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White Paper

Mobile Ad hoc Networking (MANET)

with AODV

Revision 1.0

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Mobile Ad hoc Networking (MANET) with AODV

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Table of Contents

TABLE OF CONTENTS ........................................................................................ 3

TABLE OF FIGURES ............................................................................................ 4

WHAT IS MANET? ................................................................................................ 5

MANET Protocols............................................................................................ 5

AODV..................................................................................................................... 7

Hello Beacons.............................................................................................. 7

Route Creation ................................................................................................ 7

Route Deletion............................................................................................... 10

Sequence Numbers ......................................................................................11

Stationary Wireless Infrastructure .............................................................. 12

GLOSSARY ......................................................................................................... 16


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Table of Figures

Figure 1: RREQ is sent by source node 8

Figure 2: Intermediate Node 1 rebroadcasts RREQ 8

Figure 3: Intermediate Node 2 sends RREP. 9

Figure 4: Destination Node has been separated from the network. 11

Figure 5: Wireless network using multiple towers to extend range 13

Figure 6: Vehicle has weak link to Tower A and strong link to Tower B 14


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What is MANET?

The Internet Engineering Task Force (http://www.ietf.org) has defined a Mobile

Ad hoc Network (MANET) as:

an autonomous system of mobile routers (and associated hosts)

connected by wireless links--the union of which form an arbitrary graph.

The routers are free to move randomly and organize themselves

arbitrarily; thus, the network's wireless topology may change rapidly and

unpredictably. Such a network may operate in a standalone fashion, or

may be connected to the larger Internet.

Simply put, a MANET is a wireless mobile network that is self-forming, self-

maintained, and self-healing. Nodes stay connected even as the network

topology changes.

MANET Protocols

Numerous MANET protocols exist; yet, very few have been implemented outside

of the research community. Some of the better known MANET protocols are

AODV, TORA, DSR, TBRPF and OLSR. Each protocol has evolved over time to

better suit the particular requirements of various types of mobile ad hoc networks.

MANET protocols are typically categorized as either proactive or on-demand

(reactive). Proactive MANET protocols update routing information in a proactive

manner by exchanging route information at periodic intervals. The exchange of

table-based route information is evenly distributed across the wireless network.

As a result, routes are established prior to being needed, providing a wireless

network that is low in latency, at the expense of increased overhead.

Rather than distribute all route information across the entire network, On-demand

MANET protocols perform route maintenance only when required. On-demand


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protocols create less network overhead since the exchange of routing information

is localized rather than evenly distributed. The result is a network with less

overhead, at the expense of increased latency due to the route discovery

process.


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AODV

The NovaRoam Mobile Router is unique as it uses an embedded MANET

protocol called AODV (Ad hoc On-demand Distance Vector) that works

dynamically to establish and maintain routes, adapting quickly to changing link

conditions. As its name implies, AODV is an on-demand routing protocol.

Routes between nodes are built only as requested by source nodes. These

routes are maintained locally until they are no longer needed by the source nodes

or a link breakage occurs.

Hello Beacons

AODV uses hello beacons to determine which NovaRoams are capable of direct

(one hop) communication. These beacons are broadcast periodically by all

NovaRoams. Any node from which a NovaRoam receives a beacon is

considered a neighbor and added to the local neighbor list.

Route Creation

When a source node does not have a route for a required destination, AODV

initiates a route request/route reply cycle by broadcasting a route request (RREQ)

packet across the wireless network. Upon receiving the RREQ, nodes must

determine whether or not they are the destination node. If a node is not the

destination and does not have a route to the destination, it will rebroadcast the

RREQ to its neighbors and update its route table to include a reverse pointer to

the source node. This process will continue until a route to the destination node

is found, or the route request process times out.

If a node is the destination node, or has a route to the destination node, it will

respond by sending a route reply (RREP) to the source node. Intermediate nodes


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update their route information about the source and destination nodes. Upon

receiving the RREP, the source node can forward data to the destination node

using the newly created route. If the RREP is not received within a certain time

frame, the source node will retry the RREQ.

Figures 1-3 demonstrate the Route Request-Route Reply query cycle.

Figure 1: Source node sends RREQ


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Figure 2: Intermediate Node 1 rebroadcasts RREQ


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Figure 3: Intermediate Node 2 sends RREP

Route Deletion

A route will remain active as long as data continues to travel across the route. If

a route becomes inactive for a period of time, the route will be deleted. A user-

defined timeout value determines the time period for which a route must be

inactive in order to be deleted from the route table. Each time a packet is sent

across a route, the timer is reset.

Routes can also be deleted based on hello beacons. A route will be deleted if a

node fails to receive some number of consecutive hello beacons from its next hop


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neighbor. The number of consecutive hello beacons, that when missed causes a

route to be deleted, is a user-definable parameter.

Any time a link breakage occurs, a route error (RERR) is propagated to mark the

unusable route as invalid. Upon detecting a broken link, a node will send a

RERR to any neighbors that had been using the node as the next hop for the

route. After receiving the RERR, each node deletes the invalid route from its

route table. If a route to the destination is still required, the source node will

reinitiate the route discovery process. RERR packets are not sent if the route

timeout expires, as all intermediate routers will have timed out as well.

Figure 4: Destination Node has been separated from the network.


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Sequence Numbers

AODV uses sequence numbers to avoid routing loops and to measure the

freshness of route information. Prior to broadcasting RREQ, RREP, and RERR

packets, AODV must increment its sequence number. Each route maintains a

sequence number, with higher sequence numbers indicating fresher routes.

When multiple routes are available to a destination node, the route with the

greatest sequence number is used. Packets with lower sequence numbers are

ignored and dropped.

Stationary Wireless Infrastructure

While many mobile ad hoc networks do not rely on any stationary infrastructure,

many applications require the use of towers/repeaters to extend the range of the

wireless network. While using stationary infrastructure provides a more cellular-

like wireless network, this type of infrastructure can create issues that MANET

protocols do not typically take into account.

Figure 5 displays a network in which two towers are used to extend the range of

the wireless network. The towers have a strong signal between them at all times.

All application data from the mobile vehicles is sent to the wired network

connected to the NovaRoam at Tower A.

In Figure 5, the vehicle has a solid connection to Tower A. In this case, routing

works great and data flows as if on a wired network.


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Figure 5: Wireless network using multiple towers to extend range

In Figure 6, the vehicle has moved away from Tower A. At some point, the signal

to Tower A has become too weak for application data to successfully traverse the

network. The signal between the vehicle and Tower A is barely strong enough for

the AODV control messages to reach Tower A. If enough control messages are

received between the vehicle and the tower, Tower A may remain listed as a

viable route to the network to which it is connected. The vehicle now has a strong

link to Tower B, which in turn has a strong link to Tower A.

So why doesnt the route switch over to use Tower B as an intermediate node to

Tower A? Tower A will remain listed as a viable gateway to the network as long

as the vehicle receives the minimum amount of control messages required to

keep a route in the route table. Once the vehicle gets completely out of range

from Tower A, Tower B will become the intermediate node. This issue is

commonly known as Gray Zone.

Tower ATower B

Hub Router

Internet


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Figure 6: Vehicle has weak link to Tower A and strong link to Tower B

To combat the Gray Zone issue, the NovaRoam implementation of AODV

accounts for control packet signal strength information when making routing

decisions. Embedded within each control packet is a signal strength

measurement. NovaRoam has a user-definable signal strength threshold for

control packets. If the control packet signal strength from Tower A falls below the

user-defined threshold, the vehicles direct route to Tower A will be dropped,

forcing the vehicle data to find a new route. As a result, the vehicle would route

data to Tower A by using Tower B as an intermediate node. Even though this

creates an extra hop for the route, the route is more efficient since the signal

remains strong.

Tower ATower B

Hub Router

Internet


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About NovaRoam

The NovaRoam Mobile Router is a wireless device capable of long

communication ranges and high data rates. Its unique embedded Mobile Ad hoc

Networking capabilities allow the NovaRoam to be used in the most dynamic

environments. Available in Enterprise and Tactical configurations, NovaRoam

can be used for most any application where wireless data connectivity is critical.

Customized versions are also available.

Please see http://www.novaroam.com for additional information.


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Glossary

AODV Ad hoc On-demand Distance Vector

IETF Internet Engineering Task Force

MANET Mobile Ad hoc Network

RERR Route Error

RREP Route Reply

RREQ Route Request


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Nova Engineering, Inc. 5 Circle Freeway Drive

Cincinnati, OH 45246 USA

1-800-341-NOVA (6682) +1-513-642-3000

FAX +1-513-642-3300

www.novaroam.com [email protected]

Part #: Mobile Ad hoc Networking (MANET) with AODV

Revision: Version 1.0

2003 Nova Engineering, Inc. All Rights Reserved.

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