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ZIGBEE TECHNOLOGY

1. What is ZigBee ?

ZigBee is a technological standard, based on the IEEE 802.15.4

standard, which was created specifically for control and sensor

networks. Within the broad organization of the Institute of

Electrical and Electronics Engineers (IEEE), the 802 group is

the section that deals with network operations and technologies.

Group 15 works more specifically withwireless

networking, andTask Group 4 drafted the 802.15.4 standard for a low data rate

wireless personal area network (WPAN). The standard for this

WPAN specifies not only a low data rate but also low power

consumption and low complexity, among other things.

The ZigBee standard is built on top of this IEEE standard,

addressing remote monitoring and control for sensory network applications. This standard was created by an organization

known as the ZigBee Alliance, which is composed of a large

number of companies and industry leaders striving to enable

such control devices based on said standard.

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2.ZigBee/IEEE 802.15.4 - General

Characteristics

• Dual PHY (2.4GHz and 868/915 MHz)

• Data rates of 250 kbps (@2.4 GHz), 40 kbps (@ 915

MHz), and 20kbps (@868 MHz)

• CSMA-CA channel access

- Yields high throughput and low latency for low duty

cycle devices like sensors and controls

• Low power (battery life multi-month to years)

• Multiple topologies: star, peer-to-peer, mesh

• Addressing space of up to:

- 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)

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3. Network Topologies

3 types of topologies that ZigBee supports: star topology, peer-

to-peer topology and cluster tree.

3.1 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. After an FFD is activated for

the first time, it may establish its own network and become the

PAN coordinator. Each start network chooses a PAN identifier,

which is not currently used by any other network within theradio sphere of influence. This allows each star network to

operate independently.

Star Topology

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3.2 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 from such a topology. It also allows multiple hops

to route messages from any device to any other device in thenetwork. It can provide reliability by multipath routing.

Peer-Peer Technology

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3.3. Mesh Topology

In a mesh topology, there is one coordinator and aset of FFD nodes associated to it. Each node is arouter and permits other nodes to associate to it.Each node can communicate directly with othernodes if they are in its POS “Personal OperatingSpace” i.e. in-range transmission, or, passing by

other nodes (acting as routers in this case) to reachits target node. RFD devices can be attached to thenetwork as leaves.

Mesh Topology

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ZigBee/IEEE802.15.4 - Typical Traffic Types

Addressed

Periodic data

Application defined rate (e.g., sensors)

Intermittent data

Application/external stimulus defined rate (e.g., light

switch)

Repetitive low latency dataAllocation of time slots (e.g., mouse)

Each of these traffic types mandates different attributes from

the MAC. The IEEE802.15.4 MAC is flexible enough to

handle each of these types.

• Periodic data can be handled using thebeaconing system whereby the sensor will wakeup for the beacon, check for any messages andthen go back to sleep.

• Intermittent data can be handled either in abeaconless system or in a disconnectedfashion. In a disconnected operation the devicewill only attach to the network when it needs tocommunicate saving significant energy.

• Low latency applications may choose to theguaranteed time slot (GTS) option. GTS is amethod of QoS in that it allows each device aspecific duration of time each Superframe to do

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whatever it wishes to do without contention orlatency.

4. ZigBee Architecture

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5.1 Physical(PHY) and MAC layers

The PHY service enables the transmission and reception of PHY

protocol data units (PPDU) across the physical radio channel.

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.

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.

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Link Quality Indication (LQI)

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.

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 anyenergy 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 themodulation and spreading characteristics of IEEE

802.15.4 with energy above the ED threshold.

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Two device types

Full function device (FFD)

• Any topology

• Network coordinator capable

• Talks to any other device

Other Full Function Devices (FFD's) may be found in the

network, and these devices support all of the 802.15.4

functions. They can serve as network coordinators, network

routers, or as devices that interact with the physical world.

Reduced function device (RFD)

• Limited to star topology

• Cannot become a network coordinator

• Talks only to a network coordinator

• Very simple implementation

The final device found in these networks is the Reduced

Function Device (RFD), which usually only serve as devices

that interact with the physical world

5.2 Media access control (MAC) layer:

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5.2.1 Data Transfer modes

ZigBee operates in two main modes:

• non-beacon mode

• beacon mode.

Beacon mode is a fully coordinated mode in that all the device

know when to coordinate with one another. In this mode, the

network coordinator will periodically "wake-up" and send out abeacon to the devices within its network.

This beacon subsequently wakes up each device, who must

determine if it has any message to receive. If not, the device

returns to sleep, as will the network coordinator, once its job is

complete.

Non-beacon network

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Non-beacon mode, on the other hand, is less coordinated, as anydevice can communicate with the coordinator at will. However,

this operation can cause different devices within the network to

interfere with one another, and the coordinator must always be

awake to listen for signals, thus requiring more power. In any

case, ZigBee obtains its overall low power consumption because

the majority of network devices are able to remain inactive over

long periods of time.

Beacon network

5.2.2 Super Frame Structure

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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.

The beacon frame is transmitted in the first slot of each

superframe. If a coordinator does not wish to use a superframe

structure it may turn off the beacon transmissions. The beacons

are used to synchronize the attached devices, to identify the

PAN, and to describe the structure of the superframes. Any

device wishing to communicate during the contention accessperiod (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.

For low latency applications or applications requiring specific

data bandwidth, the PAN coordinator may dedicate portions of

the active super frame to that application. These portions arecalled guaranteed time slots (GTSs).

The guaranteed time slots comprise the contention free period

(CFP), which always appears at the end of the active super

frame starting at a slot boundary immediately following the

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CAP, 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 CAP 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.

5.2.3 The four basic frame types defined in

802.15.4: Data, ACK, MAC command, and beacon

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

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ensures that packets are received without error.

This frame structure improves reliability in difficult

Conditions.

data frame

Another important structure for 802.15.4 is the

acknowledgment (ACK) frame. It provides feedback

from the receiver to the sender confirming that the

packet was received without error. The device takesadvantage of specified "quiet time" between frames

to send a short packet immediately after the datapacket

transmission.

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A MAC command frame provides the mechanism

for remote control and configuration of client

nodes. A centralized network manager uses MAC

to configure individual clients' command frames no matter how

large the network.

MAC command frame

Finally, the beacon frame wakes up client devices,

which listen for their address and go back to sleep

if they don't receive it. Beacons are important for

mesh and cluster-tree networks to keep all thenodes synchronized without requiring those nodes

to consume precious battery energy by listening for

long periods of time.

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beacon frame

5.3 Network Layer

The responsibilities of the ZigBee NWK layer

include:

• Starting a network: The ability to

successfully establish a new network.

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• Joining and leaving a network : The ability to

gain membership (join) or relinquish

membership (leave) a network.

• Configuring a new device: The ability to

sufficiently configure the stack for operation as

required.

• Addressing: The ability of a ZigBee

coordinator to assign addresses to devices

joining the network.• Synchronization within a network: The

ability for a device to achieve synchronization

with another device either through tracking

beacons or by polling.

• Security: applying security to outgoing frames

and removing security to terminating frames

Routing: routing frames to their intended

destinations.

Network Summary

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The network layer builds upon the IEEE 802.15.4 MAC’s features to allow

extensibility of coverage. Additional clusters can be added; networks can be

consolidated or split up.

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 messagesbetween bound devices. Another responsibility of the APS sub-

layer is discovery, which is the ability to determine which other

devices are operating in the personal operating space of a device.

The 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 actualapplications according to the ZigBee-defined application

descriptions

ZigBee Device ObjectDefines 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 ZigBee’s security methods such as public key, symmetric key, etc.

Application Support Layer

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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

5. SECURITY

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Security and data integrity are key benefits of the ZigBee

technology. ZigBee leverages the security model of the IEEE802.15.4 MAC sublayer which specifies four security services:

• access control -- the device maintains a list of trusted devices

within the network

• data encryption -- which uses symmetric key 128-bit

advanced encryption standard

• frame integrity -- to protect data from being modified by

parties without cryptographic keys

• sequential freshness -- to reject data frames that have been

replayed—the network controller compares the freshness value

with the last known value from the device and rejects it if the

freshness value has not been updated to a new valueThe actual security implementation is specified by

the implementer using a standardized toolbox of

ZigBee security software.

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6.Comparison Overview

Bluetooth ZigBee

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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. 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 Working Group is to bring about the

existence of a broad range of interoperable consumer devices by

establishing open industry specifications for unlicensed, 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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9.References

http://www.zigbee.org/en/documents/zigbeeovervie

w4.pdf

http://www.zigbee.org/en/about/faq.asp

http://en.wikipedia.org/wiki/ZigBee

http://www.electronicstalk.com/news/ven/ven119.h

tml

http://www.zigbee.org/imwp/idms/popups/pop_dow

nload.asp?ContentID=7092

http://www.zigbee.org/en/documents/zigbeeoverview4.pdf






http://www.electronicstalk.com/news/ven/ven119.html


http://www.zigbee.org/imwp/idms/popups/pop_download.asp?ContentID=7092











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