Net operation. To effectively achieve automatic netted data communications, all participants in the net must follow an orderly sequence of operation. A common time-shared radio frequency is used, restricting transmission to a single station within any given time. When not transmitting, each station monitors the frequency for transmissions from other stations. Control of net transmissions is provided by designating one participant as the data net control station (DNCS) and all other net units as subordinates (picket stations). While other modes are also available for test and broadcast, this description of net operation describes the roll call mode, which is the primary operational mode.
A representative TADIL-A net is shown in FIGURE 1. Each participating unit is assigned a unique address code which is used Ay the DNCS to establish an orderly net transmission sequence. The DNCS transmissions are in the format of an interrogation message or an interrogation with message, and both formats contain the address code of the next picket station in the reporting sequence. The interrogation with message format is used by the DNCS to transmit its own tactical computer output data, while the interrogation message format contains only the picket address. All picket stations receive the transmission, transfer the tactical data (if any) to their respective tactical computers, and compare the received address with their own. The picket station recognizing its own address code switches its data transmission equipment to the transmit condition and transmits its tactical computer output data in a reply message format. Each participating unit in the net receives the picket reply transmission and transfers.
TADIL-A net reporting sequence. The reporting sequence used by the DNCS may be established manually by the operator in conjunction with the address generator; or alternatively, the addresses may be supplied individually by the computer, allowing event-driven dynamic change in the reporting order. The time required for all stations to have a transmit opportunity (net cycle time) is variable and depends upon the number of net members, the amount of data each transmits, and the dynamic sequencing of the station reports when under computer control. The number of active participants in a TADIL-A net is limited by the number of unique address codes (62) available in the address code library. In a manually established sequence, it is limited by the number of addresses capable of being queued by the DNCS station equipment, which varies from 15 to 64 in current equipment types. TADIL-A employs a parallel transmission of data and control frames, with each frame containing 30 bits of binary coded information. A frame may be either 13.33 ms or 22 ms.
FIGURE 1. Representative TADIL-A net. TADIL-A equipment. A representative TADIL A system configuration is illustrated in FIGURE 10. The DTS interfaces with a radio set and performs all the control functions related to net operation as described.
Transmission. The DTS requests and accepts tactical data in the form of a 24-bit data word, encodes the word with a 6-bit EDAC code (Hamming), and phase modulates 15 internally generated audio tones with the newly formed 30-bit word. The 15 phase-modulated audio tones and a Doppler correction tone are combined to form the composite audio signal that is applied to either the HF or UHF radio equipment for transmission. For HF transmission, suppressed carrier with ISB (DIV mode) or SSB modulation on either USB or LSB can be used. For UHF transmission, the audio output is used to frequency modulate the UHF carrier. Tye TAOIL A radio equipment transmit and receive operation i s control 1 ed by the DTS.
Reception. For HF operation in the DIV mode, the radio equipment at the receiving stations translates the RF signals to audio tones that are applied to the data terminal equipment as USB and LSB composite signals. By separate demodulation of each sideband and a DIV combination, three versions of the 30-bit word are created. Each is checked for errors by Hamming tests. The tactical data selected for transfer to the computer is either manually or automatically selected. When the choice is automatic, the first source with no errors is sent to the tactical computer. If all versions have errors, the DIV combination is sent. The data from the selected source is stripped of its six EDAC bits, corrected (if required), and transferred to the tactical computer along with flag bit(s) that indicate its error content. For HF operation in the SSB mode or for UHF operation, only one sideband of the DTS is utilized.