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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
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