ZigBee Wireless Networks and Transceivers

Shahin Farahani

• Éditeur: Newnes
• Année: 2011
• ISBN:
  9780080558479
• Sujet: Technology & Engineering
• Sous-sujet: Mobile & Wireless Communications

ZigBee is a short-range wireless networking standard backed by such industry leaders as Motorola, Texas Instruments, Philips, Samsung, Siemens, Freescale, etc. It supports mesh networking, each node can transmit and receive data, offers high security and robustness, and is being rapidly adopted in industrial, control/monitoring, and medical applications. This book will explain the ZigBee protocol, discuss the design of ZigBee hardware, and describe how to design and implement ZigBee networks. The book has a dedicated website for the latest technical updates, ZigBee networking calculators, and additional materials. Dr. Farahani is a ZigBee system engineer for Freescale semiconductors Inc. The book comes with a dedicated website that contains additional resources and calculators: http://www.learnZigBee.com

• 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

Chapter 1

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 [1]. ZigBee-based wireless devices operate in 868 MHz, 915 MHz, and 2.4 GHz frequency bands. The maximum data rate is 250 K 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 [2].

Another example of a ZigBee application is monitoring the structural health of large-scale buildings [3]. 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 [4], 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 [5]. 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 60 ms, 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.

Figure 1.1 Comparing the ZigBee Standard with Bluetooth and IEEE 802.11b

IEEE 802.11 is a family of standards; IEEE 802.11b is selected here because it operates in 2.4 GHz band, which is common with Bluetooth and ZigBee. IEEE 802.11b has a high data rate (up to 11 Mbps), 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 3 Mbps) 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 [6]. 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) [5].

WPANs are divided into three classes (see Figure 1.2): high-rate (HR) WPANs, medium-rate (MR) WPANs, and low-rate (LR) WPANs [7]. An example of an HR-WPAN is IEEE 802.15.3 with a data rate of 11 to 55 Mbps [8]. 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.

Figure 1.2 Short-range Wireless Networking Classes

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 [9]. 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.

Figure 1.3 ZigBee Wireless Networking Protocol Layers

As shown in Figure 1.3, the bottom two networking layers are defined by the IEEE 802.15.4 standard [5]. 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...