AD HOC WIRELESS NETWORKS

Unit 1

INTRODUCTION

The principle behind ad hoc networking is multi-hop relaying, which traces its roots back to 500 B.C. Darius

I (522-486 B.C.), the king of Persia, devised an innovative communication system that was used to send

messages and news from his capital to the remote provinces of his empire by means of a line of shouting men

positioned on tall structures or heights. This system was more than 25 times faster than normal messengers

available at that time. The use of ad hoc voice communication was used in many ancient/tribal societies with

a string of repeaters of drums, trumpets, or horns. In 1970, Norman Abramson and his fellow researchers at

the University of Hawaii invented the ALOHAnet, an innovative communication system for linking together

the universities of the Hawaiian Islands. ALOHAnet utilized single-hop wireless packet switching and a

multiple access solution for sharing a single channel. Even though ALOHAnet was originally implemented

for a fixed single-hop wireless network. The success and novelty of ALOHAnet triggered widespread interest

in different directions of computer communication, including the work that led to the development of Ethernet

by Robert Metcalfe and the packet radio network (PRNET) project sponsored by the Defense advanced

research projects agency (DARPA). The PRNET project was aimed at developing a packet wireless network

for military applications.

The initial attempt had a centralized control, it quickly evolved into a distributed multi-hop wireless

communication system that could operate over a large geographical area. Each mobile node had a broadcast

radio interface that provided many advantages such as the use of a single channel, simpler channel

management techniques, and the ease of supporting mobility.

PRNET

1. ALOHA and carrier sense multiple access (CSMA) for access to the shared radio channel.

2. The radio interface employed the Direct-sequence (DS) spread spectrum scheme.

3. The system was designed to self-organize, self-configure, and detect radio connectivity for the

dynamic operation of a routing protocol without any support from fixed infrastructure.

4. The major issues that the PRNET project faced include those of obtaining, maintaining, and utilizing

the topology information, error and flow control over the wireless links, reconfiguration of paths

to handle path breaks arising due to the mobility of nodes and routers, processing and storage capability

of nodes, and distributed channel sharing.

5. The successful demonstrations of the PRNET proved the feasibility and efficiency of infrastructureless

networks and their applications for civilian and military purposes. DARPA extended the work on

multi-hop wireless networks through the survivable radio networks (SURAN) project that aimed at

providing ad hoc networking with small, low-cost, low-power devices with efficient protocols and

improved scalability and survivability (the ability of a network to survive the failure of network nodes

and links).

6. Internet Engineering Task Force (IETF), termed the mobile ad hoc networks (MANET) working group

was formed to standardize the protocols and functional specifications of ad hoc wireless networks.

7. The IETF effort in the MANET working group is to provide improved standardized routing

functionality to support self-organizing mobile networking infrastructure.

In 1994, the Swedish communication equipment maker Ericsson proposed to develop a short range, low-

power, low-complexity, and inexpensive radio interface and associated communication protocols referred to

as Bluetooth for ubiquitous connectivity among heterogeneous devices. This effort was later taken over by a

Special Interest Group (SIG) formed by several major computer and telecommunication vendors such as

3Com, Ericsson, IBM, Intel, Lucent, Microsoft, Motorola, Nokia, and Toshiba.

The Bluetooth SIG aims at delivering a universal solution for connectivity among heterogeneous devices.

This is one of the first commercial realizations of ad hoc wireless networking. Bluetooth standardizes the

single-hop point-to-point wireless link that helps in exchanging voice or data, and formation of piconets that

are formed by a group of nodes in a smaller geographical region where every node can reach every other node

in the group within a single-hop. Multiple piconets can form a scatternet, which necessitates the use of multi-

hop routing protocols.

Even though ad hoc wireless networks are expected to work in the absence of any fixed infrastructure,

recent advances in wireless network architectures reveal interesting solutions that enable the mobile ad hoc

nodes to function in the presence of infrastructure. Multi-hop cellular networks (MCNs) and self-organizing

packet radio ad hoc networks with overlay (SOPRANO) are examples of such types of networks.

Cellular and Ad Hoc Wireless Networks

The following shows a representation of different wireless networks.

⮚ The current cellular wireless networks are classified as the infrastructure-dependent network.

⮚ The path-setup for a call between two nodes, say, node C to E, is completed through base-station

figure 2

⮚ The current cellular wireless networks (depicted in Figure 2) are classified as the infrastructure

dependent networks. The path setup for a call between two nodes, say, node C to node E, is completed

through the base station as illustrated in Figure 2. Figure 1 . Cellular and ad hoc wireless networks.

Figure 2. A cellular network.

Ad hoc wireless networks

1. Adhoc wireless networks are defined as a category of wireless network.

2. They utilize multi-hop radio replaying.

3. They are capable of operating without the support of any fixed infrastructure.

4. Absence of any central co-coordinator (or base station) makes the routing complex.

5. Adhoc network topology for the cellular network is illustrated below (Fig 3).

6. The path-setup for a call between 2 nodes, say, node C to E, is completed through the intermediate

mobile node F.

7. Wireless mesh networks and wireless sensor networks are specific examples of ad hoc wireless

networks.

Figure 3. An ad hoc wireless network.

Differences between cellular networks and ad hoc wireless networks Parameter’s Cellular Networks Adhoc wireless networks

1 Infrastructure Fixed Infrastructure –less

2 Number of Hops Single –hop Multi –Hop

3 Bandwidth Guaranteed BW(voice traffic) Shred radio channel(Best effort Data

traffic)

4 Switching Circuit Switched Packet –Switched

5 Connectivity Seamless connectivity(Low call drops) Frequent path breaks (Due to mobility)

6 Cost High cost and time for deployment Quick and cost effective

7 Frequency Reuse of Frequency spectrum through

geographical channel reuse

Dynamic frequency reuse (carrier sense

modulation)

8 Synchronization Easy to achieve Time synchronization is difficult and

consumes BW

9 BW reservation Easier to employ BW reservation BW reservation requires complex

medium access protocols

10 Application

Domains

Civilian and commercial sectors Battle field, Emergency and rescue

operations and collaborative computing

11 Maintenance High cost of network Maintenance Self organization and maintenance

properties are included in the network

12 Complexity Mobile nodes are relatively Low complex Mobile hosts require more intelligence

13 Routing Major goals of routing and call admission

are to maximize the call acceptance ration

and minimize call drop ratio.

Major aim of routing is to find paths with

minimum overhead and also quick

configuration of broken paths

14 Deployment Widely deployed and 3G of evolution Several issues needs to be addressed for

commercial deployment

⮚ The presence of base stations simplifies routing and resource management in a cellular network as

the routing decisions are made in a centralized manner with more information about the destination

node.

⮚ But in an ad hoc wireless network, the routing and resource management are done in a distributed

manner in which all nodes coordinate to enable communication among themselves. This requires each

node to be more intelligent so that it can function both as a network host for transmitting and receiving

data and as a network router for routing packets from other nodes. Hence the mobile nodes in ad hoc

wireless networks are more complex than their counterparts in cellular networks.

5.1.2 Applications of Ad Hoc Wireless Networks

1. Military Application

• Adhoc networks can be used to establish communication among a group of soldiers for tactical

operations.

• Setting up of a fixed infrastructure for communication among group of soldiers in enemy territories

(or in inhospitable terrains) may not be possible. In such a case, adhoc networks can be used to

provide required communication quickly.

• The military application enforces following requirements

* Reliability * Efficiency

* Secure communication * Support for multicast routing

2. Collaborative & Distributed Computing

• Adhoc network can be used to establish temporary communication infrastructure among a group of

people in a conference.

• In distributed file sharing application, reliability is of high importance which would be provided by

adhoc network.

• Other applications such as streaming of multimedia objects require support for soft real-time

communication.

• Devices used can be

→ laptops with add-on wireless interface cards

→ enhanced PDA (Personal Digital Assistant) or

→ mobile devices with high processing power

3.Emergency Operations

• Adhoc networks can be used in emergency operations such as

* search and rescue

* crowd control &

* commando operations

• The major factors that favor adhoc networks for such tasks are

→ self-configuration of system with minimal overhead

→ independent of fixed infrastructure

→ freedom and flexibility of mobility &

→ unavailability of conventional communication infrastructure

• In environments where the infrastructure-based communication facilities are destroyed due to

natural calamities (or due to a war), immediate deployment of adhoc networks would be a good

solution for co-coordinating rescue activities.

Wireless Mesh Network (WMN)

• WMN can be formed to provide an alternate communication-infrastructure for mobile or fixed

nodes,

* without the spectrum reuse constraint &

* without the requirement of network planning of cellular network

• It provides many alternate paths for data transfer between source & destination, which results in

quick reconfiguration of path when the existing path fails due to node-failure.

• Major advantages are

* low cost of deployment

* high scalability

* support for a high data-rate

* easy extendibility

* high availability &

* low cost/bit

• Since the infrastructure-built is in the form of small radio relaying-devices, the investment required

is much less when compared to cellular-network counterpart.

• The possible deployment scenarios include

* residential zones

* highways

* business zones

* important civilian regions &

* university campuses

• It should be capable of self-organization and maintenance.

• It operates at license-free ISM band around 2.4 GHz & 5 GHz.

Figure 4. Wireless mesh network operating in a residential zone.

Figure 5.5. Wireless mesh network covering a highway.

Wireless Sensor Networks (WSN) ∙ These are used to provide a wireless communication infrastructure among the sensors deployed in a

specific application

domain.

∙ Sensor-nodes are tiny devices that have capability of

→ sensing physical parameters

→ processing the data gathered &

→ communicating to the monitoring system

The issues that make sensor network a distinct category of adhoc network are the following

1. Node Mobility

∙ Mobility of nodes is not a mandatory requirement in sensor-networks.

∙ For example,

→ the nodes used for periodic monitoring of soil properties are not required to be mobile

→ the nodes fitted on the bodies of patients are designed to support partial mobility

∙ In general, sensor-networks need not in all cases be designed to support mobility of nodes.

2. Network Size

∙ The number of nodes in sensor network can be much larger than that in a typical adhoc network

3. Density of Deployment

∙ The density of nodes varies with the domain of application.

∙ For example, military applications require high availability of network, which makes redundancy a high

priority.

4. Power Constraints

∙ The power constraints in sensor networks are much more severe than those in adhoc networks. This is

mainly because the nodes are expected to operate in harsh environmental conditions, with minimum human

supervision & maintenance.

∙ In certain case, the recharging of the energy source is impossible.

∙ Running such a network demands very efficient protocol at network, data link, and physical layer.

∙ The power sources can be classified into following 3 categories

→ Replenishable Power Source: The power source can be replaced when the existing source is fully

drained.

→ Non-replenishable Power Source: The power source cannot be replenished once the network has

been deployed. The replacement of node is the only solution.

→ Regenerative Power Source: Here, power source have the capability of regenerating power from

the physical parameter under measurement.

5. Data/Information Fusion

∙ Data fusion refers to the aggregation of multiple packets into one before relaying it.

∙ Data fusion is used

→ to reduce bandwidth consumed by redundant headers of packets &

→ to reduce delay involved in transmitting multiple packets

∙ Information fusion is used

→ to process sensed data at intermediate nodes &

→ to relay the outcome to the monitoring system

6. Traffic Distribution

∙ The communication traffic pattern varies with the domain of application.

∙ For example,

→ Environmental sensing application generates short periodic packets indicating the status of

environmental parameter. This kind of traffic requires low bandwidth

→ Military applications generally carry user traffic such as digitized & packetized voice stream

This kind of traffic requires high bandwidth.

Hybrid Wireless Networks

∙ One of the major application areas of adhoc network is in the hybrid wireless architecture such as Multi-

hop Cellular Network[MCN] & Integrated Cellular Adhoc Relay[iCAR].

∙ The primary concept behind cellular networks is geographical channel-reuse.

∙ Several techniques like cell sectoring, cell resizing and multi tier cells increase the capacity of cellular

networks.

∙ MCNs combine

→ reliability & support of fixed base-station of cellular network with

→ flexibility & multi-hop relaying adhoc networks

∙ Major advantages are as follows

→ Higher capacity than cellular networks due to the better channel reuse

→ Increased flexibility & reliability in routing

→ Better coverage & connectivity in holes of a cell can be provided by means of multiple hops through

intermediate nodes in a cell

The MCN Architecture

The MCN architecture is depicted in Figure In this architecture, when two nodes (which are not in direct

transmission range) in the same cell want to communicate with each other, the connection is routed through

multiple wireless hops over the intermediate nodes. The base station maintains the information about the

topology of the network for efficient routing. The base station may or may not be involved in this multi-hop

path. Suppose node A wants to communicate with node B. If all nodes are capable of operating in MCN mode,

node A can reach node B directly if the node B is within node A's transmission range. When node C wants to

communicate with node E and both are in the same cell, node C can reach node E through node D, which acts

as an intermediate relay node. Such hybrid wireless networks can provide high capacity resulting in lowering

the cost of communication to less than that in single-hop cellular networks

Fig. MCN architecture

ISSUES IN AD HOC WIRELESS NETWORKS

The major issues that affect the design, deployment, & performance of an ad hoc wireless network system

are:

1. Medium Access Scheme 2. Transport Layer Protocol 3. Routing 4. Energy Management 5. Multicasting

6 .Self-Organization 7. Scalability 8. Addressing & Service discovery 9. Pricing Scheme 10. Deployment

considerations 11.QoS Provisioning

1 Medium Access Scheme The primary responsibility of a medium access control (MAC) protocol in ad hoc wireless networks is the

distributed arbitration for the shared channel for transmission of packets. The performance of any wireless

network hinges on the MAC protocol, more so for ad hoc wireless networks.

The major issues to be considered in designing a MAC protocol for ad hoc wireless networks are as

follows:

• Distributed operation:

⮚ The ad hoc wireless networks need to operate in environments where no centralized coordination is

possible.

⮚ The MAC protocol design should be fully distributed involving minimum control overhead. In the

case of polling-based MAC protocols, partial coordination is required. • Synchronization:

⮚ The MAC protocol design should take into account the requirement of time synchronization.

⮚ Synchronization is mandatory for TDMA-based systems for management of transmission and

reception slots.

⮚ Synchronization involves usage of scarce resources such as bandwidth and battery power. The control

packets used for synchronization can also increase collisions in the network. • Hidden terminals:

⮚ Hidden terminals are nodes that are hidden (or not reachable) from the sender of a data transmission

session, but are reachable to the receiver of the session ,here the hidden terminal can cause collisions

at the receiver node.

⮚ The presence of hidden terminals can significantly reduce the throughput of a MAC protocol used in

ad hoc wireless networks. Hence the MAC protocol should be able to alleviate the effects of hidden

terminals. • Exposed terminals:

⮚ Exposed terminals, the nodes that are in the transmission range of the sender of an on-going session,

are prevented from making a transmission.

⮚ To improve the efficiency of the MAC protocol, the exposed nodes should be allowed to transmit in a

controlled fashion without causing collision to the on-going data transfer. • Throughput:

⮚ The MAC protocol employed in ad hoc wireless networks should attempt to maximize the throughput

of the system.

⮚ Enhancement are minimizing the occurrence of collisions, maximizing channel utilization, and minimizing control overhead.

• Access delay:

⮚ The access delay refers to the average delay that any packet experiences to get transmitted. The MAC

protocol should attempt to minimize the delay. • Fairness:

⮚ Fairness refers to the ability of the MAC protocol to provide an equal share or weighted share of the

bandwidth to all competing nodes.

⮚ Fairness can be either node-based or flow-based. The former attempts to provide an equal bandwidth

share for competing nodes whereas the latter provides an equal share for competing data transfer

sessions.

⮚ In ad hoc wireless networks, fairness is important due to the multi-hop relaying done by the nodes. An

unfair relaying load for a node results in draining the resources of that node much faster than that of other nodes.

• Real-time traffic support:

⮚ In a contention-based channel access environment, without any central coordination, with limited

bandwidth, and with location-dependent contention, supporting time-sensitive traffic such as voice,

video, and real-time data requires explicit support from the MAC protocol. • Resource reservation:

⮚ The provisioning of QoS defined by parameters such as bandwidth, delay, and jitter requires

reservation of resources such as bandwidth, buffer space, and processing power.

⮚ The inherent mobility of nodes in ad hoc wireless networks makes such reservation of resources a

difficult task.

⮚ A MAC protocol should be able to provide mechanisms for supporting resource reservation and QoS

provisioning. • Ability to measure resource availability:

⮚ The resources can be handled such as bandwidth efficiently and perform call admission control based

on their availability

⮚ The MAC protocol should be able to provide an estimation of resource availability at every node. This

can also be used for making congestion-control decisions. • Capability for power control:

⮚ The transmission power control reduces the energy consumption at the nodes, causes a decrease in

interference at neighbouring nodes, and increases frequency reuse.

⮚ Support for power control at the MAC layer is very important in the ad hoc wireless environment. • Adaptive rate control:

⮚ The variation in the data bit rate achieved over a channel.

⮚ A MAC protocol that has adaptive rate control can make use of a high data rate when the sender

and receiver are nearby and adaptively reduce the data rate as they move away from each other.

• Use of directional antennas:

⮚ Include increased spectrum reuse, reduction in interference, and reduced power consumption.

⮚ Most of the existing MAC protocols that use omnidirectional antennas do not work with directional

antennas.

5.2.2 Routing A routing protocol include exchanging the route information; finding a feasible path to a destination based

on criteria such as hop length, minimum power required, and lifetime of the wireless link; gathering

information about the path breaks; mending the broken paths expending minimum processing power and

bandwidth; and utilizing minimum bandwidth.

The major challenges that a routing protocol faces are as follows:

• Mobility:

⮚ The mobility of nodes results in frequent path breaks, packet collisions, transient loops, stale routing

information, and difficulty in resource reservation.

⮚ A good routing protocol should be able to efficiently solve all the above issues. • Bandwidth constraint:

⮚ The channel is shared by all nodes in the broadcast region (any region in which all nodes can hear all

other nodes), the bandwidth available per wireless link depends on the number of nodes and the traffic

they handle, only a fraction of the total bandwidth is available for every node. • Error-prone and shared channel:

⮚ The bit error rate (BER) in a wireless channel is very high (of the order of 10-5 to 10-3) compared to

that in its wired counterparts (of the order of 10-12 to 10-9). ⮚ The state of the wireless link, signal-to-noise ratio, and path loss for routing in ad hoc wireless

networks can improve the efficiency of the routing protocol. • Location-dependent contention:

⮚ The load on the wireless channel varies with the number of nodes present in a given geographical

region. This makes the contention for the channel high when the number of nodes increases.

⮚ The high contention for the channel results in a high number of collisions and a subsequent wastage

of bandwidth.

⮚ A good routing protocol should have built-in mechanisms for distributing the network load uniformly

across the network so that the formation of regions where channel contention is high can be avoided. • Other resource constraints:

⮚ The constraints on resources such as computing power, battery power, and buffer storage also limit

the capability of a routing protocol.

The major requirements of a routing protocol in ad hoc wireless networks are the following:

• Minimum route acquisition delay:

⮚ The route acquisition delay for a node that does not have a route to a particular destination node should

be as minimal as possible. This delay may vary with the size of the network and the network load. • Quick route reconfiguration:

⮚ The unpredictable changes in the topology of the network require that the routing protocol be able to

quickly perform route reconfiguration in order to handle path breaks and subsequent packet losses. • Loop-free routing:

⮚ This is a fundamental requirement of any routing protocol to avoid unnecessary wastage of network

bandwidth. In ad hoc wireless networks, due to the random movement of nodes, transient loops may

form in the route thus established. A routing protocol should detect such transient routing loops and

take corrective actions. • Distributed routing approach:

⮚ An ad hoc wireless network is a fully distributed wireless network and the use of centralized routing

approaches in such a network may consume a large amount of bandwidth. • Minimum control overhead:

⮚ The control packets exchanged for finding a new route and maintaining existing routes should be kept

as minimal as possible.

⮚ The control packets consume precious bandwidth and can cause collisions with data packets, thereby

reducing network throughput. • Scalability:

⮚ Scalability is the ability of the routing protocol to scale well (i.e., perform efficiently) in a network

with a large number of nodes which requires minimization of control overhead and adaptation of the

routing protocol to the network size. • Provisioning of QoS:

⮚ The routing protocol should be able to provide a certain level of QoS as demanded by the nodes or the

category of calls.

⮚ The QoS parameters can be bandwidth, delay, jitter, packet delivery ratio, and throughput. Supporting

differentiated classes of service may be of importance in tactical operations. • Support for time-sensitive traffic:

⮚ Tactical communications and similar applications require support for time-sensitive traffic. The

routing protocol should be able to support both hard realtime and soft real-time traffic. • Security and privacy:

⮚ The routing protocol in ad hoc wireless networks must be resilient to threats and vulnerabilities.

⮚ It must have inbuilt capability to avoid resource consumption, denial-of-service, impersonation, and

similar attacks possible against an ad hoc wireless network.

3 Multicasting In emergency search-and-rescue operations and military communication. In such an environment, nodes form

groups to carry out certain tasks that require point-to-multipoint and multipoint-to-multipoint voice and data

communication. The arbitrary movement of nodes changes the topology dynamically in an unpredictable

manner.

The mesh-based multicast routing structure may work well in a high-mobility environment. The major issues

in designing multicast routing protocols are as follows:

• Robustness:

⮚ The multicast routing protocol must be able to recover and reconfigure quickly from potential

mobility-induced link breaks thus making it suitable for use in highly dynamic environments. • Efficiency:

⮚ A multicast protocol should make a minimum number of transmissions to deliver a data packet to all

the group members. • Control overhead:

⮚ The scarce bandwidth availability in ad hoc wireless networks demands minimal control overhead for

the multicast session. • Quality of service:

⮚ QoS support is essential in multicast routing because, in most cases, the data transferred in a multicast

session is time-sensitive. • Efficient group management:

⮚ Group management refers to the process of accepting multicast session members and maintaining the

connectivity among them until the session expires.

⮚ The process of group management needs to be performed with minimal exchange of control messages. • Scalability:

⮚ The multicast routing protocol should be able to scale for a network with a large number of nodes. • Security:

⮚ Authentication of session members and prevention of non-members from gaining unauthorized

information play a major role in military communications.

4 Transport Layer Protocols ⮚ The main objectives of the transport layer protocols include setting up and maintaining end-to-end

connections, reliable end-to-end delivery of data packets, flow control, and congestion control. There

exist simple connectionless transport layer protocols (e.g., UDP) which neither perform flow control

and congestion control nor provide reliable data transfer.

⮚ Unreliable connectionless transport layer protocols do not take into account the current network status

such as congestion at the intermediate links, the rate of collision, or other similar factors affecting the

network throughput. This behaviour of the transport layer protocols increases the contention of the

already-choked wireless links.

⮚ For example, in an ad hoc wireless network that employs a contention-based MAC protocol, nodes in

a high-contention region experience several backoff states, resulting in an increased number of

collisions and a high latency. Connectionless transport layer protocols, unaware of this situation,

increase the load in the network, degrading the network performance.

⮚ The major performance degradation faced by a reliable connection-oriented transport layer protocol

such as transmission control protocol (TCP) in an ad hoc wireless network arises due to frequent path

breaks, presence of stale routing information, high channel error rate, and frequent network partitions.

⮚ Due to the mobility of nodes and limited transmission range, an existing path to a destination node

experiences frequent path breaks. Each path break results in route reconfiguration that depends on the

routing protocol employed. The process of finding an alternate path or reconfiguring the broken path

might take longer than the retransmission timeout of the transport layer at the sender, resulting in

retransmission of packets and execution of the congestion control algorithm.

⮚ The congestion control algorithm decreases the size of the congestion window, resulting in low

throughput. In an environment where path breaks are frequent, the execution of congestion control

algorithms on every path break affects the throughput drastically.

⮚ The latency associated with the reconfiguration of a broken path and the use of route caches result in

stale route information at the nodes. Hence the packets will be forwarded through multiple paths to a

destination, causing an increase in the number of out-of-order packets.

⮚ Out-of-order packet arrivals force the receiver of the TCP connection to generate duplicate

acknowledgments (ACKs). On receiving duplicate ACKs, the sender invokes the congestion control

algorithm.

⮚ Wireless channels are inherently unreliable due to the high probability of errors caused by interference.

In addition to the error due to the channel noise, the presence of hidden terminals also contributes to

the increased loss of TCP data packets or ACKs. When the TCPACK is delayed more than the round-

trip timeout, the congestion control algorithm is invoked.

⮚ Due to the mobility of the nodes, ad hoc wireless networks frequently experience isolation of nodes

from the rest of the network or occurrence of partitions in the network. If a TCP connection spans

across multiple partitions, that is, the sender and receiver of the connection are in two different

partitions, all the packets get dropped. This tends to be more serious when the partitions exist for a

long duration, resulting in multiple retransmissions of the TCP packets and subsequent increase in the

retransmission timers. Such a behaviour causes long periods of inactivity even when a transient

partition in the network lasts for a short while.

5 Pricing Scheme Assume that an optimal route from node A to node B passes through node C, & node C is not

powered on.

• Then node A will have to set up a costlier & non-optimal route to B.

• The non-optimal path consumes more resources & affects the throughput of the system.

• As the intermediate nodes in a path that relay the data packets expend their resources such as

battery charge & computing power, they should be properly compensated.

• Hence, pricing schemes that incorporate service compensation or service reimbursement are

required.

6 Quality of Service Provisioning

⮚ QoS provisioning often requires negotiation between the host and the network, resource reservation

schemes, priority scheduling, and call admission control.

⮚ QoS in ad hoc wireless networks can be on a per flow, per link, or per node basis.

⮚ In ad hoc wireless networks, the boundary between the service provider (network) and the user (host)

is blurred, thus making it essential to have better coordination among the hosts to achieve QoS.

• QoS parameters:

Applications Corresponding QoS parameter

1.Multimedia application 1. Bandwidth & Delay.

2.Military application 2.Security & Reliability.

3.Defense application 3.Finding trustworthy intermediate hosts & routing

4.Emergency search and rescue operations 4.Availability.

5.Hybrid wireless network 5.Maximum available link life, delay, bandwidth &

channel utilization.

6.communication among the nodes in a sensor

network

6.Minimum energy consumption, battery life &

energy conservation

• QoS-aware routing:

i. Finding the path is the first step toward a QoS-aware routing protocol.

ii. The parameters that can be considered for routing decisions are,

⮚ Network throughput.

⮚ Packet delivery ratio.

⮚ Reliability.

⮚ Delay.

⮚ Delay jitter.

⮚ Packet loss rate.

⮚ Bit error rate.

⮚ Path loss.

• QoS framework:

I. A framework for QoS is a complete system that attempts to provide the promised services to each user or

application. All the components within this subsystem should cooperate in providing the required services.

II. The key component of QoS framework is a QoS service model which defines the way user requirements

are served.

III The key design issue is whether to serve the user on a per-session basis or a per-class basis. Each class

represents an aggregation of users based on certain criteria.

IV The other key components of this framework are QoS routing to find all or some feasible paths in the

network that can satisfy user requirements, QoS signaling for resource reservation required by the user or

application, QoS medium access control, connection admission control, and scheduling schemes pertaining

to that service model.

V The QoS modules such as routing protocol, signaling protocol, and resource management should react

promptly according to changes in the network state (topology change in ad hoc wireless networks) and flow

state (change in end-to-end view of service delivered).

6 Self-Organization

• One very important property that an ad hoc wireless network should exhibit is organizing & maintaining

the network by itself.

• The major activities that an ad hoc wireless network is required to perform for self-organization are,

Neighbour discovery.

Topology organization &

Topology reorganization (updating topology information )

🡺 The ad hoc wireless networks require updating the topology information by incorporating the

topological changes occurred in the network due to the mobility of nodes, failure of nodes, or complete

depletion of power sources of the nodes. 🡺 The reorganization consists of two major activities. First is the periodic or aperiodic exchange of

topological information. Second is the adaptability (recovery from major topological changes in

the network).

7 Security ⮚ The security of communication in ad hoc wireless networks is very important, especially in

military applications.

⮚ The lack of any central coordination and shared wireless medium makes them more vulnerable to

attacks than wired networks.

⮚ The attacks against ad hoc wireless networks are generally classified into two types: passive and active

attacks. Passive attacks refer to the attempts made by malicious nodes to perceive the nature of

activities and to obtain information transacted in the network without disrupting the operation. Active

attacks disrupt the operation of the network. Those active attacks that are executed by nodes outside

the network are called external attacks, and those that are performed by nodes belonging to the same

network are called internal attacks. Nodes that perform internal attacks are compromised nodes. The major security threats that exist in ad hoc wireless networks are as follows:

• Denial of service:

⮚ The attack effected by making the network resource unavailable for service to other nodes, either by

consuming the bandwidth or by overloading the system, is known as denial of service (DoS).

⮚ A simple scenario in which a DoS attack interrupts the operation of ad hoc wireless networks is by

keeping a target node busy by making it process unnecessary packets. • Resource consumption:

⮚ The scarce availability of resources in ad hoc wireless network makes it an easy target for internal

attacks, particularly aiming at consuming resources available in the network. The major types of resource-consumption attacks are the following:

– Energy depletion: Since the nodes in ad hoc wireless networks are highly constrained by the energy source,

this type of attack is basically aimed at depleting the battery power of critical nodes by directing unnecessary

traffic through them.

– Buffer overflow: The buffer overflow attack is carried out either by filling the routing table with unwanted

routing entries or by consuming the data packet buffer space with unwanted data. Such attacks can lead to a

large number of data packets being dropped, leading to the loss of critical information. Routing table attacks

can lead to many problems, such as preventing a node from updating route information for important

destinations and filling the routing table with routes for nonexistent destinations.

• Host impersonation: A compromised internal node can act as another node and respond with appropriate

control packets to create wrong route entries, and can terminate the traffic meant for the intended destination

node.

• Information disclosure: A compromised node can act as an informer by deliberate disclosure of confidential

information to unauthorized nodes. Information such as the amount and the periodicity of traffic between a

selected pair of nodes and pattern of traffic changes can be very valuable for military applications. The use of

filler traffic (traffic generated for the sole purpose of changing the traffic pattern) may not be suitable in

resource-constrained ad hoc wireless networks.

• Interference: A common attack in defense applications is to jam the wireless communication by creating a

wide-spectrum noise. This can be done by using a single wide-band jammer, sweeping across the spectrum.

The MAC and the physical layer technologies should be able to handle such external threats.

8 Addressing and Service Discovery ⮚ Addressing and service discovery assume significance in ad hoc wireless networks due to the absence of any centralized coordinator.

⮚ An address that is globally unique in the connected part of the ad hoc wireless network is required for

a node in order to participate in communication.

⮚ Auto-configuration of addresses is required to allocate non-duplicate addresses to the nodes.

⮚ In networks where the topology is highly dynamic, frequent partitioning and merging of network components require duplicate address-detection mechanisms in order to maintain unique addressing

throughout the connected parts of the network.

⮚ Nodes in the network should be able to locate services that other nodes provide. Hence efficient service

advertisement mechanisms are necessary. Topological changes force a change in the location of the

service provider as well, hence fixed positioning of a server providing a particular service is ruled out.

Rather, identifying the current location of the service provider gathers importance. The integration of

service discovery with the route-acquisition mechanism, though it violates the traditional design

objectives of the routing protocol, is a viable alternative. However, provisioning of certain kinds of

services demands authentication, billing, and privacy that in turn require the service discovery

protocols to be separated from the network layer protocols.

10 Energy Management

Energy management is defined as the process of managing the sources & consumers of energy in a node

or in the network for enhancing the lifetime of a network.

Features of energy management are :

→Shaping the energy discharge pattern of a node’s battery to enhance battery life.

→Finding routes that consumes minimum energy.

→Using distributed scheduling schemes to improve battery life.

→Handling the processor & interface devices to minimize power consumption.

Energy management can be classified into the following categories : a. Transmission power management :

o The power consumed by the Radio Frequency (RF) module of a mobile node is determined by

several factors such as

* The state of operation.

*The transmission power and

*The technology used for the RF circuitry.

o The state of operation refers to transmit ,receive, and sleep modes of the operation.

o The transmission power is determined by

* Reachability requirement of the network.

* Routing protocol and

* MAC protocol employed.

b. Battery energy management :

o The battery management is aimed at extending the battery life of a node by taking advantage of its chemical

properties, discharge patterns, and by the selection of a battery from a set of batteries that is available for

redundancy.

c. Processor power management :

o The clock speed and the number of instructions executed per unit time are some of the processor parameters

that affect power consumption.

o The CPU can be put into different power saving modes during low processing load conditions.

o The CPU power can be completely turned off if the machines is idle for a long time.In such a cases, interrupts

can be used to turn on the CPU upon detection of user interaction or other events.

d. Devices power management :

o Intelligent device management can reduce power consumption of a mobile node significantly.

o This can be done by the operating system( OS) by selectively powering down interface devices that are not

used or by putting devices into different power saving modes, depending on their usage.

12 Deployment Considerations

⮚ The deployment of ad hoc wireless networks involves actions different from those of wired networks.

⮚ It requires a good amount of planning and estimation of future traffic growth over any link in the

network.

⮚ The time-consuming planning stage is followed by the actual deployment of the network. The cost and

time required for laying copper cables or fiber cables make it difficult to reconfigure any partial

deployment that has already been done.

⮚ The deployment of a commercial ad hoc wireless network has the following benefits when compared

to wired networks:

• Low cost of deployment: The use of multi-hop wireless relaying essentially eliminates the requirement of

laying cables and maintenance in a commercial deployment of communication infrastructure. Hence the cost

involved is much lower than that of wired networks.

• Incremental deployment: In commercial wireless WANs based on ad hoc wireless networks, deployment

can be performed incrementally over geographical regions of the city. The deployed part of the network starts

functioning immediately after the minimum configuration is done. For example, during the deployment

process for covering a highway, whenever each radio relaying equipment is installed on the highway side, it

can be commissioned.

• Short deployment time: Compared to wired networks, the deployment time is considerably less due to the

absence of any wired links. Also, wiring a dense urban region is extremely difficult and time-consuming in

addition to the inconvenience caused.

• Reconfigurability: The cost involved in reconfiguring a wired network covering a metropolitan area

network (MAN) is very high compared to that of an ad hoc wireless network covering the same service area.

Also, the incremental deployment of ad hoc wireless networks might demand changes in the topology of the

fixed part (e.g., the relaying devices fixed on lamp posts or rooftops) of the network at a later stage.

The following are the major issues to be considered in deploying an ad hoc wireless network:

a) Scenario of deployment :

⮚ The scenario of deployment has significance because the capability required for a mobile node

varies with the environment in which it is used.

⮚ The following are some of the different scenarios in which the deployment issues vary widely : - military deployment :

It can be either,

Data-centric network : Handle a different pattern of data traffic & can be partially comprised of

static nodes. Eg : a wireless sensor network.

User-centric network: Consists of highly mobile nodes with or without any support from any

infrastructure. Eg :soldiers or armored vehicles carrying soldiers equipped with wireless

communication devices.

- Emergency operations deployment :

Demands a quick deployment of rescue personnel equipped with hand-held communication

equipment.

The network should provide support for time-sensitive traffic such as voice & video.

Short data messaging can be used in case the resource constraints do not permit voice

communication.

- Commercial wide-area deployment :

Eg : wireless mesh networks.

The aim of the deployment is to provide an alternate communication infrastructure for

wireless communication in urban areas & areas where a traditional cellular base station

cannot handle the traffic volume.

- Home network deployment :

Deployment needs to consider the limited range of the devices that are to be connected by

the network.

Eg : short transmission range avoid network patitions.

b) Required longevity of network :

If the network is required for a short while, battery-powered mobile nodes can be used.

If the connectivity is required for a longer duration of time, fixed radio relaying equipment with

regenerative power sources can be deployed.

c) Area of coverage :

Determined by the nature of application for which the network is set up.

Eg : the home area network is limited to the surroundings of a home.

The mobile nodes’ capabilities such as the transmission range & associated hardware, software, &

power source should match the area of coverage required.

d)Service availability :

Defined as the ability of an ad hoc wireless network to provide service even with the failure of certain

nodes.

Has significance in a Fully mobile ad hoc wireless network used for tactical communication & in

partially fixed ad hoc wireless network used in commercial communication infrastructure such as

wireless mesh networks.

e)Operational integration with other infrastructure :

Considered for improving the performance or gathering additional information, or for providing better

QoS.

In military environment, integration of ad hoc wireless networks with satellite networks or unmanned

aerial vehicles(UAVs) improves the capability of the ad hoc wireless networks.

f)Choice of protocol :

The choice of protocols at different layers of the protocol stack is to be done taking into consideration

the deployment scenario.

A TDMA-based & insecure MAC protocol may not be the best suited compared to a CDMA-based MAC

protocol for a military application

AD HOC WIRELESS INTERNET

Adhoc wireless internet extends the services of the internet to the end-users over an adhoc network (Fig

7).

Some of the applications are

* Wireless mesh network

* Provisioning of temporary internet services to major conference venues

* Sports venues

* Temporary military settlements

* Battlefields &

* Broadband internet services in rural regions

Figure 7 Schematic diagram of adhoc wireless internet

The major issues to be considered for a successful adhoc wireless internet are the following:

1. Gateway

∙ They are the entry points to the wired-internet.

∙ Generally, they are owned & operated by a service-provider.

∙ They perform following tasks

* Bandwidth management

* Load balancing

* Traffic shaping

* Packet filtering &

* Address, service & location discovery

2. Address Mobility

∙ This problem is worse here as the nodes operate over multiple hops.

∙ Solution such as Mobile IP can provide temporary alternative.

3. Routing

∙ It is a major problem due to

* dynamic topological changes

* presence of gateways

* multi-hop relaying &

* hybrid character of network

∙ Possible solution is to: use separate routing protocol for the wireless part of adhoc wireless internet.

4. Transport Layer Protocol

∙ Several factors are to be considered here, the major one being the state-maintenance-overhead at the

gateway-nodes.

5. Load Balancing

∙ It is essential to distribute the load so as to avoid the situation where the gateway-nodes become

bottleneck-nodes.

6. Pricing/Billing

∙ Since internet-bandwidth is expensive, it is very important to introduce pricing/billing strategies for

the adhoc network.

7. Provisioning of Security

∙ Security is a prime concern, since the end-users can utilize the adhoc network to make e-commerce

Transaction.

8. QoS Support

∙ Provisioning of QoS-support is a very important issue because of

* widespread use of voice over IP(VOIP) &

* growing multimedia applications over the internet

9. Service, Address & Location Discovery

∙ Service discovery refers to the activity of identifying the party which provides the service( or

resource).

∙ Address discovery refers to the services such as those provided by ARP or DNS operating within

the wireless domain.

∙ Location discovery refers to different activities such as

* detecting location of a particular mobile-node in network or

* detecting geographical location of nodes