Medium Access Control and WPAN Technologies

Chaiporn Jaikaeo(chaiporn.j@ku.ac.th)

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

2

Overview•Principal options and difficulties

•Contention-based protocols

• Schedule-based protocols

•Wireless Personal Area Networks Technologies

3

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!

4

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

5

Main OptionsWireless medium access

Centralized Distributed

Contention-based

Schedule-based

Fixedassignment

Demandassignment

Contention-based

Schedule-based

Fixedassignment

Demandassignment

6

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

7

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

8

Overview•Principal options and difficulties

•Contention-based protocols

• Schedule-based protocols

•Wireless Personal Area Networks Technologies

9

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

12

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