ZigBee and IEEE 802

7/28/2019 ZigBee and IEEE 802

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Abstract

In todays modern world technological advancements are improving and developing

at a very fast pace. In this scenario the world is also facing a bigger crisis and that is power

crisis. The world today is in intense need of technologies that can save power and yet

enhance the efficiency. At this juncture ZigBee technology presents tremendous

opportunities design almost infinite life batteries.


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INTRODUCTION

ZigBee technology is a low data rate, low power consumption, low cost, wireless

networking protocol targeted towards automation and remote control applications. IEEE

802.15.4 committee started working on a low data rate standard a short while later. Then the

ZigBee Alliance and the IEEE decided to join forces and ZigBee is the commercial name for

this technology. 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. In addition, ZigBee can be

implemented in mesh networks larger than is possible with Bluetooth. ZigBee compliant

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 or 868 MHz Europe). The datarate is 250kbps at 2.4GHz, 40kbps at 915MHz and 20kbps at 868MHz. 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, provide interoperability

compliance testing, marketing of the standard, advanced engineering for the evolution of the

standard. This will assure consumers to buy products from different manufacturers with

confidence that the products will work together.

IEEE 802.15.4 is now detailing the specification of PHY and MAC by offering

building blocks for different types of networking known as star, mesh, and cluster tree.Network routing schemes are designed to ensure power conservation, and low latency

through guaranteed time slots. A unique feature of ZigBee network layer is communication

redundancy eliminating singlepoint of failure in mesh networks. Key features of PHY

include energy and link quality detection, clear channel assessment for improved coexistence

with other wireless networks.

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.

Components of WPANA ZigBee system consists of several components. The most basic is the device. A device can

be a full-function device (FFD) or reduced-function device (RFD). A network shall include at

least one FFD, operating as the PAN coordinator.

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).


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The Reduced Function Device (RFD) carries limited (as specified by the standard)

functionality to lower cost and complexity. It is generally found in network-edge

devices. The RFD can be used where extremely low power consumption is a

necessity.

Fig1 Zigbee network model

Network Topologies

Star TopologyIn 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 the radio sphere of influence. This allows each star network to

operate independently.

Peer-to-peer TopologyIn 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 the network. It can provide reliability by multipath routing.

Cluster-tree TopologyCluster-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. Only one of these coordinators however is the PAN

coordinator. The PAN coordinator forms the first cluster by establishing itself as the cluster

head (CLH) with a cluster identifier (CID) of zero, choosing an unused PAN identifier, and

broadcasting beacon frames to neighbouring devices. A candidate device receiving a beacon

frame may request to join the network at the CLH. If the PAN coordinator permits the device

to join, it will add this new device as a child device in its neighbour list. The newly joined

device will add the CLH as its parent in its neighbour list and begin transmitting periodicbeacons such that other candidate devices may then join the network at that device. Once


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application or network requirements are met, the PAN coordinator may instruct a device to

become the CLH of a new cluster adjacent to the first one. The advantage of this clustered

structure is the increased coverage area at the cost of increased message latency.

Fig 2 different type of network topologies

ZigBee Architecture

ZigBee Stack System Requirements

Full protocol stack


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Network Layer

The responsibilities of the ZigBee NWK layer include:

Starting a network: The ability to successfully establish a new network.

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.

The network layer builds upon the IEEE 802.15.4 MACs features to allow

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

consolidated or split up.

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 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 actual applications according to the ZigBee-defined application descriptions.

PHYSICAL LayersThe PHY layer defines the physical and electrical characteristics of the network. The basic

task of the PHY layer is data transmission and reception. At the physical/electrical level, this

involves modulation and spreading techniques that map bits of information in such a way as

to allow them to travel through the air. Specifications for receiver sensitivity and transmit

output power are in the PHY layer.

The PHY layer is also responsible for the following tasks:

Enable/disable the radio transceiver

Link quality indication (LQI) for received packets

Energy detection (ED) within the current channel

Clear channel assessment (CCA)


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MAC Layer

The MAC layer defines how multiple 802.15.4 radios operating in the same area will share

the airwaves. This includes coordinating transceiver access to the shared radio link and the

scheduling and routing of data frames. There are network association and disassociation

functions embedded in the MAC layer. These functions support the self-configuration and

peer-to-peer communication features of a ZigBee network.

The MAC layer is responsible for the following tasks:

Beacon generation if device is a coordinator

Implementing carrier sense multiple access with collision avoidance (CSMA-CA)

Handling guaranteed time slot (GTS) mechanism

Data transfer services for upper layers

IEEE802.15.4 MAC is flexible enough to handle each of these types.

Periodic data can be handled using the beaconing system whereby thesensor 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 adisconnected 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 in that it allows each device a specific

duration of time each Super frame to do whatever it wishes to do without

contention or latency.

Non-beacon network communication Beacon network communication

Fig 3 MAC Data Service Diagrams

Frame Structure


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The frame structures have been designed to keep the complexity to a 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. A data frame, used for all transfers of data. An acknowledgment frame, used for confirming successful frame reception. A MAC command frame, used for handling all MAC peer entity control

transfers.

SecurityWhen security of MAC layer frames is desired, ZigBee uses MAC layer security to

secure MAC command, beacon, and acknowledgement frames. ZigBee may secure messages

transmitted over a 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 usesthe 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.

When transmitting a frame, if integrity is required, the MAC header and payload dataare used in calculations to create a Message Integrity Code (MIC) consisting of 4, 8, or 16

octets. The MIC is right appended to the MAC payload. If confidentiality is required, the

MAC frame payload is also left appended with frame and sequence counts (data used to form

a nonce). The nonce is used when encrypting the payload and also ensures freshness to

prevent replay attacks. Upon receipt of a frame, if a MIC is present, it is verified and if the

payload is encrypted, it is decrypted. Sending devices will increase the frame count with

every message sent and receiving devices will keep track of the last received count from each

sending device. If a message with an old count is detected, it is flagged with a security error.

The MAC layer security suites are based on three modes of operation. Encryption at the

MAC layer is done using AES in Counter (CTR) mode and integrity is done using AES in

Cipher Block Chaining (CBCMAC) mode. A combination of encryption and integrity is doneusing a mixture of CTR and CBC- MAC modes called the CCM mode.


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Properties of 802.15.4

802.15.4 defines operation in three license-free industrial scientific medical (ISM) frequency

bands. Below is a table that summarizes the properties of IEEE 802.15.4 in two of the ISM

frequency bands: 915 MHz and 2.4 GHz.

Zigbee features

(1) Low power consumption, simply implemented.

Users expect batteries to last many months to years! Consider that a typicalsingle 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.

Bluetooth has many different modes and states depending upon latencyand power requirements such as sniff, park, hold, active, etc.; ZigBee/IEEE

802.15.4 has active (transmit/receive) or sleep. Application software needs

to focus 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. Consider a futurehome with 100 wireless control/sensor devices.

(2) Low cost (device, installation, maintenance)Low cost to the users means low device cost, low installation cost and low


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maintenance. ZigBee devices allow batteries to last up to years using

primary cells (low cost) without any chargers (low cost and easy

installation). ZigBees simplicity allows for inherent configuration and

redundancy of network devices provides low maintenance.

(3) High density of nodes per networkZigBees 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.

(4) Simple protocol, global implementation

ZigBees protocol code stack is estimated to be about 1/4th of Bluetooths or

802.11s. Simplicity is essential to cost, interoperability, and maintenance.

The IEEE 802.15.4 PHY adopted by ZigBee has been designed for the 868

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.

ZigBee Applications

Home and office automation

Industrial automation

Medical monitoring

Low-power sensors

HVAC control

Plus many other control and monitoring uses


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Comparison of wireless standards

The demand for wireless solutions continues to grow and with it new standards have come

forward and other existing standards have strengthened their position in the marketplace. This

section compares three popular wireless standards being used today and lists some of the

design considerations that differentiate them.


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Whether ZigBee and Bluetooth are competitors or complements?

Bluetooth seems best suited for:

Synchronization of cell phone to PDA Hands-free audio PDA to printerWhile ZigBee is better suited for:


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Controls Sensors Lots of devices Low duty cycle Small data packets Long battery life is critical

Bluetooth Battery Drain

Packet length can affect battery drain. Typically the shorter the packet the

quicker the device can go to sleep. Bluetooth is a slotted protocol.

Communication can occur in either: 625 S, 1875 S, or 3125 S slots.

The following graph showing effective data rate was based upon the

transmissions speeds stated in Bluetooth v1.1 and IEEE 802.15.4 draft 18,

using the 250 kb/s rate. The general trend is that at larger packet sizes the

effective data rate approaches the raw data rate. The peaks for the Bluetooth rate are a result

of the three slot sizes, when a packet becomes too big for one slot it must increment to thenext slot even though it doesnt fill the whole slot allocation.

ZigBee Battery DrainA typical scenario for sensors and control devices is to remain connected to the network.

We use connected to mean that the device periodically listens for incoming packets. In this

manner the devices behaviour may be altered or at least checked to verify correctness.

Timing Considerations

ZigBee devices can quickly attach, exchange information, detach, and then

go to deep sleep to achieve a very long battery life. Bluetooth devices

require about ~100X the energy for this operation.

Power Considerations

ZigBee

2+ years from normal batteries Designed to optimize slave power requirements

Bluetooth

Power model as a mobile phone (regular daily charging) Designed to maximize ad-hoc functionality

Comparison Summary

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.

Minor changes to Bluetooth or ZigBee wont change their inherent behaviour orcharacteristics. The different behaviours come from architectural differences.

Conclusion


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It is likely that ZigBee will increasingly play an important role in the future of

computer and communication technology. In terms of its low cost and less power

consumption. In upcoming days ZigBee is used for several application, it has that much level

of security (AES). Bluetooth and ZigBee not at all a competitors both are designed for

different purpose.

References

ZigBee Technology: Wireless Control that Simply works by Patrick Kinney ZigBee Specifications v1.0 Designing with 802.15.4 and ZigBee, Presentation Slides, available on ZigBee.org


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ZigBee Tutorial,http://www.tutorial-reports.com/wireless/zigbee IEEE 802.15.4 Specification
http://www.tutorial-reports.com/wireless/zigbeehttp://www.tutorial-reports.com/wireless/zigbeehttp://www.tutorial-reports.com/wireless/zigbeehttp://www.tutorial-reports.com/wireless/zigbee

ZigBee and IEEE 802

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