Network Kernel Architectures

and Implementation(01204423) )

Medium Access Controland WPAN Technologies

Chaiporn Jaikaeochaiporn.j@ku.ac.th

Department of Computer EngineeringKasetsart University

Materials taken from lecture slides by Karl and Willig

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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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Requirements for Energy-Efficient MAC Protocols 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

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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Contiki LPL and LPP Low-Power Listening (LPL)

Also known as ContikiMAC Similar to B-MAC, but allowing packet-

based MAC such as IEEE 802.15.4 Low-Power Probing (LPP)

Receivers periodically broadcast a probe

Sender listens for probes from receivers before transmitting

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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 26B 33 64 8 12 44 6C 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

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-enabledhttps://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