Medium Access Control and WPAN Technologies 01204525 Wireless - - PowerPoint PPT Presentation

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Medium Access Control and WPAN Technologies 01204525 Wireless - - PowerPoint PPT Presentation

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Medium Access Control and WPAN Technologies 01204525 Wireless Sensor Networks and Internet of Things Chaiporn Ja Jaikaeo (c (chaiporn.j@ku.ac.th) Department of f Computer Engineering Kasetsart University Materials taken from lecture slides

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Medium Access Control and WPAN Technologies

Chaiporn Ja Jaikaeo (c (chaiporn.j@ku.ac.th) Department of f 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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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 Options

Wireless medium access Centralized Distributed Contention- based Schedule- based Fixed assignment Demand assignment Contention- based Schedule- based Fixed assignment Demand assignment

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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

NAV NAV

NAV → Network Allocation Vector (Virtual Carrier Sensing)

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Problems Solved?

  • RTS/CTS helps, but do not solve hidden/exposed terminal

problems

A B C D

RTS CTS Data

A B C D

RTS RTS CTS RTS RTS CTS CTS Data 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
  • wn 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 E E 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 CSMA Self-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 MHz 902 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