W.A Loviyantara

7/28/2019 W.A Loviyantara

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Introduction to IEEE 802.15.4LR-WPANs/ZigBee


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OutlineIntroduction

General Description

Network topologies

PHY Sublayer

MAC Sublayer

Superframe Structure

Frame Structure

PHY Specification

2450 MHz Mode

868/915 MHz Mode


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Introduction


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The IEEE 802 Family802.1 => Spanning Tree Bridge

802.2 => Logical Link Control (LLC) Protocol

802.3 => CSMA/CD Networks (Ethernet) MAC Protocol802.4 => Token Bus Networks MAC Protocol

802.5 => Token Ring Networks MAC Protocol

802.6 => Metropolitan Area Networks (MAN)

802.11 => WLAN (wireless local area network)802.11b => 2.4GHz Band; 11 Mbps; direct-sequence

802.11a => 5.0GHz Band; 54 Mbps; OFDM

802.11g => 2.4GHz Band; 54 Mbps; OFDM

802.15 => WPAN (wireless personal area network)

802.15.3 UWB (Ultra Wide Band)

802.15.4 LR-WPAN (low rate wireless PAN)

802.16 => WLL (wireless local loop)

LAN


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OverviewLR-WPANs stands forlow-rate wireless personal area

networks.

Wireless personal area networks (WPANs) are used to

convey information over relatively short distance.

Unlike wireless local area networks (WLANs),

connections effected via WPANs involve little or no

infrastructure. This feature allows small, power-efficient,

inexpensive solutions to be implemented for a wide

range of devices.

Typically operating in thepersonal operating space

(POS) of 10m.


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ZigBee & IEEE 802.15.4


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ZigBee MembershipZigBee Alliance grows to over 90 members (August 16,

2004)

Promoter

Ember

Honeywell

Invensys

Mitsubishi Electric

Motorola

Philips

Samsung


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Traffic TypesPeriodic data

Sensors

Intermittent data

Light switch

Repetitive, low-latency data

Mouse

The raw data rate will be high enough (maximum of 250

kb/s) to satisfy a set of simple needs such as interactivetoys, but scalable down to the needs of sensor and

automation needs (20 kb/s or below) for wireless

communications.


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General Description


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General DescriptionA LR-WPAN is a simple, low-cost communication

network that allows wireless connectivity in applications

with limited power and relaxed throughput requirements.

Some of the characteristics of an LR-WPAN are:

Over-the-air data rates of 250 kb/s, 40 kb/s, and 20 kb/s.

Star or peer-to-peer operation

Allocated 16 bit short or 64 bit extended addresses

Allocation ofguaranteed time slots (GTSs)

Carrier sense multiple access with collision avoidance

(CSMA-CA) channel access

Fully acknowledged protocol for transfer reliability


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General DescriptionLow power consumption

Energy detection (ED)

Link quality indication (LQI)16 channels in the 2450 MHz band, 10 channels in the 915

MHz band, and 1 channel in the 868 MHz band

Two different device types can participate in an LR-WPAN network:

Full-function device (FFD)

Can talk to RFDs or other FFDs.Reduced-function device (RFD)

Can only talk to an FFD.

Intended for applications that are extremely simple.


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Components of the IEEE 802.15.4 WPANThe most basic component in the IEEE 802.15.4 WPAN

is the device.

A device can be an RFD or an FFD.

Two or more devices within a POS communicating on

the same physical channel constitute a WPAN.

A network shall include at least one FFD, operating as

the PAN coordinator.

An IEEE 802.15.4 network is part of the WPAN familyof standards.


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Network TopologiesDepending on the application requirements, the LR-

WPAN may operate in either of two topologies: the star

topology or thepeer-to-peer topology.Each independent PAN will select a unique identifier.


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Star TopologyThe communication is established between devices and

a single central controller, called the PAN coordinator.

A PAN coordinator is the primary controller of the PAN.

The PAN coordinator may be mains powered, while the

devices will most likely be battery powered.

Applications that benefit from a star topology include

home automation, personal computer (PC) peripherals,

toys and games, and personal health care.


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Star Network FormationAfter an FFD is activated for the first time, it may

establish its own network and become the PAN

coordinator.

All star networks operate independently from all other

star networks currently in operation. This is achieved by

choosing a PAN identifier, which is not currently usedby other network within the radio sphere of influence.

Once the PAN identifier is chosen, the PAN coordinator

can allow other devices to join its network; both FFDs

and RFDs may join the network.


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Peer-to-Peer TopologyThe peer-to-peer topology also has a PAN coordinator.

Any device can communicate with any other device as

long as they are in range of one another.Allows more complex network formations to beimplemented, such as mesh networking topology.

Applications such as industrial control and monitoring,wireless sensor networks, asset and inventory tracking,intelligent agriculture, and security would benefit from

such a network topology.Can be ad hoc, self-organizing and self-healing.

Allow multiple hops to route messages from any device

to any other device on the network.


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Peer-to-peer Network FormationEach device is capable of communicating with any other

device within its radio sphere of influence.

One device will be nominated as the PAN coordinator,for instance, by virtue of being the first device to

communicate on the channel.

An example of the use of the peer-to-peercommunications topology is the cluster-tree.

The cluster-tree network is a special case of a peer-to-peer

network in which most devices are FFDs.

An RFD may connect to a cluster tree network as a leave node

at the end of a branch, because it may only associate with one

FFD at a time.


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Topology Models

Full function device

Reduced function device

PAN Coordinator

Cluster treeStar Mesh


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LR-WPAN Device ArchitectureThe upper layers consist of

a network layer, which

provides networkconfiguration,

manipulation, and

message routing.

an application layer

provides the intended

function of the device.

LLC: logical link control.

SSCS: service specific

convergence sublayer.


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PHY SublayerThe PHY provides two services

The PHY data service

The PHY management service interfacing to thephysical layermanagement entity (PLME).

The PHY data service enables the transmission and

reception ofPHY protocol data units (PPDUs) acrossthe physical radio channel.

The features of the PHY are activation and deactivation

of the radio transceiver, ED, LQI, channel selection,clear channel assessment(CCA), and transmitting as

well as receiving packets across the physical medium.


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ZigBee Operating Bands

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


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Frequency Band and Data Rate

BPSK-92 dbm1040 kbpsAmericasISM915 MHz

BPSK-92 dbm120 kbpsEurope868 MHz

O_QPSK-85 dbm16250 kbpsWorldwideISM2.4 GHz

ModulationRxSensitivity

# ofChannels

DataCoverageBandFrequency


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MAC SublayerThe MAC sublayer provides two services:

The MAC data service

The MAC management service interfacing to theMACsublayer management entity (MLME) service access point(SAP).

The MAC data service enables the transmission andreception of MAC protocol data units (MPDUs) acrossthe PHY data service.

The features of the MAC sublayer are beacon

management, channel access, GTS management, framevalidation, acknowledged frame delivery, association,and disassociation.


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Superframe StructureThe LR-WPAN standard allows the optional use of a superframe

structure.

The format of the superframe is defined by the coordinator.The superframe is bounded by network beacons, is sent by the

coordinator, and is divided into 16 equally sized slots.

The beacon frame is transmitted in the first slot of eachsuperframe.

If a coordinator does not wish to use a superframe structure, it

may turn off the beacon transmissions.

The beacons are used to synchronize the attached devices, to

identify the PAN, and to describe the structure of the superframes.


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Superframe Structure without GTSs


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Superframe Structure with GTSs


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Frame StructureThe LR-WPAN defines four frame structures

A beacon frame, used by a coordinator to transmit beacons

A data frame, used for all transfers of data

An acknowledgement frame, used for confirming successful

frame reception

A MAC command frame, used for handling all MAC peerentity control transfers


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Schematic View of the Beacon Frame


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Schematic View of the Data Frame


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Schematic View of the AcknowledgementFrame


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Schematic View of the MAC CommandFrame


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Concept of Primitives


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PHY Specification


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Introduction

The PHY is responsible for the following tasks:

Activation and deactivation of the radio transceiver

Energy detection (ED) within the current channel

LQI for received packets

CCA for CSMA-CA

Channel frequency selectionData transmission and reception


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Operating Frequency Range

Frequency bands and data rates


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Channel Assignments and Numbering

A total of 27 channels, numbered 0 to 26, are available

across the three frequency bands.

Sixteen channels in the 2450 MHz band.Ten channels in the 915 MHz band.

One channels in the 868 MHz band.

The center frequency of these channels is defined as

follows:


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Receiver Sensitivity Definition


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General Packet Format

Each PPDU packet consists of the following basic

components:

A SHR (synchronization header), which allows a receivingdevice to synchronize and lock onto the bit stream.

A PHR (PHY header), which contains frame length information.

A variable length payload, which carriers the MAC sublayerframe.

General packet format


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Packet Fields

Preamble field

Used by the transceiver to obtain chip and symbol

synchronization with an incoming message.Composed of 32 binary zeros.

SFD (start-of-frame delimiter) fieldAn 8 bit field indicating the end of the synchronization

(preamble) field and the start of the packet data.

Format of the SFD field


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Packet Fields

Frame length field

7 bits in length and specifies the total number of octets

contained in the PSDU.

PSDU field

Has a variable length and carries the data of the PHY packet.For all packet types of length five octets or greater than seven

octets, the PSDU contains the MAC sublayer frame (i.e.,

MPDU).


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PHY Constants


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PHY PIB Attributes

PIB: PAN information base.


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2450 MHz PHY Specifications

Data rate: 250 kb/s.

Modulation and spreading

Employs a 16-ary quasi-orthogonal modulation technique.

During each data symbol period, four information bits are used

to select one of 16 nearly orthogonal pseudo-random noise

(PN) sequences to be transmitted.The PN sequences for successive data symbols are

concatenated.

The aggregate chip sequence is modulated onto the carrier

using offset quadrature phase-shift keying (O-QPSK)


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Reference modulator diagram

Reference transmitter diagram


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Symbol to Chip Mapping


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O-QPSK modulation

The chip sequences representing each data symbol are

modulated onto the carrier using O-QPSK with half-sinepulse shaping.

Pulse shape

( )sin 0 2

2

0

c

c

t t Tp t T

otherwise

=


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Sample baseband chip sequences with pulse shaping

Symbol rate

The 2450 MHz PHY symbol rate shall be 62.5 ksymbol/s.

Receiver sensitivityA compliant device shall be capable of achieving a sensitivityof -85 dBm or better.


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868/915 MHz PHY Specifications

868/915 MHz band data rates

868 MHz: 20 kb/s.

915 MHz: 40 kb/s.

Modulation and Spreading

The 868/915 MHz PHY shall employ direct sequence spreadspectrum (DSSS).

The binary phase-shift keying (BPSK) is used for chip

modulation.Differential encoding is used for data symbol encoding.


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Reference modulator diagram

Differential encoding

Differential encoding is the modulo-2 addition (exclusive or)of a raw data bit.

1

1

is the raw data bit being encoded,

is the corresponding differentially encoded bit,

is the previous differentially encoded bit.

n n n

n

n

n

E R E

R

E

E

=


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For each packet transmitted,R1 is the first raw bit to be

encoded andE0 is assumed to be zero.

Conversely, the decoding process, as performed at thereceiver, can be described by:

For each packet received,E1 is the first bit to be decoded,

andE0 is assumed to be zero.

Bit-to-chip mapping

Each input bit shall be mapped into a 15-chip PN sequence

1n n nR E E=


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BPSK modulation

The chip sequences are modulated onto the carrier using

BPSK with raised cosine pulse shaping (roll-off factor = 1).The chip rate is 300 kchip/s for the 868 MHz band and 600

kchip/s in the 915 MHz band.

Pulse shape

The raised cosine pulse shape (roll-off factor = 1) used to

represent each baseband chip is described by

( )( ) ( )

( )2 2sin / cos /

/ 1 4 /

c

c

t T t T p t

t T t T

=


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Symbol rate

868 MHz: 20 ksymbol/s

915 MHz: 40 ksymbol/s

Receiver sensitivity

A compliant device shall be capable of achieving a sensitivityof -92 dBm or better.


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Receiver Architecture

Half-sine

Matched Filter

A/D

Packet

Detection

Fine Syn.

and/or

Start of Data

Down

Sampling

to Chip Rate

RF

Over-Sampling

Rate (nchip rate)

Coarse

Synchronization

Despreading

to

(Sym. Rate)

OQPSK

Demodulation

(Sym. Rate)

Detection

(Sym. Rate)

Data Stream

D d d D d l


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Despreading and Demodulation

CI1 CI2 CI3 CI4 CI5 CI6 CI16

CQ1 CQ2 CQ3 CQ4 CQ5 CQ6 CQ16

F

i

n

d

M

a

x

i

mu

m

+

-

+

+

CSMA/CA Al i h


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CSMA/CA Algorithm

The CSMA/CA algorithm shall be used before the

transmission ofdata or MAC command frames

transmitted within the CAP, and shall not be used for

the transmission ofbeacon frames, acknowledgment

frames or data frames transmitted in the CFP.

NB is the number of times the CSMA/CA algorithmwas required to backoff.

CWdefines the number ofbackoffperiods that need

to be clear of channel activity.BEis related to how many backoffperiods a device

shall wait before assess a channel.backoff = 20 symbols

CSMA-CA


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Slotted

BE=lesser of (2,macMinBE)

BE=macMinBE

Delay for random

unit backoff periods

Locate backoffperiod boundary

(2 1)BE

Battery life

extension?

NB=0,CW=2

Performance CCA on

backoff period boundary

Channel

idle?

CW=2,NB=NB+1,

BE=min(BE+1,aMaxBE)

CW=CW-1

NB>macMaxCS

MABackoff? CW=O?

Failure Success

N

Y

Y

N

N

N

Y Y

CSMA-CA


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Unslotted

NB=0,

BE=macMinBE

Delay for random

unit backoff periods

(2 1)BE

Perform CCA

Channel

idle?

NB=NB+1,

BE=min(BE+1,aMaxBE)

NB>macMaxCS

MABackoffs?

Failure Success

N

N

Y

Y

W.A Loviyantara

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