Wavecom W61PC V7.5.0 - Manual (Page 144)

134  Transmission Modes WAVECOM Decoder W61PC/LAN Manual V7.5
Compressed mode specifics
For point-to-point channels CODAN modems utilize a dictionary based compression method, which allows
compression to be independent of the statistical characteristics of the source data. The dictionaries are
dynamically built depending on the data previously transmitted. Each modem uses one dictionary for re-
ception and another one for transmission in such a way that a transmission dictionary at one end of the
link corresponds to a reception dictionary at the opposite end of the link. The ARQ protocol ensures that
the dictionaries are updated and synchronized. If the dictionaries deviate decompression becomes impos-
sible as the buffer contents cannot be reconstructed. It is thus clear that for non-cooperative interception
deviating dictionaries pose a major problem.
CODAN decoding at the practical level
When engaging in non-cooperative monitoring a number of issues arise:
 All frames and the sequence counters contained in therein must be received without errors. Re-
transmitted packets must be discarded
 Missing packets or packets with a failed CRC must not be accepted; otherwise data output will be
incomplete
In case of uncompressed transmissions packets with a failed CRC may be marked as such. In case of
compressed transmissions decompression consequently becomes impossible due to the deviating diction-
aries built during the non-cooperative interception session. Only when a new link is set up, it is possible to
start over with empty dictionaries. It is important to understand that in case of compressed transmissions
exclusively the data from one party may be forwarded to the de-compressor. This understanding is espe-
cially important if during non-cooperative interception both parties in a “Chat Session” are received as
forwarding the data from both parties will immediately poison the dictionary.
HF transmission issues
The CODAN ARQ protocol is adapted to the propagation conditions in the HF range, but obviously relies on
an active link between the two communicating parties. This link does not exist in the case of non-
cooperative monitoring and thus complete and correct data output is fully dependent on error-free recep-
tion – there is no way to ask for re-transmission.
As transmissions do not contain source or destination addresses the monitor in case of single-frequency
half-duplex traffic must manually select the party to monitor. This may be achieved by adjusting the re-
ceiver squelch, adjusting the receiver HF gain or the use of directional antennas to suppress the unwanted
signal.
Real-time monitoring issues
To capture the contents of a CODAN modem data transmission the monitoring equipment should be placed
as close as possible to the transmitter site of the interesting party. This is the only way to ensure that the
quality of the signal is sufficient to secure that all transmitted packets are correctly captured.
To facilitate the decoding of data the WAVECOM decoder in addition to outputting the decoded data in the
decoder window also saves raw channel data with sequence numbers and including bad packets to a text
file. The text file may be processed manually or by a user provided application.
COQUELET-13
COQUELET-13 is an asynchronous system and uses a start and idle tone of 1052 Hz. As with COQUELET-8,
the first group contains 8 tones for keying, and the second group 4 tones.