An open communication protocol dedicated to Home Automation is European Home Systems (EHS), which has now entered a mature phase: hardware and software products.
The EHS specification has been defined in order that home appliances can communicate and share each other's resources. EHS protocol is based on a shared communication system and on unambiguous definitions of the device functionality.
The EHS communication model follows the structure of the Open Standard Interconnection (OSI) reference model.
EHS specifies the physical layer, the data link layer, the network layer and the application layer. Due to the fact that the message length is limited, the dialogue session is short and the application layer manages the command language, the transport, session and presentation layer has been omitted.
The application layer translates the application language in data frame able to circulate over the network. The Data Link Layers, separated in two sub-layers Medium Access Control (MAC) and Logical Link Control (LLC), handles the bit stream conversion, the rules for accessing the network, the recognition of frames and provides acknowledgement and repetition mechanism. The network layer is identical for all the EHS units and manages data related to the route (i.e. the addresses to reach a unit over several sub-layers).
Animation 5.13 Levels of processing of messages in the case of EHS protocol. Application layer, Network layer, Data link Layer.
Several physical layers are already defined taking into account the variety of application requirements. Power Line (PL), Infra Red (IR) or Radio Frequency (RF) physical layers can be used in an existing home for low speed communication without any cabling cost.
Twisted Pair (TP) cabling solutions or coaxial cable can be used when higher speed is required or when transmitting analogue signals. For example, communication over power line is well suited between energy management system and electrical heaters, in opposite way; communication over twisted pair is safer for a security system.
Designing a "Plug & Play" home automation system with EHS gives the following benefits to the customer:
- Fast integration of elements into the system
- Allow using common models and software for manages the devices through EHS network, that means that is allowed to use standard methods for a large variety of devices and configuration.
- The effort needed by the engineering phase is reduced
- Facilitate the inter-operability of devices produced by different manufacturers
- Facilitate the scalability of home automation system
Inter-operability: this general need of communication systems is fundamental in Home Automation. It involve that appliances developed independently by different manufacturers are able to co-operate. Thus, the direct benefit for the customer is that he can choose the product brand he wants.
Scalability, represent the feature that assure the modification of size of the system without fundamental changes from one stage to the other.
Flexible positioning, imply the feature of elements to be place in a vary kind of position or places without modification of them.
Automatic configuration is strong related to the “plug and play” feature and represent the capability of devices that are included in the EHS system to recognize automatically the environment and the connection to the network, taking the right parameterization automatically.
The configuration of the network can be done easily either automatically (Plug & Play) or manually to respect different zones in the home or building. For example, a heating control system has to take into account different heating zones (living room, bedroom).
A Home Automation system based on EHS is easily expandable. New applications and new physical layers can be added at any time in an existing network. This modular functionality allows the customer to start with the units needed for the application and to add more units when he wants.
Inter-working of applications means that different applications are able to communicate over the same network without interfering each other. For example, an energy management system does not interfere and disrupt a security system. Units can also use the facilities located in other units.
The architecture of EHS network is based on the notion of controllers and devices shared into application domains.
The controller named feature controller (FC) controls the application and provides features and intelligence of the application such as resources monitoring, control algorithm or decision making process. A controller defines one application domain but can cover several application domains by sharing their resources.
A device provides and manages application resources. For example, an electrical heater manages the heating resource; a thermostat manages a threshold temperature. A device belongs to a single application domain but may be shared or controlled by several controllers.
A Device Descriptor (DD) codified by the specification describes each device. Thus, devices having the same DD are interchangeable. This two-byte DD gives the necessary information in order that a controller knows what resources are available on the network and how to reach those resources.
The first byte is the application domain; the second byte gives the description of the device itself (DD = 1611 represents a room temperature sensor in the heating application domain). Controllers and devices may establish a logical link, named enrolment, between them to define an application domain. A device able to be enrolled by a controller is named complex device (CoD).
Application control units and resources units interact through commands. A command language based on the definition of standardized objects and associated services managing these objects is specified. The basic structure of a command is: Application domain, object, service, and parameters.
Animation 5.14 Structure of commands that are supported by EHS and the corresponding device descriptor
The architecture of the EHS network is based on the notions of controller devices and on the notion of application domains.
A Device Descriptor (DD) describes an application resource, which means the capabilities of devices to realize some specific action characterized by the parameters of this. The application domain is represented by the environment where a device can act. Commands exchange between controller and devices of the same application domain are based on EHS codified objects and services.
EHS specifies several physical layers. The structure of the network can consist of different sub-networks based on these physical layers. Up to seven different physical layers may be associated. Routers allow appliances to communicate over EHS sub-network. In animation5.15 is illustrated which is the format address unit accepted by EHS protocol.
Animation 5.15 Hierarchical addressing through EHS network
Gateways allow inter-connection of EHS network to non-EHS network. Animation 5.16 illustrates a possible structure of the network. Router and gateway allow appliances to communicate whatever their location on sub-network is.
Animation 5.16 EHS network can be composed on several EHS or non-EHS sub-network.
Each unit connected to a sub-network has its own sub-network address. The expandability of the network is guaranteed because a unit connected to one sub-network is able to communicate with other units located in other sub-networks. This is achieved by an addressing scheme standardized for each unit.
A unit address is composed of the sub-network address of the destination unit, the number of routes and of the addresses of the different routers to reach the destination sub-network. Animation 5.15 shows an example of this addressing scheme.
This sub-network address can be fixed at application level (for example by using dip switches) or can be dynamically acquired by a procedure called registration. In animation 5.15 Address of A seen from B. The address is composed of the address of the destination unit and the address of the route to reach this unit.
The animation 5.13 shows the structure of an EHS frame. The different layers from the application layer to the network layer elaborate the frame. Then, the data link layer dedicates the frame for the physical layer.
On power line, the EHS frame is composed of several fields. Some of them are more dedicated to power line transmission. The preamble is used to train the receiver modem. During reception of this field, the receiver modem checks if the received signal is an EHS conformant FSK signal and synchronize itself to this signal.
The header really marks the beginning of useful data. The receiver takes it into account if the difference between the received value and the expected one does not exceed 2 bits. The house address is used to discard frames coming from other homes.
The last two bytes of the EHS frame contain the Frame Check Sequence (FCS). This is a Cycle Redundancy Check (CRC) computed from the house address field to the data field. The receiver computes the CRC of the received frame and compares it to the received value of the CRC. A difference between these two values indicates that an error occurred during the transmission. The receiver discards the frame.
To secure the transmission on power line, EHS implements a Forward Error Correction (FEC). A 6-bit FEC code is added to each byte from the house address to the end of the frame. Thus, each byte is encoded with a 14-bit value. The FEC method is able to correct up to three consecutive bits. This method is well suited to correct transmission errors due to most of the electrical noises encountered on power line generated by switch mode power supply, triac or switches commutation. When the signal is too noisy, the errors are detected by the frame check sequence.