Wavecom W40PC, Manual

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Transmission Modes WAVECOM Decoders v6.15 W40PC, W41PC, W51PC
on two different frequencies, one representing "Mark" level and the other "Space" level. The
difference between the two frequencies (frequency deviation) is called the "Shift" and may for
instance be 170, 425 or 850 Hz.
One-Way Traffic, Simplex and Duplex
Traffic between users may be handled in a number of ways depending on requirements and
equipment available.
If information is sent only in one direction it is referred to as
one-way traffic
.
If information is sent in both directions, but one in direction at a time it is referred to as
simplex
.
If information is sent in both directions simultaneously it is referred to as
duplex
.
Increasing the Data Throughput
Ongoing efforts are being made to exploit as much as possible of a given channel’s capacity.
One way is to process data to be transmitted in such a way that redundant information is
removed before transmission. Another method is to transmit more than one channel on a radio
link. This may be achieved either in the frequency or time domain or a combination hereof.
The removal of redundant information is called compression . The ratio between the size of
the original data and the compressed data depends on the nature of the data and the efficiency
of the compression technique. These techniques are used in commonly known compression
software like PKZIP, ARJ and LHARC. Compression is used in the PACTOR mode.
In
frequency multiplex (FDM)
a carrier frequency is modulated with a number of sub-
carriers within a standard telephony channel from 0.3 kHz to 3 kHz. Each sub-carrier carries a
data signal. The sub-carriers may be amplitude, frequency or phase modulated. The more
common is narrow shift frequency modulation. Each channel is independent of the other ones
and may transmit with a different speed or use a different alphabet or system.
In
time multiplex (TDM)
each data source is allowed access to the
aggregate channel
(line or radio link) in well-defined time slots. To keep pace with the incoming bitstream, the
aggregate channel speed must be the sum of the speed of the individual channels. All channels
must have identical speeds. However, a channel subdivision scheme has been standardized so
that up to four sub-channels may share one channel.
The overwhelming majority of radio data systems will transmit the individual bits of a
codeword one after the other in
serial transmission
. But real-time or high volume data
systems like digitized secure voice, computer network access and image or file transfer often
uses
parallel transmission . The serial codewords are fed to a serial-to-parallel converter
and then to the sub-carrier modulators of a FDM.
Encryption
To protect data transmissions against interception various methods are in use.
Encryption may be on-line or off-line . On-line encryption takes places at transmission
time, whereas off-line encryption is done before transmission, usually in the form of coding the
clear language message into five letter or five figures groups. This is done by a key sequence.
Bit inversion
inverts logical zeroes of a codeword with logical ones and vice versa either in a
static pattern, e.g. bit 3 and bit 5 or in a dynamic pattern depending on the value of the
codeword.
Bit transposition replaces bits in one position in a codeword with bits in another position.
Shift-register encryption
uses one or more shift-registers into which the clear language
message is shifted and ex-ored with a key or part of itself. Taps in various positions of the
registers may feed bits back to the input to complicate decryption by interceptors. The shift-
registers of the transmitting and receiving equipment must be initialized to the same value - the
seed.