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Transmission Modes WAVECOM Decoder W74PC, W-PCI/e, W-CODE, W-CLOUD Manual V9.1.0
Protocol overview
The CODAN-9001 modem uses 16 DQPSK carriers for the transport of payload data. Each carrier is inde-
pendently modulated with data.
Each individual channel carries a channel packet. All 16 concurrent channel packets constitute a frame and
a number of frames constitute a multi-frame.
CODAN-9001 modem frame structure:
Each payload data packet has a constant length and a sequence number. However, the numbering in the
figure above only serves as an example, and due to the use of ARQ-based retransmissions the numbering
may not be sequential.
For the purpose of this explanation payload data is considered to be unprocessed user data and channel
encoded user data.
Independent of the payload data field, the sequence number field has its own error detecting and correct-
ing code. Payload data in each channel packet is protected by a cyclic redundancy code (CRC). This fea-
ture is included in order to allow the ARQ protocol to request retransmission of packets received in error.
A session consists of one or more multi-frames. Depending on the amount of data queued for transfer the
length of a multi-frame may vary. The receiving modem will extract the frames from the multi-frame de-
termining the number of channel packets and checking whether payload data was received without errors.
If a channel packet was received in error a re-transmission is requested. It should be clear from this that a
multi-frame may consist of a mixture of new data and re-transmitted data. Re-transmitted data may ap-
pear on any channel and in any position within a multi-frame. Additionally the transmitting modem may
opt to send ALE-like parity bit packets in a separate frame and even on another channel within the same
multi-frame as the payload data packet to which it belongs. This is indicated by the two packets belonging
together carrying the same sequence number. This mechanism is predominantly seen when the link quali-
ty deteriorates and consequently the number of re-transmissions increases.
In the “Broadcast” and “Group” modes multi-frames with new data appear for every fourth multi-frame –
in between data and coding information is repeated in the remaining three multi-frames.
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
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