Tutorial POCSAG

Tutorial: POCSAG

What it is. POCSAG is the long-standing pager protocol (512, 1200 and 2400 baud) that hospitals, fire services, utilities and some businesses still use. A pager transmitter sends short numeric or text messages addressed to a pager number (the RIC, or address). OnAir shows the pages it can read on one channel, with the address, the function bits, the message type and the text.

Read this first. Pager messages are often addressed to real people and can contain names, medical or other personal details. Whether you may listen to them, store them or pass them on differs from country to country, and in some places it is not allowed. Check your local law before you use this mode, and do not publish what you receive. See Legal and Privacy.

What you need

• A radio that tunes the pager band you want. An RTL-SDR is enough: the mode asks for 2 Msps and works from 1 Msps. The tuning range of the mode is 25 to 1000 MHz and the channel width is 25 kHz.
• A VHF/UHF antenna. A simple telescopic or discone antenna is fine for strong local transmitters.
• A frequency. The mode starts on 466.075 MHz. The frequency database (data/frequencies/data.tsv) also lists, among others: 466.230 MHz (Germany), 439.9875 MHz (Skyper, Germany), 153.025 MHz and 138.275 MHz (UK paging and hospital paging), and 929.6625 MHz and 931.4375 MHz (US 929/931 MHz bands, FLEX and POCSAG). Which of these is active near you depends on your country; the lists are starting points, not a guarantee.

Before you start

• Keep the gain moderate. Pager transmitters are strong and close to you, so too much gain overloads the front end and produces more bad decodes, not more pages.
• Put the transmitter on the channel centre. The narrower 12.5 kHz filter rejects a neighbouring channel better but needs the signal exactly in the middle.

Step by step

1. In the mode list open the group Pagers and pick Pagers, 512-2400 bd.
2. Choose your radio under Source and start the receiver.
3. Type the pager frequency into the tuner (or pick one from the frequency list).
4. Open the Messages tab. In the filter row, leave POCSAG ticked. Tick FLEX as well if you want both protocols on the same frequency.
5. Under Channel choose 25 kHz (more forgiving) or 12.5 kHz (better against a neighbour).
6. Tick the baud rates that really are on air: 512 bd, 1200 bd, 2400 bd. The tooltip for 2400 bd says to turn it off when only a slower speed is used, to avoid false decodes.
7. Wait. Pagers are quiet for minutes at a time on some channels.

What you should see

New pages appear as rows in the Messages table. A good channel shows clean text or digits and few damaged words in the Statistics tab. Short bursts of tone with no text are normal for tone-only pagers.

Reading the screen

Messages columns: Time UTC, Protocol, Address, Fn (function bits 0 to 3), Type and Message. When the receiver covers a span, a Frequency MHz column is added. A message marked incomplete had damaged words. Hover over a row to see how the text would look if read as alphanumeric and as numeric.

Statistics shows, per speed, Good (code words that passed the BCH check as received), Repaired (passed after one or two bits were corrected) and Lost (could not be repaired), with the number of pages and batches.

Options and settings

• Filter: shows only pages whose address or text contains the text you type.
• Decoded only: hides tone-only pages and pages whose text could not be decoded. A grey line under the table tells you how many were hidden.
• Message type: how the message is read: Auto (the Message type setting), Always alphanumeric or Always numeric. POCSAG does not say whether a message is text or digits, so this is a guess. Right-click an address in the table to fix the reading for that one address.
• Copy: copies the pages that match the filter to the clipboard, one per line.

Troubleshooting

• No pages at all. Check the frequency and that the receiver is running. Listen to the spectrum for the characteristic short FSK bursts. Try the other protocol: the channel may be FLEX.
• Garbage text. Wrong message type or wrong baud rate. Switch Message type between alphanumeric and numeric, or turn off the speeds you do not need.
• Many Lost words. Signal too weak or overloaded. Lower the gain, move the antenna, or switch the channel width.
• Pages show only digits when you expect text. The sender may use numeric pages. Try Always numeric and compare.
• Nothing visible when the pager band is empty near you. Many countries have moved to other systems; try a different frequency from the list.

Tips

Leave the receiver running for a while before judging a channel. Use Filter to follow one address. For both protocols at once use the same mode with both boxes ticked.

Legal and privacy notes

Pager traffic is not meant for you. Treat any message with personal content as confidential, never share it, and follow the law of your country. OnAir only receives.

Under the hood

POCSAG (ITU-R M.584) uses 2-level FSK at 512, 1200 or 2400 bit/s. A batch of 16 code words follows the sync code word 0x7CD215D8. Each code word is 32 bits with BCH(31,21) and an even parity bit; OnAir corrects up to two bit errors. An address word carries the 18 high bits of the RIC and the lowest three bits are given by the frame position.

Related

Mode-POCSAG, Tutorial-FLEX, Tutorial-Multitone-Paging, Tutorial-Pagers-POCSAG-and-FLEX, Modes, Main-window, Supported-radios, Troubleshooting, FAQ, Legal.