# Home Automation System Based on Intelligent Transducer Enablers
## 3 Plug-and-Play Mechanisms for Transducer Networks
### The ISO/IEC/IEEE 21451 Standard  
The most relevant initiative for adding plug-and-play capabilities to intelligent transducers is the ISO/IEC/IEEE 21451 standard [31; 32] (previously known as IEEE 1451). This standard constituted the basis and inspiration when designing HASITE's architecture. The ISO/IEC/IEEE 21451 standard is aimed at providing sensors and actuators with a series of features like self-identification, self-description, self-calibration and standard data formats. These mechanisms facilitate interoperability between networks with heterogeneous technologies. The Transducer Electronic Data Sheets (TEDS) are one of the most important components of any system based on ISO/IEC/IEEE 21451. TEDS are similar to the traditional data sheets provided by hardware suppliers, containing information such as the manufacturer's name, the type of transducer, the serial number, the measurement range, the accuracy, the calibration data and the supported data formats. The standard also supports the implementation of new kinds of TEDS for proprietary solutions. The complete ISO/IEC/IEEE 21451 family of standards defines a set of common, open, network-independent communication interfaces for connecting smart transducers to digital systems and instruments in network-independent environments. ISO/IEC/IEEE 21451.0 defines a set of common functionalities, TEDS and commands for accessing the transducers connected in various physical configurations (e.g., point-to-point, distributed multi-drop and wireless configurations) to fulfill different application requirements. The standard family presents three possible ways for accessing sensors and actuators from a network: IEEE 21451.1, IEEE 21451.0 Hyper Text Transfer Protocol (HTTP) and the Smart Transducer Web Services (STWS) [33; 34]. Figure 1 shows the architecture of an ISO/IEC/IEEE 21451 system. It is divided into two subcomponents: the Network Capable Application Processor (NCAP) and the Transducer Interface Module (TIM). The NCAP is a special network node that performs processing and communication tasks. Its radio interface allows it to communicate with intelligent transducers while offering different web services through their interfaces. The NCAP may run an HTTP server to provide the sensor network with access to each of the transducers via the Internet. The NCAP has a network interface that provides the services offered by the transducers. When a request is received, the NCAP uses a service to send the necessary commands to the transducer module through the communications module. Then, the transducer module processes the request and sends back the response to the NCAP. Finally, the NCAP uses its HTTP server to communicate the response to the remote client. The intelligent transducer module, referred to as TIM, Smart TIM (STIM) or Wireless TIM (WTIM) in the standards, acquires sensor signals and controls the actuators. It is able to provide sensor data or actuate over remote elements through services. The communication with the rest of the network and with remote users is conducted through an embedded air interface. The TIM is divided into two interfaces: the Network Interface (NI) (a network communications protocol for connecting the NCAP to the network) and the Transducer Independent Interface (TII) (a protocol for transferring the information about the transducer). The ISO/IEC/IEEE 21451 standard specifies the basis for interoperating and scaling heterogeneous sensor networks. Even though it provides a high degree of flexibility, it also requires carrying out a complex deployment and implementation. Such a complexity and the cost of meeting all functionality  
Figure 1: ISO/IEC/IEEE 21451 architecture. often involves the use of a separate microcontroller or a processor in the TIM, leading to a significant increase in both cost and power consumption. This issue constitutes a problem, especially in the case of creating scalable transducer networks.