The IEEE 802.15.4 standard

Credits to:Yao Liang (IUPUI, Indianapolis USA)

Wireless Simplified Stack

802.15.4

Principal options and difficulties

• Medium access in wireless networks is difficult mainly because of– Impossible (or very difficult) to send and receive at the same time– Interference situation at receiver is what counts for transmission

success, but can be very different from what sender can observe– High error rates compound the issues

• Requirement– As usual: high throughput, low overhead, low error rates, …– Additionally: energy-efficient, handle switched off devices!

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 and high BERs – wasted effort when two packets collide

or corrupted packet– Overhearing – waste effort in receiving a packet destined for another

node – Idle listening – sitting idly and trying to receive when nobody is

sending – Protocol overhead

• Always nice: Low complexity solution

Schedule- vs. contention-based MACs

• Schedule-based MAC – A schedule exists, regulating which participant may use which

resource at which time (TDMA component) – Schedule can be fixed or computed on demand

• Usually: mixed – difference fixed/on demand is one of time scales

– Usually, collisions, overhearing, idle listening no issues – Needed: time synchronization!

• Contention-based MAC– Risk of colliding packets is deliberately taken – Hope: coordination overhead can be saved, resulting in overall

improved efficiency– Mechanisms to handle/reduce probability/impact of collisions required

– Usually, randomization used somehow

Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)

Submission Title: [IEEE 802.15.4 Tutorial]

Date Submitted: [4 January, 2003]

Source: [Jose Gutierrez] Company: [Eaton Corporation]

Address: [4201 North 27th Street, Milwaukee WI. 53216]

Voice:[(414) 449-6525], FAX: [(414) 449-6131], E-Mail:[josegutierrez@eaton.com]

Re: [IEEE 802.15.4 Overview; Doc. IEEE 802.15-01/358r0, TG4-Overview; Doc IEEE 802.15-01/509r0]

Abstract: [This presentation provides a tutorial on the 802.15.4 draft standard.]

Purpose: []

Notice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.

Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15.

www.IEEE802.org

• Home Networking

• Automotive Networks

• Industrial Networks

• Interactive Toys

• Remote Metering

802.15.4 Applications Space

802.15.4 Applications Topology

Cable replacement - Last meter connectivity

Virtual Wire

Wireless Hub

Stick-On Sensor

Mobility

Ease of installation

Some needs in the sensor networks

Thousands of sensors in a small space Wireless

but wireless implies Low Power!

and low power implies Limited Range.

Of course all of these is viable if a Low Cost transceiver is required

Solution:

LR-WPAN Technology!

By means of

IEEE 802.15.4

802.15.4 General Characteristics

Data rates of 250 kb/s, 40 kb/s and 20 kb/s.

Star or Peer-to-Peer operation.

Support for low latency devices.

CSMA-CA/TDMA channel access.

Dynamic device addressing.

Fully handshaked protocol for transfer reliability.

Low power consumption.

Frequency Bands of Operation

16 channels in the 2.4GHz ISM band

10 channels in the 915MHz ISM band

1 channel in the European 868MHz band.

IEEE 802.15.4 MAC

Upper Layers

IEEE 802.2 LLC Other LLC

IEEE 802.15.4

2400 MHz

PHY

IEEE 802.15.4

868/915 MHz

PHY

802.15.4 Architecture

IEEE 802.15.4 PHY OverviewOperating Frequency 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

IEEE 802.15.4 PHY Packet Structure

PreambleStart ofPacket

Delimiter

PHYHeader

PHY ServiceData Unit (PSDU)

PHY Packet Fields• Preamble (32 bits) – synchronization • Start of Packet Delimiter (8 bits)• PHY Header (8 bits) – PSDU length• PSDU (0 to 1016 bits) – Data field

6 Octets 0-127 Octets

IEEE 802.15.4: PHY Layer

Bit to symbolSymbol to

chipModulation

Input Bit Output signal

PHY FrequencyChannel

numbering

Spreading Data parameters

Chip rate ModulationBit rate

Symbol rate

Mappatura bit a simbolo

800/915 MHz

868-870 MHz

0300

kchip/sBPSK

20 kb/s

20 kbaud Binary

902- 928 MHz

Da 1 a 10600

kchip/sBPSK

40 kb/s

40 kbaud Binary

2.4 GHz2.4-2.4835

GHzDa 11 a 26

2.0 Mchip/s

O-QPSK250 kb/s

62.5 kbaud16-ary

Orthogonal

16 channels, 5 MHz each

IEEE 802.15.4 PHY Primitives

PHY Data Service• PD-DATA – exchange data packets between MAC and PHY

PHY Management Service• PLME-CCA – clear channel assessment• PLME-ED - energy detection • PLME-GET / -SET– retrieve/set PHY PIB parameters• PLME-TRX-ENABLE – enable/disable transceiver

Extremely low cost

Ease of implementation

Reliable data transfer

Short range operation

• Very low power consumption

Simple but flexible protocol

IEEE 802.15.4 MAC OverviewDesign Drivers

IEEE 802.15.4 MAC OverviewTypical Network Topologies

• 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

IEEE 802.15.4 MAC OverviewDevice Classes

Full function device

Reduced function device

Communications flow

Master/slave

PANCoordinator

IEEE 802.15.4 MAC OverviewStar Topology

Full function device Communications flow

Point to point Cluster tree

IEEE 802.15.4 MAC OverviewPeer-Peer Topology

Full function device

Reduced function device

Communications flow

Clustered stars - for example,cluster nodes exist between roomsof a hotel and each room has a star network for control.

IEEE 802.15.4 MAC OverviewCombined Topology

• All devices have IEEE addresses• Short addresses can be allocated• Addressing modes:

– Network + device identifier (star)– Source/destination identifier (peer-peer)

IEEE 802.15.4 MAC OverviewAddressing

IEEE 802.15.4 MAC OverviewGeneral Frame Structure

Payload

PH

Y L

ayer

MA

CLa

yer

MAC Header(MHR)

MAC Footer(MFR)

MAC Protocol Data Unit (MPDU)

MAC Service Data Unit(MSDU)

PHY Header(PHR)

Synch. Header(SHR)

PHY Service Data Unit (PSDU)

4 Types of MAC Frames:

• Data Frame

• Beacon Frame

• Acknowledgment Frame

• MAC Command Frame

15ms * 2n

where 0 n 14

Network beacon

Contention period

Beacon extensionperiod

Transmitted by network coordinator. Contains network information,frame structure and notification of pending node messages.

Space reserved for beacon growth due to pending node messages

Access by any node using CSMA-CA

GTS 2 GTS 1

GuaranteedTime Slot

Reserved for nodes requiring guaranteed bandwidth [n = 0].

IEEE 802.15.4 MAC OverviewOptional Superframe Structure

Contention Access Period Contention Free Period

• Periodic data– Application defined rate (e.g. sensors)

• Intermittent data– Application/external stimulus defined rate (e.g. light switch)

• Repetitive low latency data– Allocation of time slots (e.g. mouse)

IEEE 802.15.4 MAC OverviewTraffic Types

OriginatorMAC

RecipientMAC

MCPS-DATA.request

Data frame

MCPS-DATA.confirmMCPS-DATA.indication

Acknowledgement(if requested)

Channelaccess

IEEE 802.15.4 MAC OverviewMAC Data Service

Orig

inat

orR

ecipient

IEEE 802.15.4 PHY OverviewMAC Primitives

MAC Data Service• MCPS-DATA – exchange data packets between MAC and PHY

MAC Management Service• MLME-ASSOCIATE/DISASSOCIATE – network association• MLME-SYNC / SYNC-LOSS - device synchronization• MLME-SCAN - scan radio channels• MLME-GET / -SET– retrieve/set MAC PIB parameters• MLME-START / BEACON-NOTIFY – beacon management• MLME-POLL - beaconless synchronization• MLME-GTS - GTS management• MLME-ORPHAN - orphan device management• MLME-RX-ENABLE - enabling/disabling of radio system

21/04/23 29/31

A numerical example

• Adopting beacon-enabled networks;

• Data transfer protocols (e.g. towards PANC);

• Maximum bandwidth 250 kb/s = 62.5 ksym/s (16-ary coding, 1sym = 4 bits);

• Maximum number of GTS (Guaranteed Time Slots) = 7.

AP= 16 (slots) * 960 (baseslotduration) * 2SO цs

Transfer of large data sets (1)

• Suppose you want to transmit 1 picture (P2P), use the lowest resolution (80 * 64 pel):

is 1.6 kBytes

• Maximum MAC MSDU (payload) is 102 bytes, i.e. 16 MAC frames each resulting in 132 bytes = 264 sym at the PHY layer;

• The average amount of time to transmit the data in CSMA is (BE=2, default) w/o taking into account traffic and different sources of overheads:– 16 * [(1.5 (avg BT) * BP) + 2 * SP (CCA) + 264 ] sym/ 62.5 Ksym/s =

80 ms (optimistic);

• In free access:– 16 * 264 sym / 62.5 Ksym/s = 68 ms (but pay attention at the reservation

cost).

Transfer of large data sets (2)

• To fit the transmission into 6 slots of CAP we have to use SO = 4:– 960 цs * 24 * 6 > 80 ms;

• If we want to use the GTSs:– we have an overhead of 1 superframe + minimum

CAP (440 symbols) = 16 * 960 * 24 цs + 7 ms = 100 ms (maximum) !!!!!