13 March 2016

Sending files using MS188-110A and FED-STD 1052 App.B (H520) Data Link Protocol

FED-STD-1052 Appendix B (FS-1052B) specifies a first generation Data Link Protocol (DLP) layer with priority messaging and multiple pre-emptive resume queuing ARQ (it is basically the equivalent of MS188-110A Serial Tone as to the modems). The FS-1052B HF DLP as designed will work with other data modems and not just the FS-1052 and MS110A ST modems, as in this sample. However FS-1052 it is optimized for use with a data modem having those same data rates from 75-2400bps and also supporting auto-baud.
FS-1052B provides three modes of operation:
ARQ mode The primary mode of operation is the automatic repeat request (ARQ) mode, which provides for error-free point-to-point data transfer and
employ a control frame acknowledgment scheme.
Broadcast mode A secondary mode of operation is the Broadcast (non-ARQ) mode. The Broadcast mode allows unidirectional data transfer using fixed-length frames to multiple (as well as to single) receivers. No transmissions from the receiving terminal are desired or required.
Circuit mode The other secondary mode, the Circuit mode, allows a link to be established and maintained in the absence of traffic. The ARQ variable-length frame protocol is used along with a technique to maintain the data link connection in the absence of user data.  

In the samples below, signals come from the real-world, for example the above picture  is related to a link between the callsigns BS008CB and CS003A: after the link setup, performed by MS188-141 2-G, they go into MS 188-110 for data transfer. 
After removed MS188-110A headers and other stuff (scramber, interleave and FEC coding) the resulting bistream exhibits a clear 520 bits period that is characteristic of FED-1052 App.B or H520 protocol (pic 2)

pic. 1 - over-the-air bitstream, as demodulated by SA

In the case shown in pic.2, the transmission is performed through an exchange of protocol frames so here we face the primary ARQ mode.

pic.2 - FS-1052B in ARQ mode
If the data are corrupted (strong fading, interferences, too weak signal,...) the analyzer will print out the "CRC Error" message, moreover the sessions should be taken from the beginning to the end since any lack of bits affects the integrity of the data.

pic. 3 - FS-1052B in Broadcast mode
Each new transmission begins with a three byte (24 bits) frame synchronization pattern to identify the following traffic as DLP processed traffic. The frame synchronization sequence in hexadecimal format is "5C5C5C". The sync pattern is 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 (pic. 4). 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.4 - frame synchronization pattern
The Frame Header fields (consisting of the Sync Mismatch Bit and the Frame Type bit) and then Control Frame Header fields (pic. 5) follow the 3-byte sync pattern: their possible values and meanings are illustrated in paragraph 50.1.2 of "FS-1052: Letter of Promulgation" (see the link at the end).
pic.5 - Frame Header and Control Frame Header fields

In Broadcast mode (the receiver does not send acknowledgments) the transmit peer sends 520 bits (or 65 bytes) fixed-length frames structured as (pic. 6):

a) 40-bits header
8-bits synchronization, depends on the communication line (observed patterns: "10010000", "10110000" 10001000");
8-bits descending counter indicating the frame number inside the block (pic. 7);
24-bits offset (in bytes) from the beginning of the message;
b) 448-bits information field;
c) 32-bits CRC field (computed over the preeceding 448-bits data).

pic.6 - 40 bits header
pic. 7 - the 8-bits descending counter
The information field can be obscured using cryptographic encoders such as KG-84A, KG-84C, KY-99, KY-57, KIV-7, KY-58 and  KY-68.

With this protocol, E-mail, files in MS-TNEF format, text messages, graphics extensions (TIF, GIF, JPG, BMP), Microsoft Word and Excel documents, Power Point presentations, PDF documents, HTML and other types of file can be transmitted: unfortunately, my analyzer does not have all handlers so, once removed FS-1052B protocol (pic. 8) the resulting stream/file may contain other protocols that will require further processing.
pic. 8 - after FS-1052B removal
FS-1052B is limited to a 2400bps maximum data rate by design, whereas the newer STANAG-5066 (second generation Data Link Protocol) has no such limitation.
An interesting  E-mail Performance comparison with 2nd and 3rd Generation Data Links protocols can be seen here (.pdf file).

About the user equipment, the popular Harris family of tactical HF radios includes models that implement a draft Proposed FS-1052B DLP (pFS-1052) they adopted and fielded years prior (!) to the published 1996 FS-1052 standard. So, more likely, the heard trasmissions just come from Harris tactical radios.


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