Features of the PS-18 (HF pager) mode.
1. Used simple but robust modulation (18-tone IFSK). It can be transmit and decoded easy, but it is resistant to noice, fading (amplitude change) and slow frequency deviation (inaccurate tuning).
2. An effective FEC is used based on the Reed-Solomon code RS(15.7) with fourfold interleaving, which allows you to work through channels with deep and long fading with up to 25% of raw errors.
3. The baud rate (5.86 or 1.46 Bd) is selected by the operator, the receiver decodes both speeds simultaneously.
4. An exchange of address messages up to 252 characters long without establishing a connection is provided.
5. In response to a received message positive (ACK) or negative (NAK) acknowledgment can be automatically sent.
6. The message may be automatically repeated until confirmation is received or a timeout occurs.
7. It is possible to automatically transmit periodic messages (beacons) to identificate station presence on air. A beacon can be accompanied by telegraphic CW-ID at a speed of 100 LPM (20 WPM).
8. The complete set of data from the GPS receiver may be included in a beacon.
9. The identifier of the terminal is a two-byte integer (1 ... 65534), amateur callsign can be included if necessary in the message text, beacon or in CW-D.
10.A specially formatted amateur callsign in a text of a beacon or a message can be used to identify the operator during forwarding a beacon or message to the APRS network.
11.The protocol itself allows you to transmit a full set of single-byte characters (0 ... 255), the text part of the message displays an almost complete set (1 ... 255). Android version of software displays characters as transmitted in Windows-1251 encoding.

HFpager brochure