Decoder

There are long packets containing data and short packets containing control or request information for the ARQ process.

The decoder can recognize and decode the following packet types:

RQ, FORCED OVER and END ACK are short packets. The others are long packets.

The decoded packet types are shown in the upper status bar.

In the next lower status bar the following information is shown:

•      Number of carriers/channels

•      For long packets: Number of channels with CRC OK and CRC FAILED

•      For RQ packets: Number of channels with ACK, NAK, and wrongly decoded channels (UNKN)

Due to the compression employed by GW-OFDM, decoded data is not available until the end of a transmission. The end of a transmission is indicated by an END or END ACK packet, or after a timeout of several seconds. After a transmission ends, the decoded data packets are put together and further processed to a text file which contains information about the transmitting station and which can further contain compressed emails and/or several commands. If the processed data packets yield evaluable data, the following information is shown:

•      Transmitting station information

•      If emails are found and extraction is successful (possibly with a CRC-Error): Data transmission information and the extracted emails are printed

•      If emails are found but extraction fails, the cause for the failure is printed

•      Otherwise if only commands are found, they are printed

In addition to the decoded output, several files are generated in the Global Data (RW) directory. In the subdirectory “GWOFDM[SN]”, all decoded emails are stored, either as text file (OutFileTXT) or in a folder as .SSP-file (OutDir). The .SSP files can be opened with any standard text editor. The subdirectory RAW_DATA saves for every transmission a log file with all detected packets and – if a transmission contains data packets – a data file with the decoded data packets put together is created. These files can be used for advanced examination of the transmission. In the subdirectory TRANSMISSION_DATA data packets of aborted transmissions are saved, which are used in case of a continuation of the transmission. Files in the RAW_DATA and TRANSMISSION_DATA directory are deleted after two hours, or when the mode is closed.

As the GW-OFDM-signal contains no redundancy, sufficient reception quality is important: every DATA packet must be received at least one time without corruption. In the case of packet repetition, the decoder can handle multiple received packets. It should be noted that the decompressor has no error correction - even one wrong bit can produce a completely false decoded output.