it is turned on, on a specific frequency, or within range. The radio will respond with an 'ack' if it hears the request containing its individual ID. The radio can also be programmed to ignore these packets and not acknowledge them. It can also be programmed to perform this acknowledgement 'silently' (the radio user never knows that his radio has been 'pinged'). Selective calling MDC-systems have an option for selective calling. By pressing a series of keys on the encoder-decoder, the base station operator can send a data packet that activates a lighted indicator or makes the radio beep. On some radio models with alphanumeric displays, the display may flash CALL until a reset button is pressed. On radio models with ten key pads, mobile radios may be programmed to selectively call one another in this manner. In order to call a radio, the calling party must know the four-digit MDC identity of the radio to be called. In a CAD environment, a tow truck could be automatically called at the moment the driver was dispatched to respond to a call for service. The CAD system could manage the four-digit IDs so that the user did not need to know them. Patent and manufacturers MDC-1200 is patented under U.S. patents 4,457,005; 4,517,561;4,590,473; and 4,517,669. During the patent period, Motorola charged heavy licensing fees for the MDC-1200 protocol. As a result, few other manufacturers produced equipment compatible with the system. This tended to produce a lock-in situation where many companies and public safety agencies were restricted to using mostly Motorola radios, in order to ensure compatibility with existing systems. Manufacturers of third-party MDC-1200 hardware, including in-radio encoder/decoder modules (as aftermarket add-ons for non-Motorola radios), display units for dispatch centers, etc. include Midian Electronics, Control Signal Corporation and Cimarron Technologies. Some of these products have display screens, some are 'black boxes' with inputs for power and receiver audio, and an RS232 serial data output. As of February 2008, there is some discussion as to the status of the above-mentioned patents. While the USPTO still shows them as active patents, a number of other radio manufacturers have begun including MDC-1200 support in their products. Vertex Standard manufactures OEM add-on boards for their mobile and portable Land Mobile Radios that support MDC-1200 ANI (or PTT ID). Icom Incorporated includes built-in MDC-1200 features on some of their newer radios including the IC-F3021/4021 portables and IC-F1721/1821