Furuno Felcom 82, Руководство оператора

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6. 9.6k DATA COMMUNICATIONS
6-4
Error correction
During data transmissions, data errors occur for various reasons. In text data
such as a sentence, a minor error does not hinder the reader from presuming the
correct meaning in the context. In binary data such as a program, however, even
a small error is not permitted. For this reason, the sender previously adds
information used to detect any data error and if an error is found in the received
data, the receiver requests the sender to retransmit the data. This procedure
ensures the error-free transmission of data. It is called error corrections.
The 9.6K DATA offers the ARQ mode in which error corrections are made over
the satellite link and the non-ARQ mode in which no error correction is made. It is
optionally possible to tell the LES whether error corrections should be made over
the terrestrial link regardless of either of the above two modes.
Normally, error corrections should be made for both the satellite and terrestrial
links. In some cases, however, error corrections are not recommended. For
example, when files are transferred, data transfer protocols such as ZMODEM
are used and they make error corrections by themselves. This means that if error
corrections are also made over a link, the transfer throughput decreases because
of the dual error corrections. In such a case, the use of the non-ARQ mode is
rather recommended. In addition, it may be better to determine whether error
corrections should be made for each of the satellite and terrestrial links, in
accordance with the states of those links.
Terminal speed (DTE speed)
A communication speed between the DTE and the MES. Data communications
occur at 9600 bps between the MES and the LES, but this speed is not always
applicable to the communications between the LES and a remote station. Since
error corrections and flow control are performed in the communications between
the MES and the LES and between the LES and a remote station, data is not
sent without them between the DTE and a remote station. Therefore, the
communication speed between terminals, that between the MES and the LES,
and that between the LES and a remote station do not need to be identical. A
given communication speed can be selected for communications with any remote
station.