Appendix A

2-TONE AND 5/6-TONE FORMATS

In this format, a sequence of audio tones is used to identify each pager. As an example, we can use 3 tones, f1, f2 and f3, combined two at a time, to represent six pagers' identities as: f1f2, f1f3, f2f1, f2f3, f3f1 and f3f2.

These tone sequences cannot be transmitted directly to the pagers for a number of reasons, an important one being that audio frequencies cannot travel far and another that it will require extremely large antennas. The usual method used to convey them to the pagers is through frequency modulation (FM) of the transmitter's carrier frequency. This carrier is a very high, inaudible frequency also known as an RF (radio frequency). In frequency modulation, the frequency of the carrier is varied according to the tone sequence. Each unique tone sequence will cause its unique variation of the carrier frequency and, therefore, at the paging receiver, the reverse process of demodulation can identify which particular tone sequence is being sent.

The total number of audio tones used and the number of these tones used in each sequence will determine the maximum number of pagers which can be supported by the system. For example, in 2-tone paging where each sequence is comprised of two tones, using 60 different tones will give 60 x 59 or 3,540 unique paging addresses.

2-TONE FORMAT

Two sequential audio tones, representing a unique pager address, are used to modulate the transmitted carrier. The two tones vary in both frequency and duration. In the paging receivers, the tone modulated carrier is decoded and if the transmitted tones match the code (as determined by plug-in filters in the receiver) of that receiver, the user will be alerted.

The 2-tone format can support tone-only alert and tone and voice paging. Battery Saver and Group Call options can also be provided with this encoding format.

An example of this 2-tone format used in Motorola's tone and voice pagers is shown below: Tone A is transmitted for one second followed by the B tone for three seconds. The carrier is then sent with a silent (unmodulated) space of 0.7 seconds before the voice message. This 0.7 second space allows the pager alert tone to cease before the voice transmission begins. If the caller does not leave a voice message, the terminal will automatically insert an interpage gap of 1.3 seconds to prevent B tone/A tone falsing.

The 2-tone format suffers the following disadvantages:

Excessive long tone and gap periods

Limited number of pages that a carrier frequency can support

However, many paging systems still support this format because of existing old pagers. Other newer formats can transmit up to 60 times faster and have very high code capacities.

5/6-TONE FORMAT

This is an improvement on the 2-tone format. In this format, the transmitted carrier is frequency modulated by a preamble tone (optional) and five or six sequential pager address code tones.

Twelve frequencies are used to represent digits 0 through 9, repeat tone R and special tone X. Up to 100,000 pagers with address code numbers from 00,000 to 99,999 can be supported with these tones.

To alert a pager, the five tones corresponding to its code number are transmitted sequentially. The R (repeat) tone is substituted automatically for the second of two identical successive digits whenever they appear in an address code. For example, an address code of 13387 is converted to 13R87. This ensures that two identical, successive digits will not be decoded as a single digit only. Tone X is used to implement call two of a dual-address pager, for example, indicating that a different pattern should sound.

Battery-Saver Option

In this option, five-tone pagers are grouped into 10 battery-saver groups. Before transmitting the pages, the paging terminal batches together all pages in the same battery-saver group. A preamble will be sent out ahead of a batch of pages to "wake up" the group of pagers for which the batch of pages are intended. Ten audio tones corresponding to the 10 groups are used for the preambles. Pagers belonging to a group where the battery-saver tone is not heard can continue to sleep for a while, occasionally looking to see if its preamble tone is being transmitted.

The use of an additional 10 battery-saver tones permits one million pagers to be supported. The 5/6-tone format is about 121/2 times faster than the standard 2-tone paging format. Tone-only as well as tone and voice paging can be supported by the 5/6-tone format.

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