P O Z NA N UN I V E R S ITY O F TE C H N O LO GY A C A D E M IC J O U R N AL S
No 54 Electrical Engineering 2007
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Piotr WASILEWSKI*
WIRELESS SENSOR NETWORK STUDENT’S LAB PROJECT
In the article the student’s lab for evaluating and testing wireless sensor networks based on IEEE 802.15.4/ZigBee protocol is presented. The first part contains basis information about wireless sensor networks, and IEEE 802.15.4 standard as well ZigBee protocol, network topologies, their properties and limitations. Next, the structure of the network node modules is described, and finally the sample exercises and projects are presented.
Keywords: wireless sensor network, ZigBee, IEEE 802.15.4, student’s lab
1. INTRODUCTION
A wireless sensor network (WSN) is a wireless network consisting of spatially distributed nodes with sensors used for monitoring physical or environmental conditions, such as temperature, sound, vibration, pressure, motion or pollutants, at different locations [6]. Wireless sensor networks are today used in many application areas, including environment and habitat monitoring, healthcare applications, home and industry automation, and traffic control.
The official standards adopted for wireless sensor networks are: ISO 18000-7, 6lowpan (IPv6 over Low Power Wireless Personal Area Networks) and Wire-lessHART [1, 3], however the networks build with ZigBee standard can be found more frequently.
ISO 18000-7 based networks use ISM 433 MHz band. All other mentioned above standards work in ISM 2,4 GHz band utilizing physical layer and media access control described in IEEE 802.15.4 standard.
Because ZigBee and IEEE 802.15.4 are rapidly becoming the standards of choice for wireless sensor networks, the student’s lab was designed, where the properties and limitations of WSN networks can be examined.
2. IEEE 802.15.4 STANARD
IEEE 802.15.4 is a standard which specifies the physical layer and medium access control for low-rate wireless personal area networks. IEEE 802.15.4
2007
Poznańskie Warsztaty Telekomunikacyjne Poznań 6 - 7 grudnia 2007 POZNAN UNIVERSITY OF TECHNOLOGY ACADEMIC JOURNALS
standard offers the fundamental lower network layers of a type of wireless personal area network (WPAN) which focuses on low-cost, low-speed and low-power communication between devices [2].
The basic framework defines a 10-meter communications area with a transfer rate of 20, 40, 100 and 250 kbit/s. Important features include real-time suitability by reservation of guaranteed time slots, collision avoidance through CSMA/CA and integrated support for secure communications. Devices also include power management functions such as link quality and energy detection. IEEE 802.15.4 based devices may use one of three possible frequency bands for operation: 868 MHz (Europe), 915 MHz (America, Australia) and 2,4 GHz (worldwide).
The standard defines two types of network node. The first one is the full-function device (FFD). It can act as the coordinator of a network and it may function as a common node. It implements a general model of communication which allows it to communicate with any other device; it may also relay messages. On the other hand there are reduced-function devices (RFD). These are meant to be extremely simple devices and they can only communicate with FFD's and can never act as coordinators. Networks can be built as either point-to-point or star networks. However, every network needs at least an FFD to work as the coordinator of the network. Networks are thus formed by groups of devices separated by suitable distances.
Fig. 1 Star network [5]
The physical medium is accessed through a CSMA/CA (Carrier sense multiple access with collision avoidance) protocol. Networks which are not using beaconing mechanisms utilize an unslotted variation which is based on the listening of the
medium, leveraged by a random exponential backoff algorithm. Common data transmission utilizes unallocated slots when beaconing is in use.
Regarding secure communications, the MAC sublayer offers facilities which can be harnessed by upper layers to achieve the desired level of security. Higher-layer processes may specify keys to perform symmetric cryptography to protect the payload and restrict it to a group of devices or just a point-to-point link. Furthermore, MAC computes freshness checks between successive receptions to ensure that presumably old frames, or data which is no longer considered valid, does not transcend to higher layers.
3. ZIGBEE PROTOCOL
ZigBee is the name of a specification for a suite of high level communication protocols published in 2004 by ZigBee Alliance [7]. The relationship between ZigBee and IEEE 802.15.4 is shown on figure 2.
Fig. 2 ZigBee software architecture [5]
A ZigBee standard has the capacity to address up to 65535 nodes in a single network. However, there are only three general types of node: coordinator, router and end device.
All ZigBee networks must have one and only one coordinator irrespective of the network topology. At the network level, the coordinator is mainly needed at system initialization. The coordinator provides services at the application layer and if these services are being used (for example, coordinator binding), the coordinator must be able to provide them at all times.
The tasks of the coordinator at the network layer are: selects the frequency channel to be used by the network, starts the network, allows child nodes to join
the network. The coordinator can also provide message routing and security management.
A ZigBee network which uses tree or mesh (Fig. 3, 4) topology requires the presence of at least one router. The main tasks of a router are: relays messages from one node to another, allows child nodes to connect to it.
The main tasks of an end device at the network level are sending and receiving messages. An end device can often be battery-powered and, when not transmitting or receiving, can sleep in order to conserve power.
A ZigBee network can have one of three topologies: star, tree and mesh.
Star topology is defined in IEEE 802.15.4 (Fig. 1) and is the simplest and most limited of the possible topologies. A star topology consists of a coordinator and a set of end devices. Each end device can communicate only with the coordinator. Therefore, to send a message from one end device to another, the message must be sent via the coordinator, which relays the message to the destination. A disadvantage of this topology is that there is no alternative route if the radio link fails between the coordinator and the target device. In addition, the coordinator can be a bottleneck and cause congestion. It is possible to use nodes with router functionality in a star network in place of end devices. However, in this case the routers are not allowed to have child nodes attached and so their routing capability is not used.
A tree topology consists of a coordinator, and a set of routers and end devices. The coordinator is linked to a set of routers and end devices; its children. A router may then be linked to more routers and end devices; its children. This can continue to a number of levels. This hierarchy can be visualized as a tree structure with the coordinator at the top, as illustrated in the figure 3.
The communication rules in a tree topology are: – a child can only directly communicate with its parent,
– a parent can only directly communicate with its children and with its own parent,
– in sending a message from one node to another, the message must travel from the source node up the tree to the nearest common ancestor and then down the tree to the destination node.
A disadvantage of this topology is that there is no alternative route if a necessary link fails.
A mesh topology consists of a coordinator, and a set of routers and end devices. The network structure is the same as for the tree topology. However, the communication rules are more flexible in that router nodes within range of each other can communicate directly. This gives rise to more efficient message propagation and means that alternative routes can be found if a link fails or there is congestion. The mesh topology is illustrated in the figure 4.
Fig. 4 Mesh network [5]
In the mesh topology, a “route discovery” feature is provided which allows the network to find the best available route for a message.
4. THE LAB
The students’ lab is build with 15 identical programmable modules which can be configured as coordinators, routers and end devices. The block diagram of the
module is shown in Fig. 5. The heart of each module is the ZigBee controller JN5139-Z01-M00 from Jennic [4]. ZigBee controller LCD display USB interface RS 232 interface 5V/3V converter temperature and light sensors, pushbuttons
Fig. 5 Block diagram of the designed lab module
It is equipped with a fully compliant 2.4 GHz IEEE 802.15.4 transceiver with ceramic antenna onboard, 16MHz 32-bit RISC processor, 96kB RAM, 192kB ROM pre-programmed with a ZigBee network stack, 4-input 12-bit ADC, 2 11-bit DACs, 2 comparators, temperature sensor, timers, UARTs and general purpose input/outputs (Fig. 6).
The security coprocessor provides hardware-based 128-bit AES-CCM, CBC, CTR and CCM processing as specified by the IEEE 802.15.4 standard. It does this in-band on packets during transmission and reception, requiring minimal intervention from the CPU. It is also available for off-line use under software control for encrypting and decrypting packets generated by software layers such as ZigBee and user applications. This means that these algorithms can be off-loaded by the CPU, increasing the processor bandwidth available for user applications.
The photo of the complete laboratory module is presented in Fig. 7.
Fig. 7 ZigBee module photo
ZigBee controller can acquire data from external temperature and light sensors and can be programmed by RS232 interface which can be also used for communication with the host computer. USB interface is a primary communication media with the host computer; it is also used as a power source for the module. The state of the node and some transmission parameters can be displayed on the LCD alphanumerical display.
5. SAMPLE EXERCISES
Described above modules allow testing networks build with IEEE 802.15.4 and ZigBee standards. Students can write their own programs or program them with already designed configurations.
Sample lab exercises:
– Examining average transmission speed and bit error rate as a function of the noise level in the transmission channel
– Examining average transmission speed and bit error rate as a function of the transmitter power and the distance between nodes
– Examining average transmission speed and bit error rate as a function of the packet size
– Examining average transmission speed and bit error rate in two, three and four networks sharing the same channel as a function of the packet size in beacon and non-beacon networks
– Examining average transmission speed in different network topologies (star, tree, mesh) of varying complexity
– Examining the influence of data encryption (AES) on the average transmission speed and bit error rate.
– Examining the influence of the different node type failure on the transmission parameters
– Examining route discovery algorithms in the mesh networks with moving nodes
6. CONCLUSIONS
Presented wireless sensor networks lab is a flexible and inexpensive tool for evaluating networks build over IEEE 802.15.4 and ZigBee protocols. Students can discover network abilities and limitations, examine different network topologies and even write their own stacks build over IEEE 802.15.4 standard.
REFERENCES
[1] HART Communication, http://www.hartcomm2.org/hcf/press/pr2007/ hart7released.html
[2] IEEE 802.15 Working Group for WPAN, http://www.ieee802.org/15/
[3] IPv6 over Low power WPAN, http://www.ietf.org/html.charters/6lowpan-charter.html [4] Jennic, Data Sheet — JN5139 IEEE 802.15/4/ZigBee Wireless Microcontrollers,
www.jennic.com
[5] Jennic, ZigBee Stack User Guide, 2007, www.jennic.com
[6] Römer K., Friedemann M., The Design Space of Wireless Sensor Networks, IEEE Wireless Communications 11 (6), 2004
