eBook
Related collections and offers
Overview
- Provides a comprehensive overview of ZigBee technology and networking, from RF/physical layer considerations to application layer development
- Discusses ZigBee security features such as encryption
- Describes how ZigBee can be used in location detection applications
- Explores techniques for ZigBee co-existence with other wireless technologies such as 802.11 and Bluetooth
- The book comes with a dedicated website that contains additional resources and calculators: http://www.learnZigBee.com
Product Details
| ISBN-13: | 9780080558479 |
|---|---|
| Publisher: | Elsevier Science |
| Publication date: | 04/08/2011 |
| Sold by: | Barnes & Noble |
| Format: | eBook |
| Pages: | 360 |
| File size: | 6 MB |
Read an Excerpt
ZigBee Wireless Networks and Transceivers
By Shahin Farahani
Newnes
Copyright © 2008 Elsevier Ltd.All right reserved.
ISBN: 978-0-08-055847-9
Chapter One
ZigBee Basics
This chapter is an introduction to the ZigBee standard for short-range wireless networking. The goal of this chapter is to provide a brief overview of ZigBee's fundamental properties, including its networking topologies, channel access mechanism, and the role of each protocol layer. The topics discussed in this chapter are covered in more detail in the reminder of this book.
1.1 What Is ZigBee?
ZigBee is a standard that defines a set of communication protocols for low-data-rate short-range wireless networking. ZigBee-based wireless devices operate in 868 MHz, 915 MHz, and 2.4 GHz frequency bands. The maximum data rate is 250K bits per second. ZigBee is targeted mainly for battery-powered applications where low data rate, low cost, and long battery life are main requirements. In many ZigBee applications, the total time the wireless device is engaged in any type of activity is very limited; the device spends most of its time in a power-saving mode, also known as sleep mode. As a result, ZigBee-enabled devices are capable of being operational for several years before their batteries need to be replaced.
One application of ZigBee is in-home patient monitoring. A patient's blood pressure and heart rate, for example, can be measured by wearable devices. The patient wears a ZigBee device that interfaces with a sensor that gathers health-related information such as blood pressure on a periodic basis. Then the data is wirelessly transmitted to a local server, such as a personal computer inside the patient's home, where initial analysis is performed. Finally, the vital information is sent to the patient's nurse or physician via the Internet for further analysis.
Another example of a ZigBee application is monitoring the structural health of largescale buildings. In this application, several ZigBee-enabled wireless sensors (e.g., accelerometers) can be installed in a building, and all these sensors can form a single wireless network to gather the information that will be used to evaluate the building's structural health and detect signs of possible damage. After an earthquake, for example, a building could require inspection before it reopens to the public. The data gathered by the sensors could help expedite and reduce the cost of the inspection. A number of other ZigBee application examples are provided in Chapter 2.
The ZigBee standard is developed by the ZigBee Alliance, which has hundreds of member companies, from the semiconductor industry and software developers to original equipment manufacturers (OEMs) and installers. The ZigBee Alliance was formed in 2002 as a nonprofit organization open to everyone who wants to join. The ZigBee standard has adopted IEEE 802.15.4 as its Physical Layer (PHY) and Medium Access Control (MAC) protocols. Therefore, a ZigBee-compliant device is compliant with the IEEE 802.15.4 standard as well.
The concept of using wireless communication to gather information or perform certain control tasks inside a house or a factory is not new. There are several standards, reviewed in Chapter 9, for short-range wireless networking, including IEEE 802.11 Wireless Local Area Network (WLAN) and Bluetooth. Each of these standards has its advantages in particular applications. The ZigBee standard is specifically developed to address the need for very low-cost implementation of low-data-rate wireless networks with ultra-low power consumption.
The ZigBee standard helps reduce the implementation cost by simplifying the communication protocols and reducing the data rate. The minimum requirements to meet ZigBee and IEEE 802.15.4 specifications are relatively relaxed compared to other standards such as IEEE 802.11, which reduces the complexity and cost of implementing ZigBee compliant transceivers.
The duty cycle is the ratio of the time the device is active to the total time. For example, if a device wakes up every minute and stays active for 60ms, then the duty cycle of this device is 0.001, or 0.1%. In many ZigBee applications, the devices have duty cycles of less than 1% to ensure years of battery life.
1.2 ZigBee versus Bluetooth and IEEE 802.11
Comparing the ZigBee standard with Bluetooth and IEEE 802.11 WLAN helps us understand how ZigBee differentiates itself from existing established standards. (A more comprehensive comparison is provided in Chapter 9.) Figure 1.1 summarizes the basic characteristics of these three standards.
IEEE 802.11 is a family of standards; IEEE 802.11b is selected here because it operates in 2.4GHz band, which is common with Bluetooth and ZigBee. IEEE 802.11b has a high data rate (up to 11Mbps), and providing a wireless Internet connection is one of its typical applications. The indoor range of IEEE 802.11b is typically between 30 and 100 meters. Bluetooth, on the other hand, has a lower data rate (less than 3Mbps) and its indoor range is typically 2–10 meters. One popular application of Bluetooth is in wireless headsets, where Bluetooth provides the means for communication between a mobile phone and a hands-free headset. ZigBee has the lowest data rate and complexity among these three standards and provides significantly longer battery life.
ZigBee's very low data rate means that it is not the best choice for implementing a wireless Internet connection or a CD-quality wireless headset where more than 1Mbps is desired. However, if the goal of wireless communication is to transmit and receive simple commands and/or gather information from sensors such as temperature or humidity sensors, ZigBee provides the most power and the most cost-efficient solution compared to Bluetooth and IEEE 802.11b.
1.3 Short-Range Wireless Networking Classes
Short-range wireless networking methods are divided into two main categories: wireless local area networks (WLANs) and wireless personal area networks (WPANs).
WLAN is a replacement or extension for wired local area networks (LANs) such as Ethernet (IEEE 802.3). A WLAN device can be integrated with a wired LAN network, and once the WLAN device becomes part of the network, the network treats the wireless device the same as any other wired device within the network. The goal of a WLAN is to maximize the range and data rate.
WPANs, in contrast, are not developed to replace any existing wired LANs. WPANs are created to provide the means for power-efficient wireless communication within the personal operating space (POS) without the need for any infrastructure. POS is the spherical region that surrounds a wireless device and has a radius of 10 meters (33 feet).
WPANs are divided into three classes (see Figure 1.2): high-rate (HR) WPANs, medium-rate (MR) WPANs, and low-rate (LR) WPANs. An example of an HR-WPAN is IEEE 802.15.3 with a data rate of 11 to 55Mbps. This high data rate helps in applications such as real-time wireless video transmission from a camera to a nearby TV. Bluetooth, with a data rate of 1 to 3Mbps, is an example of an MR-WLAN and can be used in high-quality voice transmission in wireless headsets. ZigBee, with a maximum data rate of 250Kbps, is classified as an LR-WPAN.
1.4 The Relationship Between ZigBee and IEEE 802.15.4 Standards
One of the common ways to establish a communication network (wired or wireless) is to use the concept of networking layers. Each layer is responsible for certain functions in the network. The layers normally pass data and commands only to the layers directly above and below them.
ZigBee wireless networking protocol layers are shown in Figure 1.3. ZigBee protocol layers are based on the Open System Interconnect (OSI) basic reference model. Dividing a network protocol into layers has a number of advantages. For example, if the protocol changes over time, it is easier to replace or modify the layer that is affected by the change rather than replacing the entire protocol. Also, in developing an application, the lower layers of the protocol are independent of the application and can be obtained from a third party, so all that needs to be done is to make changes in the application layer of the protocol. The software implementation of a protocol is known as protocol stack software.
As shown in Figure 1.3, the bottom two networking layers are defined by the IEEE 802.15.4 standard. This standard is developed by the IEEE 802 standards committee and was initially released in 2003. IEEE 802.15.4 defines the specifications for PHY and MAC layers of wireless networking, but it does not specify any requirements for higher networking layers.
The ZigBee standard defines only the networking, application, and security layers of the protocol and adopts IEEE 802.15.4 PHY and MAC layers as part of the ZigBee networking protocol. Therefore, any ZigBee-compliant device conforms to IEEE 802.15.4 as well.
IEEE 802.15.4 was developed independently of the ZigBee standard, and it is possible to build short-range wireless networking based solely on IEEE 802.15.4 and not implement ZigBee-specific layers. In this case, the users develop their own networking/application layer protocol on top of IEEE 802.15.4 PHY and MAC (see Figure 1.4). These custom networking/application layers are normally simpler than the ZigBee protocol layers and are targeted for specific applications.
One advantage of custom proprietary networking/application layers is the smaller size memory footprint required to implement the entire protocol, which can result in a reduction in cost. However, implementing the full ZigBee protocol ensures interoperability with other vendors' wireless solutions and additional reliability due to the mesh networking capability supported in ZigBee. The decision of whether or not to implement the entire ZigBee protocol or just IEEE 802.15.4 PHY and MAC layers depends on the application and the long-term plan for the product.
Physical-level characteristics of the network are determined by the PHY layer specification; therefore, parameters such as frequencies of operation, data rate, receiver sensitivity requirements, and device types are specified in the IEEE 802.15.4 standard.
This book covers the IEEE 802.15.4 standard layers and the ZigBee-specific layers with the same level of detail. The examples given throughout this book are generally referred to as ZigBee wireless networking examples; however, most of the discussions are still applicable even if only IEEE 802.15.4 PHY and MAC layers are implemented.
1.5 Frequencies of Operation and Data Rates
There are three frequency bands in the latest version of IEEE 802.15.4, which was released in September 2006:
• 868–868.6MHz (868MHz band)
• 902–928MHz (915MHz band)
• 2400–2483.5MHz (2.4GHz band)
The 868MHz band is used in Europe for a number of applications, including short-range wireless networking. The other two bands (915MHz and 2.4GHz) are part of industrial, scientific, and medical (ISM) frequency bands. The 915MHz frequency band is used mainly in North America, whereas the 2.4GHz band is used worldwide.
Table 1.1 provides further details regarding the ways these three frequency bands are used in the IEEE 802.15.4 standard. IEEE 802.15.4 requires that if a transceiver supports the 868MHz band, it must support 915MHz band as well, and vice versa. Therefore, these two bands are always bundled together as the 868/915MHz frequency bands of operation.
IEEE 802.15.4 has one mandatory and two optional specifications for the 868/915MHz bands. The mandatory requirements are simpler to implement but yield lower data rates (20Kbps and 40Kbps, respectively). Before the introduction of two optional PHY modes of operation in 2006, the only way to have a data rate better than 40Kbps was to utilize the 2.4GHz frequency band. With the addition of two new PHYs, if for any reason (such as existence of strong interference in the 2.4GHz band) it is not possible to operate in the 2.4GHz band, or if the 40Kbps data rate is not sufficient, the user now has the option to achieve the 250Kbps data rate at the 868/915MHz bands.
If a user chooses to implement the optional modes of operation, IEEE 802.15.4 still requires that it accommodate the low-data-rate mandatory mode of operation in the 868/915MHz bands as well. Also, the transceiver must be able to switch dynamically between the mandatory and optional modes of operation in 868/915MHz bands.
A 2.4GHz transceiver may support 868/915MHz bands, but it is not required by IEEE 802.15.4. There is room for only a single channel in the 868MHz band. The 915MHz band has 10 channels (excluding the optional channels). The total number of channels in the 2.4GHz band is 16.
(Continues...)
Excerpted from ZigBee Wireless Networks and Transceivers by Shahin Farahani Copyright © 2008 by Elsevier Ltd. . Excerpted by permission of Newnes. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
Table of Contents
1. ZIGBEE™ BASICS 2. ZIGBEE™/ IEEE 802.15.4 NETWORKING EXAMPLES 3. ZIGBEE / IEEE 802.15.4 PROTOCOLS 4. TRANSCEIVER REQUIREMENTS 5. RF PROPAGATION, ANTENNAS, AND REGULATORY REQUIREMENTS 6. BATTERY LIFE ANALYSIS 7. LOCATION ESTIMATING USING ZIGBEE 8. ZIGBEE™ COEXISTENCE 9. RELATED TECHNOLOGIES APPENDIX A. PSSS Code Tables B. The MAC Constants and Attributes C. The NWK Constants and Attributes D. The APS Constants and Attributes E. The ZigBee Device Profile Services F. DSSS Symbol-to-Chip Mapping Tables G. Pulse Shaping FiltersWhat People are Saying About This
A complete guide to the ZigBee protocol, hardware, and networking for RF/wireless professionals