Transparent Data Transmission
Radio Protocol Support
16
SRL-35 Operator’s Manual P/N: 1003224-01
Timing and Delays During Data Transmission
When using a radio modem for data transmission, certain delays will be formed through the use of a radio
interface and from the radio modem circuitry itself. These delays exist when the radio modem switches
from Standby Mode to Data Transfer Mode and during reception and transmission of data. For detailed
delay values in each case see Appendix B.
Data Buffering in the Radio Data Modem
Whenever the radio modem is in Data Transfer Mode it monitors both the radio channel and the serial
interface. When the terminal device starts data transmission the radio modem switches to transmission
mode. At the beginning of each transmission a synchronization signal is transmitted and this signal is
detected by another radio modem, which then switches into receive mode. During the transmission of the
synchronization signal the radio modem buffers data into its memory. Transmission ends when a pause is
detected in the data sent by the terminal device, and after all buffered data has been transmitted. When
the serial interface speed is the same or slower than the speed of the radio interface, the internal transmit
buffer memory cannot overflow. However, when the serial interface speed exceeds the speed of the radio
interface, data will eventually fill transmit buffer memory. In this instance, it will take a moment after the
terminal device has stopped transmission of data for the radio modem to empty the buffer and before the
transmitter switches off. The maximum size of transmit buffer memory is one kilobyte (1 kB). If the terminal
device does not follow the status of the CTS-line and transmits too much data to the radio modem, the
buffer will be emptied and the transmission is restarted.
In the receive mode, the buffer works principally in the above described way thus evening out differences
in data transfer speeds. If the terminal device transmits data to a radio modem in receive mode, the data
will go into transmit buffer memory. If the modem is set to TX priority mode the transmission starts
immediately as soon as data packet arrives to serial interface TD line.
Pause Length
The modem recognizes a pause on the serial line (a pause is defined as a time with no status changes on
the RS-232 interface TX-line). The pause detection is used as criteria for:
•
End of radio transmission - When the transmit buffer is empty and a pause is detected, the modem
stops the transmission and will then change the radio to the receive mode.
•
SL-command recognition - For a SL-command to be valid, a pause must be detected before the
actual “SL…” character string.
•
User address recognition - In order for the start character to be detected, a pause must precede it in
transmission.
Traditionally, in asynchronous data communication, pauses have been used to separate serial messages
from each other. However, the use of non-real-time operating systems (frequently used on PC-type
hardware) often adds random pauses, which may result in the user data splitting into two or more separate
RF transmissions. This may cause problems especially in the systems including repeater stations.
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