Wavecom W74PC V8.7.0 - Manual (Page 157)

WAVECOM Decoder W74PC, W-PCI/e, W-CODE, W-CLOUD Manual V8.7.0 Transmission Modes
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CH Frame 1 Frame 2 Frame 2 ... Frame n
2 Chnl packet 02 Chnl packet 18 Chnl packet 34 ... ...
3 Chnl packet 03 Chnl packet 19 Chnl packet 35 ... ...
... ... ... ... ... ...
16 Chnl packet 16 Chnl packet 32 Chnl packet 48 ... Chnl packet m
Multi-frame 1
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
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-