adhoc networks(unit iii)

7/28/2019 Adhoc Networks(UNIT III)

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AD HOC NETWORKS

by

Dr.P.SAMUNDISWARY

Dept. of Electronics Engineering

Pondicherry University


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DIFFERENCE BETWEEN CELLULAR

NETWORK ANDAD HOC NETWORK

CELLULAR NETWORK AD HOC NETWORK

Fixed, pre-located cell sites

and base stations

No fixed base stations, very

rapid deployment

Static backbone networktopology High dynamic networktopologies with multi-hop

Relatively benign

environment and stable

connectivity

Hostile environment(losses,

noise) and sporadic

connectivity

Detailed planning before

base stations can be

installed

Ad hoc network

automatically forms and

adapts to changes


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AD HOC NETWORK

Collection of mobile wireless nodes forming a

network without the aid of any infrastructure or

centralized administration

Nodes have limited transmission range

Nodes act as a routers


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CHARACTERISTICS OFAD HOC NETWORKS

Dynamic topologies

Limited channel bandwidth

Variable capacity links

Energy-constrained operation

Limited physical security


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APPLICATIONS

Military battlefield networks

Personal Area Networks (PAN)

Disaster and rescue operation

Peer to peer networks


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Challenges

Limited wireless transmission range

Broadcast nature of the wireless medium

Packet losses due to transmission errors

Mobility-induced route changes Mobility-induced packet losses

Battery constraints

Potentially frequent network partitions

Ease of snooping on wireless transmissions(security hazard)


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TYPES OF ROUTING PROTOCOLS


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DESTINATION-SEQUENCED DISTANCE-

VECTOR ROUTING (DSDV)

Table-driven algorithm based on the classical

Bellman-Ford routing mechanism

Improvementsfreedom of loops in routing

tables

Routing is achieved by using routing tables

maintained by each node

The main complexity in DSDV is in generating

and maintaining these routing tables


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CLUSTER-HEAD GATEWAYSWITCH

ROUTING (CGSR)

Uses DSDV as an underlying protocol andLeast Cluster Change (LCC) clusteringalgorithm

A cluster-head is able to control a group ofad-hoc hosts

Each node maintains 2 tables:1. A cluster member table, containing the cluster

head for each destination node2. A DV-routing table, containing the next hop to

the destination

The routing principle: Lookup of the cluster-head of the destination

node Lookup of next hop Packet send to destination Destination: cluster-head delivers packet


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CGSR


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CGSR

Drawbacks:

Too frequent cluster head selection can be an

overhead and cluster nodes and Gateway can be

a bottleneck


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WIRELESS ROUTING PROTOCOL (WRP)

Table-based protocol with the goal of maintainingrouting information among all nodes in the network

Each node is responsible for four tables: Distance table

Routing table

Link-cost table Message retransmission list (MRL) table

Link exchanges are propagated using updatemessages sent between neighboring nodes

Hello messages are periodically exchanged between

neighbors This protocol avoids count-to-infinity problem by

forcing each node to check predecessor information

Drawbacks: 4 tables requires a large amount ofmemory and periodic hello message consumes powerand bandwidth


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Source-Initiated On-Demand

Routing Protocols


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DYNAMIC SOURCE ROUTING (DSR)

Based on the concept of source routing Mobile nodes are required to maintain route caches

that contain the source routes of which the mobile isaware

2 major phases: Route discovery uses route request and route reply

packets Route maintenance uses route error packets and

acknowledgments

Advantages: No periodic hello message and fastrecovery - cache can store multiple paths to adestination

Drawbacks: the packets m ay be forwarded along stale cached routes. It has a

major scalability problem due to the nature of sourcerouting. Same as AODV, nodes use the routing cachesto reply to route queries


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ROUTE DISCOVERY WITH ROUTE REQUEST

N2

N1

N5

N3

N4

N6

N8

N7

N1-N2 N1-N2-N5

Destination

N1-N3-N4-N7

N1-N3-N4-N6

N1-N3-N4

SourceN1-N3-N4

N1

N1-N3-N4

N1-N3

N1


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ROUTE DISCOVERY WITH ROUTE REPLY

N2

N1

N5

N3

N4

N6

N8

N7

N1-N2-N5-N8

Destination

Source

N1-N2-N5-N8

N1-N2-N5-N8


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AD-HOC ON-DEMAND DISTANCEVECTOR

ROUTING (AODV)

Builds on DSDV algorithm and the improvementis on minimising the number of requiredbroadcasts by creating routes on an on-demandbasis (not maintaining a complete list of routes)

Broadcast is used for route requestAdvantages: uses bandwidth efficiently, is

responsive to changes in topology, is scalable andensures loop free routing

Drawbacks: nodes use the routing caches to replyto route queries. Result: uncontrolled repliesand repetitive updates in hosts caches yet earlyqueries cannot stop the propagation of all querymessages which are flooded all over the network


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TEMPORALLY-ORDERED ROUTING

ALGORITHM (TORA)

Highly adaptive, loop-free, distributed routingalgorithm based on the concept of link reversal

Proposed to operate in a highly dynamic mobilenetworking environment

It is source initiated and provides multiple routesfor any desired source/ destination pair

This algorithm requires the need forsynchronized clocks

3 basic functions: Route creation

Route maintenance

Route erasure


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TEMPORALLY-ORDERED ROUTING

ALGORITHM (TORA)

Advantages: provides loop free paths at allinstants and multiple routes so that if onepath is not available, other is readilyavailable. It establishes routes quickly sothat they may be used before the topologychanges.

Drawbacks: exhibits instability behaviorsimilar to "count-to-infinity" problem indistance vector routing protocols.


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ASSOCIATIVITY-BASED ROUTING (ABR)

Free from loops, deadlock, and packet duplicates,

and defines a new routing metric for ad-hoc

mobile networks

Each node generates periodic beacons (hello

messages) to signify its existence to the neighbors

These beacons are used to update the

associativity table of each node

With the temporal stability and the associativity

table the nodes are able to classify each neighbor

link as stable or unstable


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ABR


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ABR

Advantages: free from duplicate packets

Drawbacks: Short beaconing interval to reflect

association degree precisely


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SIGNAL STABILITYROUTING (SSR)

descendent of ABR and ABR predates SSR

it selects routes based on signal strength between

nodes and on a nodes location stability thus

offers little novelty

SSR route selection criteria has effect of choosing

routes that have stronger connectivity and it can

be divided into:

Dynamic Routing Protocol (DRP) or

Static Routing Protocol (SRP)


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SBR

DRP is responsible for maintenance of signalstability table and routing table

SRP processes packets by passing the packets upthe stack if it is the intended receiver and

forwarding the packet if it is notAdvantages: to select strong connection leads to

fewer route reconstruction

Drawbacks: long delay since intermediate nodescant answer the path (unlike AODV, DSR)


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CLASSIFICATION OF MAC

PROTOCOLS

Contention free MAC

TDMA,FDMA,CDMA divides the channel by

time, frequency and code.

o Contention based MAC

Nodes compete to access the shared medium

(channel) through random access


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CLASSIFICATION


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Other criteria for classification

Power-aware

Directional or omnidirectional antennas

QoS-aware


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NON QOS MAC PROTOCOLS

General MAC protocols MACA (Multiple Access Collision Avoidance)

IEEE 802.11

MACA-BI

Power aware MAC protocols PAMAS (Power aware medium access control with

signaling)

PCM (Power control medium access control)

PCMA (Power controlled multiple access)

Multiple channel protocols

DBMA (Dual busy tone multiple access),

Multichannel CSMA MAC protocol, etc.


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MACA

If node A wants to transmit to B, it first sends anRTS packet to B, indicating the length of the datatransmission to follow

B returns A a CTS packet with the expected

length of the transmissionA starts transmission when it receives CTS

RTS, CTS packets are much shorter than datapackets

A neighboring node overhearing an RTS defersits own transmission until the corresponding CTSwould have been finished

A node hearing the CTS defers for the expected

length of the data transmission


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Contd..

MACA can handle hidden node & exposed node

problems unsolved by CSMA

Hidden node: A sends to B; C sends to B -> Collision

at B -> In MACA, B sends CTS to A; C can hear

the CTS & defer its own transmission to B in MACA Exposed node: B sends to A; C unnecessarily delays

transmission to B -> In MACA, C can overhear Bs

RTS sent to A but C cannot hear CTS from A; So, C

transmits to B


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Contd..

Limitations

MACA does not provide ACK

RTS-CTS approach does not always solve the hidden

node problem

Example

A sends RTS to B

B sends CTS to A; At the same time, D sends

RTS to C

The CTS & RTS packets collide at C

A transmits data to B; D resends RTS to C;

C sends CTS to D

The data & CTS packets collide at B


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MACAW (MACA for Wireless)

RTS-CTS-DS-DATA-ACK

RTS from A to B

CTS from B to A

Data Sending (DS) from A to B

Data from A to B

ACK from B to A

Random wait after any successful/unsuccessful

transmission

Significantly higher throughput than MACA Does not completely solve hidden & exposed node

problems


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IEEE 802.11 MAC

Very popular wireless MAC protocol

Two modes: DCF (distributed coordination function) & PCF

(point coordination function)

DCF is based on CSMA/CA CSMA + MACA

RTS-CTS-DATA-ACK Physical carrier sensing + NAV (network allocation vector)

containing time value that indicates the duration up to

which the medium is expected to be busy due to

transmissions by other nodes

Every packet contains the duration info for the remainderof the message

Every node overhearing a packet continuously updates its

own NAV


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MACA-BI

Receiver initiated

Reduce number of control packets

RTR (Ready To Receive) & DATA rather than RTS-

CTS-DATA

Receiver needs a traffic prediction algorithm

Works well given predictable traffic patterns


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POWER AWARE MAC PROTOCOLS

Minimize expensive retransmissions due tocollisions

Transceivers should be kept in standby mode asmuch as possible

Switch to low power mode sufficient for thedestination to receive the packet

Two categories

Alternate between sleep and awake cycles

Vary transmission power


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PAMAS (Power aware medium accesscontrol with signaling)

RTS-CTS exchanges over a signaling channeling Data transmission over a separate data channel

Receiver sends out a busy tone, while receiving adata packet over the signaling channel

Nodes listen to the signaling channel todetermine when it is optimal to power downtransceivers

A node powers itself off if it has nothing totransmit and its neighbor is transmitting

A node powers off if at least one neighbor istransmitting and another is receiving

Use of ACK and transmission of multiple packetscan enhance performance

Radio transceiver turnaround time was not

considered


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PCM: POWER CONTROL MEDIUM

ACCESS CONTROL

Send RTS & CTS packets using max availablepower

Send DATA & ACK with the min power requiredto communicate between the sender and receiver

Based on the received signal strength of theRTS/CTS packet, adjust the power level forDATA transmission

Drawbacks

Requires rather accurate estimation of thereceived signal strength, which is hard inwireless communication

Difficult to implement frequent changes in thetransmission power level


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PCMA: POWER CONTROLLED MULTIPLE

ACCESS

Control transmit power of the sender The receiver is just able to receive the packet

Avoid interfering other neighboring nodes notinvolved in the packet exchange

Two channels: one for busy tone & another fordata

Request Power To Send (RTPS) & AcceptPower To Send (APTS) on the data

channelEvery receiver periodically sends out a

busy tone

Sender does carrier sensing


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QOS-AWARE MAC PROTOCOLS

For real-time (RT) applications, MAC protocolsshould support resource reservation for RT trafficin addition to addressing hidden/exposedterminal problems

Synchronous schemes: TDM variations requiringtime synchronization

Asynchronous approaches: No need for globaltime synchronization


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Categories of QoS-aware MAC protocols

Use shorter inter-frame spacing & smaller backoff contentionwindow for RT traffic

Extension of 802.11 DCF (e.g., 802.11e)

Black burst contention

RT nodes jam the channel in proportion to waiting time Observe the channel

Node with the longest jam transmits

Use reserved time slots to provide bounded & requiredbandwidth for RT traffic; Non-RT traffic is treated like

802.11


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SECURITY GOALS

Authentication

Confidentiality

Integrity

FreshnessAccess control

Privacy


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Security Threats in Ad hoc Networks

Passive Attacks

Eavedropping

Traffic analysis

i) traffic analysis at the physical layerii) traffic analysis at the MAC layer

iii) Traffic analysis by event correlation

Active Attacks Physicali) Tampering and ii) EMI

Masquerade, Message modification

Denial of service


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Difference between ad hoc networks and

sensor networks

Factor Ad hoc Network Sensor Network

Networking regime random oneto-one One to many

Many to many

Traffic Random, Multimedia Temporally and

spatially correlateddata

Mobility Mobile Generally fixed

Scalability Order of hundreds Order of thousands

Fault tolerance No critical point of

failure

High fault-tolerance

requirement

Operating

Environment

Day-to-day Hostile and harsh

adhoc networks(unit iii)

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