ALIS (Automatic Link Set-up) is a simplex ARQ system and operates at a Symbol rate of 228.66 Baud on the radio link.
ALIS is described in report 551-2 of the ITU "Reports of the CCIR 1990 - Fixed Services at Frequencies below about 30 MHz".
|
Parameter |
Value |
|
Frequency range |
HF |
|
Operation modes |
FSK, Simplex ARQ |
|
Modulation |
FSK |
|
Symbol rate |
228.66 Bd, variable 30-650 Bd |
|
Receiver settings |
DATA, CW, LSB or USB |
|
Input format(s) |
AF, IF |
|
Additional Info |
ITA-2 with block coding |
The transmission block of the standard ARQ system consists of 2 identification bits, 30 data bits and 16 CRC bits. Data transmission is transparent for ALIS. Known systems are however structured around six ITA-2 characters.
The two identification bits indicate one of four possible system states. The CRC checksum enables detection and correction of transmission errors.
The acknowledgement block is 16 bits in length. The total transmit-receive cycle for ALIS is 111 bits, which corresponds to a duration of 485.4 ms. An error free transmission is equivalent to a terminal baud rate of 100 Baud.
The ALIS system automatically determines the optimal operating frequency after having received a CALL command. The station then sends a synchronization word, address, block counter and a status word. The receiving station correlates this bit sequence and synchronizes itself. If the data transmission link fails, ALIS will search for a new frequency to re-establish the link.