SENSE: Scalable and Efficient Networking of

Sensor Elements

J.J. Garcia-Luna-AcevesCCRG

Computer Engineering DepartmentUniversity of California, Santa Cruz

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Discussion Topics

Implications of fundamental limitations to the scaling of ad hoc networks Cross-layer optimization

Impact of the physical layer on communication protocol stack.

Importance of modular protocol stacks and good understanding of their distributed algorithms.

Scaling

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Definition: A source-destination throughput of λ(n) bits/sec is feasible if every source node can send information at a rate of λ(n) bits/sec to its destination.

nn

(n)

nnD

nnn

log)( and 0

)log(

1)(

)()( and 1)( nnDn

Gupta and Kumar (for static networks)

Grossglauser and Tse (Multiuser diversity: One-copy two phase packet relay to nearest neighbor strategy for mobile networks)

Known Results on Network Capacity

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Multiuser diversity with multi-copy two- phase packet relay to close neighbors strategy for mobile networks where

2)( and )()( , 1)( nnVarnnDn

upspeedstimeflooding

delaybounded

reductiondelay %69 For fixed n

Interference analysis: cteSIR n 2 ,

Preliminary Results

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n totalusers

r0

Only one relay looking for destination

Single-copy forward

r0

n totalusers

r0

First relay reaching destinationdelivers the packet

(More than one relay looking for destination)

Multi-copy forward

r0

tt '

Preliminary Results:Node Trajectories Are IID

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How can we reduce interference subject to multiple constraints (power consumption, e-t-e delays, bandwidth requirements)?

Exploit diversity (user, space, time, code, freq) and cross-layer optimization!

S

DConventionalclose straightline path

Outlook:

Need More than Min-Hop Routing

Path of least interference subject to constraints

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Need for Cross-Layer Optimization

scheduling establishes links and decides which nodes are awake; needs multicast group

affiliations and routes to destinations of flows

routing needs links for collision-free transmission of control packets;packet forwarding

needs links for collision-free

transmission of data packets

Multicasting needs a convenient

topology

topology control determines nodes & links that can be

used for certain functions; needs links for collision-free transmission of control packets, and dissemination of neighborhood

data

S

T R

Scalable & Efficient

Network Control

Signaling to support functions

should not be redundant

Importance of analytical models

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Why Do We Need Analytical Models?

Simulations: Specific to each

scenario and setup Results for each

parameter value of interest

Statistical fitting not a trivial task

Many physical layer features not readily available

Physical layer has to be implemented

How far can we go?

Analytical Models: Aim to cover different

scenarios: general behavior!

Quick answers for the impact of different parameter values on system performance

Upper/lower bounds Insights: help in the

design Physical layer issues

at least as accurate as in simulations

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Limits of Simulation Effort

Consider executing a simulation in a Sun blade 100 running Solaris 5.8

50 seeds of a 100-node, 5-min data traffic scenario required 16.41 hours for a given set of PHY-level parameters.

Analyzing the impact of different combinations of PHY-level parameters will take a very long time, and testbeds are hard to control.

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Multihop Networks

RTS

CTS

Interference is network-wide!

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Previous Work

Single-hop (mostly) or “weak-interactions” approach (to avoid interference from distant nodes)

Scheduling rates are independent Poisson point processes

Packet lengths exponentially distributed and independently generated at each transmission attempt = backoff retransmissions ignored!

Instantaneous acknowledgments Error-free Links Assumptions on spatial distributions (e.g., Poisson)

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Modeling the Effect of the PHY:

Highlights [Mobicom 04]

Framework for any MAC protocol in ad hoc networks Focus on PHY / MAC layer interactions No assumptions on spatial probability distributions or

specific arrangement of nodes Individual (per-node) performance metrics for any given

network topology (node location) and radio channel model Linear model that provides remarkable correlation with

simulation results. Key Benefit: Analytical results are obtained much faster

than in simulations (same example as before takes 0.44 sec in Matlab).

M. Carvalho and J.J. Garcia-Luna-Aceves, " A Scalable Model for Channel Access Protocols in Multihop Ad Hoc Networks," Proc. ACM Mobicom 2004, Philadelphia, Pennsylvania, Sept. 26--Oct. 1, 2004.

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Modeling Rationale

Focus on the essentials of MAC and PHY layers: PHY: Ensure that frames are correctly received MAC: Scheduling discipline to share the channel

MAC/PHY interactions depend on connectivity among the nodes:

Network topology is key! Model each layer’s functionality in probabilistic terms:

PHY: successful frame reception probability MAC: transmission probability

Model topology with an interference matrix

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Application: Modeling IEEE 802.11 [Mobicom 04]

Based on the works by M. Carvalho and J. J. Garcia-Luna-Aceves,

“Delay Analysis of IEEE 802.11 in Single-Hop Networks,” Proc. ICNP, Atlanta, 2003.

G. Bianchi, “Performance Analysis of the IEEE 802.11 Distributed Coordination Function,” IEEE JSAC, 2000.

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Application: Modeling IEEE 802.11 [Mobicom 04]

Per-node performance metric: throughput

Simulator used: Qualnet 3.5

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Percentage of Prediction Error [Mobicom 04]

Sample topologiesHistogram over 10 random topologies

(100 nodes)

Modular protocols and distributed algorithms

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PHYSICAL

LINK

NETWORK

TRANSPORT

APPLICATION

synchronizationneighborhood

discoverytransmissionscheduling

prototyperadios

simulatedPHY

node interconnection

collaborative sensorprocessing applications…

end-to-end transport protocols…

routing-structuremaintenance

opportunisticpacket forwarding

Modular Protocol Stack

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Routing Issues Routing protocols are monolithic

One flavor of signaling for all destinations One flavor of routes (single path) for all traffic to

destinations. Routing layer in MANETs assumes that routing

takes place over a given topology, just like Internet routing protocols like OSPF and RIP do.

The existence of radio connectivity does not imply the availability of a logical link in a MANET.

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Image from sensor

command center

Not All Nodes and Traffic Are Created Equal!

Most communication is multipoint and for particular

purposes

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Need for Cross-Layer Optimization

scheduling establishes links and decides which nodes are awake; needs multicast group

affiliations and routes to destinations of flows

routing needs links for collision-free transmission of control packets;packet forwarding

needs links for collision-free

transmission of data packets

Multicasting needs a convenient

topology

topology control determines nodes & links that can be

used for certain functions; needs links for collision-free transmission of control packets, and dissemination of neighborhood

data

S

T R

Scalable & Efficient

Network Control

Signaling to support functions

should not be redundant

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Routing Issues Timers and sequence numbers can be a problem

when the networks become very large and partitions can happen (disruption tolerance): How long should a node remember its “state” for a

destination? What are the implications of forgetting?

Similarly, path information becomes obsolete very quickly in large dynamic/disrupted networks. How should path information be used to ensure

correct routing? Same mechanisms repeated in different protocols.

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Outlook:Develop Flow Adaptive Routing

Mechanisms (FARM)

Develop routing techniques that are “role”-centric (no clusters) and adapt dynamically to the flows in the network.

How a routing table entry for a destination is obtained and maintained is a function of the type of flow towards the destination.

Proactive and on-demand mechanisms used according to flow types.

Different flows are given resources (paths) according to their types and priorities.

Routing works in coordination with scheduling and topology management.

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Outlook:Integrated Routing and Multicasting

f

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g

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h

i

dc

b

a

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C1

C2

Each common node keeps paths to the cores of groups and well-known nodes.

Paths to common nodes are found on demand.Much of the traffic in sensor nets is to groups and common

nodes!

special services, sink of data

multicast group

Thanks!