3 September 2018

LINK-11 SLEW, transmission format

The SLEW waveform transmission format consists of an acquisition preamble followed by two or more fields, each field followed by a reinsertion probe. 
The first field immediately following the preamble is the header field and contains information that is used by the Combat Data System (CDS) and the encryption device. If a network PU (Partecipating Unit) has data to transmit, successive data fields follow the reinsertion probe of the preceding fields. These data fields consist of track data and other user data. The last field to be transmitted is the end-of-message (EOM) field. The transmission ends with a reinsertion probe.

Fig. 1

Data are accommodated using 3 different types of fields: header field, CDS data field and EOM field. The acquisition preamble, a very interesting topic, will be discussed in a next post.

The structure of the header field  consists of 33 data bits appended with 12 error detection bits (CRC). This 45 bit sequence is encoded with a 1/2 rate error correction code resulting in a 90 bit field. The header field contains information to define (Figure 2): 
the transmission type T (1 bit),
the Picket address ADDR (6 bits), 
the KG-40 Message Indicator MI (24 bits), 
the NCS/Picket designation N (1 bit),
a spare field SP (1 Bit).
Fig. 2 - Link-11 SLEW header field
The transmission type (T) indicates the format of the transmission to follow: is set to 0 to indicate an NCS Interrogation Message (IM) and is set to 1 to indicate a NCS Interrogation with Message (IWM) or a Picket reply transmission (the term picket indicates a PU on the network that is not the NCS).
The KG-40 Message Indicator subfield contains the sequence generated by the KG-40 crypto device. Cryptographic synchronization is achieved when the receiver acquires the correct MI. Since 24 bits is the length used by the Golay code, I tried to verify if the KG-40 MI was really coded using the extended Golay (24,12) ...but without success. For an NCS interrogation transmission (tramission type subfield = 0), this subfield will contain all zeros since no message is carried.
The address subfield  ADDR contains either the address of the next Picket to be interrogated or the address of the Picket that initiated the current transmission: note that only Pickets addresses are exposed.
The NCS/Picket designation (N)  identifies whether the current transmission originates from the NCS or from a Picket: 0 indicates an NCS transmission, 1 indicates a Picket transmission.

The structure of the CDC data field consists of 48 data bits (two standard 24 bit CDS frames seen in CLEW waveform) appended with 12 error detection bits that are encoded with 2/3 rate error correction code resulting in a 90 bit data field. 
The EOM field is used to indicate the end of the transmission and consists of a sequence of 90 bits. No error detection or correction bits are applied to this field. The sequence depends on the unit that is transmitting:
An EOM from the NCS is a 90 bit sequence of all “0”
An EOM from a Picket is a 90 bit sequence of all “1”
Below an example where all the SLEW fields are visible:

100101000110111110001110000011111 000001111100
100010100010001011000001101110010000101011110011 011001011010
111000010110000011110010111001001000000001110000 111100101001
011000001000110111001101100000001100011010110011 001111011101
111001010110100001101001111101011000101011010100 001110110011

100101010001100001001000111101001 001101011111
111111010001010101000001001101001111111111101001 001110110000
000111000100111011000110010010001001111110011110 100010111111
001100111011011100100100010000110000001100101110 011101010010
111001100100110100111100010001100010100100011101 100100001111

100101001110110010000010101000011 001111010111
100101110000001100001111011000110111011101110111 001111001101
110011011001010000111110011001100101100111110000 011111110010
000111101110101111010000101001011010010100010010 010100001110
100011010110110101111100001110111000011011111010 100010011011

It's interesting to analyze the headers related to the SLEW transmissions shown in Figure 3 

Fig. 3 - SLEW transmissions headers
In all the headers the transmission type subfields (T) are set to 1 to indicate that the following data sub-fields are NCS transmissions or Picket reply transmissions.
In the first header the NCS/Picket designation subfield (N) is et to 0 to indicate an NCS transmission: in this case the 6-bit subfield address identifies the address of the Picket to be interrogated (010100). In the second header the NCS/Picket designation is set to 1 to idicate the Picket reply transmission: in this case the 6-bit address identifies the address of the Picket which initiated the transmission (010100). You may check that
the other headers are interpretable in the same way. So, the headers indicate a series of IWMs and replies between the NCS and the Picket station addressed by 010100. 

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