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Introduction to Mobile Ad hoc Networks

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Introduction
Applications
Challenges
Medium Access Control
Routing
- Proactive routing protocols
- Reactive routing protocols

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Introduction to Mobile Ad hoc Networks

  1. 1. 한국해양과학기술진흥원 Introduction to Mobile Ad hoc Networks 2013.10.6 Sayed Chhattan Shah, PhD Electronics and Telecommunications Research Institute, Korea https://sites.google.com/site/chhattanshah/
  2. 2. 한국해양과학기술진흥원 2 Acknowledgements David B. Johnson, Rice University, “Multihop Wireless Ad Hoc Networking: Current Challenges and Future Opportunities” Carlos Pomalaza-Ráez, University of Oulu, Finland, “MAC protocols for Mobile Ad hoc Network” Jeroen Hoebeke, Ingrid Moerman, Bart Dhoedtand Piet Demeester, Ghent University, “An Overview of Mobile Ad Hoc Networks: Applications and Challenges”
  3. 3. 한국해양과학기술진흥원 Outline  Introduction  Applications  Challenges  Medium Access Control  Routing  Proactive routing protocols  Reactive routing protocols
  4. 4. Introduction
  5. 5. 한국해양과학기술진흥원 Wireless Networks  Any type of computer network that utilizes some form of wireless network connection  Types Cellular Network Wireless LAN
  6. 6. 한국해양과학기술진흥원 Wireless Networks Types  Several types but all have similar architecture  Relies on a fixed infrastructure • Centralized base station or access point • All users within wireless range of it • Communicate with an access point or base station • Need planning, installation and management
  7. 7. 한국해양과학기술진흥원 Wireless Ad Hoc Network  A decentralized type of wireless networks  Ad hoc because it does not rely on a pre existing infrastructure such as routers or access points  Each node participates in routing by forwarding data of other nodes
  8. 8. 한국해양과학기술진흥원 Mobile Ad Hoc Network  A type of wireless ad hoc network  Infrastructureless network of mobile devices  Nodes are free to move independently in any direction  Links to other devices are changed frequently
  9. 9. 한국해양과학기술진흥원 Mobile Ad Hoc Network  Used when  Infrastructure is not available • Remote areas • Unplanned meetings • Disaster relief • Military operations  May not want to use the available infrastructure • Time or cost to access service  Dynamically extend coverage of infrastructure • Allow users to be further away from infrastructure
  10. 10. Mobile Ad Hoc Network Applications
  11. 11. 한국해양과학기술진흥원 Challenges  Nodes  Battery-powered  Limited processing power  Wireless Network  Packet loss due to transmission errors  Variable capacity links  Shared Bandwidth  Limited Bandwidth and High Latency
  12. 12. 한국해양과학기술진흥원 Challenges  Dynamic Network Environment  Nodes may move any time  May join and leave the network  Self-organized Network  No one is in charge  No one to provide standard service  Security
  13. 13. Medium Access Control
  14. 14. 한국해양과학기술진흥원 Medium Access Control In a given area, only one speaker is allowed to talk at a time, Else, listener would hear noise To avoid conflict  Wait for a coordinator to ask them to speak  Wait different time before talking
  15. 15. Classification of MAC Protocols Wireless MAC protocols Distributed MAC protocols Centralized MAC protocols Random access Random access Guaranteed access Hybrid access Since we are interested in Ad hoc networks we will focus our discussions on distributed type protocols
  16. 16. 한국해양과학기술진흥원 17 Medium access methods from fixed networks  Carrier Sense Multiple Access with Collision Detection • A collision is detected, whenever a transmitting node senses a different signal on the same channel it has transmitted • CSMA/CD widely used in 802.3 and Ethernet  Problems • Signal strength decreases proportional to square of the distance • CD is not possible in wireless channel due to half duplex operation • Sender would apply CS and CD, but collisions happen at receiver • Sender may not “hear” the collision, i.e., CD does not work • CS might not work, e.g. if a terminal is “hidden” Medium Access Control
  17. 17. Hidden and Exposed Node Problems A is transmitting to B C is out of range of A and is unaware of the transmission If C transmits to B it will cause a collision at B B is transmitting to A C wants to transmit to D C senses transmission & declines even if its transmission will not cause any collision at A Hidden node Exposed node
  18. 18. Capture Problem A B C D dA B dCB If A and C transmit simultaneously to B then the signal power of C, received at B, is higher than the one from A (because dCB < dAB) and there is a good probability that C’s signal can be correctly decoded in the presence of A’s transmission This capture of C’s signal can improve protocol performance, but it results in unfair sharing of the channel with preference given to nodes closer to the receiver. Wireless MAC protocols need to ensure fairness under such conditions
  19. 19. Multiple Access with Collision Avoidance  Uses signaling packets for collision avoidance  Request to send RTS  Sender request the right to send from a receiver with a short RTS packet before it sends a data packet  Clear to send CTS  Receiver grants the right to send as soon as it is ready to receive
  20. 20. Multiple Access with Collision Avoidance  Avoids the problem of hidden terminal  A and C want to send to B  A sends RTS  C waits after receiving CTS from B
  21. 21. Multiple Access with Collision Avoidance  Avoids the problem of exposed terminal  B wants to send to A, C to another terminal  Now C does not have to wait, as it cannot receive CTS from A
  22. 22. Multiple Access with Collision Avoidance  Reliability  Solution is to use acknowledgements  If fail to receive acknowledgements, retransmit  Power Saving  Solution is to turn of radio when not needed  IEEE 802.11 Wireless MAC  Distributed and centralized MAC components  Distributed Coordination Function (DCF)  Point Coordination Function (PCF)  DCF suitable for multi-hop ad hoc networking
  23. 23. 한국해양과학기술진흥원 Key Performance Metrics  Delay  Average time spent by a packet in the MAC queue  Fairness  A fair MAC protocol does not give preference to any single node when multiple nodes are trying to access the channel  Power Consumption  Important to design MAC protocols that have power saving features  QoS Support  Protocols need to treat packets from various applications based on their delay constraints • Common methods are the use of access priorities and scheduling
  24. 24. Routing Protocols
  25. 25. 한국해양과학기술진흥원 Routing  Routing process of selecting paths in a network along which to send network traffic  Routing algorithms determine the specific choice of route
  26. 26. 한국해양과학기술진흥원 Distance Vector and Link State Routing Protocol  Routing information is only exchanged between directly connected neighbors  Router knows from which neighbor a route was learned, but it does not know where that neighbor learned the route  Link state routing requires that all routers know about the paths reachable by all other routers in the network
  27. 27. 한국해양과학기술진흥원 Routing in MANET  Local Node Mobility  Global Node Mobility  Limited Resources  Constrained Communication Environment  Limited Power
  28. 28. 한국해양과학기술진흥원 Routing in MANET  Proactive or table-driven routing protocols  Maintain routes  Based on periodic updates  DSDV (Destination sequenced distance vector)  Advantages  Low routing latency  State information  Disadvantages  High routing overhead  Maintain routes which may never be used  Route repair depends on update frequency
  29. 29. 한국해양과학기술진흥원 Routing in MANET  Reactive or On-demand routing protocols  Determine route when needed  Source initiates route discovery  DSR (Dynamic Source Routing)  Advantages  No overhead from periodic update  Disadvantages  High routing latency
  30. 30. 한국해양과학기술진흥원 Proactive Routing Protocol  DSDV (Destination Sequenced Distance Vector)  Each node maintains a routing table which stores  Destination ID  Next hop  Cost metric  Sequence No to determine freshness of route  Each node periodically forwards routing table to neighbors  Each node increments and appends its sequence number when sending its local routing table  Each route is tagged with a sequence number  routes with greater sequence numbers are preferred
  31. 31. 한국해양과학기술진흥원 Proactive Routing Protocol  DSDV (Destination sequenced distance vector)  When X receives information from Y about a route to Z • Let destination sequence number for Z at X be S(X) • S(Y) is sent from Y  If S(X) > S(Y), then X ignores the routing information received from Y  If S(X) = S(Y), and cost of going through Y is smaller than the route known to X, then X sets Y as the next hop to Z  If S(X) < S(Y), then X sets Y as the next hop to Z, and S(X) is updated to equal S(Y)
  32. 32. 한국해양과학기술진흥원 Reactive Routing Protocol  AODV (On-Demand Distance Vector Routing)  Establishes a route to a destination only on demand  A node that needs a connection broadcast a route request RREQ packet  Nodes receiving RREQ packet update their information for the source node and set up backwards pointers to the source node in the route tables  A node receiving the RREQ may send a route reply (RREP)  If it is either the destination or it has a route to the destination  Otherwise, it rebroadcasts the RREQ  If a node receives a RREQ which it has already processed  it discards the RREQ and does not forwards  As the RREP propagates back to the source, nodes set up forward pointers to the destination  A route is considered active as long as there are data packets periodically travelling from the source to the destination along that path
  33. 33. 한국해양과학기술진흥원 Routing in MANET  Hybrid routing protocols  Cluster routing protocols  Geographic routing protocols  Which protocol to use?  Depends on traffic and mobility patterns?
  34. 34. 한국해양과학기술진흥원 Vehicular Ad hoc Networks  Used for communication among vehicles and between vehicles and roadside equipment  Vehicles tend to move in an organized fashion

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