25 April 2016

playing with MARS-ALE v3.00 and FS-1052 DLP messaging


MARS-ALE is a Personal Computer based 2G-ALE Modem and Controller that in conjunction with a supported computer controllable HF SSB transceiver allows for HF ALE communications. MARS-ALE also provides an implementation of FED-STD 1052 (FS-1052) Appendix B, DLP Data Link Protocol, running over the integral software defined MS188-110A modem on the PC Sound Device. The latest MARS-ALE v3.00 and MS-DMT (MIL-STD Data Modem Terminal) versions can be downloaded from here: 

When used in conjunction with other programs such as Audacity, Signals Analyzer, MS-DMT and BEE (a bitstream editor), MARS-ALE is a great tool for practicing with MS188-110 ST and FS-1052 DLP waveforms: digital signal enthusiasts can prepare clear and clean waveforms for study, analysis and comparisons with real-world signals. Below is a  short FS-1052 DLP messaging example just to show and understand the educational possibilities offered by MARS-ALE.

Modem Type and Data Mode settings are configured as follows (pic. 1,2)

Pic. 1 - MS188-110 messaging settings
Pic. 2 -FS-1052 DLP messaging settings
As expected, since the overhead added by FS-1052 DLP the lasting of this transmission is longer than MS-188-110 SYNC sending the same message and at the same data rate (pic. 3)
 
Pic. 3 - lasting of the two transmissions
 
In pictures 4,5 I run two MARS-ALE sessions, acting as the transmitting an receiving modems,  linked by a virtual cable: since FS-1052 DLP is a Data Terminal protocol, no portions of the message are printed until the entire message is received error free.

Pic. 4 - entering the message to be sent via MS188-110 + FS-1052 DLP
Pic. 5 - transmitting and receiving modems
For the same reason, it's very interesting to note that a plain MS188-110 receiving modem, as for example MS-DMT modem, does not print any message in its output screen unless the HEX string "5C 5C 5C D3 00 00 00 00 00 00 00 00" (pic. 6) and that is just the frame sync pattern used to mark DLP processed traffic.

Pic. 6 - MS-DMT modem facing a FS-1052 DLP transmission
From FED-STD 1052 App.B "50.1.1.1 Frame sync pattern":  Each new transmission over the physical channel shall begin with a three byte (24-bit) frame synchronization pattern to identify the following traffic as DLP processed traffic. The frame synchronization sequence in hexadecimal format, shall be "5C5C5C". The sync pattern shall be transmitted such that the first eight bits in order of transmission are "00111010". Note: As shown here in transmission sequence, the left-most bits are the LSBs.


The FS-1052 DLP frame sync pattern can be easily seen by processing the ASCII-bits file obtained from the output of a common MS188-110 decoder as Sorcerer used in this example (pics 7,8): this way we get the bistream after removed the MS188-110 stuff

Pic. 7 - bistream after MS188-110 removal
Pic. 8 - FS-1052 DLP frame sync pattern
If a transmission contains more than one frame, a two-byte sync sequence shall be inserted between each pair of adjacent frames: this pattern (hexadecimal) is "5C5C" (pic. 9)

Pic. 9 - two-byte sync sequence between frames

The sync sequences, along with other control bits and CRC, are also visible looking at the output of the ASCII parser of the bitstream editor: note the lack of such bits in a plain MS188-110 transmission (pic. 10)

Pic. 10
From FED-STD 1052 App.B "50.1.1.5 CRC error control checksum": a 32-bit CRC following the Frame Headers and Data field shall conclude each protocol frame. After initially setting all 32 bits to one, the CRC shall be calculated using all bits of the frame starting with the Sync Mismatch bit and ending with the last bit of the Frame Headers and Data field. The generator polynomial for the CRC calculation shall be:

 x^32+x^26+x^23+x^22+x^16+x^12+x^11+x^10+x^8+x^7+x^5+x^4+x^2+x+1

After removed the sync patterns out from the bitstream, I tried to find all the possible 32-bit polynomial which match with the data length and CRC bits.. but it's a long time consuming procedure and I stopped it just after the first results (pic. 11)

Pic. 11 - find the CRC polynomial

Another interesting MARS-ALE tool is the TRACING dialogue (pic. 12). This feature provides seven check boxes to enable/disable the display of data related to Received (RX) Words, Transmitted (TX) Words, program States, program Events, program Commands, program Timers and FS-1052 parameters as shown in pic. 13.

Pic. 12 - the "tracing" dialogue
Pic. 13 - tracing FS-1052 DLP at work

FS-1052 DLP is typically implemented in computer software applications and not in the hardware modem, however some tactical radios do implement an embedded 1052 ARQ and Broadcast capability. 
Historically, in MIL-STD-187-721C, “Interface and Performance Standard for Automated Control Applique for HF Radio,” Appendix A (USAISEC TECHNICAL REPORT “ HFDLP HF DATA LINK PROTOCOL”) is provided which specifies HFDLP for use with MS188-110A.

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