gr-mixalot

gr-mixalot is intended to be a set of GNU Radio blocks/utilities to encode pager messages. It supports the POCSAG, FLEX, and Golay/GSC protocols.

Here is a breakdown of the blocks:

pocencode / "Single-Page POCSAG Xmit": Given parameters (capcode, baud rate, message, message type), it generates a stream of symbols that can be modulated as seen in examples/pocsagtx.grc.
gscencode / "Single-Page GSC Xmit": Like the above, but for GSC pagers. There is an example in examples/gsctx.grc.
flexencode / "PDU-driven POCSAG/Flex Encoder": Unlike the above, this does not send just one page -- it runs continuously, watching for PDUs on input to specify pages, and then modulates them. The example flowgraph (examples/pagerserver.grc) uses a "Socket PDU" source to run as a TCP-based server. See below for the commands.

PDU Commands and Responses

Commands sent to the PDU-driven encoder use the following form:

<protocol> <messagetag> <frequency_hz> <alpha|numeric> <capcode> <hexl-encoded message>

'protocol' is one of:

flex
pocsag512
pocsag1200
pocsag2400

'messagetag' is an arbitrary string (no spaces allowed) that will be sent back in responses.

'frequency_hz' is the frequency in Hz. Note: this requires an SDR that can be tuned live; this works for the USRP sinks as shown in examples/pagerserver.grc, but will probably not work elsewhere without some extra work.

'alpha' generates an alphanumeric message; 'numeric' generates a numeric message.

'capcode': the numeric (decimal) capcode of the pager.

The message is hexl-encoded (even for numeric messages); so the message HELLO would be 48454C4C4F.

Responses from the server will be of the form '<messagetag> OK'.

Examples (with responses):

Send a numeric message '0123' to a 1200-baud POCSAG pager with capcode 1615132 on 931.8625 MHz:

pocsag1200 0 931862500 numeric 1615132 30313233
0 OK

Send the alphanumeric message 'HACK THE PLANET' to a FLEX pager with capcode 1337000 on 931.3375MHz:

flex doit 931337500 alpha 1337000 4841434b2054484520504c414e4554
doit OK

Compatibility

The POCSAG encoder has been tested against many variants of the Motorola Bravo and Advisor series, as well as the ATI Wireless CCP-6000.

The FLEX encoder has been tested against a wide variety of Motorola pagers and a small numer of NEC pagers.

The Golay encoder is the least tested.

Please get in touch if you find a device that doesn't work properly.

Building

Like many other out-of-tree modules, gr-mixalot uses cmake. This branch is targeted toward GNU Radio 3.8.x+ -- previous branches should run on 3.6-3.7. The IT++/ITPP library is also a dependency. To use the HackRF sink, you'll also need gr-osmosdr installed.

To build, create a new directory and run:

% cmake PATH-TO-GR-MIXALOT-ROOT
% make

To install (may require root), run from the same directory: % make install

On UNIX systems, ensure that the install destination lib folder is added to your LD_LIBRARY_PATH, or add it to your /etc/ld.so.conf. A default install will place library files in /usr/local/lib, which is not searched by default on many Linux distributions.

Examples:

% export LD_LIBRARY_PATH /usr/local/lib
or
% echo /usr/local/lib | sudo tee -a /etc/ld.so.conf

WARNING

Pagers are still in production use today. Although it's possible to acquire working pagers from a variety of sources (often for little or no cost), they may be tuned to an active channel. Transmitting over an existing station could cause legitimate users to silently lose messages. It is your responsibility to ensure that this doesn't happen.

Usage

To send a page to a device, you'll need to know:

the encoding scheme in use (FLEX, POCSAG, or GSC)
in the case of POCSAG, its baud rate
the frequency it's tuned to (often printed on the device)
its cap code (often printed on the device)
whether it wants alphanumeric or numeric pages

In the case of Motorola devices, some of these parameters can be discovered from decoding the model number. See pagercodes.md to decode yours.

To use the single-shot flowgraphs:

After installing, start gnuradio-companion and open the example flowgraph. These are in examples/ -- for POCSAG, use pocsagtx.grc; for GSC, use gsctx.grc.

Double-click the "Single-Page Xmit" block to modify the baud rate, capcode, and message. After saving, double-click the "pagerfreq" Variable block to change the tunning frequency. (This should be specified in Hz). Change the samp_rate and sink parameters as required by your setup. (If you're not using a USRP, you'll want to swap out that sink for one appropriate for your SDR.) Build and run the flow graph; it should send the page, then exit.

Basic technical walkthrough (POCSAG)

POCSAG is plain FSK with a deviation of 4500 Hz around the center frequency.

The pocencode block ("Single-Page POCSAG Xmit" in grc) encodes the message as a POCSAG bitstream. Then, it converts the bitstream into a stream of samples, running at a rate that is an integer multiple of the POCSAG baud rate. (In my example flowgraph, I've used 38400, which is divided by 512, 1200, and 2400 evenly. This seems to work fine.)

Next, the GNU Radio FM block is used in the flowgraph to modulate samples at the intermediate sample rate. Note that the output of pocencode is not a stream of (0, 1) bits, but (1, -1) samples. This is for the benefit of the math inside the FM block.

Finally, the stream is interpolated to the sample rate desired by the sink.
(In my example, there is also a const multiplication to avoid clipping; this may not be necessary for your sink, so feel free to remove it.)

Although this works for me, I wrote this in order to teach myself the basics of GNU Radio and working with signal processing through it; for that reason, the code is not as efficient as it could be. Holler if you see anything seriously wrong or broken, or if you have any suggestions!

Protocol documentation and other references

POCSAG

POCSAG is an extremely simple protocol; the spec is just a few pages long. You can grab it from the ITU here:

http://www.itu.int/dms_pubrec/itu-r/rec/m/R-REC-M.584-2-199711-I!!PDF-E.pdf

Several folks have also written useful pages that clarify or explain certain points of the spec, including:

FLEX

The full FLEX specification is included in the ARIB standard (RCR STD-43A), as Reference Document A; this is the best place to look.

This can be found at http://www.arib.or.jp/english/html/overview/doc/1-STD-43_A-E1.pdf.

Other folks have implemented FLEX to some extent as well:

https://github.com/chris007de/flex

GSC

The GSC (Golay Sequential Code) protocol is defined in the ITU Report 900-2, starting on Page 9: https://www.itu.int/dms_pub/itu-r/opb/rep/R-REP-M.900-2-1990-PDF-E.pdf.

There is a lot of information about GSC at Brad Dye's excellent paging information site: http://www.braddye.com/, including this Motorola document: http://braddye.com/golay.pdf

Current todo list/Coming soon

Add GSC to the PDU-driven encoder
Eliminate dependency on the ITPP library (currently only used for BCH)
Eliminate dependency on third-party code to do Golay coding

Credits

gr-mixalot was written by Brandon Creighton [email protected]. The name is an homage to Sir Mix-a-Lot, who wrote the inspirational track "Beepers".

Suggestions and flames are equally welcome; as are additional pager-protocol documents.

Thanks to the following for testing and general awesomeness, in alphabetical order:

arko
banasidhe
docwho
dragorn
jeffE
the Ninjatel crew; barkode, cnelson, eliot, falconred, far_call, kubla
Mike Ossmann
pinguino
vyrus

unsynchronized/gr-mixalot

unsynchronized

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