Ad Hoc Wireless Networks

Lec#11

Mobile Wireless Network

Need: Access computing and communication services, on the move

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Mobile Networks

Hosts and routers maintains a static topology with respect to each other

Mobile network is moving with respect to the overall network Mobile router can connect to foreign agent as network roams

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Multi-hop Wireless

• May need to traverse multiple links to reach destination

• Mobility causes route changes

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Mobile Ad Hoc Networks

Routers and hosts move, so topology is dynamic and must support multi-hop paths

Hosts and routers move relative to each other, as well as relative to the Internet, in mobile ad hoc networks• Multiple hops over wireless links

• No (or little) fixed infrastructure

Connection point to the Internet may also change Incorporate routing functionality into mobile nodes --

more than just mobile hosts

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MANET vs Infrastructure Networks

Wired networks

• Symmetric links, usually with respect to both capacity and quality

• Limited planned redundancy for reliability and load sharing

• Planned links, typically of uniformly high quality, in a fixed topology

Ad hoc wireless networks

• Asymmetric links

• High degree of random redundancy in connectivity between wireless

nodes

• Unplanned, dynamic links with quality that may vary greatly due to

interference, signal, etc

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MANET vs Infrastructure Networks

Asymmetric links

• Asymmetric optimal routes

• Asymmetric delays

High level of connectivity

Links of varying strength

Dynamic topology as nodes move

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MANET vs Infrastructure Networks

Infrastructure-based wireless network

• Access points or base stations define cells or service areas

• Routing is relatively simple -- there is just a single hop from the

access point to the wireless node

Ad hoc wireless network

• There is no pre-defined or static network structure imposed by

infrastructure

• Wireless nodes are not necessarily all adjacent, so a node may need to

forward data for other nodes (i.e., to participate in routing)

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Why Ad Hoc Networks ?

Setting up of fixed access points and backbone infrastructure is not always viable• Infrastructure may not be present in a disaster area or war zone

• Infrastructure may not be practical for short-range radios; Bluetooth (range ~ 10m)

Ad hoc networks:• Do not need backbone infrastructure support

• Are easy to deploy

• Self-configure

• Useful when infrastructure is absent, destroyed or impractical

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How a Mobile Ad hoc NETwork works

User

Radio signals

Connection

Network cloud

Shared data

Physical wireless local connectivity

Two-way communicationMulti-hop

communication

Resource sharing

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A MANET

No need for fixed infrastructure Each node equipped with one

or more radios Radios can be heterogeneous Each node free to move about

while communicating Paths between nodes can be

multi-hop

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Mobile Ad Hoc Networks (MANETs)

Host movement frequent

Topology change frequent

No cellular infrastructure, only multi-hop wireless links

Data must be routed via intermediate nodes

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Mobile Ad Hoc Networks

• Rapidly deployable, without prior planning or any existing infrastructure

• Routers are free to move randomly, so topology may change rapidly and unpredictably

• Low speed, long range networks for military use

• But high speed, short range networks for commercial applications

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Advent of Ad hoc Wireless Networks

Principle behind ad hoc networking is multi-hop relaying 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

Relaying nodes are generally mobile, and communication needs are primarily between nodes within the same network

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Ad Hoc vs Cellular Wireless Nets

Multihop (Ad Hoc)

Single Hop (Cellular)

Base BaseBaseBase

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Comparisons between Cellular and Ad Hoc Wireless Networks (I)

Cellular Networks Ad Hoc Wireless Networks

Fixed infrastructure-based Infrastructureless

Guaranteed bandwidth (designed for voice traffic)

Shared radio channel (more suitable for best-effort data traffic)

Centralized routing Distributed routing

Circuit-switched (evolving toward packet switching)

Packet-switched (evolving toward emulation of circuit switching)

Seamless connectivity (low call drops during handoffs)

Frequent path breaks due to mobility

High cost and time of deployment Quick and cost-effective deployment

Reuse of frequency spectrum through geographical channel reuse

Dynamic frequency reuse based on carrier sense mechanism

Easier to employ bandwidth reservation

Bandwidth reservation requires complex medium access control protocols 16

Comparisons between Cellular and Ad Hoc Wireless Networks (II)

Cellular Networks Ad Hoc Wireless Networks

Application domains include mainly civilian and commercial sectors

Application domains include battlefields, emergency search and rescue operations, and collaborative computing

High cost of network maintenance (backup power source, staffing, etc.)

Self-organization and maintenance properties are built into the network

Mobile hosts are of relatively low complexity

Mobile hosts require more intelligence (should have a transceiver as well as routing/switching capability)

Major goals of routing and call admission are to maximize the call acceptance ratio and minimize the call drop ratio

Main aim of routing is to find paths with minimum overhead and also quick reconfiguration of broken paths

Widely deployed and currently in the third generation of evolution

Several issues are to be addressed for successful commercial deployment even though widespread use exists in defense

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Applications of Ad hoc Wireless Networks

Military applications• Ad hoc wireless networks is useful in establishing communication in a

battle field

Collaborative and Distributed Computing• A group of people in a conference can share data in ad hoc networks

• Streaming of multimedia objects among the participating nodes

Emergency Operations• Ad hoc wireless networks are useful in emergency operations such as search

and rescue, and crowd control

A Wireless Mesh Network is a mesh network that is built upon wireless communications and allows for continuous connections and reconfiguration around blocked paths by "hopping" from node to node until a connection can be established

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Wireless Mesh Networks Wireless mesh networks are ad hoc networks that are formed to

provide an alternate communication infrastructure for mobile or fixed nodes/users, without the spectrum reuse constraints and the requirements of network planning of cellular networks

Provides many alternate paths for a data transfer session between a source and destination resulting in quick reconfiguration of the path when the existing path fails due to node failures

Provides the most economical data transfer capability coupled with the freedom of mobility

In a wireless mesh network, multiple nodes cooperate to relay a message to its destination

Mesh topology enhances the overall reliability of the network, which is particularly important when operating in harsh industrial environments 19

Wireless Mesh Networks

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Wireless Mesh Networks Investment required in wireless mesh networks is much less than

in the cellular network counterparts Such networks are formed by placing wireless relaying equipment

spread across the area to be covered by the network The possible deployment scenarios include:

• Residential zones (where broadband Internet connectivity is required)

• Highways (where a communication facility for moving automobiles is required)

• Business zones (where an alternate communication system to cellular networks is required)

• Important civilian regions (where a high degree of service availability is required)

• University campuses (where inexpensive campus-wide network coverage can be provided)

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Wireless Mesh Network Covering a Highway

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Wireless Mesh Networks

Wireless mesh networks should be capable of self-organization and maintenance

Advantages• High data rate

• Quick and low cost of deployment

• Enhanced services

• High scalability

• Easy extendibility

• High availability

• Low cost per bit

• High availability

• Low cost per bit

It operates at 2.4 GHz or 5 GHz Data rates of 2 Mbps to 60 Mbps can be supported 23

Properties of MANETs

Mobile nodes include hosts and routers• Routing and other core network functions are implemented in a fixed network

in Mobile IP

May have gateways to fixed network, but mobile ad hoc

network is a “stub network”• Carry only traffic to or from nodes in the MANET

Scalability is likely an issue (as it is with almost any

network)• 10’s to 100’s of nodes per routing zone

Dynamic, random, multi-hop topologies• Nodes are free to move arbitrarily

• Links may be bidirectional or unidirectional24

Properties of MANETs

Operation is bandwidth constrained

• Wireless links typically offer lower capacity than wired counterparts

• Congestion is likely to be a normal situation

Operation is energy constrained

• Some or all nodes may be battery powered

• Energy conservation may be an important design criteria

There is limited physical security

• Increased possibility of eavesdropping, spoofing, and denial-of-

security attacks

• Distributed nature provides robustness against single-point failures

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Limitations of the Wireless Network

Packet loss due to transmission errors Variable capacity links Frequent disconnections/partitions Limited communication bandwidth Broadcast nature of the communications Heterogeneity of fragmented networks

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Limitations Imposed by Mobility

Lack of mobility awareness by system/applications Dynamically changing topologies/routes Route breakages

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Limitations of the Mobile Computer

Short battery lifetime

Limited capacities

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Issues in Ad Hoc Wireless Network Medium access scheme Routing Multicasting Transport layer protocol Pricing QoS provisioning Self-organization Security Energy management Addressing and service discovery Scalability Deployment considerations

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Medium Access Scheme• Distributed operation

• Required because of the absence of any central authority

• Minimum control overhead

• In case of polling-based MAC protocols, partial coordination is required

• Synchronization

• Mandatory for TDMA-based systems for management of transmission and reception slots

• Effects the usage of BW, battery power

• Control packets can also increase collisions

• Hidden terminals

• Hidden from sender but reachable by the recevier

• Increase the collision at the receiver

• Can significantly reduce the throughput of the MAC protocol

• Should be able to alleviate the effects of hidden nodes

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Medium Access Scheme• Exposed terminals

• Transmission range of the sender and prevented from making a transmission• Exposed nodes should be allowed to transmit in a controlled fashion without

causing collision to the on-going data transfer

• Throughput• Needs to be maximized by minimizing the occurrence of collisions, channel

utilization, and minimizing control overhead

• Access delay• Refers to the average delay that any packet experiences to get transmitted• Should be minimized

• Fairness• Refers to provide an equal share to all competing nodes• Can be either node based or flow based

• Real-time traffic support is explicitly required for voice, video, and real-time data

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Medium Access Scheme• Resource reservation is required for QoS provisioning (BW, processing

space, and processing power etc)

• Ability to measure resource availability handles the availability resources at different nodes

• Capability for power control• Transmission power control reduces the energy consumption at the nodes, causes a

decrease in interference at neighboring nodes, and increases in frequency reuse

• Adaptive rate control• Refers to the variation in the data bit rate

• Use high data rate when sender and receiver are nearby and adaptively reduce the data rate as they move away from each other

• Use of directional antennas has advantages including increased spectrum reuse, reduced interference, and reduced power consumption

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Routing• Mobility – frequent path breaks, packet collisions, transient loops, stale routing

information, and difficulty is resource reservation

• Bandwidth constraint – generally divided by the total number of nodes

• Error-prone and shared channel

• Wireless channel (10-5 to 10-3), wired channel (10-12 to 10-9)

• Consideration of the state of wireless link, signal-to-noise ratio, and path loss can improve the efficiency

• Location-dependent contention

• Load on the wireless channel varies with the number of nodes present in a given geographical region

• Makes the contention for the channel high, increases collision and waste the BW

• A routing protocols should be able to distribute the network load uniformly across the network so that the formation of regions where channel contentions is high can be avoided

• Other resource constraints such as computing power, battery power, buffer storage

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Routing• Minimum route acquisition delay – the delay may vary with the size of

network and the network load

• Quick route reconfiguration – require to handle path breaks and subsequent packet losses

• Loop-free routing – form due to random movements of nodes

• Distributed routing approach – centralized routing approaches may consume a large amount of BW

• Minimum control overhead – control packets consumes BW and can cause collisions with data packets

• Scalability – perform efficiently with large number of nodes

• Provisioning of QoS – BW, delay, jitter, packet delivery ratio, throughput

• Support for time-sensitive traffic

• Security and privacy34

Transport Layer Protocols The 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

• Congestion control

Connectionless transport layer protocol (UDP), unaware of high contention, increases the load in the network

Major performance degradation of TCP arises due to frequent path breaks, presence of stale routing information, high channel rate, and frequent network partitions

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Quality of Service Provisioning In ad hoc networks, the boundary between the service provides

(network) and the user (host) is blurred, thus making it essential to have better coordination among the hosts to achieve QoS

Can be per flow, per link, or per node basis QoS parameters based on different applications

• Multimedia traffic, defense application etc

QoS-aware routing uses QoS parameters to find a path – throughput, packet delivery ratio, reliability, delay jitter, packet loss rate, bit error rate, path loss etc

QoS framework is a complete system that aims at providing the promised services to each users

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Self-Organization

Ability to organize and maintain the network by itself• Neighbor discovery – through beacon, or promiscuous

snooping on the channel for detecting activities of neighbors

• Topology organization – every node gathers information about the entire network or part of the network

• Topology reorganization

• Update topology information

• May be periodic or aperiodic exchange of topology update messages

• Must be quick and efficient in a way transparent to the user and application

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Addressing and Service Discovery

Globally unique address is required for a node in the connected part of the ad hoc wireless network• Auto-configuration of addresses

• Detection of duplicate addresses

Nodes in the network should be able to locate services that other nodes provide

Efficient service advertisement mechanisms are necessary Service discovery protocols are normally separate from

the network routing protocols

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Energy Management Process of managing the sources and consumers of energy in a node or in

a network as a whole for enhancing the lifetime of the network

Transmission power management • Determined by state of operation (transmit, receive, sleep mode)

• Reachability requirement of the network

• Application layer – power consumption varies with application

• Transport layer – reducing the number of retransmissions, recognizing and handling the reasons behind the packet loss locally

• Network layer – can consider battery life and relaying load of nodes while selecting a path so that the load can be balanced across the network, optimizing and reducing the size and frequency of control packets

• Data link layer – by designing a data link protocol that reduces unnecessary transmissions, by preventing collisions, by switching to standby mode or sleep mode whenever possible

• RF hardware design should ensure minimum power consumption 39

Energy Management Battery energy management

• Aimed at extending the battery life by taking advantage of its chemical properties, discharge pattern

• Pulsed discharge gives longer life then continuous discharge

• Operating temperature

Processor power management• CPU can be put into different power saving modes

Devices power management• Turning of services which is not required

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Scalability

Density of nodes Geographical region

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Deployment Considerations• Low cost of deployment

• Incremental deployment

• Short deployment time

• Reconfigurability

• Scenario of deployment

• Military deployment

• Emergency operations deployment

• Commercial wide-area deployment

• Home network deployment

• Required longevity of network

• Area of coverage

• Service availability

• Operational integration with other infrastructure

• Choice of protocols at different layers should be taken into consideration

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