Lecture 13:

Mobile Ad Hoc Networks

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Contents

Introduction Ad-Hoc typical applications Characteristics and requirements Routing Protocols Ad-Hoc performance Conclusion

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Introduction

Two types of wireless networks: Infrastructured network:

base stations are the bridges a mobile host will communicate with th

e nearest base station handoff is taken when a host roams fro

m one base to another

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Ad hoc network: infrastructureless: no fixed base stations without the assistance of base stations for c

ommunication Due to transmission range constraint,

two MHs need multi-hop routing for communication

quickly and unpredictably changing topology

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Infrastructured network

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MANET = Mobile Ad Hoc Networks a set of mobile hosts, each with a transceiver no base stations; no fixed network infrastructure multi-hop communication needs a routing protocol which can handle changing

topology

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Single-Hop Ad Hoc

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Multi-hop Ad Hoc

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Single-hop Vs. Multi-hop systems

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Ad-Hoc typical applications

Personal area networking cell phone, laptop, ear phone, wrist watch

Military environments soldiers, tanks, planes

Civilian environments car network

meeting rooms sports stadiums boats, small aircraft

Emergency operations search-and-rescue policing and fire fighting

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Peer-to-Peer

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Multi-hop Peer-to-Peer

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Multi-hopping via Wireless Router

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Hopping on the Network

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Supports Mobility

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Supports Non Line-of-Sight

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Military applications Situational Awareness (SA) and Command and Con

trol (C2) for military.

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Nokia RoofTop Wireless Routing A wireless broadband solution for residential marke

ts, based on a multihop Ad-Hoc (mesh) networking.

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Related Research

IEEE 802.11 for Wireless LANs MAC PHY

IETF manet group to stimulate research and discuss possible standards in t

his area

Routing Protocols: unicast – AODV, DSR, ZRP, TORA, CBRP, CEDAR multicast – AMRoute, ODMRP, AMRIS

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Characteristics and requirements

Distributed operation Dynamic network topology Fluctuating link capacity Low-power devices

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Challenges Broadcast nature of the wireless medium

Hidden terminal problem Packet losses due to transmission errors Mobility-induced route changes Mobility-induced packet losses Battery constraints Potentially frequent network partitions Ease of snooping on wireless transmissions

(security hazard) Quality of Service

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Routing Protocols

We need a new routing and multicasts protocols that perform the following functions :

Ensure routing in a dynamic, Ad-Hoc network through automatic detection of new or missing links.

Automatically select the highest quality, least congested paths.

Provide an efficient multicast mechanism across the wireless broadcast channel.

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Overview of Current Routing Protocols

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On-demand vs. Table-driven

Table-Driven Routing Protocol: proactive!! continuously evaluate the routes attempt to maintain consistent, up-to-date routing

information when a route is needed, one may be ready

immediately when the network topology changes

the protocol responds by propagating updates throughout the network to maintain a consistent view

Example DSDV

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On-Demand Routing Protocol: reactive!! on-demand style: create routes only when it is desired

by the source node route discovery: invoke a route-determination

procedure the procedure is terminated when

a route has been found no route is found after all route permutations are

examined longer delay: sometimes a route may not be ready for

use immediately when data packets come Example AODV

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Table-Driven Routing Protocols

DSDV (Destination Sequence Distance Vector)

“Highly Dynamic Destination-Sequence Distance-Vector Routing (DSDV) for Mobile Computers”

Charles E. Perkins & Pravin Bhagwat Dated: 1994 Computer Communications Review, ‘94 pp. 234-244

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DSDV

Based on the Bellman-Ford algorithm Each node keeps a routing table to all other

nodes. based on next-hop routing

Path discovery through packet caching and header examination.

Entries have a sequence number. Incremental updates possible.

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A

B

1B

1A

>B

>AC

2C

1C1B

2A

>B

>B

>B>C

D

3D

2D1D

2B

1C

3A

>B

>D

>C>C

>C

>C

3

3E

A

2B

2D

1C

2E1E

2E E

>B

>D

>C

>C

>C

>C>C

>C

Routing TableRouting Table broadcast broadcast

Node

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Once its routing table changes, a node broadcasts its table to other nodes.

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On-Demand Routing Protocol

Ad-Hoc On-Demand Distance Vector (AODV) Routing

Protocol overview and objectives Path Discovery Reverse Path Setup Forward Path Setup Route Table Management Path Maintenance Local Connectivity Management

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Outline of Basic Functions of a Node

AODV Nodes Status

Source Node Intermediate Node Destination Node

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AODV UNICSAT ROUTE ROUTE ESTABLISHMENT

Source Node

•Initiate a path discovery process by broadcasting a RREQ packet to its neighbors.

•Reinitiate the path discovery process when it moves.

•Reinitiate the path discovery process when it receives a failure notification message from nodes along the path to the destination.

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Intermediate Nodes•Reply to the RREQ packet by unicasting a RREP packet back t

o the source, only when they have a route to the destination whose seq. # is greater or equal to the seq. # contained in the RREQ packet.

•Store in their routing tables the address of the neighbor from which the RREQ is received, thereby establishing a reverse path back to the source.

•Discard additional copies of the same RREQ latter received ( Broadcast ID and source IP address uniquely specify the RREQ packet).

•Send failure notification messages to the source (route erase ) when moving away.

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Destination Node

•Respond to the RREQ packet by unicasting a route reply (RREP) packet back to the neighbor it has first received a RREQ packet. This packet is routed along the already established reverse path.

•Reinitiate the path discovery process when it moves.

•Reinitiate the path discovery process when it receives a failure notification message from nodes along the path to the destination.

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Basic Functions of a Node

• Maintain route information in its route table.

• Information obtained through RREQ & RREP messages is kept with other routing information in the route table.

• Update the Seq. # to eliminate unused routes

• Manage aging process of routes with old seq. #

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AODV Route Discovery

• Initiated by a node wishing to communicate. Reinitiate the RREQ for rreq_retries incase of a lost RREQ

• Route Request packets are broadcast• RREQ format

< source_addr, current source_sequence-# , broadcast_id, dest_addr, dest_sequence_#, hop_cnt >

• RREQ uniquely identified by <source_addr , broadcast_id>

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• Broadcast ID is incremented with every RREQ

• Set a timer to wait for a reply.

• Neighboring nodes maintain a record of every RREQ for a specified period of time.

• Setup a reverse path entry for the source node. This entry contains the source IP, Source_Seq.#, #hops to the source and IP’s addresses of neighbor from which it received the RREQ.

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• Set a lifetime for the reverse path.• Neighboring nodes satisfy RREQ by sending RRE

P or broadcast RREQ after incrementing hop_cnt• In summary, each intermediate node keeps the fol

lowing information for a particular RREQ– Destination Address and Seq.#

– Source node’s sequence number and IP address– # hops to the source and the IP’s of neighboring

nodes– Broadcast_ID

– Expiration time for reverse path entry

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Route Discovery Flow Chart (1)

Existence of active route

Yes No

A source wishing to transmit

Check the route table

Forward packet

to next hop

Initiate a Route

Discovery Process

Create RREQ

packet

Broadcast RREQ &

Set the timer for a reply

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(2)

Check its record to see if It had seen it.

Intermediate node receiving the RREQ

Record the RREQ Information & broadcast

Setup reverse route entry

Discard the packet

Unicast a REPP

Increment hop count and broadcast

Did it see the RREQ before?

NoYes

Reverse path expired

Yes

No

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D

S

X

XX

UNICAST AODV

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Optimization: Expanding Ring Search

• Route Requests are initially sent with small Time-to-Live (TTL) field, to limit their propagation.

• If no Route Reply is received within the ttl_start value, then larger TTL is tried unlit the threshold value is reached, beyond which the RREQ is broadcast through the entire network to rreq_retries.

• After the route is established, distance to the destination is used to reinitiate TTL to this value.

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• RREQ arrives at a node that has current route to the destination ( larger/same sequence number).

• Unicast request reply (RREP)<source_addr, dest_addr, dest_sequence_#, hop_cnt,lifetime> to neighbor from which it has received the RREQ.

• Intermediate node respond the same but with hop_cnt set to its distance from the destination.

• RREP travels back to the source along the reverse path

• Each upstream node updates dest_sequence_#, sets up a forward pointer to the neighbor who transmit the RREP

AODV Forward Path Setup

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• Updates information in the forward path entry of subsequent on reception RREPs.

• Multiple arrivals of RREPs at a node will be discarded provided that their sequence # are less than the destination seq. of the first RREP or have higher hops count.

• Source can begin transmission as soon as it receives the first RREP.

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Forward Path Setup in AODV

Forward links are setup when RREP travels alongthe reverse path

Represents links on Reverse Path

Represents a link on the forward path

N1

S

D

N11

N2

N3

N4

N5

N6

N7

N8

N10

N9

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AODV Reverse Path Setup

• Source sequence number is used to maintain freshness about reverse route to the source

• Destination sequence number specified for freshness of route before accepted by source

• Reverse path setup by having link to neighbor from which it received RREQ

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AODV Route Table Management

• Route Request Expiration Timer for purging reverse paths which do not lie on source-destination route

• Route Caching Timeout for time after which the route is considered invalid

• Active_timeout Period used to determine if neighboring node is active

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AODV Route Table Management Cont.

• Route Table entry – Destination– Next Hop– Number of hops (metric)– Sequence numbers of Destination– Active Neighbors for this route– Expiration time for the route table entry

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AODV Route Maintenance

• Node movement affect only the routes containing these nodes.

• Source movement reinitiates the path discovery process

• When destination node or intermediate node moves a Route Error (RERR) message, initiated from upstream nodes of the source node , is sent to the affected source nodes.

• When next hop become unreachable the upstream node propagates RERR to neighbor with fresh sequence number and hop cnt

• restart route discovery process from source on receipt of RERR

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Summary of Route Error

• When a node is unable to forward packet (from node S to node D) on link (L), it generates a RERR message

• The node increments the destination sequence number for D cached at its route information

• The incremented sequence number N is included in the RERR

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When node S receives the RERR, it initiates a new route discovery for D using destination sequence number at least as large as N

When node D receives the route request with destination sequence number N, node D will set its sequence number to N, unless it is already larger than N

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S

N1N2

N3

N3’

D

N4N5

RERRRERR

AODV Route Maintenance

S- Source Node

D- Destination Node

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S

N1N2

N3

N3’

D

N4N5

AODV Route Maintenance Cont.

New Route Found through Node 4

X

X

S- Source Node

D- Destination Node

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AODV Local Connectivity Management

• Node learn of their neighbors in following way– on receipt of broadcast message, the node

updates the lifetime associated with that neighbor.

– on receipt of hello message

•Hello messages are only local (TTL=1) and it contains <IP_Address, Noed_Sequence_#>

•Failure to receive Hello message during (hello_interval) indicate that the route information of the neighbor need to be updated.

•Hello messages are sent from neighbor during an active path

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Local Link Failure Detection

• Hello messages: Neighboring nodes periodically exchange hello message

• Absence of ((1+allowed_hello_loss)*hello_lifetime) of hello message is used as an indication of link failure

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AODV Rebooting

To prevent routing loops during node rebooting, the node will wait for delete_period, during which it does not respond to any routing packet.

Update its Seq.# whenever it receives a RREQ from its neighbor

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Link Failure Reporting

• A neighbor of node N is considered active for a routing table entry if the neighbor sent a packet within active_route_timeout interval

• When the next hop link in a routing table entry breaks, all active neighbors are informed

• Link failures are propagated by means of Route Error messages, which also update destination sequence numbers

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Ad-Hoc performance

Simulation Environment

Simulation tools used: GloMoSim

Size of Network: 50,100,250,500

Fixed area L*L:1*1km:1.5*1.5km: 2.4*2.4:3.45:3.45km

Range: 250 meters

Data rate: 2Mbps

Packet size: 64bytes

Node Speed: 0~10 m/s

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Rest period at the random spot: 60~300 second Carrier sensing is performed by each node prior

to transmission Simulation time: 300 second

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Simulation Results

Fig.1 Neighbors vs. Speed

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Fig.2 Packets Delivered vs. Speed

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Fig.2 Packets Delivered vs. Speed

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Fig.4 Route Acquisition vs. Speed

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Comparison Of Table Driven Routing Protocols

Parameters DSDV CGSR WRP

Time Complexity O(d) O(d) O(h)

Route Complexity O(x = N) O(x = N) O(x = N)

Route Philosophy Flat Hierarchical Flat

Loop Free Yes Yes Yes, but instantenous

Multicast Capability No No No

Required Tables 2 2 4

Freq Of Updated Transmission Periodically and as needed

Periodically Periodically and as needed

Updates Transmitted to Neighbors Neighbors and cluster heads

Neighbors

Utilize Seq. No / Hello Packets Yes/No Yes/No Yes/Yes

Critical Nodes No Yes(cluster head) No

Routing Metric Shortest path Shortest path Shortest path

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Comparison Of On Demand Protocols

Performance Parameters AODV DSR TORA ABR

Time Complexity (start) O(2d) O(2d) O(2d) O(d+x)

Time Complexity (after fail) O(2d) O(2d) or 0(cache hit)

O(2d) O(l+x)

Communication Complexity O(2N) O(2N) O(2N) O(N+y)

Communication Complexity O(2N) O(2N) O(2x) O(x+y)

Routing / Loop Free Flat / Yes Flat / Yes Flat / Yes Flat / Yes

Multicast Capability Yes No No No

Beaconing Requirements No No No Yes

Multiple Route Possibilities No Yes Yes No

Route Reconfiguration Erase Route, Notify Source

Erase Route, Notify Source

Link reversal, route repair

Localized broadcast query

Route Maintained in Route table Route cache Route table Route table

Routing Metric Fresh and shortest

Shortest Shortest Associativity and shortest

path

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Comparison of On-Demand v/s Table Driven Routing

Parameters On Demand Table Driven

Availability of Routing Information

Available when needed Always available regardless of need

Routing Philosophy Flat Mostly Flat except for CGSR

Periodic route updates Not Required Yes

Coping with Mobility Using Localized route discovery in ABR

Inform other nodes to achieve consistent routing

tables

Signaling Traffic Generated

Grows with increasing mobility of active nodes

as in ABR

Greater than that of On Demand Routing

QoS Support Few Can Support QoS Mainly Shortest Path as QoS Metric