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Wireless Networks

Lecture 28

Mobile Ad hoc Network

Dr. Ghalib A. Shah

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Outlines

Introduction► What is Ad hoc networks?

► Characteristic

►  Ad hoc vs. cellular networks

►  Application► Challenges

Routing Protocol► Expected Properties of Ad-hoc Routing Protocols

►  A taxonomy for routing protocols in Mobile ad

► Some common protocols (DSDV, AODV, DSR, ZRP,TORA)

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Last Lecture Review

Problems with DCF

Virtual Carrier Sensing

RTC/CTS Protocol

Interframe Spacing PCF

Fragmentation / Reassembly

MAC Frame Format Frame Types

Physical Media in Original IEEE 802.11

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What is Ad hoc

 Ad hoc► For a specific purpose of occasion

► For this case alone

IEEE802.11► a network composed solely of stations within mutual

communication range of each other via the wireless

media.

► an independent basic service set

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Mobile distributed multi-hop wireless network(manet)

► a group of mobile, wireless nodes which

cooperatively and spontaneously form a network

independent of any fixed infrastructure or centralizedadministration

►  A node communicates

• directly with nodes within wireless range

• indirectly with all other destinations using a dynamicallydetermined multi-hop route though other nodes in the

manet

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The characteristic of ad hoc networks

Heterogeneous nodes

Self-creating

► not rely on a pre-existing fixed infrastructure

Self-organizing► no predetermined topology

Self-administering

► no central control

creating a network “on the fly” 

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 Ad hoc networks► infrastructureless

► multiple hop

• Radio power limitation, channel utilization, and power-

saving concerns

► DCF(distributed coordination function)

Cellular networks

► infrastructure-based

► one hop(uplink or downlink)

► PCF(pointed coordination function)

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Challenges

1. Spectrum allocation2. Self-configuration

3. Medium access control (MAC)

4. Energy efficiency

5. TCP Performance6. Mobility management

7. Security & privacy

8. Routing protocols

9. Multicasting10. QoS

11. Service Location, Provision, Access

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Routing Protocols

Expected Properties of Ad-hoc RoutingProtocols

 A taxonomy for routing protocols in Mobile ad

hoc networks► Reactive or On-demand routing protocols

► Proactive or Table-driven

► Hybrid

► Hierarchical► Geographical

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Expected Properties of Routing

Ideally an ad hoc network routing protocolshould

► be distributed in order to increase reliability

► assume routes as unidirectional links

► be power efficient.

► consider its security

► be hybrid protocols

► be aware of Quality of Service

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Taxonomy

Communication model► Multi-channel: Channel assignment using low-layer info

► Single channel model

Structure

►  Are all nodes treated uniformly?► How are distinguished nodes selected (neighbors or cluster-

based)?

State information

► Is network-scale topology obtained at each node?

Scheduling

► Is route information continually maintained for each destination

(proactive or reactive)?

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DSDV

is based on the idea of Ballman-Ford routing algorithm Every mobile station maintains a routing table that lists

► all available destinations

► the number of hops to reach the destination

► the sequence number assigned by the destination node

 A station transmits its routing table

► periodically

► if a significant change has occurred in its table from the last

update sent

The routing table updates can be sent in two ways

► full dump

► incremental update

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Put figure with same illustration of DSR

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 AODV

It borrows► the basic on-demand mechanism of route discovery

and route maintenance from DSR

► the use of hop-by-hop routing, sequence numbers,

and periodic beacons from DSDV

 A node periodic broadcasts hello information to

maintain the local connectivity

It only supports the use of symmetric links

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TORA

is based on the concept of link reversal

finds multiple routes from a source node to a

destination node

the control messages are localized to a verysmall set of nodes near the occurrence of a

topological change

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DSR

 A node maintains route caches containing the source routes that itis aware of 

The node updates entries in the route cache as and when it learnsabout new routes

route discovery

► route request packet contains• the address of the source

• the destination

• a unique identification number 

► route reply is generated by

• the destination

• an intermediate node with current information about the destination route maintenance

► Route error packets are generated at a node when the data link layer encounters a fatal transmission problem

►  Acknowledgements, including passive acknowledgments

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OLSR

OLSR uses multipoint relays to reducesuperfluous broadcast packet retransmission

and also the size of the LS packets

OLSR thus leads to efficient flooding of control

messages in the network

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OLSR (cont’d) 

Only the multipoint relays nodes (MPRs) needto forward LS updates

OLSR is particularly suited for dense networks

In sparse networks, every neighbor becomes amultipoint relay, then OLSR reduces to pure LS

protocol

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ZRP

 A hybrid routing protocol that combines bothproactive and on-demand routing strategies

Each node has a predefined zone

Inside zones: proactive routing Outside zones: on-demand routing

ZRP provides more flexibility

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Outlines

Introduction► What is Ad hoc networks?

► Characteristic

►  Ad hoc vs. cellular networks

►  Application► Challenges

Routing Protocol► Expected Properties of Ad-hoc Routing Protocols

►  A taxonomy for routing protocols in Mobile ad

► Some common protocols (DSDV, AODV, DSR, ZRP,TORA)