ADHOC NETWORKS

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ADHOC NETWORKS Evolution Computer
Networks Wireless Mobile
Networks Adhoc Networks Sensor
Networks
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Networks provide users access to information
communication Computer Networks Interconnected
collection of autonomous Computers 1. Wired
Network Physically connected through cables 2.
Wireless Mobile Networks Supported by a fixed
wired infrastructure
A single hop wireless radio communication to
access a base station that connects it to the
wired infrastructure 3. Adhoc Networks Does
not use any fixed infrastructure High
mobility. Therefore Mobile Adhoc Networks
(MANET)

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Network Classification by Size

• Classification of interconnected processors by
  scale.

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Client-Server Model

• The client-server model involves requests and
  replies.

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Peer-to-Peer Applications

• In a peer-to-peer system there are no fixed
  clients and servers.

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Wireless LANs

• (a) Wireless networking with a base station.
  (b) Ad hoc networking.

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• What is an ad hoc network
• A collection of nodes that can communicate with
  each other without the use of existing
  infrastructure
• Each node is a sender, a receiver, and a relay
• There are no special nodes
• No specialized routers
• Nodes can be static or mobile
• Can be thought of as peer-to-peer communication

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• Ad hoc networks are wireless networks
• Decentralized networks, in which each node acts
  as both an endpoint and a router for other nodes
• increase the redundancy of the network and open
  up the possibilities for network scaling as well
• self-organizing networks which automatically
  reconfigure without human intervention in the
  event of degraded or broken communication links
  between transceivers
• Also called Self Healing Networks

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• A node in an ad hoc network
• Two main components of a node
• From neighbors To neighbors
• Routing methods
• Each node maintains a table of routing
  information.
• Table entry destination X preferred neighbor.
• Data packet contains a destination ID in
  header.
• Packet received forward packet to the preferred
  neighbor. Use table entry for the destination.

Local Task Routing
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• Some features
• These networks may have bridges or gateways to
  other networks such as wired Ethernet or 802.11
• the strength of their architecture is that they
  do not require a base station or central point
  of control.
• Automated network analysis through link and route
  discovery and evaluation are the distinguishing
  features of self-healing network algorithms
• Through discovery, networks establish one or more
  routes between the originator and the recipient
  of a message.
• Through evaluation, networks detect route
  failures, trigger renewed discovery, and in some
  cases select the best route available for a
  message.

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• Mode of Operation
• Peer to peer multihop mobile wireless networks
• Information packets transmitted in a store and
  forward manner from a source to an arbitrary
  destination via intermediate nodes
• Topology information noted in each node (Mobile
  Host)
• All MHs need not be in the range of each other
  as MHs move topology changes.
• Symmetric Asymmetric links
• If MH1 is within radio range of MH3, then MH3
  is also within the Radio range of MH1
  Communication links are symmetric or
  bidirectional.
  Asymmetric links are unidirectionaL

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Mobile Adhoc Network (MANET) An autonomous system
of Mobile Hosts (MH also serving as routers)
connected by wireless links, forming a
communication network. Contrast In Cellular
networks, communication between two mobile nodes
completely rely on the wired backbone and the
fixed Base Stations In a MANET no such
infrastructure exists. Network topology may
dynamically change in an unpredictable manner
since the nodes are free to move
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• Example Ad hoc network
• Nodes have power range
• Communication happens between nodes within range

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What Is Different Here?

• Broadcasts of nodes can overlap -gt collision
• How do we handle this?
• A MAC layer protocol could be the answer
• If one node broadcasts, neighbors keep quiet
• Thus, nearby nodes compete for air time
• This is called contention

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The Hidden Terminal Effect

• hidden terminals A, C cannot hear each other
• obstacles, signal attenuation
• collisions at B
• goal avoid collisions at B
• CSMA/CA CSMA with Collision Avoidance

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(a)The hidden station problem. (b) The
exposed station problem.
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• The Hidden Terminal Problem
• Wireless stations have transmission ranges and
  not all stations are within radio range of each
  other.
• Simple CSMA will not work!
• C transmits to B.
• If A senses the channel, it will not hear Cs
  transmission and falsely conclude that A can
  begin a transmission to B.
• The Exposed Station Problem
• This is the inverse problem.
• B wants to send to C and listens to the channel.
• When B hears As transmission, B falsely assumes
  that it cannot send to C.

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The 802.11 MAC protocol with CA
RTS
RTS
A
B
D
CTS
C
CTS

• Introduced to reduce collisions
• Sender sends Request To Send (RTS) ask
  permission
• Case A Receiver gives permission Clear To Send
  (CTS)
• Sender sends Data
• Receiver sends ACK, if received correctly
• Case B Receiver does not respond
• Sender waits, times out, exponential back-off,
  and tries again

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Why is this necessary?

• A sends RTS, and B replies with a CTS
• C hears RTS and avoids sending anything
• C could have been near B (not shown here)
• D hears CTS so it does not send anything to B

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• ROUTING ISSUES IN ADHOC NETWORKS
• Nodes liable to move
• Highly dynamic network
• Rapid topological changes causing route failures
• Wireless channel acting as shared medium
• Available bandwidth per node is lower
• Nodes run on batteries which have limited energy
  supply
• Hence routing to be bandwidth efficient, having
  low overheads, energy efficient

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Proactive Routing Maintains routes between all
pairs of nodes regardless of whether all routes
are actually used. Two optimised variations of
these protocols are
(1) Distance Vector and (2) Link State In (1) a
node exchanges with its neighbours a vector
containing the current distance information to
all known destinations the distance information
propagates across the network and routes are
computed in a distributed manner at each node. In
(2) each node disseminates the status of each of
its outgoing links throughout the network in the
form of link state updates each node locally
computes routes using the complete topology
information
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• On Demand (Reactive) Routing Find and maintain
  only needed routes
• Attractive when traffic is sporadic, bursty and
  directed mostly towards a small subset of nodes.
• Queuing delays occur at the source as routes are
  created at session initiation when need arises
• Dynamic Source routing Sender knows the complete
  hop-by-hop route to the destination. These
  routes are stored in a route cache The data
  packet carries the source route in the packet
  header.
• Route Discovery By flooding the network with
  Route Request (Query) packets. Each node
  retransmits it unless it is the destination or it
  has a route to the destination in its route
  cache. Such a node replies to the request with a
  route reply packet routed back to the original
  source. The route of the reply packet is cached
  at the source for future use.

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• Hybrid Approaches Combination of Proactive and
  Reactive
• Augments a reactive protocol with some proactive
  functionality
• Each active destination periodically refresh
  routes
• Zone Routing Protocol is the example
• Defines zones for each node X which includes all
  nodes that are within a certain distance in hops,
  around the node X
• A proactive link state protocol is used to keep
  every node it reactively initiates a zone aware
  of the complete topology within its zone
• When X needs a route to Y not in its zone, it
  reactively initiates a route discovery

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• Common feature for the 3 routing schemes
• Nodes exchange routing messages and use this
  information to guide future routing decisions.
• An entirely different routing scheme is Location
  based routing
• Assumes each node knows its own location
• Nodes use GPS GLOBAL POSITIONING SYSTEM
• Every node learns location of its immediate
  neighbours by exchanging hello messages
• Location of potential destination nodes is
  assumed to be available via this location service
• Source sending a packet to destination uses ds
  location to find a neighbour closest in
  geographic distance to d forwards packet to
  that neighbour. That neighbour repeats the
  process until reaching d

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Flooding Source Node simply broadcasts data to
neighboring nodes Each Node hearing the broadcast
for the first time rebroadcasts it Broadcast
propagates until every node has heard the packet
and transmitted it once Delivers data to every
node in the connected component of the
network Suitable when node mobility is high
otherwise inefficient A node may receive the same
packet from several neighbours Other flooding
techniques also available
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• Wireless Routing Protocol (WRP)
• An important proactive routing approach
• Table driven protocol with the goal of
  maintaining routing information among all nodes
  in the network
• Each node responsible for maintaining four tables
  
• Distance table
• Routing table
• Link-cost table
• Message Retransmission List (MRL) table

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• Mode of Operation
• Peer to peer multihop mobile wireless networks
• Information packets transmitted in a store and
  forward manner from a source to an arbitrary
  destination via intermediate nodes
• Topology information noted in each node (Mobile
  Host)
• All MHs need not be in the range of each other
  as MHs move topology changes.
• Symmetric Asymmetric links
• If MH1 is within radio range of MH3, then MH3
  is also within the Radio range of MH1
  Communication links are symmetric or
  bidirectional.
  Asymmetric links are unidirectionaL

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• SOME ROUTING ATTACKS
• Wormhole attacks Two collaborating malicious
  nodes create a tunnel to falsify the hop count
  metric.
• Rushing attack targets routing protocols that
  choose routes on what message arrives first.
  Malicious route message will be rushed to block
  legitimate messages
• Sybil attacks one malicious node takes up
  multiple identities to project a false topology

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• Sensor networks
• Low-bit-rate, low-grade data that is aggregated
  and distilled into characterized by a large
  quantity of nodes per network, where each node
  produces useful information.
• Some possible applications
• Traffic monitoring Control
• Sensors placed at strategic locations in road
  junctions assess traffic intensity/congestion and
  convey information to controlling officers so as
  to select alternative route for incoming traffic
• Goods management applications
• commercial goods equipped with inexpensive
  wireless nodes that communicate information about
  their state and place of origin.

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WIRELESS SENSOR NETWORKS WSN is a special case of
Adhoc Networks with reduced or no
mobility Combine wireless communication minimal
computation facilities with sensing of physical
phenomenon which can be easily embedded in our
physical environment A sensor node consists of a
radio front end, a microcontroller, power supply
and the actual sensor all in a single device A
sensor consists of a transducer, an embedded
processor for local processing, small memory unit
for storage of data and a wireless transceiver to
transmit or receive data, all these run on the
power supplied by the attached battery Eg The
Mica Mote
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Some advantages of WSN Ease of deployment can
be put anywhere, anytime. Extended range One
large wired-sensor can be replaced by many
smaller wireless sensors for the same cost Fault
tolerant if one macro-sensor fails, monitoring
of its area is gone. Failure of one node in WSN
does not affect operation Mobility ease of
redeployment Some Challenges Limited energy
supply, limited computing power, limited
bandwidth of the wireless connecting links Energy
management technique an important issue
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Some numbers for 802.11

• Typical radius of power-range 250m
• Interference range 500m
• At 500m one can not hear, but they are bothered!
• RTS packet 40 bytes
• CTS and ACK 39 bytes
• MAC header is 47 bytes

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• Some issues for investigative research
• Scalability
• Quality of Service
• Security
• Interoperation with the Internet
• Energy conservation
• Node cooperation
• etc

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Performance Metrics(General Definitions)

• Utilization the percentage of time a device is
  busy servicing a customer.
• Throughput the number of jobs processed by the
  system per unit time.
• Response time the time required to receive a
  response to a request (round-trip time).
• Delay the time to traverse from one end to the
  other in a system.

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Network Performance Measures

• Latency usually implies the minimum possible
  delay. Latency assumes no queuing and no
  contention encountered along the path.
• Goodput measured at the receiver rate in bits
  per second of useful traffic received. Goodput
  excludes duplicate packets and packets dropped
  along the path.
• Fairness either Jains fairness or max-min
  fairness are used to measure fair treatment among
  competing flows.
• Quality of Service a QoS measure accounts for
  importance of specific metric to one type of
  application. e.g. jitter for streaming media

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Network Performance Measures

• Channel utilization the average fraction of
  time a channel is busy e.g. Util 0.8
• when overhead is taken into account (i.e.,
  excluded from useful bits, channel utilization is
  often referred to as channel efficiency
• Throughput bits/sec. successfully transmitted
• e.g. Tput 10 Mbps

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End-to-end packet delay

• End-to-end packet delay the time to deliver a
  packet from source to destination.
• most often we are interested in the packet delay
  within the communications subnet
• This delay is the sum of the delays on each
  subnet link traversed by the packet.
• Each link delay consists of four components

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Packet Delay

• The processing delay PROC between the time the
  packet is correctly received at the head node of
  the incoming link and the time the packet is
  assigned to an outgoing link queue for
  transmission.
• The queueing delay QD between the time the
  packet is assigned to a queue for transmission
  and the time it starts being transmitted. During
  this time, the packet waits while other packets
  in the transmission queue are transmitted.
• The transmission delay TRANS between the times
  that the first and last bits of the packet are
  transmitted.
• The propagation delay PROP between the time the
  last bit is transmitted at the head node of the
  link queue and the time the last bit is received
  at the next router. This is proportional to the
  physical distance between transmitter and
  receiver.

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End-to-End Packet DelayLink packet delay
PROC QD TRANS PROP.

• end-to-end packet delay sum of ALL link packet
  delays.
• Be Careful !! end-to-end can be defined
• either from Host-to-Host or only within the
  sub-network.

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