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”
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
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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
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
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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
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
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
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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
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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
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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
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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