Integrated sensor nodes with GSM modems simplify the task of wireless data acquisition
The sensor network can thus be classified as a data acquisition network and data distribution network. The data acquisition system typically consists of sensors and circuitry to handle the real-world information available and the data distribution network involves the communication protocols, network topology, and methodology to transmit and handle the data. The basic network topologies used are star, ring, bus, and mesh, as shown in figure 1.
The choice of the sensor network topology depends upon the application and the kind of processing and data handling required. The need for improving connectivity from PCs to the real world is gaining momentum. There are a many sensors and actuators in use, and interconnecting them by integrating the data available is becoming a necessity. The numbers of nodes in a sensor network continues to increase and wired connectivity is often not an option since sensors must be placed in remote locations. The cost per node is also decreasing, enabling wider reach of sensor nodes. There are also many improvements in low power radio technologies which can be used to design more efficient systems.
Wireless networks also offer better scalability compared to wired networks and deploying a new node in a wireless network is easier. Sensor networks need to balance performance versus the lifetime of the sensor node. Wireless nodes can be configured dynamically to balance this tradeoff, as well as operate autonomously to permit local control of operation and power management. A number of wireless protocols can be considered for sensor networks namely Zigbee, Bluetooth, GSM, Wi-Fi, etc. The choice of wireless protocol depends upon the application needs for the sensor network.
Low power capability
Wireless sensor nodes require very little maintenance and must run for days and sometimes months using the same battery. Thus, low power design is critical for the design of real-world wireless sensor networks, and it is a primary requirement that sensor nodes process and transmit sensor data while consuming very little power.
As sensors in a sensor node typically measure slow varying analog quantities, nodes need only be active for a short duration to transmit data before they go back to sleep. This means that sensor nodes have to have excellent standby current capabilities. Also, most of the data transmission occurs between the sensor nodes to the base station.
Network architecture and communication protocols must exploit this asymmetry of sensor communication from sensor node to base station. Design of a low power sensor is critical. Micro Electro Mechanical Systems (MEMS) based sensors with low power capabilities are also critical. Sensor nodes may operate in an environment of densely distributed nodes from different sources. Sensor nodes may also need to transmit using very low power in noisy environments.
Aggregation of data from sensor nodes
The data from a sensor network must be aggregated and processed in a centralized location. Data handling in a sensor network can be split into data dissemination and data gathering. Data dissemination is the process by which information is routed in the sensor network. This information could be data acquired from the sensor or requests for data from other sensors. A number of algorithms are available for disseminating data across a sensor network. Data gathering algorithms maximize the number of communications that happen with a sensor node before the node dies. The trade off in this case is between delay and power consumption. In case of a direct transmission, every node sends collected data directly to a centralized network as in the case of nodes with GSM capabilities. Nodes of a wireless sensor network would have an operating system ported onto it. This enables an easy expansion through the addition of more wireless sensors. The operating systems for sensor networks resemble embedded operating systems since they are developed keeping an application in mind and are not generic. Also, since the system is built with low power and low cost capabilities, most general purpose operating systems must be eliminated. Given that most sensor networks do not require real-time capabilities, a smaller operating system such as TinyOS that has been specifically designed for sensor nodes may be used.
Figure 2 shows a typical implementation of a sensor network using a GSM (Global system for mobile communication) modem. Here all of the sensors communicate their data to a centralized server. The server has control over individual sensor nodes; however, individual sensor nodes cannot communicate between themselves. The server has to be involved for any communication between any two sensor nodes.
GSM modem
GSM (Global System for Mobile communication) is one of the standards for mobile telephony in the world. Although initially used only for voice communication, it has been adapted to include data capabilities by means of GPRS (General Packet Radio Service) and EDGE (Enhanced Data Rates for GSM Evolution). A GSM modem is a type of modem which accepts a SIM (Subscriber Identification Module) and operates like a mobile phone. GSM modems can be used in low power mode or can also be turned off when they are not in use. The cost of transmitting data by GSM networks is has been falling rapidly. What’s more, a GSM modem can easily be interfaced to microcontrollers using standard communication protocols. Mobile phones are increasingly handling data along with voice. Most GSM modems have a TCP/IP stack implemented on them and can be used to transmit data over secure channels. This also reduces the complexity of developing applications and enables the use of simple microcontrollers to interface to the GSM modem.
A GSM modem can connect to any IP (Internet Protocol) address and transmit data. Multiple modems can send data to a single IP address, and all the data can be collected and processed from a single location anywhere in the world. Users can dynamically configure each and every modem remotely based on data sent across from the network. In certain networks, a single node would have GSM capability. Other nodes would send their data to this particular node to be transmitted to the centralized server. This could reduce the cost of the overall system but would not be possible if nodes are not clustered together. GSM modems also have the ability to provide instant alerts using SMS (short messaging service) or by transmitting data to a different, high-priority IP address based on certain conditions. These features can be used for fault tolerance and redundancy checks.
Let us consider an example of a GSM modem (SIM300) where communication happens over a serial port with the microcontroller. The modem has a standard set of commands called AT commands. These commands control the operation of the modem from the microcontroller. The microcontroller sends these commands over a UART (Universal Asynchronous Receive Transmit) interface at a specified baud rate. The data sent through the serial port can be transmitted to a centralized server by configuring the modem using a specific set of commands. Thus, interfacing with the GSM modem simplifies data acquisition and processing in sensor networks.
Complete system implementation
A sensor node consists of an analog signal chain and also requires a host of digital peripheral interfaces. Also each sensor node may be interfaced to different kinds of sensors, requiring flexibility in its interfaces and I/Os. Programmability of individual nodes plays a critical role in a successful implementation of a sensor network.
Mixed-signal programmable microcontrollers like PSoC from Cypress provide analog and digital subsystems which can be configured to provide the functionality required by individual nodes. This eliminates the need for specialized hardware for each different kind of sensor. Such System-on-Chip-based (SoC) MCU also have the capability to handle all tasks required for the sensor node in a single chip, including ADC, DAC, PGA’s, comparators, op-amps, digital filtering capabilities, DMA, and LCD controllers, among others. Design of the system using SoCs allows frequent and fast changes in design to assist customization of sensors nodes. PSoC with its low power capabilities at sleep mode is very suitable for these applications.
Ajay Bharadwaj is a Senior Applications Engineer at Cypress Semiconductor — www.cypress.com. Balaji Mamidala is an Applications Engineer.
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