ABSTRACT
The last two decades have witnessed a remarkable evolution of embedded systems from being assembled from discrete components on printed circuit boards, although, they still are, to systems being assembled from IP components “dropped” on to silicon of the system on a chip. Systems on a chip offer a potential for embedding complex functionalities, and to meet demanding performance requirements of applications such asDSP, network, andmultimedia processors. Another phase in this evolution, already in progress, is the emergence of distributed embedded systems; frequently termed as networked embedded systems, where the word “networked” signifies the importance of the networking infrastructure and communication protocol. A networked embedded system is a collection of spatially and functionally distributed embedded nodes interconnected bymeans of wireline and/or wireless communication infrastructure and protocols, interacting with the environment (via a sensor/actuator elements) and each other, and, possibly, a master node performing some control and coordination functions, to coordinate computing and communication to achieve certain goal(s).The networked embedded systems appear in a variety of application domains such as automotive, train, aircraft, office building, and industrial areas-primarily for monitoring and control, environment monitoring, and, in future, control, as well. There have been various reasons for the emergence of networked embedded systems, influenced
largely by their application domains. The benefits of using distributed systems and an evolutionary need to replace point-to-point wiring connections in these systems by a single bus are some of the
most important ones. The advances in design of embedded systems, tools availability, and falling fabrication costs of
semiconductor devices and systems have allowed for infusion of intelligence into the field devices
such as sensors and actuators. The controllers used with these devices provide typically on-chip signal conversion, data and signal processing, and communication functions. The increased functionality, processing, and communication capabilities of controllers have been largely instrumental in the emergence of a widespread trend for networking of field devices around specialized networks, frequently referred to as field area networks. The field area networks, or fieldbuses [] (fieldbus is, in general, a digital, two-way, multidrop com-
munication link) as commonly referred to, are, in general, the networks connecting field devices such as sensors and actuators with field controllers (for instance, programmable logic controllers (PLCs) in industrial automation, or electronic control units (ECUs) in automotive applications), as well as man-machine interfaces; for instance, dashboard displays in cars. In general, the benefits of using those specialized networks are numerous, including increased
flexibility attained through the combination of embedded hardware and software, improved system performance, and ease of system installation, upgrade, and maintenance. Specifically, in automotive and aircraft applications, for instance, they allow for a replacement of mechanical, hydraulic, and pneumatic systems bymechatronic systems, wheremechanical or hydraulic components are typically confined to the end-effectors; just to mention this two different application areas. Unlike local area networks (LANs), due to the nature of communication requirements imposed by
applications, field area networks, by contrast, tend to have low data rates, small size of data packets, and typically require real-time capabilities which mandate determinism of data transfer. However, data rates above Mbit/s, typical of LANs, have become a commonplace in field area networks. The specialized networks tend to support various communication media like twisted pair cables,
fiber-optic channels, power line communication, radio frequency channels, infrared connections, etc. Based on the physicalmedia employed by the networks, they can be in general divided into threemain groups, namely, wireline-based networks usingmedia such as twisted pair cables, fiber-optic channels (in hazardous environments like chemical and petrochemical plants), and power lines (in building automation); wirelss networks supporting radio frequency channels and infrared connections; and hybrid networks, with wireline extended by wireless links []. Although the use of wireline-based field area networks is dominant, the wireless technology offers
a range of incentives in a number of application areas. In industrial automation, for instance, wireless device (sensor/actuator) networks can provide a support for mobile operation required in case of mobile robots, monitoring and control of equipment in hazardous and difficult to access environments, etc. In a wireless sensor/actuator network, stations may interact with each other on a peer-to-peer basis, and with a base station. The base station may have its transceiver attached to a cable of a (wireline) field area network, giving rise to a hybrid wireless-wireline system []. A separate category is the wireless sensor networks, envisaged to be largely used for monitoring purposes. The variety of application domains impose different functional andnonfunctional requirements on
to the operation of networked embedded systems. Most of them are required to operate in a reactive way; for instance, systems used for control purposes. With that comes the requirement for real-time operation, in which systems are required to respond within a predefined period, mandated by the dynamics of the process under control. A response, in general, may be periodic to control a specific physical quantity by regulating dedicated end-effector(s), or aperiodic arising from unscheduled events such as out-of-bounds state of a physical parameter or any other kind of abnormal conditions. Broadly speaking, systems which can tolerate a delay in response are called soft real-time systems; in contrast, hard real-time systems require deterministic response to avoid changes in the system dynamics which potentially may have negative impact on the process under control, and as a result may lead to economic losses or cause injury to human operators. Representative examples of sys-
tems imposing hard real-time requirement on their operation are Fly-by-Wire in aircraft control, Steer-by-Wire in automotive applications, to mention some.
