3.2
Polling
The system can be configured as a "Master-Slave" network which allows one radio modem
to control the others in the system. Slave units can communicate to the Master during the
time allocated to that Slave by the Master unit.
The advantage of the polling mode is that collisions (i.e. simultaneous transmissions) do not
occur. The disadvantage of this method is that the transmitter of the Master is switched on
half of the time in situations where there is no data transmission from mobile to Master.
A polling protocol is not included in the transparent radio modem. It is a protocol of higher
hierarchy and supplied by the system.
3.3
Multi Master
The system can also be configured as a "Multi-Master" network. In this situation any radio
data modem can start transmission after first testing either by the CTS line or the CD line
that the radio channel is free.
The advantage of this system is that the transmitter is ON only during data transmission.
The disadvantage is that if all the mobile stations do not "hear" each another collisions may
occur.
A multi master protocol is not included in the transparent radio modem. It is a protocol of
higher hierarchy and supplied by the system.
4
Planning a Radio Modem Network
4.1
Factors affecting quality and distance of the radio connection
- power of radio transmitter
- sensitivity of radio receiver
- tolerance of spurious radiations of the radio modulating signal
- amplification of transmitting and receiving antennas
- antenna cable rejection
- height
- natural obstacles
- interferences caused by radio frequencies
The transmitter power of the base model of SATELLINE-1AS/2AS is 0.5 W and sensitivity of
receiver more than -108 dBm. Thus in a flat area and in free space with a 1/4 wave
antenna (antenna amplifcation 1dBi) and an antenna height of 1 m communications from
3 km to 4 km can be achieved. Distances may be considerably shorter in situations where
there are metallic walls or other material inhibiting the propagation of radio waves.
Over long distances, problems caused by natural obstacles can often be solved by raising
the height of antennas. A ten fold increase in distance can be achieved with the use of
SATELLINE-1AS and SATELLINE-2AS
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