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Lecture 6
Mobile Ad-Hoc Networks: MAC
Reading: • “MAC Protocols for Ad Hoc Wireless Networks,” in Ad Hoc Wireless Networks: Architectures and Protocols, Chapter 6.
“A Survey, Classification and Comparative Analysis of Medium AccessControl Protocols for Ad Hoc Networks” by Raja Jurdak, Cristina VideiraLopes, and Pierre Baldi; University of California, Irvine,http://www.comsoc.org/livepubs/surveys/public/2004/jan/index.html - E. Royer, S.-J. Lee and C. Perkins, “The Effects of MAC Protocols on Ad hocNetwork Communication,” Proceedings of the IEEE Wireless Communications and Networking Conference (WCNC ’00), 2000.
MAC Protocols
Provide “rules” for channel access In MANETs, no centralized control Nodes independently determine access Local nodes elected to control channel access Nodes coordinate amongst themselves locally to determine channelaccess Goals for MAC protocols for MANETs High channel efficiency Low power Scalable Fair Support for prioritization Support for heterogeneous nodes Distributed operation QoS support Low control overhead
Cannel Separation and Access
Common channel vs. multiple channels
Typical use of channel
Data transmission RTS/CTS handshake Carrier sensing Periodic information exchange Reservations
Can use single channel for all packets
Send some packets (e.g., overhead) on one channeland other packets (e.g., data) on other(s)
Multiple channels allow more simultaneous users
Single Channel
All nodes share the medium for transmission of data and controlmessages Collisions and contention Handshake protocol ACKs Backoff protocol Examples CSMA MACA, MACAW, FAMA MACA-BI, RIMA-SP: receiver-initiated approaches MARCH: string of RTS-CTS-CTS-CTS… DPC/ALP: consecutive increase in RTS power PS-DCC: calculate sending probability based on current network load
Multiple Channels (cont.)
TDMA-based separation
Time segmented into frames, slots Nodes maintain synchronization Best with real-time, periodic data Examples FPRP, CATA, SRMA/PA: each slot has reservation andinformation subslots Markowski: traffic classes, window-splitting contentionresolution ADAPT: nodes “own” slots but others may use D-PRMA: continuous reservations for voice
Multiple Channels (cont.)
FDMA-based separation
Allows multiple nodes to transmit simultaneously Examples MCSMA: CSMA on each channel
CDMA-based separation
Simultaneous transmissions via code separation Examples MC-MAC: one common control signal code, N data codes IEEE 802.11: DSSS or FHSS channel separation RICH-DP: reserve hops in frequency hopping scheme, RTRscheme
Topology
Ad hoc network features
Mobility Heterogeneous node capabilities
Types of topologies
Centralized Base station used for network control and management Not useful for MANETs Flat: single and multi-hop Completely distributed approach Clustered Local cluster head elected and used for network control
Flat Topologies
Nodes make independent decisions to access thechannel
Local coordination via handshaking, carrier sensing
Single-hop: concerned only with immediate neighbors
Scalability issues CSMA, MACA, FAMA, MACA-BI, RIMA-SP, 802.11, etc.
Multi-hop: some notion of nodes outside localneighborhood
Can aid in scalability and power efficiency Most use multiple channels
PAMAS, DCA-PC, DCP/ALP
MARCH: directly uses notion of multi-hop path
Power Consumption
Radio operates in 3 modes: transmit, receive, standby Relative powers P TX
P RX
P
SB for long-range communication P TX ~ P RX
P SB for short-range, low power transceivers Different MAC protocols will be “low-power” depending onmodel of transceiver power dissipation Time delay and power dissipation switching between states
Reducing Energy Consumption
Reduce transmit power Use “just enough” to reach intended destination Examples GPC, DCAPC, DCA-PC, DPC/ALP Place nodes in standby mode as much as possible Nodes do not need to be on when not receiving data Requires nodes to know when they must listen to the channeland when they can “sleep” MAC protocols cannot use “promiscuous” mode to listen toother conversations Node must know when other nodes have data to tx to it Examples PAMAS, Bluetooth, HIPERLAN
Reducing Energy Consumption(cont.)
Collisions should be minimized
Retransmissions expend energy Introduce delays Reduce number of ACKs required Use contention for reservations and contention-free fordata transmission
Allocate contiguous slots for transmission/reception
Avoids power/time in switching from Tx to Rx
Have node buffer packets and transmit all packets atonce
Allows node to remain asleep for long time Trade-off in delay to receive packets and buffer size
Reducing Energy Consumption(cont.)
Make protocol decisions based on battery level
Choose cluster head to have plenty of energy Give nodes with low energy priority in contention Examples WCA, DPC/ALP, Jin GPC
Reduce control overhead
Need control to avoid collisions, but reduce as much aspossible Examples MARCH
Transmission Initiation
Sender-initiated
Most protocols follow this approach Sender attempts to access channel when it has data
Receiver-initiated
Receiver attempts to clear channel for transmissions Send request-to-transmit (RTR) to all neighbors orspecific node Polling for data Only efficient if large amount of traffic on network
Traffic Load and Scalability
Highly loaded networks
Receiver-initiated approaches Adjust sending probability based on network load Channel borrowing for non-uniform load TDMA approaches for periodic sources
Dense networks
Transmission power control Directional antennas
Voice and real-time traffic
Priorities Reservations
