ZigBee-ready modules for sensor networking Johan Lönn
Jonas Olsson
Shaofang Gong
Department of Science and Technology Linköping University SE-60174 Norrköping, Sweden +46 11 363445
Department of Science and Technology Linköping University SE-60174 Norrköping, Sweden +46 11 363446
Department of Science and Technology Linköping University SE-60174 Norrköping, Sweden +46 11 363459
[email protected]
[email protected]
[email protected]
ABSTRACT Fully functional ZigBee-ready modules have been designed, implemented, and tested. One of the modules consists of both the IEEE 802.15.4 PHY/MAC and ZigBee networking stack layers, while the other module consists of only the IEEE 802.15.4 PHY/MAC stack layers. Furthermore, a ZigBee based testapplication for temperature sensor networking has been evaluated by utilizing the developed ZigBee-ready modules. The modules can have a line-of-sight radio link up to 180 m, when having only 1-mW output power. It is shown that ZigBee is suitable for sensor networking where long battery life, large networks, and fast network establishment are the requirements.
Application Application Profiles Application Framework Network And Security Layers
PHY Layer 2.4 GHz
Categories and Subject Descriptors
Design, experimentation and verification.
Keywords ZigBee, modules, networks, sensors.
1. INTRODUCTION The past several years have witnessed a rapid growth of wireless networking. However, up to now wireless networking has been mainly focused on high-speed communication applications such as the IEEE 802.11 wireless local area network (WLAN) standard. Another well-known standard focusing on low-rate wireless personal area networks (WPAN) is Bluetooth, but it has limited capacity for networking since a Bluetooth piconet only can have up to 8 nodes. There are many wireless monitoring and control applications in the industrial and home environments that require less complexity and lower data rates than those from existing standards. For such wireless applications, a new standard called ZigBee based upon the IEEE 802.15.4 specification has been specified by the ZigBee alliance with members of Freescale, Philips, Atmel, Siemens, Samsung, Analog Devices and Chipcon, etc. Figure 1 shows the entire ZigBee stack. The ZigBee standard has the characteristics of large network capability (up to 65,000 nodes), long battery lifetime (up to a few years), short link establishment time (15-30 ms), but low data rate (up to 250 kbps) [1].
ZigBee Alliance
MAC Layer MAC Layer
IEEE 802.15.4
868/915 MHz
Figure 1. ZigBee stack.
B. [Hardware]
General Terms
Customer
2. DESIGN The main task was to develop fully functional ZigBee modules. The design of the modules is made by using the CAD program Protel DXP 2004, having the schematic and layout in the same program environment. The design process is basically carried out in two steps. Firstly, a schematic diagram is designed. Secondly, the schematic is transferred to a layout. Afterwards, the layout is used to manufacture printed circuit boards (PCBs). It was decided to develop two different modules. One module consists of both a radio frequency part (RF-part) and a microcontroller unit (MCU), which is called the "RF+MCU module". The other module only consists of the RF-part of the system, hereafter called the "RF module". This RF module can be used in an embedded system with an integrated microcontroller. The RF+MCU module is preferably used when the user wants to quickly develop a new ZigBee application. The RF-part is identical in the two modules.
2.1 RF+MCU Module Figure 2 shows the block diagram of the RF+MCU module. The RF part utilizes the CC2420 chip [2] from Chipcon, while the microcontroller is the ATmega128L chip [3] from Atmel. The ATmega128L is fully capable of operating the ZigBee software stack, since it contains 128kB of Flash memory. The modules are designed to support in-system programming via the JTAG interface. The RF+MCU module has a wide range of external data interfaces, which allows the microcontroller to communicate with a wide range of different external-devices. This RF+MCU module is thus a ZigBee-ready module.
3. IMPLEMENTATION As schematically shown in Figure 5a, the PCB of the two modules utilizes a standard 4-layer FR-4 board. The board has a thickness of 0.9 mm. The two internal layers are used for power and ground, while the top and bottom layers, shown in Figures 5b and 5c, are used for routing. A copper pour connected to ground is used on both the top and bottom layers.
Figure 2. Block diagram of the RF+MCU module.
2.2 RF Module The RF module utilizes the same RF chip from Chipcon as in the RF+MCU module, but without the microcontroller chip as shown in Figure 3. In order to convert the single-ended signal from the antenna to the differential inputs of the RF-chip, a BalUn circuit is used. The BalUn circuit also matches the impedance between the antenna and the RF-chip inputs. The connector used is the same as that in the RF+MCU module. This is an IEEE 802.15.4compatible RF module.
RF
BalUn Circuit
RF Transceiver Circuit (CC2420)
(a)
Connector
(b) Figure 3. Block diagram of the RF module.
2.3 ZigBee Application Since our RF+MCU module has both a microcontroller and a connector, it is very easy to develop various ZigBee-based applications. A suitable application is to form wireless sensor networks. As shown in the block diagram in Figure 4, a sensor can be connected to the RF+MCU module. In this work, a temperature sensor has been chosen. The measured temperature is then transferred to the network coordinator that presents the value onto an LCD display. Similarly, other types of sensors can also be connected to the RF+MCU module. A larger network can then be established.
(c) Figure 5. PCB of the RF+MCU module: (a) cross-section, (b) top layer, and (c) bottom layer.
3.1 RF+MCU Module After processing the PCB, all components including the antenna, the RF and microcontroller chips, passive components, and the connector are assembled on the board. Figure 6 shows both sides of the assembled RF+MCU module, having a size of 23X40 mm.
Figure 4. Block diagram of ZigBee with a sensor.
Figure 6. Assembled RF+MCU module. (a)
3.2 RF Module Figure 7 shows both sides of the assembled RF module, having a size of 18X25 mm.
Figure 7. Assembled RF module. (b)
3.3 ZigBee Temperature Sensor Network To evaluate the developed ZigBee-ready module, a test application with a temperature sensor and a display has been developed. A ZigBee module with the display seen in Figure 8a is programmed as the network coordinator. The function flow diagram of the coordinator is shown in Figure 8b. Another ZigBee module, shown in Figure 9a, with a temperature sensor reports the current temperature value to the coordinator. Two ZigBee-ready modules are connected to the display and the sensor via the connector on the module, respectively. Figure 9b shows the function flow diagram of the ZigBee-based temperature sensor.
Figure 8. Test application coordinator: (a) ZigBee+Display, and (b) function flow diagram.
(a)
5. DISCUSSIONS The developed ZigBee-ready modules can be used to build sensor networks, and they can be programmed to the following three types of devices [1]: • Coordinator of the network, which manages and controls the network. • Full Function Device (FFD), which supports all functions and features, specified by the ZigBee and the IEEE 802.15.4 standards. It can be used as network routers to form peer-to-peer networks.
(b) Figure 9. Test application device: (a) ZigBee+Sensor, and (b) function flow diagram.
4. RESULTS This work has resulted in two different types of modules, i.e., a ZigBee-ready RF+MCU module and an IEEE 802.15.4compatible RF module. Moreover, a test application of a ZigBeebased temperature sensor with a display has been developed. The test results show that both the RF+MCU module and the RF module work properly, and the line-of-sight (LOS) radio link can reach up to 180 m when the output power from the transmitter is only 1 mW. Some main features of these developed modules and a test application are summarized below.
• Reduced Function Device (RFD), which carries out limited (as specified by the standard) functionality to reduce cost, complexity and power consumption. The RFD can be used where extremely low power consumption is a necessity.
5.1 Star Network The first FFD that is activated may establish its own network and becomes a personal area network (PAN) coordinator. Then both FFD and RFD devices can connect to the PAN coordinator. Figure 10 shows a typical star network of this type. All networks within the radio sphere must have a unique PAN address. All nodes in a PAN must talk to the PAN Coordinator.
4.1 ZigBee-Ready RF+MCU Module •
Size: 23x40mm
•
Compact module for quick prototyping
•
2 USARTs, SPI, TWI (I2C) and JTAG interfaces
•
5 digital I/O ports, and 5 pieces of 10bit ADC ports
•
32.768kHz real time clock (RTC)
•
Integral antenna (~180m LOS)
•
2.7-10V voltage supply
4.2 IEEE 802.15.4 RF Module •
18x25 mm module
•
IEEE 802.15.4 compatible
•
SPI interface
•
Integral antenna (~180m LOS)
•
2.1-3.6V power supply
•
Easy to connect to various microcontrollers
4.3 Test Application of Sensor Networking •
ZigBee-based coordinator with a display
•
The coordinator initiates the networking
•
ZigBee-based temperature sensor
•
LOS range of ~180 m
Figure 10. Star network.
5.2 Peer-to-Peer Network In the peer-to-peer topology shown in Figure 11, there is also a PAN coordinator, but it differs from the star topology in that any device can communicate with any other device as long as they are in the range of one another. The peer-to-peer topology allows more complex network formations to be implemented, such as the mesh topology shown in Figure 12.
Figure 11. Peer-to-peer network.
5.3 Mesh Network
5.4 Security and Robustness
By combining the star and peer-to-peer topologies, ZigBee can form so-called mesh networks, as shown in Figure 12, which may extend over a large area and contain thousands of nodes. Each FFD in the network also acts as a router to direct messages. The routing of the network can dynamically change, so as to take evolving conditions into account. This enables an extremely reliable network, since the network can heal itself if one node is disabled. A new network node may be recognized and associated in about 30 ms. Waking up a sleeping node takes about 15 ms, as does accessing a channel or transmitting data. ZigBee applications benefit from the ability to quickly attach information, detach, and go to deep sleep, which results in low power consumption and extended battery life.
There are three types of security modes defined [1]: •
Unsecured mode, i.e., no security is used.
• Access Control List, where no encryption is used but the network rejects frames from unknown devices. • Secured mode, where data encryption, frame integrity, sequential freshness and access control lists can be used. Figure 13 shows the bit error rate (BER) comparison between IEEE 802.11b WLAN, IEEE 802.15.1 Bluetooth, and ZigBee, etc [4]. It is seen that ZigBee is the most robust protocol as compared to others. This is a big advantage for ZigBee to be used in the industrial environment.
Figure 13. BER comparison
Figure 12. Mesh network.
5.3 Comparison
6. CONCLUSIONS
Table 1 shows a comparison between IEEE 802.11b WLAN, IEEE 802.15.1 Bluetooth and IEEE 802.15.4 ZigBee. It is seen that these three standards are aimed at different applications. The strength of ZigBee lies on its low power, low cost, and forming large networks where no higher data rate then 250 kbps is required.
• A IEEE 802.15.4-compatible module has also been developed. This module can be used in embedded systems where a different microcontroller is preferred.
Table 1. Comparison between WLAN, Bluetooth and ZigBee Market Name Standard Application Focus
Wi-Fi™ 802.11b
Bluetooth ™ 802.15.1
ZigBee™ ZigBee™ 802.15.4
Web, Email, Video
Cable Replacement
Monitoring & Control
• A ZigBee-ready module consisting of RF+MCU has been developed. The module can be used for quick prototyping of ZigBee-based applications.
• A ZigBee-based test application of temperature sensor networking has been evaluated, showing the success to use our ZigBee-ready modules for quickly development of new applications.
7. REFERENCES
1MB+
250KB+
4KB - 32KB
[1] IEEE P802.15.4, “Low Rate Wireless Personal Area Networks,” Oct. 2003, ISBN 0-7381-3677-5 SS95127.
.5 - 5
1-7
100 - 1,000+
[2] Chipcon, http://www.chipcon.com, 2005-03.
Network Size
32
8
65,000
Bandwidth (KB/s)
11,000+
720
20 - 250
Transmission Range (meters)
1 - 100
~ 10
~100
System Resources Battery Life (days)
Success Metrics
Speed, Flexibility
Cost, Convenience
Reliability, Power, Cost
[3] Atmel Corporation, http://www.atmel.com, 2005-03. [4] The official website of the ZigBee Alliance, http://www.zigbee.org, 2005-03.
