14-06-2009, 01:28 AM
Seminar Report
On
ZigBee: Next Generation Wireless Network
Submitted by
Anil Kumar K
In the partial fulfillment of requirements in degree of
Master of Technology (M-Tech)
in
SOFTWARE ENGINEERING
DEPARTMENT OF COMPUTER SCIENCE
COCHIN UNIVERSITY OF SCIENCE AND TECHNOLOGY
KOCHI-682022
2005Page 2
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ABSTRACT
ZigBee is an open technology developed by the SigBee Alliance to overcome the limitations of
BLUETOOTH and Wi-Fi. ZigBee is an IEEE 802.15.4 standard for data communications with
business and consumer devices. It is designed around low-power consumption allowing batteries to
essentially last forever. BLUETOOTH as we know was developed to replace wires and Wi-Fi to
achieve higher data transfer rate, as such till now nothing has been developed for sensor networking
and control machines which require longer battery life and continuous working without human
intervention. ZigBee devices allow batteries to last up to years using primary cells (low cost)
without any chargers (low cost and easy installation).
The ZigBee standard provides network, security, and application support services operating on top
of the IEEE 802.15.4 Medium Access Control (MAC) and Physical Layer (PHY) wireless standard.
It employs a suite of technologies to enable scalable, self-organizing, self-healing networks that can
manage various data traffic patterns. The network layer supports various topologies such star,
clustered tree topology and self healing mesh topology which is essential in Smartdust
Apart from easy installation and easy implementation ZigBee has a wide application area such as
home networking, industrial networking, Smartdust, many more, having different profiles specified
for each field. The upcoming of ZigBee will revolutionize the home networking and rest of the
wireless world. Page 3
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Table of Contents
1. Intorduction
3
2. Existing Standards
4
2.1. Wi-Fi (IEEE standard 802.11)
4
2.1.1. Standards
5
2.1.2. Network Types
5
2.2. Bluetooth (IEEE standard 802.15.1)
6
2.3. ZigBee (IEEE standard 802.15.4)
7
3. Intoduction to ZigBee
8
3.1. The ZigBee Alliance
8
3.2. The Name ZigBee
9
3.3. Why ZigBee
9
3.4. IEEE 802.15.4
11
3.5. Components of IEEE 802.15.4
11
4. ZigBee/IEEE 802.15.4 “ General Characteristics
12
4.1. ZigBee/IEEE 802.15.4 “ Typical Traffic Types Addressed
12
5. ZigBee Protocol Stack
14
5.1. The Physical Layer (PHY)
15
5.2. Media Access Layer (MAC)
16
5.2.1. Frame Structure
18
5.2.2. Super Frame Structure
20
5.3. Network and Security Layer
21
5.4. Application Layer
23
5.4.1. ZigBee Device Object
24
5.4.2. Application Support Layer
24
6. ZigBee Security
24
7. ZigBee Applications
25
8. Conclusion
28
9. Bibliography
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1.Introduction
It was in 1896 that Guglielmo Marconi invented the first wireless telegraph. In 1901 he sent
telegraphic signals across the Atlantic ocean from Cornwall to St. Johnâ„¢s Newfoundland; a distance
of 1800 miles. Over the last century, advances in wireless technologies have led to the radio, the
television, the mobile telephone, and communication satellites. All type of information can now be
send to any corner of the world.
A wireless network is a flexible data communication system, which uses wireless media such as
radio frequency technology to transmit and receive data over the air, minimizing the need for wired
connections. Wireless networks are used to augment rather than replace wired networks and are
most commonly used to provide last few stages of connectivity between a mobile user and a wired
network.
Wireless networks use electromagnetic waves to communicate information from one point to
another without relying on any physical connection. Radio waves are often referred to as radio
carriers because they simply perform the function of delivering energy to a remote receiver. The
data being transmitted is superimposed on the radio carrier so that it can be accurately extracted at
the receiving end. Once data is superimposed (modulated) onto the radio carrier, the radio signal
occupies more than a single frequency, since the frequency or bit rate of the modulating information
adds to the carrier. Multiple radio carriers can exist in the same space at the same time without
interfering with each other if the radio waves are transmitted on different radio frequencies. To
extract data, a radio receiver tunes in one radio frequency while rejecting all other frequencies. The
modulated signal thus received is then demodulated and the data is extracted from the signal.
Wireless networks offer the following productivity, convenience, and cost advantages over
traditional wired networks:
Mobility: provide mobile users with access to real-time information so that they can roam
around in the network without getting disconnected from the network. This mobility
supports productivity and service opportunities not possible with wired networks. Page 5
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Installation speed and simplicity: installing a wireless system can be fast and easy and can
eliminate the need to pull cable through walls and ceilings.
Reach of network: the network can be extended to places which can not be wired
More Flexibility: wireless networks offer more flexibility and adapt easily to changes in the
configuration of the network.
Reduced cost of ownership: while the initial investment required for wireless network
hardware can be higher than the cost of wired network hardware, overall installation
expenses and life-cycle costs can be significantly lower in dynamic environments.
Scalability: wireless systems can be configured in a variety of topologies to meet the needs
of specific applications and installations. Configurations can be easily changed and range
from peer-to-peer networks suitable for a small number of users to large infrastructure
networks that enable roaming over a broad area.
2. Existing Standards
In the world of wireless communication there are many standards existing today, each with a
specific application field and characteristics which best suites the need. However among so many
standard we will only discuss about Wi-Fi, Bluetooth and ZigBee as they are the most
complementary standards among all.
2.1.Wi-Fi (IEEE standard 802.11)
Wi-Fi is the wireless way to handle networking. It is also known as 802.11 networking and wireless
networking. The big advantage of Wi-Fi is its simplicity. Mobile connectivity for computers is a
rapidly growing requirement. Of the schemes that are available the IEEE 802.11 standard, often
termed Wi-Fi has become the de-facto standard. With peak operating speeds of around 54 Mbps it is
able to compete with many wired systems. As a result of the flexibility and performance of the
system, many Wi-Fi hotpots have been set up and more are following. These enabvle people to
use their laptop computers as they wait in hotels, airport lounges, cafes, and many other places using
a wire less page link rather that needing to use a cable. Page 6
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2.1.1. Standards
There is a plethora of standards under the IEEE 802 LMSC (LAN / MAN Standard Committee). Of
these even 802.11 has variety of standards, each with a letter suffix. These cover everything from
the wireless standards themselves, to standards for security aspects, quality of service and the like:
802.11a “ Wireless network bearer operating in the 5 GHz. ISM band with data rate up to 54 Mbps.
802.11b “ Wireless network bearer operating in the 2.4 GHz ISM band with data rates up to 11
Mbps
802.11e “ Quality of service and prioritization
802.11f “ Handover
802.11g “ Wireless network bearer operating in 24.GHz ISM band with data rates up to 54 Mbps
802.11h “ Power control
802.11i “ Authentication and encryption
802.11j “ Internetworking
802.11k “ Measurement reporting
802.11n “ stream multiplexing
802.11s “ Mesh networking
Of these the standards that are most widely known are the network bearer standards, 802.11a,
802.11b, 802.11g.
2.1.2 Network types
There are two types of network that can be formed: infrastructure networks; and ad-hoc networks.
The infrastructure application is aimed at office areas or to provide a hotspot. It can be installed
instead of a wired system, and can provide considerable cost savings, especially when used in
established offices. A backbone wired network is still required and is connected to a server. ThePage 7
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wireless network is then split up into a number of cells, each serviced by a base station or Access
Point (AP) which acts as a controller for the cell. Each Access Point may have a range of between
30 and 300 metres dependent upon the environment and the location of the Access Point.
The other type of network that may be used is termed as Ad-Hoc network. These are formed when a
number of computers and peripherals are brought together. They may be needed when several
people come together and need to share data or if they need to access a printer without the need for
having to use wire connections.
In this situation the user4s may only communicate with each other and not a larger wired network.
As a result there is no Access Point and special algorithms within the protocols are used to enable
one of the peripherals to take over the role of master to control the network with the others acting as
slaves.
2.2. Bluetooth
Bluetooth is based on IEEE standards 802.15.1. Bluetooth has now established itself in the market
place enabling a variety of devices to be connected together using wireless technology. Bluetooth
technology has come into its own connecting remote headsets to mobile phones, but it is also used in
a huge number of other applications as well.
Bluetooth technology originated in 1994 when Erricsson came up with a concept to use a wireless
connection to connect items such as an earphone and a cordless headset and the mobile phone.
The name of the Bluetooth standard originates from the Danish king Harald Blatand who was king
of Denmark between 940 and 981 AD. His name translates as Bluetooth and this was used as his
nickname. A brave warrior, his main achievement was that of uniting Denmark under the banner of
Christianity, and then uniting it with Norway that he had conquered. The Bluetooth standard was
named after him because Bluetooth endeavors to unite personal computing and telecommunications
devices. Page 8
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Bluetooth is a wireless data system and can carry data at speeds up to 721 Kbps in its basic form and
in addition to this it offers up to three voice channels. Bluetooth technology enables a user to
replace cables between devices such as printers, fax machines, desktop computers and peripherals,
and a host of other digital devices. Furthermore, it can provide a connection between an ad-hoc
wireless network and existing wired data networks.
The technology is intended to be placed in a low cost module that can be easily incorporated into
electronics devices of all sorts. Bluetooth uses the license free Industrial, Scientific and
Medical(ISM) frequency band for its radio signals and enables communications to be established
between devices up to a maximum distance of 100 metres. Running in the 2.4 GHz ISM band,
Bluetooth employs frequency hopping techniques with the carrier modulated using Gaussian
Frequency Shift Keying (GFSK).
After a network connection is established between two devices they change their frequency 1600
times per second thus leaving no time for interference, and if by chance there is interference it will
be for few microseconds. No other sub network will be working at the frequency at which other sub
networks work, thus eliminating interference.
2.3. ZigBee
ZigBee is a wireless networking standard that is aimed at remote control and sensor applications
which is suitable for operation in harsh radio environments and in isolated locations, It builds on
IEEE standard 802.15.4 which defines the physical and MAC layers. Above this ZigBee defines the
application and security layer specifications enabling interoperability between products from
different manufacturers. In this way ZigBee is a superset of the 802.15.4 specification.
With the applications for remote wireless sensing and control growing rapidly it is estimated that the
market size could reach hundreds of millions of dollars as early as 2007. This makes ZigBee a very
attractive proposition, and one, which warrants the introduction of a focused standard. Page 9
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3. Introduction to ZigBee
The past few years have witnessed a rapid growth of wireless networking. However, up to now
wireless networking has been mainly focused on high “ speed communications, and relatively long
range applications such as IEEE 802.11 wireless local area network standards. The first well known
standard focusing on low rate wireless personal area networks was BLUETOOTH. However it has
limited capacity for networking of many nodes. There are many wireless monitoring and control
applications in industrial and home environments which require longer battery life, lower data rates
and less complexity than those from existing standards. For such wireless applications, a new
standard called IEEE 802.15.4 has been developed by IEEE. The new standard is also called
ZigBee.
3.1 The Zigbee Alliance
The ZigBee standard is organized under the auspices of the ZigBee Alliance. The ZigBee alliance is
an organization of companies working together to define an open global standard for making low
power wireless networks. The intended outcome of ZigBee alliance is to create a specification
defining how to build different network topologies with data security features and interoperable
application profiles. This organization has over 150 members, of which seven have taken on the
status of what they term promoter. These seven companies are Ember, Honeywell, Invensys,
Mitsubishi, Motorola, Philips and Samsung. A big challenge for the alliance is to make the
interoperability to work among different products. To solve this problem, the ZigBee Alliance has
defines profiles, depending on what type of category the product belongs to. For example there is a
profile called home lightning that exactly defines how different brands of home lightning-products
should communicate with each other. Under the umbrella of the ZigBee Alliance, the new standard
will be pushed forward, taking on board the requirements of the users, manufacturers and the system
developers.
The Alliance has specified three profiles
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Private Profile: In this profile interoperability is not at all important. However producers
cannot use the official ZigBee stamp, but can claim that Ëœbased on ZigBee platformâ„¢.
Published Profile: A private profile is shared among other users. Still one cannot use official
ZigBee stamp, but can claim Ëœbased on ZigBee platformâ„¢.
Public profile: It is the official ZigBee profile.
3.2. The Name ZigBee
The name ZigBee is said to come from the domestic honeybee which uses a zig-zag type of dance to
communicate important information to other hive members. This communication dance (The
ZigBee Principle) is what engineers are trying to emulate with this protocol “ a bunch of separate
and simple organisms that join together to tackle complex tasks.
3.3 Why ZigBee?
There are a multitude of standards like Bluetooth and Wi-Fi that address mid to heigh data rates for
voice, PC LANs, video etc. However, up till now there hasnâ„¢t been a wireless network standard that
meets the unique needs of sensors and control devices. Sensors and controls donâ„¢t need high
bandwidth but they do need low latency and very low energy consumption for long battery lives and
for large device arrays.
There are a multitude of proprietary wireless systems manufactured today to solve a multitude of
problems that donâ„¢t require high data rates but do require low cost and very low current drain. These
proprietary systems were designed because there were no standards that met their application
requirements. These legacy systems are creating significant interoperability problems with each
other and with newer technologies.
The ZigBee Alliance is not pushing a technology; rather it is providing a standardized base set of
solutions for sensor and control systems. Here are the following points that justify the use of ZigBee
over the existing standards. Page 11
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Low power consumption, simply implemented: Users expect batteries to last many months to
years! Consider that a typical single-family house has about 6 smoke/CO detectors. If the
batteries for each one only lasted six months, the home owner would be replacing
batteries every month!
In contrast Bluetooth, which has many different modes and states depending upon your
latency and power requirements, ZigBee/IEEE 802.15.4 has two major states:
active(transmit/receive) or sleep. The application software needs to foicus on the
application, not on which power mode is optimum for each aspect of operation.
Even mains powered equipment needs to be conscious of energy. ZigBee devices will be
more ecological than their predecessors saving megawatts at it full deployment. Consider
a future home that has 100 wireless control/sensor devices,
Case 1: 802.11 Rx power is 667 mW (always on) @ 100 devices/home & 50,000
homes/city = 150 3.33 megawatts.
Case 2: 802.15.4 Rx power is 30 mW (always on) @ 100 devices/home & 50,000
homes/city = 150 kilowatts.
Case 3: 802.15.4 power cycled at .1% (typical duty cycle) = 150 watts
Low cost to the users means low device cost, low installation cost and low maintenance.
ZigBee devices allow batteries to last up to years using primary cells (low cost)
without any chargers (low cost and easy installation). ZigBeeâ„¢s simplicity allows
for inherent configuration and redundancy of network devices provides low
maintenance.
High density of nodes per network: ZigBeeâ„¢s use of the IEEE 802.15.4 PHY and MAC
allows networks to handle any number of devices. This attribute is critical for massive
sensor arrays and control networks.
Simple protocol, global implementation: ZigBeeâ„¢s protocol code stack is estimated to be
about 1/4
th
of Bluetoothâ„¢s or 802.11â„¢s. Simplicity is essential to cost, interoperability, and
maintenance. The IEEE 802.15.4 PHY adopted by ZigBee has been designed for the 868 Page 12
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MHz band in Europe, the 915 MHz band in N America, Australia, etc; and the 2.4 GHz
band is now recognized to be a global band accepted in almost all countries.
3.4 IEEE 802.15.4
IEEE 802.15 is the working group 15 of the IEEE 802 which specializes in Wireless PAN standards.
It includes four task groups (numbered from 1 to 4):
Task group 1 (WPAM/Bluetooth) deals with Bluetooth, having produced the 802.15.1
standard, published on June 14, 2002. It includes a medium access control and physical
layer specification adapted from Bluetooth 1.1.
Task group 2 (coexistence) deals with coexistence of Wireless LAN (802.11) and Wireless
PAN.
Task group 3 is in fact two groups: 3 (WPAN High Rate) and 3a (WPAN Alternate Higher
Rate), both dealing with high-rate WPAN standards (20 Mbit/s or higher).
Task group 4 (WPAN Low Rate) deals with low rate but very long battery life (months or
even years). The first edition of the 802.15.4 standard was released in May 2003. In
March 2004, after forming Task Group 4b, task group 4 put itself in hibernation.
The new Task Group 4b aims at clarifying and enhancing specific parts of the Task Group 4
standard.
3.5 Components of IEEE 802.15.4
IEEE 902.15.4 networks use three types of devices.
The network coordinator maintains the overall network knowledge. It is the most
sophisticated one of the three types and required the most memory and computing power. Page 13
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The Full Function Device (FFD) supports all IEEE 802.15.4 functions and features specified
by the standard. It can function as a network coordinator. Additional memory and
computing power make it ideal for network router functions or it could be used in
network-edge devices (where the network touches the real world).
The Reduced Function Device (RFD) carries limited (as specified by the standard)
functionality to lower cost and complexity. It us generally found in network-edge
devices.
4. ZigBee/IEEE 802.15.4 “ General Characteristics
Data rates of 250 kbps (@2.4 GHz), 40 Kbps (@ 915 MHz) and 20 kbps (@868 MHz)
Optimized for low duty-cycle applications (<0.1%).
Low power (battery life multi-month to years).
Multiple topologies: star, peer-to-peer, mesh.
CSMA-CA channel access yields high throughput and low latency for low duty cycle devices
like sensors and controls.
Addressing space of 64 bits “ 18,450,000,000,000,000,000 devices (64 bit IEEE address) “
65,535 networks.
Optional guaranteed time slot for applications requiring low latency.
Fully hand-shaked protocol for transfer reliability
Range: 50m typical (5-500m based on environment).
4.1 ZigBee/IEEE 802.15.4 “ Typical Traffic types Addressed
Following are typical traffic types specified:
Periodic dataPage 14
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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)
Each of these traffic types mandates different attributes from the MAC. The IEEE 802.15.4 MAC is
flexible enough to handle each of these types.
Periodic data can be handled using the beaconing system whereby the sensor will wake up for
the beacon, check for any messages and then go back to sleep.
Intermittent data can be handled either in a beaconless system or in a disconnected fashion.
In a disconnected operation the device will only attach to the network when it needs to
communicate saving significant energy.
Low latency applications may choose to the guaranteed time slot (GTS) option. GTS is a
method of QoS (Quality of Service) in that it allows each device a specific duration of
time each Superframe to do whatever it wishes to do without contention or latencyPage 15
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5. ZigBee Protocol Stack
The ZigBee protofol stack is 1/4
th
of that of Wi-Fi and Bluetooth. It may be helpful to think of IEEE
802.15.4 as the physical radio and ZigBee as the logical network and application software.
Following the standard Open Systems Intenconnection (OSI) reference model, ZigBeeâ„¢s protocol
stack is structured in layers. The first two layers, physical (PHY) and media access (MAC) are
defined by the IEEE 802.15.4 standard as shown in the figure Ëœfig 5.1â„¢. The layers above them are
defined by the ZigBee Alliance. The IEEE working group passed the first draft of PHY and MAC in
2003.
Fig 5.1 ZigBeeâ„¢s Protocol Stack
PHY LAYER
MAC LAYER
MAC LAYER
DATA LINK LAYER
NETWORK LAYER
APPLICATION INTERFACE
APPLICATION
ZigBee or OEM
IEEE
ZigBee
AlliancePage 16
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5.1 The Physical Layer (PHY)
ZigBee-compliant products operate in unlicensed bands worldwide, including 2.4 GHz (global), 902
to 928 MHz. (America) and 868 MHz (Europe). Raw data throughput rates of 250Kbps can be
achieved at 2.4 GHz (16 channels), 40 Kbps at 915 MHz (10 channels), and 20 Kbps at 868 MHz (1
channel). The transmission distance is expected to range from 10 to 75m, depending on power
output and environmental characteristics. Like Wi-Fi, ZigBee uses direct-sequence spread spectrum
in the 2.4 GHz band, with offset-quadrature phase shift keying modulation. Channel width is 2 MHz
with 5 MHz channel spacing. The 868 and 900 MHz bands also use direct-sequence spread
spectrum but with binary-phases shift keying modulation.
868/915 MHz Band Modulation
The transmitter must be capable of transmitting atleast “3dbm although this should be reduced when
possible to reduce interference to other users. The maximum allowable power will depend on local
regulatory bodies. The receiver must have a packet error rate of <1% for input signals at the antenna
connector of >-92dBm.
2450 MHz Band Modulation
The transmitter must be capable of transmitting at least “3dBm although this should be reduced
when possible to reduce interference to other users. The maximum allowable power will depend on
local regulations.
What is Direct Sequence Spread Spectrum (DSSS)?
In direct Sequence Spread Spectrum a bit is assigned a particular code spectrum that is transmitted
and on the destination node that code is replaced by that specific bit, this way assigning the code
spectrum utilizes bandwidth efficiently. Page 17
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The fig 5.2. shows the operating frequencies offered by the physical layer of ZigBee protocol. The
phusical also specifies other parameters for transmission such as device types that are used.
Speading Parameters
Data Parameters
PHY
Frequency
Band
Channel
Numbering Chip Rate Modulation Bit Rate
Symbol
Rate
Modulation
868 to 870
MHz
0
300
Kchip/s
BPSK
20 Kb/s 20 Kbaud
BPSK
868 to
915
MHz
902 to 928
MHz
1 to 10
600
Kchip/s
BPSK
40 Kb/s 40 Kbaud
BPSK
2.4 GHz
2.4 to 2.4835
GHz
11 to 26
2.0
Mchip/s
O-QPSK
250
Kb/s
62.5 Kbaud
16-ary
Orthogonal
Fig 5.2 Table showing ZigBeeâ„¢s operating frequency and modulation technologies used
Tow types of devices are defined: Full Function Device (FFD) and Reduced Function Device
(RFD). An FFD can serve as a coordinator or a regular device. It can communicate with any other
devices within its transmission range. An RFD is a simple device that associates and communicates
only with an FFD, The IEEE 802.15.4 PHY layer provides a parameter, Link Quality Indivation
(LQI), to characterize the quality of received signal. It can be the received power, the estimated
signal-to-noise-ration (SNR), or a combination of both. LQI is passed to MAC layer and finally
available to the network and upper layers. Other futures of PHY layer include the activation and
deactivation of the radio transceiver, channel selection, clear channel assessment, and
transmitting/receiving packets across physical medium.
5.2 Media Access Layer (MAC)
There are two channel access mechanisms used by MAC Layer:
Non-Beacon mode
Beacon mode
ZigBee networks can use beacon or non-beacon environments. Beacons are used to synchronize the
network devices, identify the PAN and describe the structure of the superframe. The beacon Page 18
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intervals are set by the network coordinator and vary from 15ms to over 4 minutes. Sixteen equal
time slots are allocated between beacons are message delivery. The channel access in each time slot
is contention-based. However, the network coordinator can dedicate up to seven guaranteed time
slots for non contention based or low-latency delivery.
The non-beacon mode is a simple, traditional multiple-access system used in simple peer and near-
pear networks. It operates like a two-way radio network, where each client is autonomous and can
initiate a conversation at will, but could interfere with others unintentionally. The recipient may not
here the call or the channel might already be in use
Beacon Mode is a mechanism for controlling power consumption in extended networks such as
cluster tree or mesh. It enables all the clients to know when to communicate with each other. Here,
the two-way radio network has a central dispatcher that manages the channel and arranges the calls.
The primary value of beacon mode is that it reduces the systemâ„¢s power consumption
Non-beacon mode is typically used for security systems where client units, such as intrusion sensors,
motion detectors, and glass-break detectors, sleep 99.999% of the time.
Remote units wake up on a regular, yet random, basis to announce their continued presence in the
network. When an event occurs, the sensor wakes up instantly and transmits the alert (Somebody is
on the front porch). The network coordinator, powered from the main source, has its receiver on all
the time and can therefore wait to hear from each of these stations. Since the network coordinator
has an infinite source of power it can allow clients to sleep for unlimited periods of time, enabling
them to save power.
Beacon mode is more suitable when the network coordinator is battery-operated. Client units listen
for the network coordinatorâ„¢s beacon (broadcast at intervals between 0.015 and 252 s). A client
registers with the coordinator and looks for any messages directed to it. If no messages are pending,
the client returns to sleep, awaking on a schedule specified by the coordinator. Once the client
communications are completed, the coordinator itself returns to sleep. Page 19
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This timing requirement may have an impact on the cost of the timing circuit in each end device.
Longer intervals of sleep mean that the timer must be more accurate or turn on earlier to make sure
that the beacon is heard, both of which will increase receiver power consumption. Longer sleep
intervals also mean the timer must improve the quality of the timing oscillator circuit (which
increases cost) or control the maximum period of time between because to not exceed 252s, keeping
oscillator circuit costs low.
5.2.1 Frame Structure
The frame structures have been designed to keep the complexity to minimum while at the same time
making them sufficiently robust for transmission on a noisy channel. Each successive protocol layer
adds to the structure with layer-specific headers and footers.
The IEEE 802.15.4 MAC defines four frame structures:
A beacon frame, used by a coordinator to transmit beacons. The beacon frame wakes up
client devices, which listen for their address and go back to sleep if they donâ„¢t receive it.
Beacons are important for mesh and cluster-tree networks to keep all the nodes
synchronized without requiring those nodes to consume precious battery energy by
listening for long periods of time.
A data frame, used for all transfers of data. The data frame provides a payload of up to 104
bytes. The frame is numbered to ensure that all packets are tracked. A frame-check
sequence ensures that packets are received without error. This frame structure improves
reliability in difficult conditions. This frame is shouwn in fig. 5.3.
An acknowledgment frame, used for confirming successful frame reception It provides
feedback from the receiver to the sender confirming that the packet was received without
error. The device takes advantage of specified quiet time between frames to send a
short packet immediately after the data-packet transmission.
A MAC command frame, used for handling all MAC peer entity control transfers. A Mac
command frame provides the mechanism for remote control and configuration of clientPage 20
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nodes. A centralized network manager uses MAC to configure individual clientsâ„¢
command frames no matter how large the network
The data frame is illustrated below in fig 5.3:
Fig 5.3 ZigBeeâ„¢s Data Frame
The Physical Protocol Data Unit is the total information sent over the air. As shown in the
illustration above the Physical layer adds the following overhead:
Preamble sequence
:
4 Octets
Start of Frame Delimiter
:
1 Octet
Frame Length
:
1 Octet
The MAC adds the following overhead:
Frame control
:
2 Octets
Data Sequence Number
:
1 Octet
Address Information
:
4 to 20 OctetsPage 21
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Frame Check Sequence
:
2 Octets
The total overhead for a single packet is therefore 15 “ 31 octets (120 bits); depending upon the
addressing scheme used (short or 64 bit addresses). These numbers do not include any security
overhead.
5.2.2 Super Frame Structure
The 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 as shown in fig 5.4. The beacons are used
to synchronize the attached devices, to identify the PAN and to describe the structure of the
superframes. Any device wishin to communicate during the contention access period (CAP)
between two beacons shall compete with other devices using a slotted CSMA-CA mechanism. All
transactions shall be completed by the time of the next network beacon.
Fig. 5.4 ZigBeeâ„¢s super frame structure bounded by two beacons
For the low latency applications or applications requiring specific data bandwidth, the PAN
coordinator may dedicate portions of the active superframe to that application. These portions are
called guaranteed time slots (GTSs). The guaranteed time slots comprise the contention free period
(CFP), which always appears at the end of the active superframe starting at a slot boundaryPage 22
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immediately following the CAP, as shown in fig. 5.5. the PAN coordinator may allocate up to seven
of these GTSs and a GTS may occupy more than one slot period. However, a sufficient portion of
the CAO shall remain for contention-based access of other networked devices or new devices
wishing to join the network. All contention-based transactions shall be complete before the CFP
begins. Also each device transmitting in a GTS shall ensure that its transaction is complete before
the time of the next GTS or the end of the CFP.
Fig. 5.5 ZigBeeâ„¢s superframe structure with contention access and free period
5.3 Network and Security Layer (NWK)
The NWK layer associates or dissociates devices using the network coordinator implements security,
and routes frames to their intended destination. In addition, the NWK layer of the network
coordinator is responsible for starting a new network and assigning an address to newly associated
devices.
The NWK layer supports multiple network topologies including star, cluster tree, and mesh as
shown in fig 5.6 and fig 5.7. In a star topology, one of the FFD-type devices assumes the role of
network coordinator and is responsible for initiating and maintaining the devices on the network.
All other devices, known as end devices, directly communicate with the coordinator. Page 23
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Fig. 5.6 Star and pear-to-pear network topology
In a mesh topology, the ZigBee coordinator is responsible for starting the network and for choosing
key network parameters, but the network may be extended through the use of ZigBee routers. The
routing algorithm uses a request-response protocol to eliminate sub-optimal routing. Ultimate
network size can reach 264 nodes (more than weâ„¢ll probably need). Using local addressing, you can
configure simple networks of more than 65,000 (2
16
) nodes, thereby reducing address overhead. Page 24
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Pan Coordinator
Cluster Head
Device
Fig 5.7 Cluster tree topology
5.4 Application Layer.
The ZigBee application layer consists of the APS sub-layer, the ZDO and the manufacturer-defined
application objects. The responsibilities of the APS sub-layer include maintaining tables for
binding, which is the ability to match two devices together based on their services and their needs,
and forwarding messages between bound devices. Another responsibility of the APS sub-layer is
discovery, which is the ability to determine which responsibilities of the ZDO include defining the
role of the device within the network (e.g. ZigBee coordinator or end device), initiating and/or
responding to binding requests and establishing a secure relationship between network devices. The
manufacturer-defined application objects implement the actual applications according to the ZigBee-
defined application descriptions. Page 25
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5.4.1 ZigBee Device Object
Defines the role of the device within the network (e.g. ZigBee coordinator or end device)
Initiates and/or responds to binding requests
Establishes a secure relationship between network devices selecting one of ZigBeeâ„¢s security
methods such as public key, symmetric key etc.
5.4.2 Application Support Layer
This layer provides the following services:
Discovery: The ability to determine which other devices are operating in the personal operating
space of a device.
Binding: The ability to match two or more devices together based on their services and their needs
and forwarding messages between bound devices.
The General Operation Framework (GOF) is a glue layer between applications and rest of the
protocol stack. The GOF currently covers various elements that are common for all devices. It
includes sub-addressing and addressing modes and device descriptions, such as type of device,
power source, sleep modes, and coordinators using an object model, the GOF specifies methods,
events, and data formats that are used by application profiles to construct set/get commands and their
responses.
Actual application profiles are defined in the individual profiles of the IEEEâ„¢s working groups.
Each ZigBee device can support up to 30 different profiles. Currently, only one profile, Commercial
and Residential Lighting, is defined. It includes switching and dimming load controllers,
corresponding remote-control devices, and occupancy and light sensors.
6. ZigBee Security
When security of MAC layer frames is desired, ZigBee uses MAC layer security to secure MAC
command, beacon, and acknowledgment frame. ZigBee may secure messages transmitted overPage 26
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single hop using secured MAC data frames, but for multi-hop messaging ZigBee relies upon upper
layers (such as the NWK layer) for security. The MAC layer uses the Advanced Encryption
Standard (AES) as its core cryptographic algorithm and describes a variety of security suites that use
the AES algorithm. These suites can protect the confidentiality, integrity, and authenticity of MAC
frames. The MAC layer does the security processing, but the upper layers, which set up the keys
and determine the security levels to use, control this processing. When the MAC layer transmits
(receives) a frame with security enabled, it looks at the destination (source) of the frame, retrieves
the key associated with that destination (source), and then uses this key to process the frame
according to the security suite designated for the key being used. Each key is associated with a
single security suite and the MAC frame header has a bit that specifies whether security for a frame
is enabled or disabled.
7. ZigBee Applications
The ZigBee Alliance targets applications Across consumer, commercial, industrial and government
markets worldwide. Unwired applications are highly sought after in many networks that are
characterized by numerous nodes consuming minimum power and enjoying long battery lives.
ZigBee technology is designed to best suit these applications, for the reason that it enables reduced
costs of development, very fast market adoption and rapid ROI..
For the last few years, we have witnessed a great expansion of remote control devices in our day-to-
day life. Five years ago, infrared (IR) remotes for the television were the only such devices in our
homes. Now the number of devices is uncountable. This number will only increase as more devices
are controlled or monitored from a distance. To interact with all these remotely controlled devices,
we will need to put them under a single standardized control interface that can interconnect into a
network, specifically a HAN or home-area network.
ZigBee applications can be divided into the following groups.
Home networking
Industrial control and management
Human and computer interfacePage 27
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Smart dust
Intrusion sensors, motion detectors and glass break detectors.
The Home is receiving a lot of attention lately as the place that could do with a lot of new
technology. Some of it seems like wishful thinking. Ideas that we want to connect all our electronic
devices at home “ from PCs, stereos, TV and DVD players to the security system, all utility meters,
microwave oven, fridge and even toaster “ to a single home network that is then connected to the
internet does not stand up to much scrutiny at this time. Why would we all want to do that?
In fact, the home networking market appears to be fragmented into four different application areas:
PC networking, connecting two or more PCs to a single broadband connection to the Internet
as well as printers and other resources that can be shared.
Home entertainment distribution, sharing content among televisions, stereos, and game
consoles around the home.
Home control, where one group of applications offers electronic control of heating, lighting
and security systems.
Home appliances, where your fridge can access recipes on the Internet or shop on your
behalf and your washing machine can call a service engineer.
Then there is Microsoftâ„¢s work on SPOT (Smart Personal Object Technology) that seems to be a
way for Microsoft to try to Improve everyday household objects like alarm clocks, key chains ad
pens.
Of these, PC networking is clearly in the ascendancy at present, as a direct result of the rollout of
broadband connections to the home. The second typically involves connecting the TC to the Stereo
system, for example, and looks a little less certain as a mass market. It may well pick up steam
though also as a result of broadband connections “transferring those music and video files from the
PC to the home entertainment system perhaps. Page 28
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Industrial automation includes extending existing manufacturing and process control systems
reliably and improve asset management by continuously monitoring critical equipment.
Using ZigBee we can reduce energy costs through optimized manufacturing process and
to identify inefficient operation or poorly performing equipment.
Smart dust an emerging technology can be used for various purposes such as surveillance,
military purposes, weather monitoring and many other things which are still beyond reach.
The goal of the Smart Dust project is to build a self-contained, millimeter-scale sensing and
communication platform for a massively distributed sensor network. This device will be
around the size of a grain of sand and will contain sensors, computational ability, bi-
directional wireless communications, and a power supply, while being inexpensive
enough to deploy by the hundreds. Smart Dust may not be the subject matter of science
fiction any longer “ the advent of ZigBee and other wireless protocols suitable for sensor
networks is pushing the technology to the next level. Page 29
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8. Conclusion
Bluetooth has already matured and graduated to version 1.2 after its initial hype. Lots of products
compliant to Bluetooth version 1.1 are available on the market. Will ZigBee be able to compete
with Bluetooth in the market? And if yes, will it replace Bluetooth? This question is asked by the
people where since ZigBee came to the market. We have already seen all the aspects of both
ZigBee and Bluetooth. And hence can be concluded that ZigBee and Bluetooth are two solutions for
two different application areas. The differences are from their approach to their desired application.
Bluetooth has addressed a voice application by embodying a fast frequency hopping system with a
master slave protocol. ZigBee has addressed sensors, controls, and other short message applications
by embodying a direct sequence system with a star or peer-to-peer protocols. Minorchanges to
Bluetooth or ZigBee wonâ„¢t change their inherent behaviour or characteristics. The different
behaviours come from architectural differences. Page 30
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9. Bibliography
[1]
http://standards.ieee.org
[2]
http://sigbeeen/about/initial_markets/1_app_home.asp
[3]
http://zigbeeen/documents/zigbeeoverview4.pdf
[4]
http://palowirelesszigbee/tutorials.asp
[5]
http://zigbeeen/resources/031418r00ZB-MG-ZigBeeTechnology.doc
[6]
http://en.wikipediawiki/Zigbee
[7]
Behrouz A. Frouzan, Data Communication, Third Edition, Tata McGraw-Hill Publishing
company Limitted, 2004
[8]
Andrew S. Tenenbaum, Computer Networks, Fourth Edition Pearson Publication
Limited, 2003
[9]
William Stalling, Wireless Communication and Networks, Fourth Edition, Pearson
Publication Limited, 2004
[10]
James Kurose & Keith W. Ross, Computer Networks, Fourth Edition, Pearson
Publication Limited, 2
