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

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. 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. 한국해양과학기술진흥원
Outline
 Introduction
 Applications
 Challenges
 Medium Access Control
 Routing
 Proactive routing protocols
 Reactive routing protocols

4. Introduction

5. 한국해양과학기술진흥원
Wireless Networks
 Any type of computer network that utilizes some form of
wireless network connection
 Types
Cellular Network Wireless LAN

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. 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. 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. 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. Mobile Ad Hoc Network Applications

12. 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

13. 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

14. Medium Access Control

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. Routing Protocols

26. 한국해양과학기술진흥원
Routing
 Routing process of selecting paths in a network along
which to send network traffic
 Routing algorithms determine the specific choice of
route

27. 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

28. 한국해양과학기술진흥원
Routing in MANET
 Local Node Mobility
 Global Node Mobility
 Limited Resources
 Constrained Communication Environment
 Limited Power

29. 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

30. 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

31. 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

32. 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)

33. 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

34. 한국해양과학기술진흥원
Routing in MANET
 Hybrid routing protocols
 Cluster routing protocols
 Geographic routing protocols
 Which protocol to use?
 Depends on traffic and mobility patterns?

35. 한국해양과학기술진흥원
Vehicular Ad hoc Networks
 Used for communication among vehicles and between
vehicles and roadside equipment
 Vehicles tend to move in an organized fashion