06-12-2012, 02:49 PM
Mobile Ad Hoc Networks
Adhoc.ppt (Size: 748.5 KB / Downloads: 22)
MANETs: Introduction
MANETs are mobile nodes that form a network in an ad hoc manner
The nodes intercommunicate using single or multi-hop wireless links
Each node in MANETs can operate as a host as well as a router
The topology, locations, connectivity, transmission quality are variable
MANET: Applications
Civil
Wireless LANs/WANs – mobile and stationary
Remote data collection and analysis
Taxi/Cabs, Buses scheduling
Disaster recovery
Communications over water using floats
Vehicular Ad Hoc Network
Defense
Battlefield communications and data transfer
Monitoring and Planning
Issues and Challenges
Operating in presence of unpredictable mobility and environmental changes
Operating in an error prone media
Low bandwidth channels
Low power devices with limited resources
Maintaining and retaining connectivity and states
Security: infrastructure and communication
MAC for MANET
Special requirements
Avoid interferences among simultaneous transmissions
Yet, enable as many non-interfering transmissions as possible
Fairness among transmissions
No centralized coordinators, should function in full distributed manner
No clock synchronization, asynchronous operations
MACs Suitable for MANET
MACA [Karn’90]
Propose to solve hidden terminal problem by RTS/CTS dialog
MACAW [Bharghavan’94]
Increase reliability by RTS/CTS/DATA/ACK dialog
IEEE 802.11 [IEEE 802.11WG]
Distributed and centralized MAC components
Distributed Coordination Function (DCF)
Point Coordination Function (PCF)
DCF suitable for multi-hop ad hoc networking
Also use RTS/CTS/DATA/ACK dialog
Disadvantage of IEEE 802.11 DCF
802.11 DCF not considered perfect for MANET
High power consumption
Hidden terminal problem not totally solved, exposed terminal problem not solved
Cause fairness problem among different transmitting nodes
Can only provide best-effort service
Active research area in MAC for MANET
Black Bursts
All nodes begin the priority contention phase together
Higher priority node transmit a longer burst than low priority node
After transmitting its burst, a node listens to the channel
If channel still busy, the node has lost contention to a higher priority node
PS Mode in WLANs
ATIM (Ad hoc Traffic Indication Map) window : short interval during which the PS hosts wake up periodically.
Assume that hosts are fully connected and synchronized.
In the beginning of each ATIM window, each mobile host will contend to send a beacon frame.
Successful beacon serve for synchronizing mobile host’s clock.
This beacon also inhibits other hosts from sending their beacon
To avoid collisions among beacons, use random back-off [0-2*CWmin –1]
MAC for Directional Antenna
Benefits of Directional Antenna
More spatial reuse
With omni-directional antenna, packets intended to one neighbor reaches all neighbors as well
Increase “range”, keeping transmit power constant
Reduce transmit power, keeping range comparable with omni mode
Reduces interference, potentially increasing spatial reuse
ORTS/OCTS – cont.
Protocol takes advantage of reduction in interference due to directional transmission/reception of DATA
All neighbors of sender/receiver defer transmission on receiving omni-directional RTS/CTS
Directional NAV
Physical carrier sensing still omni-directional
Virtual carrier sensing be directional – directional NAV
When RTS/CTS received from a particular direction, record the direction of arrival and duration of proposed transfer
Channel assumed to be busy in the direction from which RTS/CTS received