Prior to sending data between the CODAN modems, a proprietary ALE (Automatic Link Establishment) is used for link establishment consisting with an 80 Baud (symbols/sec) Chirp signal. CODAN-CHIRP uses DBPSK modulation across 32 channels with 80 Hz of spacing and a bandwidth of 2560 Hz.

CODAN-CHIRP is based on the US federal standard (FED-STD-1045). The data station originating a data call transmits a Chirp sequence with signalling information. This information causes a RING signal at the receiving node. If the receiving node accepts the call, an exchange of more signalling information follows. The signalling information contains the source and destination address, as well as the parameters of each modem, such as the serial number, the tuning and the transmission mode: secure, secure interactive and unsecure (compressed or uncompressed). In a peer-to-peer communication, the sender and the receiver exchange information about their parameters, whereas in a broadcast communication only the sender sends information about his parameters. Once all the signalling information has been sent, the link is established and the data transfer between the modems begins.

The transmission format of CODAN-CHIRP is as follows: First the Chirp signal starts with a 23 bit preamble sequence, which is followed by a 32 bit frame-indicator. The frame-indicator is followed by 96 bits of data. In some transmissions there are several pairs of frame-indicator and data. The last 48 bits among the 96 data bits are the inverse of the first 48 bits. Additionally, the data is interleaved and encoded using the Golay (24, 12) block code, which means that 12 data bits are encoded to produce 24 bit code words. So the first 48 data bits are divided into 24 bits of actual data and 24 Golay encoded bits, and the last 48 bits are the inverse of the first 48 bits.

The mode CODAN-CHIRP is implemented in HF Modes → PSK & OFDM. The decoder demodulates and decodes the Chirp signal using frequency correction. The CODAN-CHIRP decoder automatically corrects small frequency deviations from the exact center frequency of the signal. The polarity (NOR or INV) of the signal is automatically identified by the decoder. For CODAN-CHIRP, the decoder identifies the source and destination address, as well as information about the serial number, the tuning and the transmission mode (secure, secure interactive or unsecure, compressed or uncompressed). The decoded output is displayed in ASCII and HEX.

CODAN-CHIRP decoder output.