Medium Access Control and WPAN Technologiescpj/204525/slides/09-mac.pdf · 2018-11-18 · Medium...
Transcript of Medium Access Control and WPAN Technologiescpj/204525/slides/09-mac.pdf · 2018-11-18 · Medium...
Medium Access Control and WPAN Technologies
Chaiporn Jaikaeo([email protected])
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
arbitration
◦ Link-layer acknowledgement for reliability
◦ Low-power listening (LPL)◦ I.e., preamble sampling
•Currently: Often considered as the default WSN MAC protocol
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B-MAC•B-MAC does not have
◦ Synchronization
◦ RTS/CTS
◦ Results in simpler, leaner implementation
◦ Clean and simple interface
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Clear Channel Assessment• "Carrier Sensing" in wireless networks
Thresholding CCA algorithm
Outlier detection CCA algorithm
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Overview•Principal options and difficulties
•Contention-based protocols
• Schedule-based protocols
•Wireless Personal Area Networks Technologies
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LEACH• Low-Energy Adaptive Clustering Hierarchy
•Assumptions◦ Dense network of nodes
◦ Direct communication with central sink
◦ Time synchronization
• Idea: Group nodes into “clusters”◦ Each cluster controlled by clusterhead
◦ About 5% of nodes become clusterhead (depends on scenario)
◦ Role of clusterhead is rotated
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LEACH Clusterhead• Each CH organizes
◦ CDMA code for its cluster
◦ TDMA schedule to be used within a cluster
• In steady state operation◦ CHs collect & aggregate data from all cluster members
◦ Report aggregated data to sink using CDMA
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LEACH rounds
Setup phase Steady-state phase
Fixed-length round
……….. ………..
Advertisement phase Cluster setup phase Broadcast schedule
Time slot
1
Time slot
2
Time slot
n
Time slot
1…..….. …..
Clusterheads
compete with
CSMA
Members
compete
with CSMASelf-election of
clusterheads
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TRAMA•Traffic Adaptive Medium Access Protocol
•Assume nodes are time synchronized
• Time divided into cycles, divided into◦ Random access period
◦ Scheduled access period
Random Access Period Scheduled-Access Period
time cycle
• Exchange and learn two-hop neighbors
• Exchange schedules
• Used by winning nodes to transmit data
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TRAMA – Adaptive Election •How to decide which slot (in scheduled access period) a
node can use? ◦ For node id x and time slot t, compute p = h (x t)
◦ h is a global hash function
◦ Compute p for next k time slots for itself and all two-hop neighbors
◦ Node uses those time slots for which it has the highest priority
t = 0 t = 1 t = 2 t=3 t = 4 t = 5
A 14 23 9 56 3 26
B 33 64 8 12 44 6
C 53 18 6 33 57 2
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Overview•Principal options and difficulties
•Contention-based protocols
• Schedule-based protocols
•Wireless Personal Area Networks Technologies
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IEEE 802.15.4• IEEE standard for low-rate WPAN (LR-WPAN) applications
◦ Low-to-medium bit rates
◦ Moderate delays without too strict requirements
◦ Low energy consumption
• Physical layer◦ 20 kbps over 1 channel @ 868-868.6 MHz
◦ 40 kbps over 10 channels @ 905 – 928 MHz
◦ 250 kbps over 16 channels @ 2.4 GHz
• MAC protocol◦ Single channel at any one time
◦ Combines contention-based and schedule-based schemes
◦ Asymmetric: nodes can assume different roles
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868MHz / 915MHz PHY
2.4 GHz
868.3 MHz
Channel 0 Channels 1-10
Channels 11-26
2.4835 GHz
928 MHz902 MHz
5 MHz
2 MHz
2.4 GHz PHY
802.15.4 PHY Overview•Operating frequency bands
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802.15.4 Device Classes• Full function device (FFD)
◦ Any topology
◦ Network coordinator capable
◦ Talks to any other device
•Reduced function device (RFD)◦ Limited to star topology
◦ Cannot become a network coordinator
◦ Talks only to a network coordinator
◦ Very simple implementation
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802.15.4 Network Topologies
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802.15.4 Beaconed Mode
• Superframe structure
•GTS assigned to devices upon request
Active period Inactive period
Contention
access
period
Guaranteed time
slots (GTS)Beacon
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802.15.4 GTS Data Transfer
•Device → coordinator◦ If having allocated GTS, wake up and
send
◦ Otherwise, send during CAP◦ Using slotted CSMA
•Coordinator → device◦ If having allocated GTS, wake up and
receive
◦ Otherwise, see picture
Coordinator Device
Beacon
Data
request
Acknowledgement
Data
Acknowledgement
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IEEE 802.15.4 Adopters• ZigBee
◦ Requires battery life of at least two years be certified
◦ Applications: Industrial control, embedded sensing, home automation
◦ ZigBee RF4CE (Radio Frequency for Consumer Electronics)
•Nest (acquired by Google)◦ Learning thermostats,
Smoke and CO alarms
◦ WiFi- and ZigBee-enabled
https://nest.com
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Bluetooth Smart• Formally Bluetooth Low Energy (BLE)
◦ Part of Bluetooth 4.0 Specification
•Based on Nokia's Wibree technology
• First smartphones to support → iPhone 4S◦ Now supported by most recent smartphones
http://redbearlab.com/blenano/
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Bluetooth: Classic vs. Smart
Source: Bluetooth SIG
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Bluetooth Compatibility
http://blog.laptopmag.com/just-what-is-bluetooth-4-0-anyway
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Bluetooth Smart: Device Roles•Central device
◦ Serves as a hub to one or more peripheral devices
◦ Two central devices cannot directly communicate
◦ Similar to IEEE 802.15.4's FFD
•Peripheral device◦ Must be connected to a central device
◦ Two peripheral devices cannot directly communicate
◦ Similar to IEEE 802.15.4's RFD
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ANT / ANT+ / NIKE+•Primarily used for fitness
monitoring devices
•ANT / ANT+◦ open access multicast wireless
sensor network
•NIKE+◦ Proprietary protocols on 2.4 GHz
band
http://developer.sonymobile.com
Nike.com
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WiFi/ZigBee/Bluetooth Coexistence
• They all employ 2.4 GHz spectrum
http://www.digikey.com/en/articles/techzone/2011/aug/comparing-low-power-wireless-technologies
WiFi vs. Zigbee WiFi vs. Bluetooth
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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
◦ B-MAC for WSN research not focusing on MAC