User's Manual
SATELLINE-3AS
User Guide, Version 2.2
49
7.3.3 Overhop function in Source Mode Routing
Master TD
Radio
Substation RD
M
R1 R2
S
TD
RD
t
Master
R1 R2
When using repeaters, the same packet is sent via a (radio) channel several times. A radio
modem situated in the repeater chain will often hear other radio modems in addition to the
immediate neighbouring radio modems. With Source Mode Routing the complete address
information is sent together with the packets enabling the use of these secondary routes. Usually
repeaters have better antennas (and/or antennas placed higher) than normal substations, which
means that the distance between two repeaters can be much longer than the distance between a
repeater and a substation.
Connections to other than the neighbouring stations of the repeater are not necessarily reliable
under all possible (radio) conditions, but they can often be used to keep the network up and
running at least partially should a repeater somewhere in the middle of the chain fail. In addition
to this, the likelihood of a transmission error decreases if it is possible to listen to more than one
transmission since, in this case, it is more likely that at least one of them will be received error-
free. It is also possible, that the radio connection is asymmetrical due to the greater output
power of a repeater or due to local interference, in which case, data can in fact travel in
opposite directions using different routes.
When a repeater receives a message that contains its address but not as the first address in the
address field, the packet is stored in a buffer. If the relayed message from the repeater, and the
modem having sent the message originally, is not received (due for example, because of radio
modem failure or if it has an erroneous checksum), the packet already buffered into the memory
is re-sent without any change to the timing (no additional delays are introduced). Because of
this, relatively short hop distances can be used, without the likelihood of errors being increasing
due to the added number of repeats. A possible error in one of the repeaters does not
necessarily cause a total breakdown of traffic.