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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.