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HTINews Article

INTRODUCTION TO THE CEBUS^® COMMUNICATIONS PROTOCOL

Kenneth P. Wacks, Ph.D.
Home Automation Consultant

Introduction

This article is an excerpt from a report entitled, Home Automation and Utility Customer Services, written by Dr. Wacks and published by Cutter Information Corporation. Please see the Cutter web site (www.cutter.com/energy/reports/homeauto.htm) for an outline of the report and ordering information.

The full report is intended to guide energy utility companies in developing new customer services that use home automation networks. This excerpt focuses on the creation and definition of the CEBus communications standard for home automation. The full report covers additional protocols including BatiBUS, European Home Systems, European Installation Bus (EIB), the Home Bus System from Japan, Home Electronic System (HES, an international standard), LonTalk^® from Echelon Corporation, SMART HOUSE^®, and X-10^®, as shown in Figure 1.

Figure 1. The Major Home Automation Communications Protocols

A Brief History of the CEBus Standard

The CEBus communications protocol for home automation is a United States standard developed by the Electronic Industries Association (EIA). In 1984 the EIA decided to standardize the infrared signalling used for remote control of appliances to avoid incompatible or interfering formats. The motivation was consumer confusion among multiple hand-held remote control devices for televisions, VCRs, and cable television decoder boxes. This effort grew into the development of the CEBus standard for home automation. CEBus is an acronym for Consumer Electronics Bus.

The CEBus Committee has made steady progress since being formed in 1984 by the EIA. CEBus became an interim standard in 1992 and balloting to move it toward a national standard commenced in 1995. Key sections have already been voted with about 90% of the ballots affirmative. The CEBus standard is comprehensive and is intended to open new markets for the consumer electronics industry. The CEBus specifications consist of about 1000 pages.

Motivation and Goals of the CEBus Protocol

Much debate and thoughtful design have been incorporated into the CEBus communications protocol. More than 400 companies have attended CEBus committee meetings. Key among the contributors have been appliance manufacturers and utilities, including electric, gas, and telephone. They have influenced a standard with the following features that allow flexibility and cost control in appliances incorporating home automation:

• Provide home automation for retrofit into existing houses.

• Allow appliances with capabilities ranging from simple to complex to use subsets of the CEBus facilities. All devices interpret a minimum subset of commands.

• Encourage the development of low cost interface units embedded in appliances for operation on CEBus media.

• Accommodate a variety of data transmission media (to be described in this report). Most aspects of device communications do not vary by medium.

• Support the distribution of wide-band audio and video services in a variety of analog and digital formats.

• Use a distributed communications strategy for CEBus so no central controller is required for communications among appliances. The network intelligence is contained in CEBus appliances. Centralized coordination of subsystems and whole-house home automation are supported at the option of the manufacturer.

• Permit appliances and components to be added to the network or removed with no disruption and with minimal user involvement in network configuration. In consumer electronics, this feature is called plug-and-play.

• Provide a fair method for devices to access a shared medium. However, allow time critical applications a higher priority in gaining communications services.

Overview of the CEBus Protocol

Network Architecture

The CEBus standard defines a data communications network that accommodates the following variety of media:

Ÿ The electric power line
Ÿ Twisted-pair wires
Ÿ Coaxial cable
Ÿ Infrared signalling
Ÿ Radio frequency signalling
Ÿ Fiber optics
Ÿ Audio-video bus

With this choice of media, some home automation systems can be installed in existing houses with no additional wiring. Such systems might use the power line for data exchange among components and infrared or radio frequency signalling for remote control. The CEBus home automation technology would be contained entirely within the appliances that plug into conventional outlets or within the remote control units.

All media carry the CEBus control channel and transmit data at the same rate (about 8000 bits per second). They also can carry data channels with bandwidths suitable for analog or digital audio and video, depending on the medium. Commands and status reports are carried on the control channel in the form of messages, composed of packets of bytes. The bulk of the CEBus specification is devoted to specifying the control channel. Commands for allocating data channels are included. The encoding of data channels may be specified in later releases of the CEBus protocol or may be left to manufacturers or trade associations to specify.

CEBus requirements for fiber optics are released. Specifications will be developed later. The EIA is soliciting assistance from manufacturers of fiber optics components and products to develop specifications that will have wide industry support. The audio-video bus is being specified also for later release by the EIA. It is intended for interconnecting a cluster of entertainment products in one room. The bus consists of a thin cable with eight twisted-pair wires intended for interconnecting a cluster of entertainment products in one room. The cable has a maximum length of 30 feet and contains three audio channels, four video channels, and the CEBus control channel. A single connector will attach the cable to an audio-video appliance.

CEBus specifies a dual coaxial system. An up-stream cable collects in-house generated video from VCRs and cameras. A head-end combines external video signals with in-house sources, such as video tape players and cameras, for distribution on the down-stream cable to all receivers.

The format for CEBus control messages is independent of the communications medium used. Each message contains the destination address of the recipient without reference to where on the medium the sender or the receiver is located. Thus, CEBus forms a logically uniform network.

CEBus Devices and Topology

CEBus supports a flexible topology. A device may be located wherever convenient. It may connect to any medium for which it contains the appropriate CEBus interface. Messages can be sent between media via an electronic circuit called a router. A router is not necessarily a separate unit. It may be contained within an appliance.

In addition to sending individual messages, all devices or specified groups of devices may be reached with a single message containing a unique broadcast address. All CEBus devices must respond to the broadcast address. A device may be assigned membership in one or more groups. This allows one message to be sent, for example, to all alarms or to all time-keepers. The device manufacturer chooses whether a device supports group addressing and how many groups a device can support. The members of a group receive messages containing the group address.

CEBus does not use a central controller for managing the delivery of control messages. Control is distributed among the CEBus appliances and media routers. The CEBus standard does not specify a particular topology. All the appliance connection points on each medium are treated logically as if they were on a bus. This means that all the appliances on a particular medium sense a data packet about the same time. All appliances read the destination address contained in the message. Only that appliance with the matching address reads the contents and acts or responds accordingly.

Figure 2 illustrates a typical CEBus network with three media interconnected by routers. Appliances and sensors are plugged into the CEBus network wherever convenient. The cluster controller, illustrated in the figure, is responsible for organizing an application such as lighting or energy management.

Figure 2. Example of CEBus Topology

A simple demonstration of appliance interconnectivity was prepared for a Consumer Electronics Show a few years ago in Las Vegas. Figure 3 illustrates the configuration of appliances that operated CEBus on multiple media. A hand-held remote control unit was aimed at a television while the key labelled LIGHTS was pressed. The television received an infrared signal from the remote control infrared interface module, called a brick in CEBus terminology. The television interpreted the signal and recognized that this command was not addressed to the television. Therefore, the television passed the signal to a router built into the television. A power line brick inside the router impressed onto the power line a signal containing the command, which was addressed to the lighting control unit. The lighting controller received this signal and illuminated the lights, as directed by the command embedded in the signal.

Figure 3. Example of a CEBus Application

Institutional and Corporate Support for CEBus

The EIA has prime responsibility for managing refinements to EIA-600. Beyond organizing and facilitating committee meetings, the EIA is not chartered to develop specific business strategies. Therefore, in June 1994, the EIA authorized the establishment of the CEBus Industry Council (CIC) as a not-for-profit organization to expand support for CEBus and encourage the adoption of the CEBus standard by manufacturers. About 100 companies are members of the CIC.

Among the responsibilities of the CIC are:

- Product certification of conformance to the CEBus protocol.

- Development of extensions to the CEBus Common Application Language (CAL).

- Creation of guidelines for using CAL to enhance product interoperability.

- Informing manufacturers about the CEBus standard.

© Copyright 1997, Kenneth P. Wacks

Dr. Kenneth Wacks provides management and engineering consulting in home and building automation. He offers impartial and practical advice on business opportunities, network alternatives, and resources for product development. His clients have included SMART HOUSE, the Electronic Industries Association, the Intelligent Buildings Institute, electric and gas utilities, and manufacturers in the U.S., Canada, and Europe. For further information, please contact Ken at 9 Pinewood Road, Stoneham, Massachusetts 02180, USA; Tel: (617) 662-6211, Fax: (617) 665-4311 Email: kenn@MIT.EDU