introduction to mobile ad hoc networks

한국해양과학기술진흥원

Introduction to Mobile Ad hoc

Networks

2013.10.6

Sayed Chhattan Shah, PhD

Electronics and Telecommunications Research Institute, Korea

https://sites.google.com/site/chhattanshah/

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Acknowledgements

David B. Johnson, Rice University, “Multihop Wireless Ad Hoc Networking: Current Chal-lenges and Future Opportunities”

Carlos Pomalaza-Ráez, University of Oulu, Finland, “MAC protocols for Mobile Ad hoc Network”

Jeroen Hoebeke, Ingrid Moerman, Bart Dhoedtand Piet Demeester, Ghent Univer-sity, “An Overview of Mobile Ad Hoc Net-works: Applications and Challenges”

한국해양과학기술진흥원

Outline

Introduction

Applications

Challenges

Medium Access Control

Routing

Proactive routing protocols

Reactive routing protocols

Introduction

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Wireless Networks

Any type of computer network that utilizes some

form of wireless network connection

Types

Cellular Network Wireless LAN

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Wireless Networks Types

Several types but all have similar architecture

Relies on a fixed infrastructure

• Centralized base station or access point

• All users within wireless range of it

• Communicate with an access point or base station

• Need planning, installation and management

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Wireless Ad Hoc Network

A decentralized type of wireless networks

Ad hoc because it does not rely on a pre existing

infrastructure such as routers or access points

Each node participates in routing by forwarding

data of other nodes

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Mobile Ad Hoc Network

A type of wireless ad hoc network

Infrastructureless network of mobile devices

Nodes are free to move independently in any direction

Links to other devices are changed frequently

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Mobile Ad Hoc Network

Used when

Infrastructure is not available

• Remote areas

• Unplanned meetings

• Disaster relief

• Military operations

May not want to use the available infrastructure

• Time or cost to access service

Dynamically extend coverage of infrastructure

• Allow users to be further away from infrastructure

Mobile Ad Hoc Network Applications

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Challenges

Nodes Battery-powered

Limited processing power

Wireless Network Packet loss due to transmission errors

Variable capacity links

Shared Bandwidth

Limited Bandwidth and High Latency

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Challenges

Dynamic Network Environment Nodes may move any time

May join and leave the network

Self-organized Network No one is in charge

No one to provide standard service

Security

Medium Access Control

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Medium Access Control

In a given area, only one speaker is allowed to talk at a time,

Else, listener would hear noise

To avoid conflict Wait for a coordinator to ask them to speak

Wait different time before talking

Classification of MAC Protocols

Wireless MAC protocols

DistributedMAC protocols

CentralizedMAC protocols

Randomaccess

Randomaccess

Guaranteedaccess

Hybridaccess

Since we are interested in Ad hoc networks we will focus our discussions on distributed type

protocols

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Medium access methods from fixed networks

Carrier Sense Multiple Access with Collision De-tection• A collision is detected, whenever a transmitting node senses a

different signal on the same channel it has transmitted

• CSMA/CD widely used in 802.3 and Ethernet

Problems

• Signal strength decreases proportional to square of the distance

• CD is not possible in wireless channel due to half duplex operation

• Sender would apply CS and CD, but collisions happen at receiver

• Sender may not “hear” the collision, i.e., CD does not work

• CS might not work, e.g. if a terminal is “hidden”

Medium Access Control

Hidden and Exposed Node Problems

A is transmitting to B C is out of range of A and is unaware of the transmissionIf C transmits to B it will cause a collision at B

A B C

A B C D

B is transmitting to AC wants to transmit to DC senses transmission & declines even if its transmission will not cause any collision at A

Hidden node

Exposed node

Capture Problem

A B

C

D

dA

BdC

B

If A and C transmitsimultaneously to B then the signal power of C, re-ceived at B, is higher than the one from A (because dCB < dAB) and there is a good probability that C’s signal can be correctly de-coded in the presence of A’s transmission

This capture of C’s signal can improve protocol perfor-mance, but it results in unfair sharing of the channel with preference given to nodes closer to the receiver. Wireless MAC protocols need to ensure fairness under such conditions

Multiple Access with Collision Avoidance

Uses signaling packets for collision avoidance

Request to send RTS Sender request the right to send from a receiver with a

short RTS packet before it sends a data packet

Clear to send CTS

Receiver grants the right to send as soon as it is ready to receive

Multiple Access with Collision Avoidance

Avoids the problem of hidden terminal A and C want to send to B

A sends RTS

C waits after receiving CTS from B

Multiple Access with Collision Avoidance

Avoids the problem of exposed terminal B wants to send to A, C to another terminal

Now C does not have to wait, as it cannot receive CTS from A

Multiple Access with Collision Avoidance

Reliability Solution is to use acknowledgements

If fail to receive acknowledgements, retransmit

Power Saving Solution is to turn of radio when not needed

IEEE 802.11 Wireless MAC

Distributed and centralized MAC components

Distributed Coordination Function (DCF)

Point Coordination Function (PCF)

DCF suitable for multi-hop ad hoc networking

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Key Performance Metrics

Delay Average time spent by a packet in the MAC queue

Fairness A fair MAC protocol does not give preference to any single node when

multiple nodes are trying to access the channel

Power Consumption Important to design MAC protocols that have power saving features

QoS Support Protocols need to treat packets from various applications based on

their delay constraints

• Common methods are the use of access priorities and scheduling

Routing Protocols

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Routing

Routing process of selecting paths in a network along which to send network traffic

Routing algorithms determine the specific choice of route

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Distance Vector and Link State Routing Protocol

Routing information is only exchanged between directly connected neighbors Router knows from which neighbor a route was

learned, but it does  not know where that neighbor learned the route

Link state routing requires that all routers know about the paths reachable by all other routers in the network

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Routing in MANET

Local Node Mobility

Global Node Mobility

Limited Resources

Constrained Communication Environment

Limited Power

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Routing in MANET

Proactive or table-driven routing protocols Maintain routes

Based on periodic updates

DSDV (Destination sequenced distance vector)

Advantages Low routing latency

State information

Disadvantages High routing overhead

Maintain routes which may never be used

Route repair depends on update frequency

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Routing in MANET

Reactive or On-demand routing protocols Determine route when needed

Source initiates route discovery

DSR (Dynamic Source Routing)

Advantages No overhead from periodic update

Disadvantages High routing latency

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Proactive Routing Protocol

DSDV (Destination Sequenced Distance Vector)

Each node maintains a routing table which stores

Destination ID

Next hop

Cost metric

Sequence No to determine freshness of route

Each node periodically forwards routing table to neighbors

Each node increments and appends its sequence number when sending its local routing table

Each route is tagged with a sequence number

routes with greater sequence numbers are preferred

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Proactive Routing Protocol

DSDV (Destination sequenced distance vector) When X receives information from Y about a route to Z

• Let destination sequence number for Z at X be S(X) • S(Y) is sent from Y

If S(X) > S(Y), then X ignores the routing information received from Y

If S(X) = S(Y), and cost of going through Y is smaller than the route known to X, then X sets Y as the next hop to Z

If S(X) < S(Y), then X sets Y as the next hop to Z, and S(X) is updated to equal S(Y)

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Reactive Routing Protocol

AODV (On-Demand Distance Vector Routing) Establishes a route to a destination only on demand

A node that needs a connection broadcast a route request RREQ packet

Nodes receiving RREQ packet update their information for the source node and set up backwards pointers to the source node in the route ta-bles

A node receiving the RREQ may send a route reply (RREP)

If it is either the destination or it has a route to the destination

Otherwise, it rebroadcasts the RREQ

If a node receives a RREQ which it has already processed

it discards the RREQ and does not forwards

As the RREP propagates back to the source, nodes set up forward pointers to the destination

A route is considered active as long as there are data packets peri-odically travelling from the source to the destination along that path

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Routing in MANET

Hybrid routing protocols

Cluster routing protocols

Geographic routing protocols

Which protocol to use? Depends on traffic and mobility patterns?

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Vehicular Ad hoc Networks

Used for communication among vehicles and be-

tween vehicles and roadside equipment

Vehicles tend to move in an organized fashion