2 September 2018

DHFCS 1536-bit TDM protocol (2)

In the previous post I associated the 1536-bit TDM protocol to the DHFCS network, and that's correct, but I wrongly ascribed this protocol to Rockwell Collins. Indeed, looking carefully at the two slides below you can see that they refer to the products GA-123 (HF modem) and GA-205 (TDM multiplexer), both are produced by DRS Technology, a Leonardo (formerly Finmeccanica) company.

Fig. 1
Reading the GA-205 datasheet [1] we can shed a bit of light on the 1536-bit protocol: GA-205 is a 12-channel Time Division Multiplexer (TDM) that provides full-duplex and half-duplex transmission and reception of data at selectable user port rates of 75 x 2n up to 9,600 bps. The system accommodates user data that do not share common timing sources and provides for isochronous, bit stuff, synchronous and asynchronous operation.

Fig. 2 - the GA-205 multiplexer

In Time Division Multiplexing (TDM) the communication resource is shared by assigning input channels the full spectral occupancy of the system for a fixed duration of time called time slots.
Synchronous TDM works by the muliplexer giving exactly the same time slot to each device connected to it even if one or more devices have nothing to transmit. The data rates of different input devices control the number of the slots: a device may have one slot, other may have two or three according to their data rate. 
Asynchronous TDM, or statistical TDM, is a more flexible method of TDM since slots are assigned dynamically as needed, ie slots are not assigned to devices that have nothing to transmit. Variable-Length Time Slots Asynchronous TDM can accommodate traffic of varying data rates by varying the length of the time slots. Stations transmitting at a faster data rate can be given a longer slot.

Since GA-205 multiplexer can handle up to 12 channels, the four ports you see in Figure 1 can be misleading: it is possible that the "preset" shown in the screenshot (identified as TDM1 in the upper right), refers to a particular configuration used to manage only 4 input channels of the 12 available. Maybe a default? who knows, the slide dates back to 2006. Notice that in the shown preset the input channels exhibit different baud rates: 600, 300, and 75. In that condition, bit stuffig or variable length slots can be used.

Given the above considerations:
1) at most, the DHFCS 1536-bit format carries up to 12 channels (by the way, 1536 bits = 1024+512, ie 1.5 Kb);
2) managing TDM requires that some control bits (sync, device tagging, ...) be appended to the beginning of each slot and this overhead is clearly part of the raw bitstream that we get after S4285 removal;
3) since we do not manage the control bits, when the the GA-205 is used in async mode we can't say the number of the channels currently transmitted; as well as we do not know the number of "traffic" channels when GA-205 is used in sync mode.

My guess is that the 1536-bit period could be the frame length (the slots gathered in a complete cycle), no matter if GA-205 works in sync or async mode. 
Channels are encrypted individually before being applied to the multiplexer, they probably use BID-950 or KIV-7 (KIV-7 may work as KW-46).

DHFCS STANAG-4285 stations logged and DF'ed so far:
05553.2 Cyprus Is.
07937.0 Crimond 
11015.0 Crimond 
14390.0 Ascension Is. 
14548.2 Cyprus Is. 
15812.1 Cyprus Is.
16106.3 St. Eval 
16287.0 Ascension Is. 
16398.2 Cyprus Is. 
17398.2 Cyprus Is. 

[1] http://www.drs-ds.com/media/1414/ga205.pdf

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