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Challenging the Modeling Assumptions of Mobile Networks

Seminar 266Michalis Faloutsos

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Scope: Challenge the Assumptions

Targeted to graduate students (ugrad nets req.)Reading papersPresenting• Papers• Your project

Do a cool projectPublish!

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What is an ad hoc network

A collection of nodes that can communicate with each other without the use of existing infrastructure Each node is a sender, a receiver, and a relayThere are no “special nodes” (in principal)• No specialized routers, no DNS servers

Nodes can be static or mobileCan be thought of us: peer-to-peer communication

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Example: Ad hoc network

Nodes have power rangeCommunication happens between nodes within range

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What’s the problem?

There is no systematic way to model and simulate such networksNo clue what are the right assumptions Not sure how the assumptions affect the results

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Consequences

Simulation results are• Meaningless• Unrepeatable• Incomparable between different analysis• Prone to manipulation

Claim: give me any statement, I can create simulations to prove it

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What Will We Do Here?

Identify assumptions• Some of them are subtle

Characterize the scenariosStudy their effect on the performance results

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Topics Of Interest

The capacity of ad hoc networks• What is the inherent capacity of a network

Characterizing the topology Mobility and its effect• Mobility models• Characterizing the topology of mobility pattern

The effect of power-rangeSimulating TCP over ad hoc networksSimulating multicasting in ad hoc networks

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Some major assumptions

The way-point model is a good model for mobilityHomogeneity is a good assumptionLinks are bidirectional: I hear U, U hear meUniform distribution of location is good802.11 will be used at the MAC layerSpace is two dimensional

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Some “proven” claims

The smallest the range, the better the throughputMobility increases the capacity of a networkA node should aim for 6-7 neighbors

We will challenge these claims

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Some Introductory Things

The MAC layer 802.11Typical SimulationsThe routing protocolsTCP and ad hoc networks

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The 802.11 MAC protocol

Introduced to reduce collisionsSender sends Request To Send (RTS): ask permissionCase A: Receiver gives permission Clear To Send (CTS)Sender sends DataReceiver sends ACK, if received correctlyCase B: Receiver does not respond• Sender waits, times out, exponential back-off, and tries again

ADC

BRTS

CTS

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Why is this necessary?

A: RTS, and B replies with a CTSC hears RTS and avoids sending anythingD hears CTS so it does not send anything to B

ADC

BRTS

CTS

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Some numbers for 802.11

Typical radius of power-range: 250mInterference range: 500m • At 500m one can not hear, but they are

bothered!

RTS packet 40 bytesCTS and ACK 39 bytesMAC header is 47 bytes

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Typical Simulation Environment

A 2-dimensional rectangleFixed number of nodesStatic: uniformly distributedDynamic: way-point modelPower range: fixed or variableSender-receivers uniformly distributed

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Typical “Errors”

Mobility: • too slow or too fast• Mobility speed may not be the expected

Homogeneity may “hide” issues• Few nodes are responsible for most traffic• Some spots are more popular than others

Power range is too large for the area• Ie radius 250m, a grid of 1Km -> one broadcast

covers “half” the area

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Various Communication Paradigms

Broadcasting: • one nodes reaches everybody

Multicasting:• One node reaches some nodes

Anycasting:• One node reaches a subset of some target nodes (one)

Application Layer protocols and overlays• Applications like peer-to-peer

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Layered and Cross Layer Protocols

Layering: • Modular• Isolates details of each

layer

Cross Layer:• Information of other layers

is used in decisions• Pros: efficiency• Cons: deployability and

compatibility

application

transport

Network

Link

physical

application

transport

Network

Link

physical

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Example: application layer multicast

Source unicasts data to some destinationsDestinations unicast data to othersPros: easy to deploy, no need to change network layerCons: not as efficient

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Example: application layer multicast II

Members need to make multiple copies• It would happened anyway

Link A B gets two packets• Similarly in wireline

multicast

Node B sends and receives packet 4 times

s

A

B

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Contention in ad hoc networks

A major difference with wireline networksAir-time is the critical resourceFact 1: connections that cross vertically interfereFact 2: connections that do not share nodes interfereFact 3: a single connection with itself interferes!

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Example of contention

Yellow connection bothers pink connectionYellow bothers itselfWhen A-E is active• E-F is silent• F-G is silent (is it?)

A

B

CD

E GF H

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We need to model contention

First the obvious• Adjacent edges

Second, one edge away, considering RTS CTSThird, interference (500m instead of 250m)• Modeling issue

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