15 August 2015

Rohde & Schwarz ALIS (RS-ARQ selcall)

This is the Rohde & Schwarz proprietary standard "ALIS" (Automatic Link Setup) selective call, an adaptive ALE procedure based on FSK modem and ALIS processor. 
Quoting Hoka: "Rohde & Schwarz simplex ARQ, so far found in use by German, Italian, Nairobi and Turkish Diplo services, typically 228.7bd but reports of 457.0 have been noted. There appears to be no "real" name for the data system. Some people call RS-ARQ as ALIS but strictly speaking, ALIS is only the automatic link processor and frequency management system. It is not responsible for generating the traffic. ALIS is therefore somewhat of a misnomer. The modems generating the traffic are the GM857 and GM2000. Our suggestion is to stick with RS-ARQ as the system name."

The signal has been heard today afternoon at 1620z on 12270.5 KHz on USB. The waveform is an FSK 228.6Bd/170 (normal FSK modulation, ALIS basic feature), ACF value (in the analyzed sample) of 342.6 msec or 78 bit. By using the SA "scan raster" method is visible the feature called "diagonal bit" that can be used to recognize the signal.

Fig. 1

The following basic parameters are permanently stored in the communication processor unit:

- address list with geographical coordinates and distances of corresponding stations;
- frequency pool (total number of frequencies up to 100, with a maximum of
16 frequencies for each pool and a maximum of 25 different pools);
- date and time;
- sunspot number.
The calling (master) station establishes the connection. It transmits a defined number of frames on each frequency of the desired pool to enable a weighted bit addition at the receiving station. The frames are repeated because the slave station must have enough time to scan through all the programmed frequencies and receive at least three frames on each single frequency for synchronization.
After the correct reception of (at least) three frames from the master station, the slave station transmits a synchronization acknowledgement and additional information about the reception quality along with the monitored status of the sending station to ensure that the two stations are working in the same operation mode.

The selcall phase maybe followed by the data transfer phase which is performed using several waveforms, as indicated in in the field "followon"  ("status") of some decoded selcalls (Fig. 3). Note also the different "pool size" which is related to the number of the allowed frequencies for the current pool.


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