zigbee technology 2012

7/30/2019 Zigbee Technology 2012

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ZIGBEE TECHNOLOGY

Dept of ECE JNTUCEH, HYD

ABSTRACT

ZigBee is an IEEE 802.15.4 standard for data communications with business and consumer devices. It

isdesigned around low-power consumption allowing batteries to essentially last forever. 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. ZigBee is a low-cost, low-power,

wireless mesh networking standard. The low cost allows the technology to be widely deployed in wireless control and

monitoring applications, the low power-usage allows longer life with smaller batteries, and the mesh networking

provides high reliability and larger range.ZigBee has been developed to meet the growing demand for capable wireless

networking between numerous lowpower devices. In industry ZigBee is being used for next generation

automatedmanufacturing, with small transmitters in every device on the floor, allowing for communication between

devices to a central computer. This new level of communication permits finely-tuned remote monitoring and

manipulation.

Keywords: Medium Access Control (MAC), Physical Layer (PHY), Wireless Personal Area Networking

(WPAN), Open Systems Interconnection

(OSI).

1. INTRODUCTION

Evolution of Low-Rate Wireless Personal

Area Network (LR-WPAN)

Standardization

The IEEE 802.11 working group for

Wireless Local Area Network (WLAN) is

formed, to create a wireless local area network

standard. Whereas IEEE 802.11 was

concerned with features such as Ethernet

matching speed, long range(100m),

complexity to handle seamless roaming,

message forwarding, and data throughput of 2-

11Mbps.

Wireless personal area networks

(WPANs) are used to convey information over

relatively short distances. WPANs are focused

on a space around a person or object

that typically extends up to 10m in all

directions. The focus of WPANs is low-cost,

low power, short range and very small size.

The IEEE 802.15 working group create

WPAN standard.

This group has currently defined three classes

of WPANs that are differentiated by data rate,

battery drain and quality of service (QoS).

The high data rate WPAN (IEEE

802.15.3) is suitable for multi-media

applications that require very high

QoS.

Medium rate WPANs(IEEE

802.15.1/Bluetooth) will handle a



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variety of tasks ranging from cell

phones to PDA communications and

have QoS suitable for voice

communications.

The low rate WPANs (IEEE

802.15.4/LR-WPAN) is intended to

serve a set of industrial, residential

and medical applications with very

low power consumption, with relaxed

needs for data rate and QoS. The low

data rate enables the LR-WPAN to

consume very little power. This

feature allows small, power-efficient,

inexpensive solutions to be

implemented for a wide range of

devices.

1.2 Zigbee and IEEE 802.15.4

The IEEE 802.15.4 standard is a simple

packet data protocol for lightweight wireless

networks and specifies the Physical (PHY) and

Medium Access Control (MAC) layers for

Multiple Radio Frequency (RF) bands,

including 868 MHz, 915 MHz, and 2.4 GHz.

The IEEE 802.15.4 standard is designed

to provide reliable data transmission of modest

amounts of data up to 100 meters or more

while consuming very little power. IEEE

802.15.4 is typically less than 32 kb in size,

featuring a 64-bit address space, source and

destination addressing, error detection, and

advanced power management.

ZigBee technology takes full advantage

of the IEEE 802.15.4 standard and extends the

capabilities of this new radio standard by

defining a flexible and secure network layer

that supports a variety of architectures to

provide highly reliable wireless

communications in harsh or dynamic RF

environments.

ZigBee technology also offers

simplicity and a cost-effective approach to

building, construction and remodelling with

wireless technology. ZigBee is all set to

provide the consumers with ultimate

flexibility, mobility, and ease of use by

building wireless intelligence and capabilities

into every day devices.

ZigBee is expected to provide low cost

and low power connectivity for equipment that

needs battery life as long as several months to

several years but does not require data transfer

rates as high as those enabled by Bluetooth.

This kind of network eliminates use of

physical Ethernet cables. The devices could

include telephones, hand-held digital

assistants, sensors and controls located within

a few meters of each other.

Thus, ZigBee technology is a low data

rate, low power consumption, low cost;

wireless networking protocol targeted towards

automation and remote control applications.

ZigBee Alliance



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communication redundancy eliminating

“single point of failure” in mesh networks.

IEEE and ZigBee Alliance have been

working closely to specify the entire protocol

stack. IEEE 802.15.4 focuses on the

specification of the lower two layers of the

protocol (physical and data link layer). On the

other hand, ZigBee Alliance aims to provide

the upper layers of the protocol stack (from

network to the application layer) for

interoperable data networking, security

services and a range of wireless home and

building control solutions.

Zigbee characteristics

The focus of network applications under

the IEEE 802.15.4 / ZigBee standard include

the features of low power consumption,

needed for only two major modes (Tx/Rx or

Sleep), high density of nodes per network, low

costs and simple implementation.

These features are enabled by the following

characteristics

2.4GHz and 868/915 MHz dual PHY

modes.

This represents three license-free bands:

2.4-2.4835 GHz, 868-870 MHz and 902-

928 MHz The number of channels

allotted to each frequency band is fixed

at 16 channels in the 2.45 GHz band, 10

channels in the 915 MHz band, and 1

channel in the 868 MHz band

Maximum data rates allowed for each of

these frequency bands are fixed as 250

kbps @2.4 GHz, 40 kbps @ 915 MHz,

and 20 kbps @868 MHz

Allocated 16 bit short or 64 bit extended

addresses.

Allocation of guaranteed time slots

(GTSs)

Carrier sense multiple access with

collision avoidance (CSMA-CA)

channel access Yields high throughput

and low latency for low duty cycle

devices like sensors and controls.

Fully “hand-shake” acknowledged

protocol for transfer reliability.

Low power consumption with battery

life ranging from months to years.

Energy detection (ED).

Link quality indication (LQI).

Multiple topologies : star, peer-to-peer,

mesh topologies

Device Types

ZigBee devices are required to

conform to the IEEE 802.15.4-2003 Low-Rate

Wireless Personal Area Network (WPAN)

standard.

ZigBee wireless devices are expected

to transmit 10-75 meters, depending on the RF

environment and the power output

consumption required for a given application,

and will operate in the unlicensed RF

worldwide (2.4GHz global, 915MHz Americas



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or 868 MHz Europe). The data rate is 250kbps

at 2.4GHz, 40kbps at 915MHz and 20kbps at

868MHz.

There are three different ZigBee device

types that operate on these layers in any self-

organizing application network. These devices

have 64-bit IEEE addresses, with option to

enable shorter addresses to reduce packet size,

and work in either of two addressing modes –

star and peer-to-peer.

The ZigBee (PAN) coordinator node:

The most capable device, the coordinator

forms the root of the network tree and

might bridge to other networks. It is able

to store information about the network.

There is one, and only one, ZigBee

coordinator in each network to act as the

router to other network. It also acts as the

repository for security keys.

The Full Function Device (FFD): The

FFD is an intermediary router transmitting

data from other devices. It needs lesser

memory than the ZigBee coordinator

node, and entails lesser manufacturing

costs. It can operate in all topologies and

can act as a coordinator.

The Reduced Function Device (RFD) :

This device is just capable of talking in the

network; it cannot relay data from other

devices. Requiring even less memory, (no

flash, very little ROM and RAM), an RFD

will thus be cheaper than an FFD. This

device talks only to a network coordinator

and can be implemented very simply in

star topology.

Network Topologies

Three types of topologies that ZigBee

supports: star topology, peer-to-peer topology

and cluster tree.

Star Topology

In the star topology, the

communication is established between devices

and a single central controller, called the PAN

coordinator. The PAN coordinator may be

mains powered while the devices will most

likely be battery powered. Applications that

benefit from this topology include home

automation, personal computer (PC)

peripherals, toys and games.



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Peer-to-peer Topology

In peer-to-peer topology, there is also

one PAN coordinator. In contrast to star

topology, any device can communicate with

any other device as long as they are in range of

one another. A peer-to-peer network can be ad

hoc, self-organizing and self-healing.

Applications such as industrial control and

monitoring, wireless sensor networks, asset

and inventory tracking would benefit fromsuch a topology. It also allows multiple hops

to route messages from any device to any other

device in the network. It can provide reliability

by multipath routing.

Cluster-tree Topology

Cluster-tree network is a special case

of a peer-to-peer network in which most

devices are FFDs and an RFD may connect to

a cluster-tree network as a leave node at the

end of a branch. Any of the FFD can act as a

coordinator and provide synchronization

services to other devices and coordinators.

The advantage of this clustered

structure is the increased coverage area at the

cost of increased message lat

Architecture

The LR-WPAN architecture is defined in

terms of a number of blocks in order to

simplify the standard. These blocks are called

layers. Each layer is responsible for one part of

the standard and offers services to the higher

layers. The layout of the blocks is based on the

open systems interconnection (OSI) seven-

layer model. The interfaces between the layers

serve to define the logical links between

layers. The LR-WPAN architecture can be

implemented either as embedded devices or as

devices requiring the support of an external

device such as a PC.

Network and Application Support layer:

The network layer permits growth of

network sans high power transmitters. This

layer can handle huge numbers of nodes.

This level in the ZigBee architecture includes

The ZigBee Device Object (ZDO)

User-Defined Application Profile(s)

The Application Support (APS) Sub-

layer.

The APS sub-layer's responsibilities

include maintenance of tables that enable

matching between two devices and

communication among them, and also

discovery, the aspect that identifies other



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devices that operate in the operating space of

any device.

The responsibility of determining the

nature of the device (Coordinator / FFD or

RFD) in the network, commencing and

replying to binding requests and ensuring a

secure relationship between devices rests with

the ZDO (Zigbee Define Object). The user-

defined application refers to the end device

that conforms to the ZigBee Standard.

Physical (PHY) layer:

The features of the IEEE 802.15.4

PHY physical layer are Activation and

deactivation of the radio transceiver, energy

detection (ED), Link quality indication (LQI),

channel selection, clear channel assessment

(CCA) and transmitting as well as receiving

packets across the physical medium.

Media access control (MAC) layer:

The MAC service enables the

transmission and reception of MAC protocol

data units (MPDU) across the PHY data

service.

The features of MAC sub layer are

beacon management, channel access, GTS

management, frame validation, acknowledged

frame delivery, association and disassociation

The standard offers two PHY options

based on the frequency band. Both are based

on direct sequence spread spectrum (DSSS).

The data rate is 250kbps at 2.4GHz, 40kbps at

915MHz and 20kbps at 868MHz. The higher

data rate at 2.4GHz is attributed to a higher-

order modulation scheme.

There is a single channel between 868

and 868.6MHz, 10 channels between 902.0

and 928.0MHz, and 16 channels between 2.4

and 2.4835GHz as shown in Figure 3.3.

Several channels in different frequency bands

enable the ability to relocate within spectrum.

The standard also allows dynamic Channelselection, a scan function that steps through a

list of supported channels in search of beacon,

receiver energy detection, link quality

indication, channel switching.

Operating Frequency Bands



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3.1 Receiver Energy Detection (ED)

The receiver energy detection (ED)

measurement is intended for use by a network

layer as part of channel selection algorithm. It

is an estimate of the received signal power

within the bandwidth of an IEEE 802.15.4

channel. No attempt is made to identify or

decode signals on the channel. The ED time

should be equal to 8 symbol periods.

The ED result shall be reported as an

8-bit integer ranging from 0x00 to 0xff. The

minimum ED value (0) shall indicate received

power less than 10dB above the specified

receiver sensitivity. The range of received

power spanned by the ED values shall be at

least 40dB. Within this range, the mapping

from the received power in decibels to ED

values shall be linear with an accuracy of + or

− 6dB.

3.2 Link Quality Indication (LQI)

Upon reception of a packet, the PHY

sends the PSDU length, PSDU itself and link

quality (LQ) in the PD-DATA. Indication

primitive. The LQI measurement is a

characterization of the strength and/or quality

of a received packet. The measurement may be

implemented using receiver ED, a signal-to-

noise estimation or a combination of these

methods. The use of LQI result is up to the

network or application layers.

The LQI result should be reported as

an integer ranging from 0x00 to 0xff. The

minimum and maximum LQI values should be

associated with the lowest and highest quality

IEEE 802.15.4 signals detectable by the

receiver and LQ values should be uniformly

distributed between these two limits.

3.3 Clear Channel Assessment (CCA) The clear channel assessment (CCA) is

performed according to at least one of the

following three methods:

Energy above threshold. CCA shall

report a busy medium upon detecting

any energy above the ED threshold. Carrier sense only. CCA shall report a

busy medium only upon the detection

of a signal with the modulation and

spreading characteristics of IEEE

802.15.4. This signal may be above or

below the ED threshold.

Carrier sense with energy above

threshold. CCA shall report a busy

medium only upon the detection of a

signal with the modulation and

spreading characteristics of IEEE

802.15.4 with energy above the ED

threshold.

3.4 PPDU Format

The PPDU packet structure is

illustrated in Figure 3.4. Each PPDU packetconsists of the following basic components:

SHR, which allows a receiving device

to synchronize and lock into the bit

stream

PHR, which contains frame length

information



ZIGBEE TECHNOLOGY


A variable length payload, which

carries the MAC sub layer frame.

4. IEEE 802.15.4 MAC

Figure 4.1 depicts the components and

interfaces of the MAC sub layer.

The MAC sub layer provides an

interface between the SSCS and the PHY. The

MAC sub layer conceptually includes a

management entity called the MLME. This

entity provides the service interfaces through

which layer management functions may be

invoked. The MLME is also responsible for

maintaining a database of managed objects

pertaining to the MAC sub layer. This

database is referred to as the MAC sub layer

PIB.

The MAC sub layer provides two services:

The MAC data service and The MAC

management service interfacing to the MAC

sub layer management entity (MLME) service

access point (SAP) (MLMESAP).

The MAC data service enables the

transmission and reception of MAC protocol

data units (MPDU) across the PHY data

service.

The features of MAC sub layer are

beacon management, channel access, GTS

management, frame validation, acknowledged

frame delivery, association and disassociation.

4.1 Super frame Structure

LR-WPAN allows the optional use of

a super frame structure. The format of the

super frame is defined by the coordinator. Thesuper frame is bounded by network beacons

and is divided into 16 equally sized slots. The

beacon frame is sent in the first slot of each

super frame. If a coordinator does not want to

use the super frame structure, it may turn off

the beacon transmissions. The beacons are

used to synchronize the attached devices, to

Figure 3.4 Format of the PPDU

Fi ure 4.1 MAC SUBLAYER REFERENCE MODEL

MCPS SAP MLME SAP

PLME SAP

MACMAC LAYER

MLME

PD SAP



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identify the PAN and to describe the structure

of super frames.

The super frame can have an active and an

inactive portion. During the inactive portion,

the coordinator shall not interact with its PAN

and may enter a low-power mode. The active

portion portion consists of contention access

period (CAP) and contention free period

(CFP). Any device wishing to communicate

during the CAP shall compete with other

devices using a slotted CSMACA mechanism.

On the other hand, the CFP contains

guaranteed time slots (GTSs). The GTSs

always appear at the end of the active super

frame starting at a slot boundary immediately

following the CAP. The PAN coordinator may

allocate up to seven of these GTSs and a GTS

can occupy more than one slot period.

4.2 CSMA-CA Algorithm

If super frame structure is used in the

PAN, then slotted CSMA-CA shall be used. If

beacons are not being used in the PAN or a

beacon cannot be located in a beacon-enabled

network, unslotted CSMA-CA algorithm is

used. In both cases, the algorithm is

implemented using units of time called back

off periods; this is equal to a Unit Back off

Period symbols.

In slotted CSMA-CA channel access

mechanism, the back off period boundaries of

every device in the PAN are aligned with the

super frame slot boundaries of the PAN

coordinator. In slotted CSMA-CA, each time a

device wishes to transmit data frames during

the CAP, it shall locate the Boundary of

the next back off period. In unslotted

CSMA-CA, the back off periods of one device

does not need to be synchronized to the back

off periods of another device.

4.3 Traffic Types

ZigBee/IEEE 802.15.4 addresses three typical

traffic types.

IEEE 802.15.4 MAC can accommodate all the

types.

Data is periodic. The application dictates

the rate, and the sensor activates checks

for data and deactivates.

Data is intermittent. The application, or

other stimulus, determines the rate, as in

the case of say smoke detectors. The

device needs to connect to the network

only when communication is necessitated.

This type enables optimum saving on

energy.

Figure 4.2 SUPER FRAME STRUCTURE



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Data is repetitive, and the rate is fixed a

priori. Depending on allotted time slots,

called GTS (guaranteed time slot), devices

operate for fixed durations.

ZigBee employs either of two modes,

beacon or non-beacon to enable the to-and-fro

data traffic. Beacon mode is used when the

coordinator runs on batteries and thus offers

maximum power savings, whereas the non-

beacon mode finds favour when the

coordinator is mains-powered.

4.4 Data Transfer model

Three types of data transfer

transactions exist: from a coordinator to a

device, from a device to a coordinator and

between two peer devices. The mechanism for

each of these transfers depends on whether the

network supports the transmission of beacons.

The non-beacon mode will be

included in a system where devices are „asleep'

nearly always, as in smoke detectors and

burglar alarms. The devices wake up and

confirm their continued presence in the

network at random intervals.

When a device wishes to transfer data

in a no beacon-enabled network, it simply

transmits its data frame, using the unslotted

CSMA-CA, to the coordinator. On detection of

activity, the sensors „spring to attention', as it

were, and transmit to the ever-waiting

coordinator's receiver (since it is mains-

powered). There is also an optional

acknowledgement at the end as shown in

Figure 4.3.

In the beacon mode, a device watches

out for the coordinator's beacon that gets

transmitted at periodically, locks on and looks

for messages addressed to it. If message

transmission is complete, the coordinator

dictates a schedule for the next beacon so that

the device „goes to sleep'; in fact, the

coordinator itself switches to sleep mode.

While using the beacon mode, all the

devices in a mesh network know when to

communicate with each other. In this mode,

necessarily, the timing circuits have to be quite

accurate, or wake up sooner to be sure not to

miss the beacon. This in turn means an

increase in power consumption by the

coordinator's receiver, entailing an optimal

increase in costs.

When a device wishes to transfer data

to a coordinator in a beacon-enabled network,

it first listens for the network beacon. When



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the beacon is found, it synchronizes to the

super frame structure. At the right time, it

transmits its data frame, using slotted CSMA-

CA, to the coordinator. There is an optional

acknowledgement at the end as shown in

Figure 4.4.

The applications transfers are

completely controlled by the devices on a

PAN rather than by the coordinator. This

provides the energy-conservation feature of

the ZigBee network.

When a coordinator wishes to transfer

data to a device in a beacon-enabled network,

it indicates in the network beacon that the data

message is pending. The device periodically

listens to the network beacon, and if a message

is pending, transmits a MAC command

requesting this data, using slotted CSMA-CA.

The coordinator optionally acknowledges the

successful transmission of this packet. The

pending data frame is then sent using slotted

CSMA-CA. The device acknowledged the

successful reception of the data by transmitting

an acknowledgement frame. Upon receiving

the acknowledgement, the message is removed

from the list of pending messages in the

beacon as shown in Figure 4.5.

When a coordinator wishes to transfer

data to a device in a nonbeacon-enabled

network, it stores the data for the appropriate

device to make contact and request data. A

device may make contact by transmitting a

MAC command requesting the data, using

unslotted CSMA-CA, to its coordinator at an

application-defined rate. The coordinator

acknowledges this packet. If data are pending,

the coordinator transmits the data frame using

unslotted CSMA-CA. If data are not pending,

the coordinator transmits a data frame with a

zero-length payload to indicate that no data

were pending.

The device acknowledges this packet as shown

in Figure 4.6.



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In a peer-to-peer network, every

device can communicate with any other device

in its transmission radius. There are two

options for this. In the first case, the node will

listen constantly and transmit its data using

unslotted CSMA-CA. In the second case, the

nodes synchronize with each Other so that

they can save power.

.

5. ZigBee Routing Layer5.1 AODV: Ad hoc On Demand Distance

Vector

AODV is a pure on-demand route

acquisition algorithm: nodes that do not lie on

active paths neither maintain any routing

information nor participate in any periodic

routing table exchanges. Further, a node does

not have to discover and maintain a route to

another node until the two needs to

communicate, unless the former node is

offering services as an intermediate

forwarding station to maintain connectivity

between two other nodes. The primary

objectives of the algorithm are to broadcast

discovery packets only when necessary, to

distinguish between local connectivity

management and general topology

maintenance and to disseminate information

about Changes in local connectivity to those

neighbouring mobile nodes those are likely to

need the information.

When a source node needs to

communicate with another node for which it

has no routing information in its table, the Path

Discovery process is initiated. Every node

maintains two separate counters: sequence

number and broadcast id. The source node

initiates path discovery by broadcasting a route

request (RREQ) packet to its neighbours,

which includes source addr, source sequence

number, broadcast id, dest addr, dest sequence

number, hop cnt. (Source sequence number is

for maintaining freshness information about

the reverse route whereas the destination

sequence number is for maintaining freshness

of the route to the destination before it can be

accepted by the source.)The pair source addr,

broadcast id uniquely identifies a RREQ,

where broadcast id is incremented whenever



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6. SUMMARY

6.1 Technology Comparisons

6.2 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.

With ZigBee designed to enable two-

way communications, not only will the

consumer be able to monitor and keep track of

domestic utilities usage, but also feed it to a

computer system for data analysis.

A recent analyst report issued by West

Technology Research Solutions estimates that

by the year 2008, "annual shipments for

ZigBee chipsets into the home automation

segment alone will exceed 339 million units,"

and will show up in "light switches, fire and

smoke detectors, thermostats, appliances in the

kitchen, video and audio remote controls,

landscaping, and security systems."

Futurists are sure to hold ZigBee up

and say, "See, I told you so". The ZigBee

Alliance is nearly 300 strong and growing,

with more OEM's signing up. This means that

more and more products and even later, all

devices and their controls will be based on this

standard. Since Wireless personal Area

Networking applies not only to household

devices, but also to individualized office

automation applications, ZigBee is here to

stay. It is more than likely the basis of future

home-networking solutions.

The technology is designed to be

simpler and cheaper than other WPANs such

as Bluetooth. The most capable ZigBee node

type is said to require only about 10% of the

software of a typical Bluetooth or Wireless

Internet node, while the simplest nodes are

about 2%. ZigBee is aimed at applications

with low data rates and low power

consumption.

Applications areas:

Table



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Enterprise systems: health care and

patient monitoring, environmental,

Monitoring and hazard detection.

Industrial systems: remote controlled

machines such as in tracking wind

turbines.

Military and government systems:

asset tracking, personnel monitoring

and surveillance.

Transportation systems: audio control

and automation, security and access

control.

Consumer products: cellular

handsets, computer peripherals,

remote controls and other portable

devices.

Climate control: customize the

temperatures of ac machines or

thermostats as differently needed.

Home automation: turn on or off

ovens, air conditioners, geysers, lights

without any hassles only when

needed. Also sprinkle water to plants

in garden monitoring moisture

content in soil.

Private Security: this also acts like a

private security to monitor kids or

aged even from office and alert in

case of medical emergencies.

7. CONCLUSION

The ZigBee Standard enables the

broad-based deployment of reliable wireless

networks with low complexity, low cost

solutions and provides the ability for a product

to run for years on inexpensive primary

batteries (for a typical monitoring application).

It is also, of course, capable of inexpensively

supporting robust mesh networking

technologies .ZigBee is all set to provide the

consumers with ultimate flexibility, mobility,

and ease of use by building wireless

intelligence and capabilities into every day

devices.

The mission of the ZigBee WorkingGroup is to bring about the existence of a

broad range of interoperable consumer devices

by establishing open industry specifications

for unlicensed, untethered peripheral, control

and entertainment devices requiring the lowest

cost and lowest power consumption

communications between compliant devices

anywhere in and around the home.



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8. References

1. ZigBee “wireless sensors and control

networks” presented at Wireless Congress

2008.

2. ZigBee “the green wireless solution”

published by Bob Heile, Chairman, and

ZigBee alliance - December 1, 2008.

3. ZigBee Technical Overview, Tokyo

Members Meeting Open House, Tokyo,

Japan - February 28, 2008.

Websites:

1. http://www.technologyreview.com/arti

cles/zigbee

2. http://www.tutorial-

reports.com/wireless/zigbee

3. http://www.zigbee.org

4. http://www.wisegeek.com

http://www.technologyreview.com/articles/zigbee




http://www.tutorial-reports.com/wireless/zigbee








zigbee technology 2012

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