BACKGROUND OF THE INVENTION 
     The present invention relates to systems for controlling devices on a vehicle, and more particularly to wireless control systems. 
     Automobiles, trucks and trailers have numerous devices, such as lights and actuators, which are electrically operated. For example tail lights, brake lights, left and right turn signal indicators, and back-up lights are all mounted at the rear of a typical automobile. Each type of light requires that a separate power wire be run from the dashboard to control the light&#39;s operation. Similar groups of lights are mounted at the front of the vehicle which require another set of electrical wires. In addition, different actuators are located in the engine compartment and also receive control signals. In all, numerous bundles of wires run throughout the motor vehicle in order to control and operate the various devices. 
     It is desirable to merely run a pair of wires that form a power bus throughout the vehicle and provide a wireless mechanism for sending control signals to the individual devices. Such a mechanism must provide a technique by which several sets of controllers and devices can communicate simultaneously. In addition, wireless communication within a particular vehicle can not be interfered with by similar communications occurring in a nearby vehicle. Thus a robust communication protocol must be utilized. 
     Bidirectional radio frequency communication has been used for some time in cordless telephones. The term “cordless telephone” as used in the telecommunication industry, means a telephone comprising a base station and a hand-held transceiver unit. The base station is connected by wires to a terrestrial telephone line serving the owner&#39;s premises. A hand-held transceiver carried by the user communicates by radio frequency signals with the single base station that is up to approximately 300 meters away. 
     The Digital Enhanced Cordless Telecommunications (DECT) protocol was developed in the mid-1980&#39;s as a pan-European standard for cordless telephones and has been adapted for use outside the European Union. The DECT standard protocol has been used for simultaneous bidirectional communication between a base station and a hand-held transceiver of cordless telephones. This standard utilizes ten frequencies for communication. The exchange of signals over each frequency is divided into repetitive frames  10 , each being ten milliseconds in duration and subdivided into twenty-four time slots, as shown in FIG.  1 . The twelve time slots in the first half  14  of each frame are used for communication from a hand-held transceiver to the associated base station, while the twelve time slots in the second frame half  16  are used for communication from the base station and the hand-held transceiver. It should be noted that different regions of the world have implemented the DECT protocol is slightly different manners. For example, in some regions the frequencies and the number of time slots in each message frame may differ. 
     When a user desires to use activates the cordless telephone to make an outgoing call, the hand-held transceiver searches for a frequency that has a matching time slots in each frame half which are not being used by another cordless telephone system. This is accomplished by the hand-held transceiver listening for digital signals being sent in each time slot of the frame at each of the assigned frequencies. When a vacant pair of time slots, such as  18  and  19 , is found, the hand-held transceiver sends a message initiation signal on the selected frequency during time slot  18  in the first half of a message frame. 
     While the hand-held transceiver is performing these functions, the base station is scanning the ten frequencies and listening during each of the twelve time slots in the first half  14  of the message frames at each frequency. When the base station hears a message initiation signal that is addressed to it, i.e. containing the proper identification data, the base station sends a response to the transceiver in the associated time slot  19  in the second half  16  of a frame at the same frequency and bidirectional communication is established. A reverse procedure occurs when the base station receives an incoming call via the terrestrial telephone line. 
     SUMMARY OF THE INVENTION 
     A general object of the present invention is to provide an system for wireless communicaton among devices on a motor vehicle. 
     Another object is to provide a system by which the devices on a vehicle can be operated by signals sent via a wireless communication protocol. 
     A further object is to make such as system immune from interference from wireless control taking place in nearby vehicles. 
     These and other objectives are satisfied by an apparatus which responds to an operating signal that indicates an operational state for the device. A control circuit has a transmitter which wirelessly transmits messages using the Digital Enhanced Cordless Telecommunication protocol. The messages are received by a receiver that is configured for communication using that protocol and which is part of a controller connected to the device. The controller responds by controlling the device according to the messages. 
     Specifically, the control circuit responds to the operating signal by transmitting a message using the Digital Enhanced Cordless Telecommunications protocol. The message includes identification of the device and an indication of the function to be performed. The controller receives the wirelessly communicated message and recovers the identification of the device and the indication of the function. The controller processes the recovered information and responds by activating the device to perform that function. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a message frame of the Digital Enhanced Cordless Telecommunications wireless telephone protocol; 
     FIG. 2 is a represetnation of an automobile which incorporates the present invention; and 
     FIG. 3 is a block schematic diagram of a system for wireless control of devices on the automobile. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With initial reference to FIG. 2, the present wireless control system is incorporated into a motor vehicle, such as automobile  20 . The automobile  20  has brake lights  21 , front and rear turn signals  22  and  23 ,respectively, and headlights  24 . The lights  21  and  23  at the rear of the vehicle are operated by at rear controller  25  while the lights at the front are operated by a front controller  26 . The controllers govern application of electricity from a power bus  33  to the individual lights in response to wireless communications received from a control circuit  27  in the dashboard, as will be described. The automobile  20  may include additional control circuits. The power bus receives electricity from an alternator on engine  15 . One skilled in the art will appreciate that there can be additional control circuits located within the motor vehicle, for example one may be connected to driver operable controls on the dashboard, while another control circuit receives signals from a computer that contrils the engine. 
     Referring to FIG. 3, the control circuit  27  includes a microcomputer  28  with an internal microprocessor, a memory in which the control program and data are stored, and input/output circuits. A standard clock circuit  29  supplies timing pulses to the microcomputer  28 . A service technician is able to place the microcomputer into different functional modes and configurations by operating a plurality of manual switches  31 . 
     The control circuit  27  operates numerous functions on the motor vehicle  23 , such as controlling the engine  15  and operating other vehicle devices such as the lights which are controlled from the dashboard. For that functionality, the microcomputer  28  is interfaced to switches  30  that are manually operated by the driver and other control devices for activating vehicle equipment. Additional activation signals are received from other circuits in the vehicle via a parallel communication bus  36 . The control circuit  27  also can send signals over the communication bus  36  to other computer systems on the motor vehicle  20 . 
     A serial output port  32  and a serial input port  34  of the microcomputer  28  are connected to a first radio frequency transceiver  35  which utilizes the Digital Enhanced Cordless Telecommunications (DECT) protocol. In a general sense, the first radio frequency (RF) transceiver  35  contains a transmitter that modulates a standard RF frequency carrier with the serial digital data received from output port  32  and transmits that modulated radio frequency signal via an antenna  37 . The first transceiver  35  also includes a receiver that demodulates radio frequency signals received by the antenna  37  to recover serial digital data carried by that signal. The recovered data is sent to the microcomputer input port  34 . 
     The first transceiver  35  in the control circuit  27  is designed to communicate with controllers, such as rear controller  25 , located throughout the automobile  20 . The present invention will be described in the context of communication between control circuit  27  and rear controller  25  with an understanding that the vehilce has other control circuits and controllers which have similar structures and communication procedures. 
     The rear controller  25  has a second radio frequency transceiver  40  and antenna  42 . As will be described, both transceivers  40  and  35  are designed to utilize the DECT protocol and are similar to devices found in cordless telephones. The second transceiver  40  has a receiver which demodulates the received radio frequency signal to recover digital data carried by that signal and the recovered data is sent in a serial format to an input register  44 . The input register  44  converts the serial data stream from the second transceiver  40  into a parallel format which is read by a processor  46 . The processor  46  may be a hardwired device that sequentially performs the control procedure to be described or a programmable device which executes a software program to implement that procedure. The processor  46  is connected to an electrically erasable programmable read only memory (EEPROM)  48  which stores identification data to be transmitted to the control circuit  27 . A clock circuit  52  provides timing signals to the processor  46 . 
     The rear controller  25  also includes an encryptor  50  connected to the processor  46  to encrypt a security number for transmission to control circuit  27 . The encryptor  50  utilizes a secret-key cryptography algorithm to encode data for sending to the control circuit. For example the algorithm specifies a sequence of a plurality of logical operations which are performed on a known seed number and a challenge number received from the control circuit to produce a resultant number for transmission by the rear controller. Several suitable cryptography algorithms are described by Mehrdad Foroozesh in an article entitled “Protecting Your Data With Cryptography,”  UNIX Review , November 1996, volum0000e 14, number 12, page 55(6), which description is incorporated herein by reference. Such encryption techniques and algorithms are commonly used to encrypt computer data being transmitted over common carriers. It should be understood that other encryption algorithms may be used. 
     Digital output data is sent by the processor  46  in parallel form to a parallel-in/serial-out output register  56 . The serial data from the output register  56  is applied to the input of a transmitter within the second transceiver  40  which modulates a radio frequency signal with that data. The resultant RF signal is sent via the antenna  42  to the control circuit  27 . The components of the rear controller  25  are powered by a battery (not shown). 
     When a particular device or function on the automobile is desired to be activated, the driver closes the associated input switch  30  of the control circuit  27 . The microcomputer  28  responds to this signal by formulating a message to be sent to the corresponding device throughout the vehicle that perform the selected function. For example, when the driver steps on the brake pedal, closure of the brake switch causes a message to be sent to illuminate the brake lights  21 . The message contains a device identification number designating the particular device to be operated, a controller identification number for the controller associated with the particular device, and a command indicating the operation to be performed. In the present example, the device identification number designates the brakes lights and the command is to turn-on the lights. 
     Before the message may be sent, the control circuit  27  must locate a pair of DECT frame time slots which are not already in use. This process begins by scanning each of the ten DECT frequencies. If the control circuit  27  does not hear a message frame on a given frequency, then it forms a new message frame and selects an arbitrary pair of time slots to use. If a particular frequency already is carrying DECT messages, the control circuit  27  listens during the message frames for an available pair of frame slots, one that does not already contain message data. If none is found, the control circuit  27  selects the next DECT frequency. When an available pair of time slots, such as the third time slots  18  and  19  in each half of the message frame shown in FIG. 1, is found, the control circuit  27  transmits the message in the time slot  19  during the second half  16  of the message frame. The control circuit continues to transmit the command message and listens for an acknowledgment in time slot  18  during the first half of subsequent frames. As noted previously, any of several well known data encryption algorithms may be employed to exchange data between the control circuit  27  and the rear controller  25  for greater robustness against interference. 
     While this is occurring, rear controller  25 , as well as all of the other controllers, is scanning the ten DECT frequencies and each time slot in the second half  16  of the frames for a message signal which contains its controller identification number. When the rear controller  25  hears a message addressed to it, processor  46  responds by parsing the message into the device identification number and the command. The processor  46  then determines for which of its devices the command is intended and the action to be taken. Thus in the present example, the processor  46  applies a output control signal to the corresponding output driver  54  which switches electric current from vehicle power bus  33  to one of the output lines  55  that is connected to the brake lights  21 . The processor  46  also senses whether current flows to each of the brake lights, either by sensing the cumulative current magnitude or current on individual conductors for each light. This enables the processor  46  to detect a burned-out lamp. 
     The processor  46  then formulates a message containing the identification numbers of the rear controller and the brake lights and an acknowledgment code indicating that the designated operation has been performed. If the rear controller is unable to perform the designated function or encounters a malfunctioning device, such as a burned-out lamp, that fact is communicated with the acknowledgement message. The acknowledgment message then is transmitted by the rear controller  25  back to the control circuit  27 . The acknowledgment message is sent at the same frequency as the command signal and during a time slot (e.g.  18 ) in the first half of a message frame that corresponds to the time slot (e.g.  19 ) of the second frame half that contained the command message. Specifically, the processor sends the acknowledgment message via output register  56  to the second transceiver  40  from which it is transmitted to the dashboard control circuit  27 . 
     Upon receiving the acknowledgment message, the control circuit&#39;s microcomputer  28  extracts the controller and device identification numbers and determines to which of possibly several command messages that are being transmitted simultaneously the acknowledgment relates. Then the control circuit terminates further transmission of the associated command message. 
     By employing the DECT bidirectional communication protocol, numerous control signals can be transmitted simultaneously within the vehicle using the different DECT frequencies and the different frame time slots of the each frequency. Thus the likelihood of interference among the controllers on the same vehicle is minimized. In addition, the present system reduces the possibility of interference from similar control systems on nearby vehicles. Even if another vehicle is stopped alongside automobile  20 , the other vehicle will be using a different set of DECT message frame time slots and thus the two vehicle systems will be able to distinguish which messages are for its controllers. In addition, the transmission of the unique identification numbers in message to and from the rear controllers further reduces the likelihood of interference from adjacent devices and enables the control circuit to identify messages related to its components. 
     Each communication device, such as rear controller  25  and control circuit  27 , is able to measure the amplitude of the received RF signals. That amplitude measurement is sent back in the acknowledgment signal to the communication device which transmitted the original signal. The transmitter within each transceiver  35  and  40  has the capability of varying the output power used to transmit signals. Therefore, if the amplitude measurement in the acknowledgment signal indicates that the signal at the recipient device is too weak or too strong, the transmitted can adjust the output power accordingly for subsequent transmissions. This feedback process prevents the output power from being stronger than is needed for good communication throughout the vehicle and reduces the likelihood that signals from one vehicle will be transmitted to another nearby vehicle.
A device on a motor vehicle is controlled by selecting a function for the device to perform. The selection is conveyed to a control circuit which wirelessly transmits a message using the Digital Enhanced Cordless Telecommunications protocol, wherein the message identifies the device and function. A controller receives the message and recovers the identification of the device and the function and then responds by activating the device to perform that function.