[AD Hoc Networks]

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AD Hoc Networks

• by Farhad Rad

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Agenda

• Definition of an Ad Hoc Networks
• routing in Ad Hoc Networks
• IEEE 802.11
• security in Ad Hoc Networks
• Multicasting Protocols for Ad Hoc Networks

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Introductin

• MANET (Mobile Adhoc NETworks)
• An ad hoc network is a collection of wireless
  mobile hosts forming a temporary network without
  the aid of any established infrastructure or
  centralized administration
• Military Applications
• Rescue Operations
  Mobile Ad hoc Networks
• Virtual Classrooms

solution
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Routing in Ad Hoc Networks

• Challenges to Routing in MANETs
• Routing Protocols for MANETs
• Ad-hoc On Demand Distance Vector (AODV)
• Comparisons and Conclusions

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Challenges to Routing in MANETs

• Lack of a fixed infrastructure
• Each node in the network must route messages
  towards their destination
• Nodes operate on battery power (Routing of
  messages may cause faster battery consumption,
  leading to node going offline)
• Nodes are constantly moving, leaving, or joining

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Routing Protocols for Ad-hoc Networks

• Destination-Sequenced Distance Vector Protocol
  (DSDV)
• Dynamic Source Routing (DSR)
• Ad-hoc On Demand Distance Vector (AODV)

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Ad-hoc On Demand Distance Vector (AODV)

• Routes are discovered on demand
• AODV is capable of both unicast and multicast
  routing
• AODV uses sequence numbers to ensure the
  freshness of routes
• It is loop-free
• scales to large numbers of mobile nodes
• AODV maintains routes for as long as the route
  is active.

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Route Discovery

• Node can initiate route discovery by
  broadcasting a Route Request (RREQ) message
• RREQ contains
• Source and Destination addresses
• Sequence number of source
• Last known sequence number of destination
• Broadcast ID (incremented with each RREQ)
• Number of hops

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AODV introduction

• Generating Route Request
• Processing and Forwarding Route Requests
• Generating Route Replies
• Receiving and Forward Router Replies

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Aodv Algorithm
Source A Dest. I
A
B
C

• A broadcast Route Request packet.
• If the receiving node has a route to the
  destination
• Set up reverse path entry as before
• Sends back a Route Reply message
  (RREP) to the source containing
• Last known sequence number of destination
• Number of hops to destination

D
E
F
G
H
I
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Comparisons

• Percentage of Packets Received Correctly

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Routing Overhead in Packets
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Conclusions

• Routing protocols for MANETs will become
  important due to of wireless devices
• Different routing protocols for different needs
• DSDV does not perform well with highly mobile
  nodes
• DSR and AODV seem to give similar results

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An overview of IEEE 802.11

• 802.11 refers to a set of WLANs that was
  approved by IEEE in 1997.
• Specifies the lowest two layers of the OSI model

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IEEE 802.11

• IEEE 802.11 was first designed for wireless
  fixed networks
• Many problems occur when building ad hoc
  Networks with the IEEE 802.11 standard as the
  lowest two layers
• Until now, IEEE 802.11 doesnt function well in
  wireless ad hoc netwroks

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Modes of operation

• Infrastructure-based
• The main most mature technology for WLANs
• Most commonly used to construct Wi-Fi hotspots
• Costly for dynamic environments

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Modes of operation

• Infrastructureless-based
• Also called Ad Hoc mode
• Stations form an Independent Basic Service Set
  (IBSS)
• Any stations within the same transmission range
  can communicate

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IEEE 802.11 Architecture

• Physical Layer
• infrared, FHSS, or DSSS in 1997
• OFDM and HR-DSSS were added in 1999
• MAC Layer
• Distributed Coordination Function (DCF)
• Provides the basic access method to the 802.11
  MAC protocol
• Uses random backoff time following a busy signal
• Based on CSMA/CA
• Point Coordination Function (PCF)
• Only used in infrastructure-based

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Common Problems in Wireless Ad Hoc Networks

• The hidden-station problem
  The exposed-station problem
• Collision occurs Degradation in throughput

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Solution for the hidden and exposed station
problems

• Hidden Station Problem Solution
• Extension for the DCF protocol by a virtual
  carrier sensing mechanism.
• Adding two control frames Ready-To-Send (RTS),
  Clear-To-Send (CTS)
• Sending station transmits RTS to receiver and
  waits for CTS
• Receiver will not send CTS if receiving from
  another station
• Avoiding collision
• Exposed Station Problem Solution
• A node can identify itself as an exposed node if
  it hears an RTS frame but not a CTS frame from
  the other transmitting node. Therefore, it
  concludes that it can have a simultaneous
  transmission
• Avoiding the reduction in throughput

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Exposed node problem

• Add info of your choice here
• Add text, graphic or photo at left

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Mobility Problem

• In ad hoc networks nodes can change their
  positions anytime
• TCP protocol cannot distinguish between
  congestion on one hand and route failure or
  packet loss due to transmission on the other hand
  
• This results in reduction in the performance of
  the network because of the slow start mechanism
  of the TCP protocol
• Mobility Problem Solutions
• Route Failure and Rout Re-establishment
  notifications
• Explicit Link Failure Notification (ELFN) signal
• Ad hoc TCP (ATCP) by adding a thin layer
  between TCP and IP layers

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MAC Protocol TCP

• IEEE 802.11 was designed for wireless
  infrastructure LANs not for multi-hop ad hoc
  networks
• 802.11 doesnt function well ad hoc networks
  because of the TCP protocol mechanisms and the
  difference among the transmission, sensing and
  interference ranges
• Three major problems will occur
• Instability problem
• In-compatibility problem
• One hop Un-fairness problem

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Instability Problem

• If station 1 is sending to station 5, the
  throughput can drop down to zero in some
  scenarios because of the following
• The hidden and exposed station problems that may
  prevent station 2 from receiving RTS or sending
  CTS to station 1
• The random backoff time
• High window size that the TCP uses
• Solutions for the Instability Problem
• Decreasing the maximum window size of the TCP
  layer
• making the interfering range the same as the
  communication range

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In-Compatibility Problem

• This problem is defined as two simultaneous TCP
  traffics cannot coexist in the network. Once one
  session develops, the other one is shut down. The
  overturn can happen at any time randomly.
• the main causes of this problem are the hidden
  station problem, the exposed node problem and the
  exponential back-off scheme in the MAC layer.
• Solutions for the In-Compatibility Problem
• Changing the back-off policy by penalizing
  stations that transmit too much data, so the
  other stations can still use the media.
• Adjusting the interfering and the sensing range

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One-hop unfairness problem

• If there are two simultaneous TCP connections
  one is a single-hop connection and the other is a
  multi-hop connection, the single-hop connection
  will be activated even if the multi-hop
  connection started first.
• Causes are hidden station problem, the exposed
  node problem and the exponential back-off scheme

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Unicast and Multicast

• Unicast
• With n receivers, sender must replicate the
  stream n times

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Multicast

• Source transmits one stream of data for n
  receivers
• Replication happens inside routers and switches

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Multicast Routing Protocols for Ad hoc Networks

• Tree Based Protocols
• Ad hoc Multicast Routing (AMRoute)
• Ad hoc Multicast Routing Protocol utilizing
  Increasing id numberS (AMRIS)
• Mesh Based Protocols
• On-Demand Multicast Routing Protocol (ODMRP)
• Core-Assisted Mesh Protocol (CAMP)

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Multicast Routing Protocols
protocol AMRoute ODMRP AMRIS CAMP
Configuration Tree Mesh Tree Mesh
Loop - Free No Yes Yes Yes
Dependency on unicast Protocol Yes No No Yes
Periodic Messaging Yes Yes Yes Yes
Control Packet Flood Yes Yes Yes No
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On-Demand Multicast Routing Protocol (ODMRP)
Source broadcasts periodically Join Request.
R
R
Nodes receiving the request, save upstream node
id and rebroadcast the message.
S
R
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ODMRP

• When a receiver gets the request, it updates its
  member table and return message Join Table to its
  neighbors.

• Nodes that are on the path from receiver to
  source, become part of the Forwarding Group

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ODMRP

• If source wants to leave the group, simply stop
  sending JOIN REQUEST packets
• If a node wants to leave the group it stops
  sending JOIN TABLE packets for that group

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Simulation

• Metrics
• Packet Delivery Ratio The ratio of the number of
  data packets actually delivered to the
  destinations versus the number of data packets
  supposed to be received.
• Number of control packets transmitted per data
  packet delivered The ratio of control packets
  transmitted to data packets delivered gives a
  measure of efficient utilization of control
  packets in delivering data.  
• Number of data packets transmitted per data
  packet delivered
• Number of control and data packets transmitted
  per data packet delivered

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Simulation Model

• network of 50 mobile hosts
• Radio propagation range for each node was 250
  meters and channel capacity was 2 Mbits/sec.
• There are 21 nodes in the multicast group and 5
  nodes are chosen as sources
• .

ODMRP transmits more data packets than AMRIS
because it exploits multiple redundant routes for
data delivery
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Simulation Model

• AMRIS has the smallest number of packet
  transmissions because it uses a tree
• ODMRP transmits more data packets on redundant
  paths

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Network Traffic Load

• AMRIS is very sensitive to traffic load
• ODMRP is also affected at higher loads, but the
  packet loss rate is much lesser than AMRIS

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Challenges

• Security in Ad Hoc Networks
• Qos
  
• Routing Protocol
• Multicasting
• ..