The pager services introduced by a number of PTT administrations use the POCSAG (Post Office Code Standard Advisory Group) standard. Pagers are one-way devices. A base station controls a large number of receivers and a return channel for transmission of reception acknowledgements or text is not available.

POCSAG specifies four different call modes:

Tone only pagers (Mode 0 and 1)

The receiver can receive four different messages. The meaning of the four tones must be determined beforehand. The acoustic signal is supported by the display of the A, B, C or D messages on a small LCD display. All pagers must in principle have tone-only capability as a minimum.

Numeric pagers (Mode 2)

Individual parties receive their messages in the form of transmitted digits and some special characters. In this way for example, the telephone number which the POCSAG subscriber is to call, can be relayed. Messages are displayed on the pager display and may be partially stored.

Alphanumeric pagers (Mode 3)

The pager type which is easiest to use, is the alphanumeric pager where the transmitted message is displayed on the pager display. Such messages may be up to 80 characters long.

A country-wide pager network is, like a mobile telephone service, subdivided into individual cells that are each serviced by a base station.

Cells are fed with information in a time multiplex fashion i.e., messages are transmitted in cycles. This prevents that areas falling in the overlap zone of adjacent cells, receive the same message simultaneously. To prevent the condition that a base station only transmits for one third of the available time, stations are usually equipped to handle four frequencies.

Selecting Options/Message Type enables the user to select the various pager modes for display:

Data modulation is achieved by direct 2FSK carrier keying with a transmission speed of 512 bit/s. Newer nets operate at 1200 or 2400 bit/s using FFSK modulation.

For reasons of compatibility with older pagers two different bit rates (512 Bit/s and 1200 Bit/s or 1200 Bit/s and 2400 Bit/s) are often used on the same frequency. In some countries POCSAG and GOLAY systems are on the same frequency. By selecting the Auto speed function all baud rates are automatically detected and switch-over between 512, 1200 and 2400 bps is performed.

POCSAG Signaling

Generally activation of the HF carrier is followed by the transmission of a preamble or bit synchronization pattern which slaves the receiver to the clock frequency. The preamble contains at least 576 bits and represents a continuously alternating sequence of 010101...0101. Following the preamble, data batches containing the actual information are transmitted, each batch being 17 x 32 bit or 544 bits long.

The structure of a batch is fixed: Each batch starts with a 32 bit synchronization codeword with a fixed content.

The synchronization word is followed by eight frames (frame 0 - 7) containing 2 x 32 bits in total. Only the first 32 bits are however used for transmitting a pager address. It would therefore be possible to double the maximum number of subscribers in the group by utilizing the frame contents to its full extent.

The message contents of the code words may include pager addresses or messages destined for specific pager addresses. The frames contained in a batch are numbered from 0 to 7. All pagers are similarly divided into 8 groups with each pager only receiving frames for the group to which it has been assigned. A pager synchronizes itself to the batch synch codeword (SC) and ignores the seven frames which are not applicable.

Following the address at the beginning of a frame, a message of any desired length may be sent. In the case of numeric pagers, digits are sent in BCD code while for alphanumeric pagers the message is transmitted using 7 bit ASCII codes.

The example shows the transfer of a message of 350 bits in frame 3. The first 32 bits of frame 3 are the address codeword (Add). Then follow 4 1/2 frames equivalent to 9 x 32 bits which extend up to the next synch word (SC). The remaining 62 bits fit into frame 0. If the frame containing the end of a message is not required to send a new address, it is filled with IDLE codes until a new address or a new synch word must be sent.

POCSAG mode uses direct frequency modulation. Proper decoding is only possible from the receiver IF output.