Digital data transmission differs from voice communication
in the nature of the information that is transferred.
Digital data consists of a list of information bits.
A data bit can have only two distinct values labelled as "1" and "0".
These are abstract logical values that have to be mapped to two distinct physical
states when data is stored or transferred.
Since radio communication systems were principally designed for voice communication,
the value of the individual data bits may be assigned e.g. two different,
audible tones or frequencies.
This modulation format is called Frequency Shift Keying (FSK), in radio
technique referred to as F1B.
An important feature in data communication is the speed at which a data package
can be transferred.
Data speed is normally expressed in bits-per-second (bps), but since in many
systems forward-error-correction (FEC) is applied the net bit rate at the communication
level rate may differ from the gross bit rate on the transmission level.
When using FSK for data transmission as described above, the logical bit
information is assigned to two distinct frequencies.
A widely used FSK standard on VHF channels applies the frequencies
1300 Hz and 2100 Hz.
Since at least one period of an alternating signal is needed to determine
its frequency, the maximum obtainable data rate with FSK can never be higher
than the period of the signals that are used for mapping the logical bit values.
In the above mentioned standard, a data rate of 1200 baud is used.
On MF/HF communication channels, which underlay more atmospheric disturbances,
only 300 or even 100 baud are used in SITOR or RTTY systems.
For example NAVTEX broadcasts use 100 baud FSK modulation with a frequency shift of 170 Hz.
Due to radio history reasons, the frequency range (bandwidth) that can be used to map
logical bit information onto analogue signals is limited to about 3000 Hz.
For communication at data rates considerably higher than the available bandwidth,
the modulation efficiency (expressed in number of transferred data bits per Hertz bandwidth: bits/Hz)
must be increased by applying more complex modulation schemes.
This is used e.g. in analogue telephone-line modems using high-level QAM modulation schemes
or multi-carrier OFDM schemes.
But more complex modulation schemes will also make the system more vulnerable to data corruption due
to noise and other communication disturbances. To a certain degree, this data corruption
may be tackled by applying forward-error-correction schemes, but this requires a
certain amount of added redundancy in the transmitted data, which reduces the net data rate
and counteracts the introduction of the more complex modulation schemes.
In the end if system complexity is not an issue, modulation- and FEC schemes can be optimised
for the typical behaviour of the available channel to achieve the highest possible data rate.
However in HF-radio communication, an additional aspect is the dynamic nature of the
communication quality. This depends on a number of uncontrollable factors such as weather,
solar activity and ionospheric conditions. So RTTY modulation schemes for HF-radio modems
may use a variable bit rate depending on the channel conditions. For example
PACTOR transmission speeds range from 20 to 200 characters per second.
So in bandwidth limited, noisy HF-radio channels, there is a rather low limit for the
maximum data rate at which such a communication channel can perform.
As a consequence from this low data rate, the amount of data (file size)
that can be transferred in a reasonable amount of time is obviously also limited.
Shortwave Data Communication Systems
Data communication on board of cruising yachts may be divided into two types:
- reception of broadcasted telex-like information (e.g. weatherfax), and
- text-based station-to-station communication (e.g. e-mail service).
For those two application basically two techniques are in use: SITOR-B for telex-like broadcast information
and PACTOR for point-to-point data communication.
PACTOR combines the bandwidth efficiency of packet radio with the error-correction (CRC) and automatic
repeat request (ARQ) of AMTOR. PACTOR is most commonly used on HF frequencies between 3 MHz and 30 MHz.
Except for NAVTEX, for which specialised receivers are available,
all data communication will require a personal computer (PC) on board.
This PC is used for a variety of tasks such as:
- the visualization of the received data (e.g. weather charts),
- edit text-based data files (e.g. e-mail messages),
- encode and decode the transmitted digital data,
- generate and receive the audio signals (this requires a suitable sound card).
Available services are:
- HF-SSB-based Sailmail and
- satellite-based INMARSAT-C (600 bps).
- Over Amateur Radio frequencies also e.g. Winlink.
- Professional services combining satellite and HF technology to provide true global communications
from e.g. Globe Wireless.