AdHoc Networks

• Features
• Adantages and Applications
• Adhoc versus Cellular Networks
• Network Architecture
• Protocols: MAC Protocols, Routing Protocols,
Technologies
 Definition
 An Ad-hoc network is a local area network or some other small
network, especially one with wireless devices, in which some of
the network devices are the part of the network only for the
duration of a communications session.
 Ad-hoc Network is a collection of mobile wireless
nodes forming a network without the aid of any
infrastructure or centralized administration, where
nodes having limited transmission range act as a
routers.
 A network without any base stations “infrastructure-
less” or multi-hop. A collection of two or more
devices equipped with wireless communications and
networking capability infrastructure network. It
supports anytime and anywhere computing.
Types of Ad Hoc Networks
MANET
WSN
WMN
VANETs
Principle
The principle behind ad hoc networking is multi-
hop relaying in which messages are sent from
the source to the destination by relaying
through the intermediate hops (nodes).
In multi-hop wireless networks, communication
between two end nodes is carried out through a
number of intermediate nodes whose function is
to relay information from one point to another.
In the last few years, efforts have been focused
on multi-hop "ad hoc" networks, in which
relaying nodes are in general mobile, and
communication needs are primarily between
nodes within the same network.
Features
 Dynamic topology
 Variability of the radio channel
 Do not require network infrastructure
 Using multi-hop communications
 Limited bandwidth
 Allows new network devices to be quickly added.
 Each user has a unique network address that is recognized as the part of the
network.
 Collection of nodes that do not rely on a predefined infrastructure
 Auto-configurable network and Self organizing
 Nodes are mobile and hence have dynamic network topology.
 Nodes in ad-hoc networks play both the roles of routers and terminals.
 Routing protocol required
 Examples
 Classroom

◦ Ad hoc network between student PDAs and workstation of the instructor

 Large IT campus

◦ Employees of a company moving within a large campus with PDAs,

laptops, and cell phones
 Moving soldiers with wearable computers

◦ Eavesdropping, denial-of-service and impersonation attacks can be

launched
 Shopping mall, restaurant, coffee shops

◦ Customers spend part of the day in a networked mall of specialty shops,

coffee shops, and restaurants
Advantages
No expensive infrastructure must be installed
Independence from central network administration
Use of unlicensed frequency spectrum
Self-configuring, nodes are also routers
Self-healing through continuous re-configuration
Scalable: accommodates the addition of more nodes
Flexible: similar to being able to access the Internet from many
different locations
Quick distribution of information around sender
Use of ad-hoc networks can increase mobility and flexibility, as ad-hoc
networks can be brought up and torn down in a very short time.
Ad-hoc networks can be more economical in some cases, as they
eliminate fixed infrastructure costs and reduce power consumption at
mobile nodes.
Because of multi-hop support in ad-hoc networks, communication
beyond the Line of Sight (LOS) is possible at high frequencies.
Multi-hop ad-hoc networks can reduce the power consumption of
wireless devices. More transmission power is required for sending a
signal over any distance in one long hop than in multiple shorter hops
Reduced interference levels, increases spectrum reuse efficiency, and
makes it possible to use unlicensed unregulated frequency bands(Because
of short communication links radio emission levels can be kept low).
Disadvantages
Each node must have full
performance
Throughput is affected by system
loading
Reliability requires a sufficient
number of available nodes.
Sparse networks can have
problems
Large networks can have
excessive latency (high delay),
 Applications
Military Applications
◦ Establishing communication among a group of soldiers for tactical
operations
◦ Coordination of military object moving at high speeds such as fleets
of airplanes or ships
◦ Requirements: reliability, efficiency, secure communication, and
multicasting routing,
Collaborative and Distributed Computing
◦ Conference, distributed files sharing
Emergency Operations
◦ Search, rescue, crowd control, and commando operations
◦ Support real-time and fault-tolerant communication paths
PersonalArea Networks (PANs)
Environmental monitoring
◦ used to predict water pollution or to provide early a natural
catastrophe warning.
Peer to Peer Networks
• Disaster recovery
• Battlefield
• ‘Smart’ office

 Rapidly deployable
infrastructure
◦ Wireless: cabling impractical
◦ Ad-Hoc: no advance planning
 Backbone network:
wireless IP routers
• Network of access devices
• Wireless: untethered
• Ad-hoc: random deployment
• Edge network: Sensor networks,
Personal Area Networks (PANs), etc.

August 24, 2006 Talk at SASTRA 12

Adhoc vs Cellular
Networks
Ad Hoc Networks
multi-hop radio relaying and
without support of infrastructure
◦ Wireless Mesh Networks
◦ Wireless Sensor Networks
Cellular Wireless Networks
infrastructure dependent network
Cellular Network
Single hop communication (Cellular
Networks)
 Single hop wireless connectivity to the wired world

◦ Space divided into cells

◦ A base station is responsible to communicate with hosts in its

cell

◦ Mobile hosts can change cells while communicating

◦ Hand-off occurs when a mobile host starts communicating via a

new base station
Multi hop communication (AdHoc
Networks)
May need to traverse multiple
links to reach destination
Mobility causes route changes
Single hop
Architecture

Multi hop
Architecture
Differences between Ad-hoc Wireless
Networks and Cellular Networks
Network Architecture
There are two architectures
available: standalone (Ad Hoc
Mode) and
centrally coordinated
(Infrastructure Mode)
Standalone architecture
(Ad hoc mode)
 By using ad hoc mode, all devices in the
wireless network are directly
communicating with each other in peer to
peer communication mode.
 No access point (routers/switches) is
required for communication between
devices.
 For setting up ad hoc mode, we need to
manually configure the wireless adaptors
of all devices to be at ad hoc mode instead
of infrastructure mode, and all adaptors
must use the same channel name and
same SSID for making the connection
active.
 Ad hoc mode is most suitable for small group of devices
and all of these devices must be physically present in
close proximity with each other.
 The performance of network suffers while the number of
devices grows.
 Disconnections of random device may occur frequently
and also, ad hoc mode can be a tough job for network
administrator to manage the network.
 Ad hoc mode has another limitation is that, ad hoc mode
networks cannot bridge to wired local area network and
also cannot access internet if without the installation of
special gateways.
 However, Ad hoc mode works fine in small environment.
Because ad hoc mode does not need any extra access
point (routers/switches), therefore it reduces the cost.
 Ad hoc can be very useful as a backup option for time
being if network based on centrally coordinated wireless
network (infrastructure mode) and access points are
malfunctioning.
 An ad hoc mode uses the integrated
functionality of each adaptor to enable
wireless services and security
authentication.
 The characteristics of an Ad hoc wireless
network are listed as below:
◦ All access points in the network operate
independently and has own configuration file.
◦ Access point is responsible for the encryption
and decryption.
◦ The network configuration is static and does
not respond to changing network conditions.
Centrally Coordinated Architecture
(Infrastructure mode)
 The other architecture in wireless network is
centrally coordinated (infrastructure mode).
 All devices are connected to wireless network with
the help of Access Point (AP).
 Wireless APs are usually routers or switches which
are connected to internet by broadband modem.
 Infrastructure mode deployments are more suitable
for larger organizations or facility. This kind of
deployment helps to simplify network management,
and allows the facility to address operational
concerns. And resiliency is also assured while more
users can get connected to the network
subsequently.
 The infrastructure mode provides improved security,
ease of management, and much more scalability and
stability. However, the infrastructure mode incurs
extra cost in deploying access points such as routers
or switches.
 Aninfrastructure mode wireless network has the
characteristics as below:
◦ The wireless centralized controller coordinates the
activity of access point.
◦ The controller is able to monitor and control the wireless
network by automatically reconfiguring the access point
parameters in order to maintain the health of the
network.
◦ The wireless network can be easily expanded or reduced
by adding or removing access points and the network
can be reconfigured by the controller based on the
changes in RF footprint.
◦ Tasks such as user authentication, fault tolerance, control
of configuration, policy enforcement and expansion of
network are done by the wireless network controller.
◦ Redundant access points can be deployed in separate
locations to maintain control in the event of an access
point or switch failure.
Protocols: MAC Protocols, Routing
Protocols, Technologies
Routing Protocols
Pro-active (table-driven) routing
 Maintains fresh lists of destinations & their routes by periodically
distributing routing tables
 Disadvantages:

1. Respective amount of data for maintenance

2. Slow reaction on restructuring and failures
(e.g. OSLR, DSDV)

Reactive (on-demand) routing

 On demand route discovery by flooding the network with Route
Request packets
 Disadvantages:

1. High latency time in route finding

2. Flooding can lead to network clogging
(e.g. AODV, DSR)
Ad-Hoc on-demand Distance
Vector Routing (AODV)
Reactive algorithms like AODV create routes on-demand.
They must however, reduce as much as possible the
acquisition time

We could largely eliminate the need of periodically system-

wide broadcasts

AODV uses symmetric links between neighboring nodes. It

does not attempt to follow paths between nodes when one
of the nodes can not hear the other one

Nodes that have not participate yet in any packet exchange
(inactive nodes), they do not maintain routing information


They do not participate in any periodic routing table exchanges
Each node can become aware of
other nodes in its neighborhood by
using local broadcasts known as
hello messages
neighbor routing tables organized
to :
◦ optimize response time to local
movements
◦ provide quick response time for new
routes requests

AODV main features:


Broadcast route discovery mechanism


Bandwidth efficiently (small header

information)


Responsive to changes in network topology


Loop free routing