Designers of buses and protocols to automate people's homes have taken on quite a challenge. Home-automation systems handle tasks as complex as those in industry but must cost much less. Some obstacles are unique: Few home owners want electricians to tear apart existing houses to install control wiring. Reliable implementation of power-line data communication, the "obvious" alternative to rewiring, has proven devilishly tricky. Meanwhile, wireless approaches present problems of their own. Today, three fairly new home-automation standards aim to capture a market still dominated by X-10, a protocol first deployed almost two decades ago.

Home-automation proponents predict that, from the widespread use of demand management-a scheme that employs technology to help utility companies and consumers reduce peak demand-will flow a torrent of ideas for convenient and lucrative new applications. (See box, "Demand management: home automation's "killer app'?".) These applications would be impractical if the various elements of home-automation systems could not communicate freely. Open and reasonably priced communication among products from multiple vendors is what home-automation protocols are all about.

There are four major contenders among home-automation protocols:

• Echelon Corp's LONworks,
• The Electronic Industries Association's CEBus (EIA IS-60),
• Smart House, which Smart House LP (Limited Partnership) created for the National Association of Home Builders,
• X-10 (USA) Corp's X-10 protocol.

Table 1 summarizes key characteristics of the first three protocols.

Table 1 -- A comparison of CEBus, LONworks, and Smart House

	CEbus	LONworks	Smart House
Maximum number of nodes	61,000	Almost 1019	900
Media supported	Power line, twisted pair, coaxial cable, RF, IR; fiber-optic cable(eventually)	Power line, twisted pair, RF; third-party transceivers support additional media	Custom-made wiring available from three sources
Data rate for control and communication	10 kbps	610 bps to 1.25Mbps	50 kbps
Support for high-speed data	Logical and electrical-channel allocations and media specifications for video, audio, and data distribution; standardized resource-allocation protocols	No specifications for video or audio channels	Coaxial distribution system
Ease of use: development	Big learning curve for network protocol; development tools reduce learning time	Development tools easy to use; no need to learn network-protocol details	Development tools are available
Ease of use: production	Maximum hardware flexibility; µP of 8 bits or more required	Two custom ICs from two manufacturers	Two custom ICs from Smart House
Ease of use: installation	Prewired connection or on-site association- algorithms	Varies: Equipment manufacturers can sometimes preinstall; users can install; professional installation can be required	Addressing tool required to initialize some components; Databases can be established on site or remotely
Target applications	Existing and new homes	Existing and new homes, commercial and industrial buildings, industrial automation	Residential (new homes, primarily), some light commercial buildings
Relative cost	Low to moderate	Low to moderate	Moderate to high
Adapted from "Home-automation systems in North America: an analysis of the three main contenders," by Diablo Research Corp, distributed by the Gas Research Institute.

Of the four protocols, X-10 is by far the simplest and oldest. It first appeared in 1978, and field experience with it far exceeds experience with the other three technologies combined.

LONworks is the only one of these protocols that has found wide use outside of the home. Echelon reports that over 500,000 LONworks nodes have been installed, mostly in factory-automation applications, and that at least 75,000 new nodes are being installed each month.

In larger buildings, especially commercial ones, several protocols compete, including a host of "fieldbus" standards (Ref 8). One standard specifically designed for automation of commercial buildings is the building automation and control network (BACnet) protocol (Refs 1, 2, and 3). The organization behind BACnet is the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE).

In the United States, automating existing homes is-potentially, at least-a market about 100 times the size of the market for automating new homes. Because wiring installation is such a huge headache in existing homes, the industry has developed several communication techniques that don't require special wiring. Sending data over the ac power line is the oldest such approach. The earliest power-line communication systems (X-10, for example) used narrowband techniques. More recently, systems have used direct-sequence or chirp spread-spectrum modulation (Ref 10). Spread-spectrum technology is at the heart of CEBus' power-line communication capability and, along with a new, advanced narrowband design, is an option under LONworks.

Other approaches that avoid special wiring include RF and IR wireless communication. The developers of both the CEBus and LONworks standards envision RF and IR communications. X-10 Corp is active in both of these wireless technologies and has used them in some products that also incorporate the power-line-based X-10 protocol.

Many vendors and installers assemble systems using proprietary protocols. Such protocols abound in home-security applications, such as intrusion-, smoke-, and fire-detection systems. The proprietary schemes range from simple (a contact closure represents an alarm condition) to elaborate and from loosely defined to fully specified. In the installed base of home-automation systems, proprietary protocols are the norm; they've been around for decades, even if nobody dignified them with the name "protocol." Because multivendor protocols are newer, they are an exception to the norm. This article discusses only major multivendor protocols.

Authors who discuss home-automation protocols (Refs 4, 6, 7, and 9) almost always begin by declaring that comparisons are inappropriate-that comparing protocols is like comparing apples and oranges. That said, the authors proceed to compare the protocols.

X-10

Today, designers understand much better than they did in 1978 just how terrible power lines are for transmitting signals. Indeed, power lines are probably a poorer transmission medium now than they were in the late '70s. Switching power supplies, which were recently introduced into homes, inject high-frequency noise onto ac lines, thus degrading the S/N ratio. To improve reliability, X-10 has made many refinements over the years but has retained its simple modulation scheme and generally has kept new units compatible with older products. Thus, you often can't tell which improvements a unit incorporates; if you have problems, you are likely-perhaps wrongly-to indict the whole X-10 line.

The most familiar embodiment of X-10 is the family of lamp- and appliance-control modules and control centers sold under the X-10 Powerhouse brand name. X-10 (USA) Inc's Asian affiliates manufacture these units as well as ones sold under other names, including Decora Home Control from Leviton Manufacturing Co. Because X-10 is available from many companies with broad distribution, we include it with the major multivendor protocols.

Although data transmission under the X-10 protocol is not fast, it is usually fast enough for the target application. Except for the message preamble, which packs 4 bits into two ac-line cycles, transmission proceeds at 1 bit per line cycle. A message comprises 13 bits-the 4-bit preamble, a 4-bit house code (which the user sets to avoid interfering with systems in nearby houses), a 4-bit unit/function code, and 1 bit that indicates whether the unit/function code represents a unit number or a function. Examples of functions are on, off, brighter, and dimmer. This arrangement allows up to 16 modules, each of which can respond to up to 16 commands. By using multiple house codes, systems can include more than 16 modules.

To improve reliability, a packet contains two identical messages, which are concatenated without a gap. A command consists of two packets transmitted with a gap of at least three line cycles. In the first packet, the unit/function code indicates which unit is being addressed; in the second packet, the unit/function code represents the function. A command requires at least 47 cycles and takes nearly 0.8 sec at 60 Hz.

Each bit is transmitted as an [Greek];2-msec burst of an amplitude-keyed, 120-kHz carrier whose amplitude in the on state is 6V p-p in the United States and 2.8V in Europe, where electromagnetic-compatibility regulations are tougher. The actual amplitude depends strongly on the line impedance at 120 kHz, although some modules use feedback to stabilize the amplitude. The start of each 120-kHz carrier burst coincides with a line-voltage zero crossing, so the burst repeats twice per cycle in single-phase systems and up to six times per cycle in three-phase systems. To distinguish the preamble from normal data, the bits transmitted during the two halves of preamble line cycles need not be the same.

Most US houses use split-phase ac (120V from each side to neutral). When wiring houses, electricians try to balance the loads. Some 120V outlets connect from one phase to neutral; others connect between the opposite phase and neutral. High-power loads, such as electric ranges and clothes dryers, connect from phase to phase, across 240V. When the 240V loads are off, the impedance from phase to phase is quite high. Thus, if you plug an X-10 controller into a 120V outlet, the chance is only about 50% that the controller's signal can reach a control module plugged into another 120V outlet elsewhere in the house. To solve this problem, X-10 sells units that electricians can connect from phase to phase to provide a low impedance at 120 kHz.

Over the years, X-10 has made many product improvements, including adding metal-oxide varistors to prevent damage from transient spikes, changing to triacs that withstand the momentary short circuits that can occur when incandescent lamps burn out, and adding automatic gain control to some modules. X-10 has also developed an extended protocol that allows for larger systems with more functions. Moreover, certain X-10 units can now acknowledge the receipt of commands. Controllers designed to work with these modules can retransmit a command until they receive acknowledgment. This feature addresses an often-heard complaint about X-10-that modules fail to turn on and off.

Nevertheless, X-10 faces several problems in expanding its user base. The company must convince potential users that it has overcome the reliability problems that have plagued its technology. Moreover, the X-10 protocol is proprietary, and the company does not currently license others to use it. As noted, though, X-10 (USA) incorporates its technology in products it builds for other companies. It also designs custom products to its partners' specifications. However, unless their applications appear able to generate enough volume to interest X-10 in custom designs, potential users for whom existing X-10 products don't work must look to other-often more expensive-technologies.

X-10's policy of not licensing its technology contrasts sharply with the approaches used by backers of other home-automation protocols. Although only CEBus is in the public domain, manufacturers can license LONworks and Smart House. In most cases, license fees are built into the price of components used in products that support the standards. Moreover, even CEBus has some proprietary aspects. For example, it uses a patented chirp spread-spectrum power-line and RF-wireless communication scheme developed by Intellon. Like Echelon and Smart House LP, Intellon encourages others to use its technology in their products; the company receives royalties from the sale of ICs that implement its protocol.

Smart House

The heart of a Smart House system is the control/communications subsystem, which transmits signals at speeds to 50 kbps. This subsystem includes the system controller, a 12V-dc power supply, cabling that distributes signals throughout the house, sensor outlets, and switches. Within the control/communications subsystem (from a logical standpoint) are branch/slave ICs that format messages, convert between serial protocols, implement node addressing, and provide a power-control interface. Physically, the ICs reside in the devices to which they interface-120V-ac convenience outlets, lamp dimmers, lighting fixtures, and points of connection to permanently wired appliances.

Although you can use conventional appliances in a Smart House system, the Smart House standard defines three other appliance classes: simple, normal, and complex. Appliances in all three of these classes interface with the three status lines and one control line of the Smart House appliance-channel connector. The last two classes require a Smart House appliance chip, which implements the physical and data-link layers of the Smart House appliance protocol. Complex appliances also require a µP.

The system controller is an electronics module and enclosure. It can manage communication and control power on up to 30 branch networks, each of which can have up to 30 nodes or attachment points. In addition, the system controller implements all protocols, executes programmed event/action sequences, manages system databases, and coordinates other functions.

The electrical-energy subsystem, which distributes 120 and 240V ac throughout the house, incorporates surge protectors and ground-fault interrupters on each branch. In a Smart House, wall switches do not switch ac power; they switch 12V dc. The system controller senses this dc and directs ac to the appropriate wall outlets, lighting fixtures, and permanent appliances. Outlets can sense whether a load is connected and can wait until a load is connected before applying power.

The control/communications subsystem implements a pair of 9600-bps RS-232D asynchronous serial channels. One of these channels is the sole interface between the control/communications and telecommunications subsystems. The second serial port lets service technicians access the system.

Smart House defines several cable types. Most messages travel among sensors, appliances, outlets, and the system controller on six-conductor cable. The telecommunications subsystem uses four twisted pairs. This subsystem can accommodate both analog and digital telephone devices, modems, dual-tone multifrequency decoders, and voice-response devices. In conjunction with the control/communications subsystem, the telecommunications subsystem permits telephone access to home-security functions and allows using the telephone for remote control of devices throughout the house.

One of the drawbacks of Smart House has been its cost. Installing Smart House during construction can add $20,000 to the cost of a home. To mitigate this problem, companies that supply Smart House products have developed the Smart-Redi program. Under Smart-Redi, a builder installs only certain portions of a Smart House system. Later, though, the purchaser can easily replace or modify items that do not fully implement Smart House. Smart-Redi requires buyers of new homes to pay only for those Smart House features they plan to use immediately plus a modest premium for products that allow upgrading without reconstruction or rewiring.

CEBus

Because CEBus is a peer-to-peer network, it does not require a system controller of the type that is at the heart of Smart House systems. CEBus offers a wide choice of low-speed signal-transmission media: power-line carrier (PLC), twisted pair, wireless RF, and IR. Higher speed data can travel on twisted-pair, coax, or fiber-optic cable, although the fiber-optic portion of the standard is not yet complete. Telephone service can share cables that contain four twisted pairs. Bridges or routers link the various media in a network.

The PLC technology uses a chirp spread-spectrum signaling technique, which Intellon patented. The chirps cover 100 to 450 kHz. Whereas some spread-spectrum systems make it virtually impossible for outsiders to intercept messages, the receivers in such systems take many milliseconds to lock onto signals. Intellon's priority wasn't a system that could provide high security, but one that could quickly transmit many messages. So, the CEBus PLC system uses carrier-sense multiple access with collision detection to control its nodes' access to the power-line medium.

Competitor Echelon has questioned the reliability of Intellon's first-generation chirp spread-spectrum hardware and has developed a narrowband power-line modem for LONworks. Echelon claims that its new modem is more reliable than either its own spread-spectrum power-line modems or Intellon's CEBus devices. Echelon now lets LONworks users choose either narrow- band or spread-spectrum power-line modems.

Intellon has not been napping, however. The company says it has found and fixed its technology's Achilles' heel: saturation of receiver front ends by high-amplitude narrowband noise sources. Intellon will soon announce second-generation chirp spread-spectrum CEBus modems that are backward-compatible with the company's earlier units. Intellon believes these modems are even more reliable than Echelon's new narrowband units.

CEBus is more than a power-line-communications technology. ISO's seven-layer reference model fully defines the protocol. A description of a CEBus "frame," or packet, gives an idea of the thoroughness and flexibility of the standard. A normal frame comprises eight sections: a preamble (PRE), a control byte, a destination address, a destination house code, a source address, a source house code, up to 32 bytes of information, and a frame-check sequence (FCS). Addresses and house codes can contain up to 16 bits.

Acknowledgment frames include the PRE, control, information, and FCS sections. An acknowledgment frame usually contains no information and, at most, contains only 2 bytes.

CEBus is starting to gather momentum. The quarterly catalogs from Home Automation Laboratories (see box, "For free information..."), which heretofore have focused on products that support the X-10 standard, have recently started to include a few CEBus products as well. Moreover, each successive issue of the catalog includes a few more CEBus items.

LONworks

Moreover, Echelon has enrolled hundreds of partner companies (at last count, about 700) to offer LONworks hardware, development tools, and support. Part of the reason for Echelon's success in developing partnerships stems from the company's broad focus. LONworks is the only one of the multivendor home-automation protocols that aims at applications outside the home. LONworks has already achieved considerable success in industrial automation.

Like CEBus, LONworks is a peer-to-peer system and so requires no system controller. LONworks offers a wide variety of data rates and transmission media. Data rates range from hundreds of bits per second to 1.25 Mbps. Media include twisted pair, 49- and 450-MHz wireless RF, IR links, and the ac power line. One option is an RS-485 transceiver. Microsym offers a 1.25-Mbps fiber-optic transceiver.

At the heart of virtually every LONworks-compatible hardware unit is an Echelon Neuron IC. These chips, the 3120 and 3150, implement much of the LONworks protocol's lowest layers. In so doing, they greatly simplify the jobs of application developers, who need not worry about the minutiae of network interfacing.

Even though they share a peer-to-peer organization, CEBus and LONworks represent different philosophies of protocol design. CEBus appears to place high value on generality-on not establishing rules so specific to particular hardware or applications that they would limit opportunities for future growth. LONworks' developers, however, were more pragmatic. Although they did not turn their back on flexibility, they sacrificed some flexibility to achieve simplicity and ease of implementation.

Examples of LONworks application-focused approach include comprehensive sets of standard network-management and diagnostic messages and a set of standard network-variable types. These sets are extensible using a process that Echelon controls. Ref 4 notes a possible shortcoming of Echelon's focused approach-LONworks' speed of response. According to the reference, an attempt to phase-control a triac via a LONworks network provided only about 1-msec resolution-not fine enough for the application.

"For free information..." are good places to begin searching for more detailed information on home automation. Companies that want to educate groups of employees on the technology can arrange for in-plant seminars. The Training Department, a consulting company, conducts such tutorials. Company President Grayson Evans says that his company has conducted more than 100 seminars on CEBus alone.

1. Bushby, Steven T, Back to the Basics about BACnet, ASHRAE, 1993.

2. Bushby, Steven T, "A new age in building," Construction Business Review, March/April 1994, pg 60.

3. Bushby, Steven T and HM Newman, "BACnet: a technical update," ASHRAE Journal Product and Show Guide, January 1994, pg S72.

4. "Home-automation systems in North America: an analysis of the three main contenders," Diablo Research Corp, Gas Research Institute Final Report, September 1993, available from Gas Research Institute.

5. LONworks Control Network Products and Services Resource Directory, Echelon Corp, 1994.

6. Evans, Grayson, and Parks Associates, A Comparative Analysis: CEBus, Smart House, and LONworks, Parks Associates, 1991.

7. "Home automation standards," Electronic House, March/April 1994, pg 40.

8. Kerridge, Brian, "Network vendors agonize over fieldbus standards," EDN, April 28, 1994, pg 45.

9. Parks, Patricia, and G Evans, X-10: Myths and Realities, Parks Associates, 1994.

10. Strassberg, Dan, "Spread-spectrum communication rises from military roots to star in a wireless world," EDN, Dec 22, 1994, pg 59.