Medium Access Control and WPAN Technologies Chaiporn Jaikaeo ([email protected]) Department of Computer Engineering Kasetsart University Materials taken from lecture slides by Karl and Willig Cliparts taken from openclipart.org 01204525 Wireless Sensor Networks and Internet of Things Last updated: 2018-11-17
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Medium Access Control and WPAN Technologiescpj/204525/slides/09-mac.pdf · 2018-11-18 · Medium Access Control and WPAN Technologies Chaiporn Jaikaeo ([email protected]) Department
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Department of Computer EngineeringKasetsart University
Materials taken from lecture slides by Karl and WilligCliparts taken from openclipart.org
01204525 Wireless Sensor Networks and Internet of Things
Last updated: 2018-11-17
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Overview•Principal options and difficulties
•Contention-based protocols
• Schedule-based protocols
•Wireless Personal Area Networks Technologies
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Difficulties•Medium access in wireless networks is difficult, mainly
because of◦ Half-duplex communication
◦ High error rates
•Requirements◦ As usual: high throughput, low overhead, low error rates, …
◦ Additionally: energy-efficient, handle switched off devices!
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Energy-Efficient MAC: Requirements•Recall
◦ Transmissions are costly
◦ Receiving about as expensive as transmitting
◦ Idling can be cheaper but is still expensive
• Energy problems◦ Collisions
◦ Overhearing
◦ Idle listening
◦ Protocol overhead
•Always wanted: Low complexity solution
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Main OptionsWireless medium access
Centralized Distributed
Contention-based
Schedule-based
Fixedassignment
Demandassignment
Contention-based
Schedule-based
Fixedassignment
Demandassignment
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Centralized Medium Access•A central station controls when a node may access the
medium◦ E.g., Polling, computing TDMA schedules
◦ Advantage: Simple, efficient
•Not directly feasible for non-trivial wireless network sizes
•But: Can be quite useful when network is somehow divided into smaller groups
•Distributed approach still preferable
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Schedule- vs. Contention-Based• Schedule-based protocols
◦ FDMA, TDMA, CDMA◦ Schedule can be fixed or computed on demand
◦ Usually mixed
◦ Collisions, overhearing, idle listening no issues◦ Time synchronization needed
•Contention-based protocols◦ Hope: coordination overhead can be saved◦ Mechanisms to handle/reduce probability/impact of collisions
required ◦ Randomization used somehow
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Overview•Principal options and difficulties
•Contention-based protocols
• Schedule-based protocols
•Wireless Personal Area Networks Technologies
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A
Distributed, Contention-Based MAC
•Basic ideas◦ Receivers need to tell surrounding nodes to shut up
◦ Listen before talk (CSMA) ◦ Suffers from sender not knowing what is going on at receiver
B C D
Hidden terminal scenario:
Also: recall exposed terminal scenario
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How To Shut Up Senders • Inform potential interferers during reception
◦ Cannot use the same channel
◦ So use a different one◦ Busy tone protocol
• Inform potential interferers before reception◦ Can use same channel
◦ Receiver itself needs to be informed, by sender, about impending transmission
◦ Potential interferers need to be aware of such information, need to store it
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MACA
•Multiple Access with Collision Avoidance
• Sender B issues Request to Send (RTS)
• Receiver C agrees with Clear to Send (CTS)
• Potential interferers learns from RTS/CTS
• B sends, C acks
• Used in IEEE 802.11
A B C D
RTS
CTS
Data
Ack
NAV indicates
busy medium
NAV indicates
busy medium
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Virtual Carrier Sensing
RTS
CTS
Data
ACK
A B C D
NAVNAV
NAV→ Network Allocation Vector
(Virtual Carrier Sensing)
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Problems Solved?•RTS/CTS helps, but do not solve hidden/exposed terminal
problemsA B C D
RTS
CTS
Data
A B C D
RTS
RTS
CTS
RTS
RTSCTS
CTSData
Data
Ack
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MACA Problem: Idle listening•Need to sense carrier for RTS or CTS packets
◦ Simple sleeping will break the protocol
• IEEE 802.11 solution◦ Idea: Nodes that have data buffered for receivers send traffic
indicators at prearranged points in time◦ ATIM - Announcement Traffic Indication Message◦ Receivers need to wake up at these points, but can sleep otherwise
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Sensor-MAC (S-MAC)•MACA unsuitable if average data rate is low
◦ Most of the time, nothing happens
• Idea: Switch off, ensure that neighboring nodes turn on simultaneously to allow packet exchange◦ Need to also exchange
wakeup schedule between neighbors
◦ When awake, perform RTS/CTS
Wakeup period
Active period
Sleep period
For SYNCH For RTS For CTS
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Listen for SYNC
td
Schedule Assignment• Synchronizer
◦ Listen for a mount of time◦ If hear no SYNC, select its
own SYNC◦ Broadcasts its SYNC
immediately
• Follower◦ Listen for amount of time◦ Hear SYNC from A, follow
A’s SYNC◦ Rebroadcasts SYNC after
random delay td
Sleep
Listen
Go to sleep after time t
Sleep
Listen
Broadcasts
A
B
Broadcasts
Go to sleep after time t- td
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S-MAC Synchronized Islands•Nodes learn schedule from other nodes
• Some node might learn about two different schedules from different nodes◦ “Synchronized islands”
• To bridge this gap, it has to follow both schemes
Time
A A A A
C C C C
A
B B B B
D D D
A
C
B
D
E E E EE E E
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Preamble Sampling•Alternative option: Don’t try to explicitly synchronize
nodes◦ Have receiver sleep and only periodically sample the channel
•Use long preambles to ensure that receiver stays awake to catch actual packet ◦ Example: B-MAC, WiseMAC, LoRa
Check channel
Check channel
Check channel
Check channel
Start transmission:Long preamble Actual packet
Stay awake!
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B-MAC•Very simple MAC protocol
• Employs◦ Clear Channel Assessment (CCA) and backoffs for channel
Summary• Many different ideas exist for medium access control in MANET/WSN
• Comparing their performance and suitability is difficult
• Especially, clearly identifying interdependencies between MAC protocol and other layers/applications is difficult◦ Which is the best MAC for which application?
• Nonetheless, certain “common use cases” exist◦ IEEE 802.11 DCF for MANET
◦ IEEE 802.15.4 for some early “commercial” WSN variants