologies (UNISI). Development of realistic models for communication latency and packet loss characteristics of wireless links suitable for control design (TUE, KTH, ESENZA).

Measure of success: Delay and loss models that reflect actual network performance and are suitable for analysis and design of networked estimators and controllers are developed. Developed methodologies allow designing an adequate wireless infrastructure for wide-area sensor networks deployed in the water distribution demo.


WP2 update

Task 2.3 Protocols and in-network processing techniques for state estimation and monitoring

Roles: Development of protocol (UNISI) and in-network processing techniques (KTH) for networked estimation and alarm-handling.

Measure of success: Protocols that allow efficient implementation of the estimation, control, and alarm-handling methods developed in other work packages are proposed.

A measure of success common to all tasks is the quality of work judged by its adequacy for the water distribution demo, along with the scientific novelty in terms of the number of publications in peer-reviewed conferences or journals, patents, or suggestions to relevant standards committees.


Contents

Aspects of wireless communications:– The wireless landscape and the 2.4GHz ISM band– Propagation and interference– Medium access control basics– Increasing link-level reliability– Networking aspects: topology, routing, resilience.

Models for latencies and losses:– Simple channel models– Network-wide performance under scheduled access– Contention-induced loss


The wireless landscape


The 2.4GHz unlicensed ISM band


Propagation and interference

Insight 1: signal is attenuated over distance– ”Free path loss” model (single signal path)

– Path loss exponent depends on environment, see [Rap:89]

Insight 2: signal takes multiple paths, results ”stochastic”

Insight 3: Receiver noise and radio interference reduces reliability


What can we measure?

What can we measure with standard WSN hardware?

1. Packet error rate (PER)– The ratio of packets received/packets sent

2. Received signal strength indicator (RSSI)– Average received signal power (measured over eight symbols)

Closely related to SINR (see, e.g. [Zur+:06])

3. Average correlation value (CORR)– Average correlation value between eight symbols

4. Link quality indicator (LQI)– Empirical relationship between PER and CORR/RSSI [Tan+:07]


What can we expect in practice?

Static fading over bandwidth spanning multiple 802.15.4 channels [WeS:05]


Influence of spatial node distribution


Another perspective

Base station at T4, LoS!RSSI, LQI and PER (<1%)[Tan+:07]


Temporal correlations

Losses tend to come in bursts!

Average burst-length: 1.77 packets, 90% bursts <= 3 packets[WiM:06]


What about 802.11?

PER for different traces CDF for burst lengths

[Wil+02]


Co-existence

802.11 can have significant influence on 802.15.4 (when channels overlap in frequency) [Pet:06]


So far…

Channel quality varies!– In space (distance, location, …)– In time– In frequency

Quality not independent– Not in time– Not in space– Not in frequency

Significant correlations, possibly also drift


Medium access

When more than one transmitter, need to share medium!

Broadly speaking, either• Random access (Aloha, CSMA/CA)• Scheduled access (TDMA,FDMA)

Scheduled access ”deterministic” but complex to implement

Random access stochastic access latency, ”easy” to implement


Example: wirelessHART

Multi-channel TDMA, no ”spatial reuse” in same channel.

Links are assigned transmission right in time-channel schedule

L1

L2

L3

L4

L1 L2 L3 L4


The 802.15.4 MAC

Transmitting node first listens to channel, to ensure clearance• If busy, wait random time (uniform on ”contention window”)• If busy next CCA, double CW and wait random time, etc.

[Pol:08]


Contention delay

Random access methods introduce contention delay

[StP:07]


Increasing link-level reliability

Many ways of increasing reliability, not covered here

• Retransmissions (”ARQ”)• Better coding• Power control•


Networking aspects

More than ”more sensors”– Coverage extensions, resilience, …

Many possible topologies: star, line, multiline, tree, mesh, …


Network-induced latency

Networking (almost always) decreases throughput, increases delay

Simple example:

Even if scheduled access, relaying halves throughput, doubles delay.

If random access, source and relay contends:– access delay more than doubled!

Relay

Source Sink


Network diversity

Positive aspect (apart from coverage): diversity

Primary path

Source Sink

Secondary path

L1

L2 L3

L1 L1 L1 L1 L1

L1 L2

L1 L3


Control-relevant models

Modelling loss– Independent losses (Bernoulli)– Correlated losses (Gilbert-Elliot)– More advanced Markov models

Latency models– Stochastic delays

Latency vs. loss: to buffer or to drop?


Independent packet losses

Simplest model: Bernoulli independent packet losses.

Packet is lost with probability ploss:

• ”Easy” to compensate for using control theory

However, we have seen:• Loss probability should be different on different links

(PER depends on distance, fading, interference, …)• It should vary in time • It is good to consider correlated losses (”bursts”)


Correlated losses

Simplest model for correlated losses is due to Gilbert and Elliot:

Loss probabilities

Burst lengths

[ARJ:09]


More complex models

Can pose ”arbitrarily complex” Markov models for loss process– Potentially more accurate, but very hard to tune– Can be dealt with using JLS-techniques


Latency under unreliable channels

Can study delay distributionfor scheduled transmissionsover lossy links

So far, only single-packetBernoulli losses.

[ChS:07]


Latency models for scheduled access

Need to consider the influence of multiple packets!

We have some results for latency in reliable multi-hop networks

Example: minimum convergecast time [ZSJ:09]

1 1 1 1

GW


Contention-induced loss

Natural to use ”attempt and discard” (cf. TOD):– Abort transmission attempt if new sample arrives

[RSPJ:09]


Conclusions

Basic insight in wireless industrial communications– Channel quality varies in space, time and frequency– Losses are often correlated (in time, freq., space)– Not a consistent view: need to do own measurements!

From link quality and network topology to loss models– Bernoulli losses, simple Markov models– Attempts to extend from single link to network– Models for contention-induced delay and loss.


References

[WeS:05] J. Werb and D. Sexton, ”Improved quality of service in IEEE 802.15.4 mesh networks”, International Workshop on Wireless and Industrial Automation, San Francisco, CA, March 2005.

[Rap:89] T. S. Rappaport, ”Indoor radio communications for factories of the future”, IEEE Communications Magazine, Vol. 27, No. 5, pp15-24, 1989

[Zur+:06] B. Ares Zurita, P. G. Park, C. Fischione, A. Speranzon, K. H. Johansson, ”On power control for wireless sensor networks: system model, middleware component and experimental evaluation”, European Control Conference, Kos, Greece, 2006

[Tan+07] L. Tang, K.-C. Wang, Y. Huang and F. Gu, ”Channel characterization and link quality assessment of IEEE 802.15.4-compliant radio for factory environments”, IEEE Transactions on Industrial Informations, Vol. 3, Bo.2, May 2007

[WiM:07] A. Willig and R. Mitschke, ”Results of bit-error measurements with sensor nodes: consequences for energy-efficient error control schemes”, European Wireless Sensor Networking Conference 2006


References

[Pet:06] M. Petrova et al.”Performance study of IEEE 802.15.4 using measurements and simulations”, IEEE WCNC, 2006.

[Wil+:02] A. Willig, M. Kubisch, C. Hoene and A. Wolisz, ”Measurements of a wireless link in an industrial environment using an IEEE 802.11-compliant physical layer”, IEEE Transactions on Industrial Electronics, Vo. 4, No. 6, Dec. 2002

[Pol+:08] S. Pollin, M. Ergen, S. Ergen, B. Bougard, L. Van der Perre, I. Moerman, A. Bahai, P. Varaiya, F. Catthoor: Performance analysis of slotted carrier sense IEEE 802.15.4 medium access layer, IEEE Transactions on Wireless Communications 7(9), 2008.

[StP:07] L. Stabellini and A. Proutiere, ”Evaluating delay and energy in sensor networks with sporadic and correlated traffic” AdHoc Workshop, 2007.

[ARJ:09] P. Almström, M. Rabi and M. Johansson, ”Estimation under correlated packet losses”, IEEE CDC 2009, Submitted.

[RSPJ:09] M. Rabi, L. Stabellini, A. Proutiere, M. Johansson, ”Networked estimation under contention-based medium access”, International Journal of Robust and Nonlinear Control, Special Issue on Wireless Industrial Control, 2009 (to appear)


References

[ChP:07] P. Chen and S. Sastry, ”Latency and connectivity analysis tools for wireless mesh networks”, Proc. ROBOCOMM, October 2007

[Wil+:02] H. Zhang, P. Soldati and M. Johansson, ”Optimal convergecast on a line”, WiOPT 2009, Seoul, South Korea, Submitted.

WIDE meeting, Eindhoven, April 2-3 2009, Mikael Johansson [email protected] Mikael Johansson KTH ·...

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