#6 - System Enhancements

TELEPHONE INTERCONNECT

Most two-way radio systems can be enhanced by a telephone interconnect option. Telephone interconnect allows the mobile or portable radio user to place and receive standard telephone calls via the two-way radio system. In a conventional system, telephone interconnect requires a special piece of equipment called a patch, or interconnect device. This allows connection of the telephone lines to the base station or repeater. In addition, the mobile or portable radios must also have telephone interconnect capability. In trunked systems, a telephone interconnect terminal is connected to the central controller which routes any telephone calls to the appropriate repeater. A radio initiated telephone call is routed through the assigned repeater, the central controller, the interconnect device and is completed over standard land line service. A land line user can call a mobile or portable by dialing a number to access the interconnect terminal and by adding the mobile/portable identification number. The central controller then assigns a repeater to the call and sends the signal to the appropriate radio. It is also possible for a land line user to call an entire user group.

STAT-ALERT CONTROL SIGNALING

Control signaling technology uses the voice channel to quickly send and receive short bursts of data containing pre-defined information without tying up the voice channel. In conventional two-way radio systems, RF channels are most commonly used to carry voice messages. However, these same RF channels can also be used to carry digital signal transmissions. In order to send and receive digital Stat-Alert messages, the radios must contain digital encoding and decoding circuitry. A typical voice message may be 15 to 20 seconds long by the time the caller identifies him or herself, states the message, and receives an acknowledgement that the message was received and understood. That same message can be sent in fractions of a second as a digital signal. In addition, since digital signals have built-in error detection and correction capability, the messages are always accurately received. When a Stat-Alert message is sent, it also includes the sending unit's identification. When the base station receives the message, if there are no errors, it automatically sends back an acknowledgement signal to the sending unit. If there is an error, or if the message did not get through, the sending unit will not receive the acknowledgement and will automatically retransmit the message until it receives the acknowledgement.

STAT-ALERT PROVIDES THE FOLLOWING FEATURES

Vehicle ID—which displays the sending unit's identification on the dispatch control unit. Status Change—which automatically displays the unit's status, such as "at job site", on the dispatcher's screen. Emergency Alarm—which by pressing a button sends an emergency signal which alerts the dispatcher to a critical situation. Call Alert—which lets the dispatcher leave a page if the driver is away from the vehicle. Voice Selective Calling—which allows the dispatcher to send voice messages to selected individual radios in the field.

VOICE SECURITY

Once voice information is transmitted over an RF channel, it is susceptible to interception by almost anyone with an inexpensive scanner as well as other users on the same frequencies. A radio user transmitting sensitive information must either accept the risks or avoid using the radio system—which is not always practical. A solution is Motorola's SECURENET, the most sophisticated form of digital encryption commercially available for two-way radio. Digital encryption is similar to Stat-Alert signaling except the actual voice message is sent as a digital signal. Instead of sending two-way voice messages as an analog signal, SECURENET first converts the analog voice signal to a digital signal. Once the signal is in a digital format, SECURENET uses an electronic code key to encrypt, or code, the digital signal. When the encryption is completed, the digital coded message is transmitted. Receiving radios have an electronic code key that checks for the correct code and then decrypts, or decodes, the message. The radio converts the original digital signal back to an analog voice signal and the listener only hears the intended voice message. The SECURENET receiving radio can respond to the encrypted call using the same digital encryption process. With SECURENET encryption, anyone listening in on the channel will hear undistinguishable digital noise. The actual message is undetectable without the proper decoding circuitry and encryption algorithm. SECURENET equipped radios can operate in either the clear mode, for normal voice transmissions, or in the encrypted mode for secure voice transmissions.

DATA COMMUNICATIONS

Data communications provides a means of extending a user's centralized computer system, and its associated data files, to people in the field. Because of this capability, data communications can provide users with unique applications that go far beyond the realm of normal two-way radio communications. This includes: Database inquiry and update Data entry by field personnel User Status Updates and Messaging Computer-Aided Dispatch Report Writing

DATA COMMUNICATIONS SYSTEM COMPONENTS

Within a typical data communications system there are four major components: End-User Terminals Radio Communications Equipment Data Communications Infrastructure Equipment Customer's Computer Application Software required to support the data communications system The End User Terminals are all microprocessor based and consist of a keyboard, a display screen, an RF modem, and a data radio. The data radio may either be an integral part of the terminal or a separate two-way radio. Both mobile and portable data terminals are available.

HOW DATA COMMUNICATIONS WORKS

To see how a data system works, we'll follow a typical radio communications application. For example, a police officer stops a vehicle and wants to do a license check to see if there are any outstanding warrants, or if the vehicle is stolen. In a traditional voice system, the officer would call the dispatcher and give him or her the license number. The dispatcher would then go to a computer and look up the information. The dispatcher would then call back the police officer and verbally provide the information found in the computer files. This may take several minutes, and depending on the amount of voice traffic on the system, could take even longer. In a dangerous situation, even a few minutes is a long time. In a data communications system, the officer simply enters the license plate number on the terminal and hits the inquiry button. The radio sends the data request to the base station which routes it through the data infrastructure directly to the computer. The computer accesses the appropriate file and automatically sends the information back through the system directly to the officer's data terminal which displays the information on the screen. This occurs in only a few seconds. This is only one typical data communications application. There are many other areas in which data communications can be an important system enhancement.

WIDE AREA COVERAGE

Wide area coverage systems are required when radio users must operate in remote areas. Both Simulcast and Spectra-Tac wide area coverage systems can be used on both conventional and trunked systems.

SPECTRA-TAC SYSTEMS

Spectra-Tac is designed to improve inbound coverage from mobile or portable radios to the base station. Spectra-Tac receivers are strategically placed throughout the coverage area. These receivers pick-up the radio signal and feed it back to the central location over phone lines or microwave. Because multiple receivers may pick-up the same signal, a voting switch determines which receiver has the signal with the best audio quality. That signal is relayed to the central site either over phone lines or by microwave.

SIMULCAST

Simulcast utilizes multiple remote transmitter and receiver sites to extend coverage of the system. Each remote site uses repeaters with identical frequencies to those at the prime site. A dedicated microwave or fiber optic link is required for inter-site communications. Because these sites are all linked together, whenever a user transmits a signal at any one site, all of the sites automatically retransmit the signal simultaneously. In this manner, the entire coverage area receives the transmitted message.