1

IEEE 802.15.4 IEEE 802.15.4 Low-Rate Wireless PAN Low-Rate Wireless PAN (LR-WPAN)(LR-WPAN)

1

Wireless Sensor Network Wireless Sensor Network StandardsStandards

IEEE 802.15.4 Low-Rate Wireless PAN

ZigBee

6LoWPAN

IEEE 1451standards for connecting smart transducers to

networks

2

Wireless Sensor Network Wireless Sensor Network StandardsStandards

IEEE 802.15.4 PHY

IEEE 802.15.4 MAC (CPS)

ZigBee NWK

MAC (SSCS)802.2 LLC

6LowPAN

API Transport

ZA1 ZA2 … IA1 IA2 IAn

Transmission & reception on the physical radio channel

Channel access, PAN maintenance, reliable data

transport

Topology management, MAC management, routing, discovery protocol, security management

Application interface designed using

general profile

End developer applications, designed using application

profiles

3

802.15.4 with Five Key Words802.15.4 with Five Key Words

Very low costVery low power consumptionLow complexityLow rateShort range

4

Basic Radio CharacteristicsBasic Radio Characteristics

5

Home Networking

Automotive Networks

Industrial Networks

Interactive Toys

Remote Metering

802.15.4 Applications Space802.15.4 Applications Space

6

High-Level CharacteristicsHigh-Level Characteristics

7

802.15.4 Architecture802.15.4 Architecture

8

9

Device ClassesDevice Classes

Full function device (FFD)Any topologyNetwork coordinator capableTalks to any other device

Reduced function device (RFD)Limited to star topologyCannot become a network coordinatorTalks only to a network coordinatorVery simple implementation

9

Network TopologyNetwork Topology

10

Star

PANCoordinator

Point to point Cluster tree

Full function device

Reduced function device

LR-WPAN: LR-WPAN: Data RateData Rate

DSSSTx range: 10 ~ 75 m at 0 dBm (1 mW)

Band Symbol rate Modulation Bit rate channels

868 MHz 20 Ksps BPSK 20 Kbps 1

915 MHz 40 Ksps BPSK 40 Kbps 10

2.4 GHz 62.5 Ksps O-QPSK 250 Kbps 16

11

MAC FeaturesMAC Features

Generating network beacons if the device is a coordinator

Synchronizing to beaconsPAN association, disassociationOptional acknowledged frame deliveryEmploying the CSMA/CA for channel access

mechanismGuaranteed time slot management

MAC management has 35 primitives RFD has 24 primitivescf. 131 primitives of 802.15.1 / Bluetooth

12

Superframe StructureSuperframe Structure For some applications requiring dedicated bandwidth to achieve low latencies A superframe is divided in 16 time slots

CAP: • Slotted CSMA-CA channel access (beacon-enabled network)• Unslotted or standard CSMA-CA in networks (non beacon-enabled network)

CFP: Optionally, contention-free access using Guaranteed Time Slots (GTSs) in beacon-enabled netrwork

aBaseSuperframeDuration = 60 symbols/slot * 16 slots = 960 symbols 15.36 ms at 250 kbps, 24 ms at 40 kbs, 48 ms at 20 kbps

BO (Beacon Order) How often the PNC transmits a beacon, 0 ≤ BO ≤ 14 (15.36 ms ~ 251.65824 sec) 15 if non beacon

13

Backoff periods of a device not related to that of any other deviceTherefore, synchronization is not required

CCA – Clear Channel Assessment to check if channel is busy or idle

Unslotted CSMA-CAUnslotted CSMA-CA

Another Device’s Transmission

2 1 0 8 7 6 5 4 3 2 1 0 BackoffNumber

Frame Transmission

Perform CCA, and finds channel busyBE=4 8 selected

Perform CCA, and finds channel idle

Frame arrivalBE=3 2 selected

aTurnaroundTime

14

Slotted CSMA-CASlotted CSMA-CA

Backoff period boundaries aligned by the periodic beacon transmission

It also implies that they are aligned with superframe slot boundaries (for GTS) as Slot = n * aUnitBackoffPeriod

Frame Transmission

Another Device’s Transmission

2 1 0 8 7 6 5 4 3 2 1 0 BackoffNumber

Perform CCA, and finds channel busyBE=4 8 selected

Perform CCA, and finds channel idle

Frame arrivalBE=3 2 selected

BCNCW= 2 1 0

aTurnaroundTime

15

Inter-frame SpacingInter-frame Spacing

Short frame: frame size <= aMaxSIFSFrameSize

Long frame: otherwise

Long frame ACK Short frame ACK

tack LIFS tack SIFS

Acknowledged transmission

Long frame Short frame

LIFS SIFS

Unacknowledged transmission

aTurnaroundTime tack (aTurnaroundTime (12 symbols) + aUnitBackoffPeriod (20 symbols))LIFS > aMaxLIFSPeriod (40 symbols)SIFS > aMacSIFSPeriod (12 symbols)

16

MAC addressingMAC addressing

All devices have IEEE addresses (64 bits)Short addresses (16 bits) can be allocatedAddressing modes

PAN identifier (16 bits)+ device identifier (16/64 bits)

Beacon frame: no destination address

General Frame FormatGeneral Frame Format

18

Payload

PHY Header(PHR)

Synch.Header(SHR) PHY Service Data Unit (PSDU)

PH

Y L

ayer

MA

CL

ayer MAC Header

(MHR)MAC Footer

(MFR)

MAC Protocol Data Unit (MPDU)

MAC Service Data Unit(MSDU)

General MAC Frame FormatGeneral MAC Frame Format

Octets:2 1 0/2 0/2/8 0/2 0/2/8 variable 2Destination

PAN identifier

Destination address

Source PAN

identifier

Source address

MAC payload

MAC footer

Frame check

sequence

MAC header

Addressing fields

Frame control

Sequence number

Frame payload

Bits: 0-2 3 4 5 6 7-9 10-11 12-13 14-15

Frame typeSequrity enabled

Frame pending

Ack. Req. Intra PAN ReservedDest.

addressing mode

ReservedSource

addressing mode

Frame control field

Beacon frameData frameAcknowledgement frameMAC command frame

19

source PAN id is skipped

Destination in Beacon frame

Data Frame formatData Frame format

Provides up to 104 byte data payload capacity Data sequence numbering to ensure that all packets are tracked Robust frame structure improves reception in difficult conditions Frame Check Sequence (FCS) ensures that packets received are

without error

20

Acknowledgement Frame FormatAcknowledgement Frame Format

Provides active feedback from receiver to sender that packet was received without error

Short packet that takes advantage of standards-specified “quiet time” immediately after data packet transmission

21

MAC Command Frame FormatMAC Command Frame Format

Mechanism for remote control/configuration of client nodes

Allows a centralized network manager to configure individual clients no matter how large the network

22

Beacon Frame formatBeacon Frame format

Client devices can wake up only when a beacon is to be broadcast, listen for their address, and if not heard, return to sleep

Beacons are important for mesh and cluster tree networks to keep all of the nodes synchronized without requiring nodes to consume precious battery energy listening for long periods of time

Minimum beacon PPDU length = 136 bits / 250 Kbps = 544 μsec

Bits: 0-3 4-7 8-11 12 13 14 15Beacon

orderSuperframe

orderFinal CAP

slotBattery life extension

ReservedPAN

coordinatorAssociation

permit

23

MAC Data PrimitivesMAC Data Primitives

Primitive Request Confirm Indication Response

MCPS-DATA Required Required Required

MCPS-PURGEOptional for

RFDOptional for

RFD

24

Data Transfer: no-beacon modeData Transfer: no-beacon mode

Originator

MAC Recipient

MAC

Data frame

Acknowledgment (if requested)

Originator higher layer

Recipient higher layer

MCPS-DATA.request

MCPS-DATA.indication

MCPS-DATA.confirm

25

Device Coordinator

Coordinator Device

Indirecttransmission

Data Transfer: Beacon ModeData Transfer: Beacon Mode

Coordinator

MAC Device MAC

Data frame

Acknowledgment

Coordinator higher layer

Device higher layer

MCPS-DATA.request (indirect)

MCPS-DATA.indication

MCPS-DATA.confirm

Beacon frame

Data request

Acknowledgement

26

Coordinator DeviceDevice Coordinator

Management ServiceManagement Service

Access to the PIBAssociation / disassociationGTS allocationMessage pending Node notificationNetwork scanning/startNetwork synchronization/search

27

MAC Management PrimitivesMAC Management Primitives

Access to the PIB Association / disassociation GTS allocation Message pending

Node notification Network scanning/start Network

synchronization/search

Primitive Request Confirm Indication Response

MLME-GET Required Required

MLME-SET Required Required

MLME-ASSOCIATE Required Required Optional for RFD Optional for RFD

MLME-DISASSOCIATE Required Required Required

MLME-GTS Optional for RFD Optional for RFD Optional for RFD

MLME-BEACON-NOTIFY Required

MLME-POLL Required Required

MLME-COMM-STATUS Required

MLME-ORPHAN Optional for RFD Optional for RFD

MLME-SCAN Required Required

MLME-START Optional for RFD Optional for RFD

MLME-RX-ENABLE Required Required

MLME-SYNC Required

MLME-SYNC-LOSS Required

MLME-RESET Required Required

28

AssociationAssociation

Device MAC

Coordinator MAC

Association request

Acknowledgment

Device higher layer

Coordinator higher layer

MLME-ASSOCIATE.request

MLME-ASSOCIATE.indication

MLME-ASSOCIATE.response

Acknowledgement

Association response

MLME-ASSOCIATE.confirm

aResponseWaitTime

MLME-COMM-STATUS.indication

Data request

Acknowledgment

29

DisassociationDisassociation

= Originator

MAC Recipient

MAC

Disassociation notification

Acknowledgment

Originator higher layer

Recipient higher layer

MLME-DISASSOCIATE.request

MLME-DISASSOCIATE.indication MLME-DISASSOCIATE.confirm

30

Data PollingData Polling Device MAC

Coordinator MAC

Data request

Acknowledgment (FP = 0)

Device higher layer

MLME-POLL.request

MLME-POLL.confirm

No data pending at the coordinator

31

Device MAC

Coordinator MAC

Data request

Acknowledgment (FP = 1)

Device higher layer

MLME-POLL.request

MLME-POLL.confirm

Data

Acknowledgement

MCPS-DATA.indication

Data pending at the coordinator

ED SCANED SCAN

When a prospective PAN coordinator to select a channel

Measure peak energy in each requested channel

Discard every frame received while scanningReturn energy levels

Active ScanActive Scan

33

Device MAC

Coordinator MAC

Beacon request

Device higher layer

MLME-SCAN.request

MLME-SCAN.confirm

ScanDuration Beacon

Set 1st Channel

CSMA

Set 2nd Channel

Beacon request

When FFD wants to locate any coordinator within POSA prospective coordinator

selects PAN IDPrior to device association

Receive beacon frames onlymacPANId = 0xffff

Send beacon request commandDestination PAN ID = 0xffff

Return PAN descriptors

Passive ScanPassive Scan

No beacon request command

Device to prior to association

Receive beacon frames onlymacPANId = 0xffff

34

Device MAC

Coordinator MAC

Device higher layer

MLME-SCAN.request

MLME-SCAN.confirm

ScanDuration Beacon

Set 1st Channel

Set 2nd Channel

Orphan ScanOrphan Scan

Device attempts to relocate its coordinator

For each channel, send orphan notification commandDest PAN id, dest short

addr = 0xffff Only the original

coordinator will reply Receive coordinator

realignment command frame only

35

= Coordinator

MAC Device MAC

Coordinator realignment

Orphan notification

Coordinator higher layer

MLME-ORPHAN.response

MLME-COMM-STATUS.indication

MLME-ORPHAN.indication

Differences from 802.11 WLANDifferences from 802.11 WLAN

Simpler PHYOne Tx rate per channelLow Tx power

Simpler MACNo virtual carrier-senseNo worry about hidden nodesNo RTS/CTS & No fragmentationNo continuous CCARelaxed timing requirement

Extensive power saving features

36

Power Save MechanismsPower Save Mechanisms

Going to sleep state as often as possible by utilizing:Inactive mode in superframesBackoff periods when macRxOnWhenIdle is

reset.GTS for other devices

Extracting pending messages from coordinatorUsing data request commandMessage pending indicated in beacon frames

37

LR-WPAN: Low Duty CycleLR-WPAN: Low Duty Cycle

Beacon interval(max) 960 symbols * 214 = 15,728,640 symbolsAt 250 Kbps, (min) 15.36 msec ~ (max) 251.65824

sec (over 4 min)

Beacon duty cycle544 μsec / 251.65824 sec = 0.000216% (lowest

possible)Non-beacon mode is also possible

Example: 0.1% duty cycle 10 mW active, 10 μW standby → 19.99 μW average

powerAAA battery with capacity of 750mAh, regulated to 1VBattery life: 37,519 hours ≈ 4.28 years

38

LR-WPAN: LR-WPAN: Imperfect Time BasesImperfect Time Bases

“Ideal” beacon

reception time

“Ideal” beacon

transmission time

εTX TbeaconεTX Tbeacon

εRX TbeaconεRX Tbeacon

εTbeaconεTbeacon TC

receiver

transmitter

Uncertainty due to imperfect receiver time base

Uncertainty due to imperfect transmitter time base

[Guti03]

39

LR-WPAN: LR-WPAN: Duty Cycle vs. CostDuty Cycle vs. Cost

Lowest possible duty cycle of a receiver is(2ε·Tbeacon + TC) / Tbeacon

Duty cycle is limited by the time base tolerance εNo matter how long Tbeacon is made

IEEE 802.15.4 is designed to supportTime base tolerance as great as ±40 ppm

(note) lowest duty cycle = 2.16 ppmUse of inexpensive reference crystals

Lower duty cycle requires more stable time baseIncreases the cost of time base

40

IEEE 802.15.4aIEEE 802.15.4a Scope and Description:

Develop an alternate physical layer (PHY) for data communication with

• high precision ranging / location capability (1 meter accuracy and better)

• high aggregate throughput• and ultra low power• scalability to data rates• longer range• lower power consumption and cost.

The alternate PHY is an (optional) amendment to the current IEEE 802.15.4-2003 LR-WPAN standard.

802.15.4a became an official Task Group in March 2004; with its committee work tracing back to November 2002.

Current StatusThe baseline is two optional PHYs

• UWB Impulse Radio (operating in unlicensed UWB spectrum) • Chirp Spread Spectrum (operating in unlicensed 2.4GHz spectrum)

The UWB Impulse Radio will be able to deliver communications and high precision ranging.

41

IEEE 802.15.4bIEEE 802.15.4b Scope and Description

Resolve ambiguities, provide corrections, removing unnecessary complexity, and define enhancements to the current IEEE 802.15.4-2003 standard. The revised standard will be backward compatible.

Enhancementssupport for distributing a shared time-baseSupport for group addressingExtensions of the 2.4GHz derivative modulation

• Yields higher data rates at the lower frequency bandsSupport of Beacon-Enabled Cluster Tree network.

• IEEE802.15.4 does not support while 15.4b doesProtection of broadcast and multicast frames possibleEasier setup of protection parameters possiblePossibility to vary protection per frame, using a single

keyOptimization of storage for keying material

42