Once the paging data has been created by the paging terminal, this information must be forwarded to one or more radio transmitters connected to antenna sites that distribute the radio signals over different geographical areas. When transmitting, these devices can consume a lot of power that, under continual load, would reduce the overall lifetime of certain components; transmitters normally are designed to remain in a low-power mode until they need to transmit data. When paging, all transmitters in a network normally are not expected to transmit every page. Some customers may subscribe to services that are only within specific geographical areas even though the paging terminal is controlling a much larger coverage area.
In order to control the "power up" of certain sets of transmitters and the forwarding of paging information to them, paging terminals normally are connected to transmitter control equipment. Different protocols are used to communicate between the paging terminal and different manufacturers' control equipment. Tone sequences, digital codes or control signals across individual wires may be used to communicate between the paging terminal and the transmitter controller. Regardless of the protocol used, the same basic set of information must be conveyed to the transmitter network from the paging terminal. Other protocols are used to transmit between the transmitter controller and the transmitter itself. Analog and digital protocols are used between these devices.
Many times, in smaller paging networks or in private carrier paging situations, a carrier will not necessarily be granted exclusive use of a particular radio frequency. The radio channel may be shared with one or more other small carriers. Therefore, the use of the transmitter control equipment will have to be coordinated so that only one paging terminal is transmitting at a time. When the paging terminal is ready to transmit information, it must first determine that it has exclusive use of the radio channel. In a channel-sharing arrangement, external channel sharing equipment or control signals from other paging terminals are used to provide some type of channel grant signal. The paging terminal will not send while the channel is busy and will await a channel grant signal.
When paging is to begin, the paging terminal forwards "zone" information to the transmitter controller to indicate that particular set of transmitters are to be used for the following transmissions. The selected transmitters must then "key up" before transmission is started. In the key-up process, the transmitters are brought up to full power in an analog or digital mode as selected by the paging terminal. Some type of "key grant" signal indicates that the process has been completed and that paging may be initiated. The zone identification specified by the paging terminal defines the geographical area that will simultaneously receive a set of pages. Simultaneous receipt of pages is known as simulcasting. It is possible that following the forwarding of a set of page requests, another zone will be selected immediately and the same page alerts may sent again; this process is known as sequencing. This is necessary when geographical coverage is such that simulcasting from two transmitters will create a region where the pager will be unable to receive information reliably from either transmitter.