ZigBee/IEEE 802.15.

4
Overview

Y.-C. Tseng
CS/NCTU

國立交通大學 資訊工程系 曾煜棋 教授 1

New trend of wireless technology

Most Wireless industry focuses on increasing high data

throughput

A set of applications require simple wireless connectivity,
relaxed throughput, very low power, short distance and
inexpensive hardware.
 Industrial
 Agricultural
 Vehicular
 Residential
 Medical

國立交通大學 資訊工程系 曾煜棋 教授 2

What is ZigBee Alliance?

An organization with a mission to define reliable, cost effective,

low-power, wirelessly networked, monitoring and control
products based on an open global standard

Alliance provides interoperability, certification testing, and
branding

國立交通大學 資訊工程系 曾煜棋 教授 3

IEEE 802.15 working group

國立交通大學 資訊工程系 曾煜棋 教授 4

Comparison between WPAN

國立交通大學 資訊工程系 曾煜棋 教授 5

ZigBee/IEEE 802.15.4 market feature

Low power consumption

Low cost

Low offered message throughput

Supports large network orders (<= 65k nodes)

Low to no QoS guarantees

Flexible protocol design suitable for many applications

國立交通大學 資訊工程系 曾煜棋 教授 6

 ZigBee network applications

monitors TV VCR
sensors DVD/CD
INDUSTRIAL CONSUMER
automation & ELECTRONIC
Remote
control COMMERCIAL S control

monitors ZigBee mouse

diagnostics PERSONAL LOW DATA-RATE PC & keyboard
HEALTH RADIO DEVICES PERIPHERAL
sensors joystick
CARE S

security
consoles
HVAC
portables TOYS & HOME lighting
educational GAMES AUTOMATION closures

國立交通大學 資訊工程系 曾煜棋 教授 7

Wireless technologies
Range
Meters
GSM GPRS EDGE 3G 2000
10,000
2003-4
2005

1,000

802.11b 802.11a/g
100 ZigBee

Hiper
Bluetooth 2.0 LAN/2
Bluetooth
Bluetooth 1.5 WiMedia Bandwidth
10 kbps
10 100 1,000 10,000 100,000

國立交通大學 資訊工程系 曾煜棋 教授 8

ZigBee/802.15.4 architecture

ZigBee Alliance
 45+ companies: semiconductor mfrs, IP providers, OEMs, etc.
 Defining upper layers of protocol stack: from network to application, including
application profiles
 First profiles published mid 2003

IEEE 802.15.4 Working Group
 Defining lower layers of protocol stack: MAC and PHY

國立交通大學 資訊工程系 曾煜棋 教授 9

How is ZigBee related to IEEE
802.15.4?

ZigBee takes full advantage of a powerful physical radio
specified by IEEE 802.15.4

ZigBee adds logical network, security and application
software

ZigBee continues to work closely with the IEEE to ensure an
integrated and complete solution for the market

國立交通大學 資訊工程系 曾煜棋 教授 10

IEEE 802.15.4 overview

國立交通大學 資訊工程系 曾煜棋 教授 11

General characteristics

Data rates of 250 kbps , 20 kbps and 40kpbs.

Star or Peer-to-Peer operation.

Support for low latency devices.

CSMA-CA channel access.

Dynamic device addressing.

Fully handshaked protocol for transfer reliability.

Low power consumption.

Channels:
 16 channels in the 2.4GHz ISM band,
 10 channels in the 915MHz ISM band
 1 channel in the European 868MHz band.

Extremely low duty-cycle (<0.1%)

國立交通大學 資訊工程系 曾煜棋 教授 12

IEEE 802.15.4 basics

802.15.4 is a simple packet data protocol for lightweight

wireless networks
 Channel Access is via Carrier Sense Multiple Access with
collision avoidance and optional time slotting
 Message acknowledgement
 Optional beacon structure
 Target applications

Long battery life, selectable latency for controllers, sensors, remote
monitoring and portable electronics
 Configured for maximum battery life, has the potential to last as
long as the shelf life of most batteries

國立交通大學 資訊工程系 曾煜棋 教授 13

IEEE 802.15.4 Device Types

There are two different device types :

 A full function device (FFD)
 A reduced function device (RFD)

The FFD can operate in three modes by serving as

 Device
 Coordinator
 PAN coordinator

The RFD can only serve as:

 Device

國立交通大學 資訊工程系 曾煜棋 教授 14

FFD vs RFD

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

國立交通大學 資訊工程系 曾煜棋 教授 15

Star topology

Network
coordinator

Master/slave

Full Function Device (FFD)

Reduced Function Device (RFD)
Communications Flow

國立交通大學 資訊工程系 曾煜棋 教授 16

Peer to peer topology

Point to point Tree

Full Function Device (FFD)

Communications Flow

國立交通大學 資訊工程系 曾煜棋 教授 17

Device addressing

Two or more devices communicating on the same physical

channel constitute a WPAN.
 A WPAN includes at least one FFD (PAN coordinator)
 Each independent PAN will select a unique PAN identifier

Each device operating on a network has a unique 64-bit
extended address. This address can be used for direct
communication in the PAN

A device also has a 16-bit short address, which is allocated by
the PAN coordinator when the device associates with its
coordinator.

國立交通大學 資訊工程系 曾煜棋 教授 18

IEEE 802.15.4 physical layer

國立交通大學 資訊工程系 曾煜棋 教授 19

IEEE 802.15.4 PHY overview

PHY functionalities:
 Activation and deactivation of the radio transceiver
 Energy detection within the current channel
 Link quality indication for received packets
 Clear channel assessment for CSMA-CA
 Channel frequency selection
 Data transmission and reception

國立交通大學 資訊工程系 曾煜棋 教授 20

IEEE 802.15.4 PHY Overview

Operating frequency bands

868MHz/ Channel 0 Channels 1-10

2 MHz
915MHz
PHY
868.3 MHz 902 MHz 928 MHz

2.4 GHz
PHY Channels 11-26 5 MHz

2.4 GHz 2.4835 GHz

國立交通大學 資訊工程系 曾煜棋 教授 21

Frequency Bands and Data Rates

The standard specifies two PHYs :

 868 MHz/915 MHz direct sequence spread spectrum (DSSS)
PHY (11 channels)

1 channel (20Kb/s) in European 868MHz band

10 channels (40Kb/s) in 915 (902-928)MHz ISM band
 2450 MHz direct sequence spread spectrum (DSSS) PHY (16
channels)

16 channels (250Kb/s) in 2.4GHz band

國立交通大學 資訊工程系 曾煜棋 教授 22

PHY Frame Structure

PHY packet fields

 Preamble (32 bits) – synchronization
 Start of packet delimiter (8 bits) – shall be formatted as
“11100101”
 PHY header (8 bits) –PSDU length
 PSDU (0 to 127 bytes) – data field

Sync Header PHY Header PHY Payload

Start of Frame Reserve PHY Service
Preamble Packet Length (1 bit) Data Unit (PSDU)
Delimiter (7 bit)
4 Octets 1 Octets 1 Octets
0-127 Bytes

國立交通大學 資訊工程系 曾煜棋 教授 23

IEEE 802.15.4 MAC

國立交通大學 資訊工程系 曾煜棋 教授 24

Superframe
Beacon Beacon

CAP CFP

GTS GTS
Inactive
0 1

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

SD = aBaseSuperframeDuration*2 SO symbols (Active)

BI = aBaseSuperframeDuration*2 BO symbols

A superframe is divided into two parts

 Inactive: all station sleep
 Active:

Active period will be divided into 16 slots

16 slots can further divided into two parts
 Contention access period
 Contention free period

國立交通大學 資訊工程系 曾煜棋 教授 25

Superframe

Beacons are used for

 starting superframes
 synchronizing with other devices
 announcing the existence of a PAN
 informing pending data in coordinators

In a “beacon-enabled” network,
 Devices use the slotted CSMA/CA mechanism to contend for the
usage of channels
 FFDs which require fixed rates of transmissions can ask for
guarantee time slots (GTS) from the coordinator

國立交通大學 資訊工程系 曾煜棋 教授 26

Superframe

The structure of superframes is controlled by two parameters:

 beacon order (BO) : decides the length of a superframe
 superframe order (SO) : decides the length of the active potion in
a superframe

For a beacon-enabled network, the setting of BO and SO

should satisfy the relationship 0≦SO≦BO≦14

For channels 11 to 26, the length of a superframe can range

from 15.36 msec to 215.7 sec (= 3.5 min).

國立交通大學 資訊工程系 曾煜棋 教授 27

 Superframe

Each device will be active for 2-(BO-SO) portion of the time, and
sleep for 1-2-(BO-SO) portion of the time

Duty Cycle:

BO-SO 0 1 2 3 4 5 6 7 8 9 ≧10
Duty cycle (%) 100 50 25 12 6.25 3.125 1.56 0.78 0.39 0.195 < 0.1

國立交通大學 資訊工程系 曾煜棋 教授 28

Data Transfer Model (I)

Data transferred from device to coordinator
 In a beacon-enable network, a device finds the beacon to
synchronize to the superframe structure. Then it uses slotted
CSMA/CA to transmit its data.
 In a non-beacon-enable network, device simply transmits its data
using unslotted CSMA/CA

Communication to a coordinator
In a non beacon-enabled network
Communication to a coordinator
In a beacon-enabled network
國立交通大學 資訊工程系 曾煜棋 教授 29
Data Transfer Model (II-1)

Data transferred from

coordinator to device in a
beacon-enabled network:
 The coordinator indicates in
the beacon that some data
is pending.
 A device periodically listens
to the beacon and transmits
a Data Requst command
using slotted CSMA/CA.
 Then ACK, Data, and ACK
follow … Communication from a coordinator
In a beacon-enabled network

國立交通大學 資訊工程系 曾煜棋 教授 30

Data transfer model (II-2)

Data transferred from
coordinator to device in a
non-beacon-enable network:
 The device transmits a
Data Request using
unslotted CSMA/CA.
 If the coordinator has its
pending data, an ACK is
replied.
 Then the coordinator Communication from a coordinator
transmits Data using in a non beacon-enabled network
unslotted CSMA/CA.
 If there is no pending data,
a data frame with zero
length payload is
transmitted.

國立交通大學 資訊工程系 曾煜棋 教授 31

Channel Access Mechanism

Two type channel access mechanism:

 beacon-enabled networks  slotted CSMA/CA channel access
mechanism
 non-beacon-enabled networks  unslotted CSMA/CA channel
access mechanism

國立交通大學 資訊工程系 曾煜棋 教授 32

Slotted CSMA/CA algorithm

In slotted CSMA/CA
 The backoff period boundaries of every device in the PAN shall
be aligned with the superframe slot boundaries of the PAN
coordinator

i.e. the start of first backoff period of each device is aligned with the
start of the beacon transmission
 The MAC sublayer shall ensure that the PHY layer commences
all of its transmissions on the boundary of a backoff period

國立交通大學 資訊工程系 曾煜棋 教授 33

Slotted CSMA/CA algorithm (cont.)

Each device maintains 3 variables for each transmission
attempt
 NB: number of times that backoff has been taken in this attempt
(if exceeding macMaxCSMABackoff, the attempt fails)
 BE: the backoff exponent which is determined by NB
 CW: contention window length, the number of clear slots that
must be seen after each backoff

always set to 2 and count down to 0 if the channel is sensed to be
clear

The design is for some PHY parameters, which require 2 CCA for
efficient channel usage.

Battery Life Extension:
 designed for very low-power operation, where a node only
contends in the first 6 slots

國立交通大學 資訊工程系 曾煜棋 教授 34

Slotted CSMA/CA (cont.) need 2 CCA to
ensure no
collision

國立交通大學 資訊工程系 曾煜棋 教授 35

Why 2 CCAs to Ensure Collision-Free

Each CCA occurs at the boundary of a backoff slot (= 20

symbols), and each CCA time = 8 symbols.

The standard species that a transmitter node performs the
CCA twice in order to protect acknowledgment (ACK).
 When an ACK packet is expected, the receiver shall send it after
a tACK time on the backoff boundary

tACK varies from 12 to 31 symbols
 One-time CCA of a transmitter may potentially cause a collision
between a newly-transmitted packet and an ACK packet.
 (See examples below)

國立交通大學 資訊工程系 曾煜棋 教授 36

 Why 2 CCAs (case 1)
Backoff boundary

Existing
session

New CCA
transmitter
Backoff Detect
end here an ACK

New
transmitter CCA CCA

Backoff Detect
end here an ACK
國立交通大學 資訊工程系 曾煜棋 教授 37
 Why 2 CCAs (Case 2)
Backoff boundary

Existing
session

New CCA
transmitter
Backoff Detect
end here an ACK

New
transmitter CCA

Backoff Detect
end here an DATA
國立交通大學 資訊工程系 曾煜棋 教授 38
 Why 2 CCAs (Case 3)
Backoff boundary

Existing
session

New CCA CCA

transmitter
Backoff Detect
end here an ACK

New
transmitter CCA

Backoff Detect a
end here DATA
國立交通大學 資訊工程系 曾煜棋 教授 39
Unslotted
CSMA/CA
only one
CCA

國立交通大學 資訊工程系 曾煜棋 教授 40

GTS Concepts (I)

A guaranteed time slot (GTS) allows a device to operate on

the channel within a portion of the superframe

A GTS shall only be allocated by the PAN coordinator

The PAN coordinator can allocated up to 7 GTSs at the same
time

The PAN coordinator decides whether to allocate GTS based
on:
 Requirements of the GTS request
 The current available capacity in the superframe

國立交通大學 資訊工程系 曾煜棋 教授 41

GTS Concepts (II)

A GTS can be deallocated

 At any time at the discretion of the PAN coordinator or
 By the device that originally requested the GTS

A device that has been allocated a GTS may also operate in
the CAP

A data frame transmitted in an allocated GTS shall use only
short addressing

國立交通大學 資訊工程系 曾煜棋 教授 42

GTS Concepts (III)

Before GTS starts, the GTS direction shall be specified as

either transmit or receive
 Each device may request one transmit GTS and/or one receive
GTS

A device shall only attempt to allocate and use a GTS if it is
currently tracking the beacon

If a device loses synchronization with the PAN coordinator, all
its GTS allocations shall be lost

The use of GTSs be an RFD is optional

國立交通大學 資訊工程系 曾煜棋 教授 43

Association Procedures (1/2)

A device becomes a member of a PAN by associating with its

coordinator

Procedures
Coordinator Device

Scan
Association req. channel

ACK

Make Beacon Wait for

decision (pending address) response

Data req.

ACK

Association resp.

ACK

國立交通大學 資訊工程系 曾煜棋 教授 44

Association Procedures (2/2)

In IEEE 802.15.4, association results are announced in an

indirect fashion.
 A coordinator responds to association requests by appending
devices’ long addresses in beacon frames

Devices need to send a data request to the coordinator to

acquire the association result

After associating to a coordinator, a device will be assigned a

16-bit short address.

國立交通大學 資訊工程系 曾煜棋 教授 45

ZigBee Network Layer Protocols

國立交通大學 資訊工程系 曾煜棋 教授 46

ZigBee Network Layer Overview

Three kinds of networks are supported: star, tree, and mesh

networks

國立交通大學 資訊工程系 曾煜棋 教授 47

ZigBee Network Layer Overview

Three kinds of devices in the network layer

 ZigBee coordinator: responsible for initializing, maintaining, and
controlling the network
 ZigBee router: form the network backbone
 ZigBee end device: must be connected to router/coordinator

In a tree network, the coordinator and routers can announce

beacons.

In a mesh network, there is no regular beacon.
 Devices in a mesh network can only communicate with each
other in a peer-to-peer manner

國立交通大學 資訊工程系 曾煜棋 教授 48

Address Assignment

In ZigBee, network addresses are assigned to devices by a

distributed address assignment scheme

ZigBee coordinator determines three network parameters
 the maximum number of children (Cm) of a ZigBee router
 the maximum number of child routers (Rm) of a parent node
 the depth of the network (Lm)

A parent device utilizes Cm, Rm, and Lm to compute a parameter
called Cskip
 which is used to compute the size of its children’s address pools

1 + Cm ⋅ ( Lm − d − 1), if Rm = 1



(a)


Cskip ( d ) = 1 + Cm − Rm − Cm ⋅ Rm Lm − d −1
 , Otherwise



(b)
1 − Rm

國立交通大學 資訊工程系 曾煜棋 教授 49

 Cskip=31 Total:127

For node C 0 1 32 63 94

125,126
node A 32

If a parent node at depth Cm=6
Addr = 64,
Cskip = 1
d has an address Aparent, Rm=4
Lm=3
Addr = 92
 the nth child router is Addr = 125

assigned to address Addr = 63,

Aparent+(n- Addr = 30
Cskip = 7

Addr = 0,
1)×Cskip(d)+1 C Cskip = 31

 nth child end device is Addr = 1,

Addr = 126
Cskip = 7
assigned to address A
Addr = 40,
Addr = 32,
Aparent+Rm×Cskip(d)+n Addr = 31
B Cskip = 7
Cskip = 1

Addr = 33,
Cskip = 1
C Addr = 45
Addr = 38
Addr = 39

國立交通大學 資訊工程系 曾煜棋 教授 50

ZigBee Routing Protocols

In a tree network
 Utilize the address assignment to obtain the routing paths

In a mesh network:

Routing Capability: ZigBee coordinators and routers are said to have
routing capacity if they have routing table capacities and route
discovery table capacities

There are 2 options:
 Reactive routing: if having “routing capacity”
 Tree routing: if having no routing capacity

國立交通大學 資訊工程系 曾煜棋 教授 51

ZigBee Tree Routing

When a device receives a Cm=6

Addr = 64,
Cskip = 1
packet, it first checks if it is the Rm=4
Lm=3
destination or one of its child Addr = 92
Addr = 125
end devices is the destination
 If so, accept the packet or Addr = 63,
Cskip = 7
forward it to a child Addr = 30
Addr = 0,
 Otherwise, relay it along the Cskip = 31

tree Addr = 126

Addr = 1,
Cskip = 7
A

Example: Addr = 31
Addr = 32,
Addr = 40,
Cskip = 1
Cskip = 7
B
 38  45 Addr = 33,
Cskip = 1
 38  92 C Addr = 45
Addr = 38
Addr = 39

國立交通大學 資訊工程系 曾煜棋 教授 52

ZigBee Mesh Routing

Route discovery by AODV-like routing protocol
 The cost of a link is defined based on the packet delivery
probability on that link

Route discovery procedure

 The source broadcasts a route request packet
 Intermediate nodes will rebroadcast route request if

They have routing discovery table capacities

The cost is lower
 Otherwise, nodes will relay the request along the tree
 The destination will choose the routing path with the lowest cost and
then send a route reply

國立交通大學 資訊工程系 曾煜棋 教授 53

Routing in a Mesh network: Example

國立交通大學 資訊工程系 曾煜棋 教授 54

Summary of ZigBee network layer
Pros Cons
Star 1. Easy to synchronize 1. Small scale
2. Support low power
operation
3. Low latency
Tree 1. Low routing cost 1. Route reconstruction is
2. Can form superframes to costly
support sleep mode 2. Latency may be quite long
3. Allow multihop
communication
Mesh 1. Robust multihop 1. Cannot form superframes
communication (and thus cannot support
2. Network is more flexible sleep mode)
3. Lower latency 2. Route discovery is costly
3. Needs storage for routing
table
國立交通大學 資訊工程系 曾煜棋 教授 55