User's Manual

In Packet Mode, selection of the serial port baud-rate is important. As shown in
Figure 1 (Packet Mode of Operation) above, if the serial port baud-rate is the
same as the over-the-air baud rate and the packets are short, the channel
utilization is only about 50%. But, if the serial port baud rate is set much
higher, say 2-8X the over-the air rate, the channel utilization becomes near
100%.
Because the FireLine can handle serial-port data rate far in excess of the
over-the-air rate, the efficiency of the FireLine in Packet Mode is
approximately the same as other brand modems that cannot operate in a
Packet Mode — with the added benefit or ARQ, error-free data, and
addressing.
Note that many Windows applications which use the serial port, such as
HyperTerminal, put large gaps between the bytes of data they send out the
serial port. If an application is not getting the desired throughput, verify that it
is not an artifact of the Operating System or the computer.
Flow Control
If large amounts of data will be sent with the FireLine, it may be possible to
overflow the internal data buffer. To ensure the transmit buffer does not
overflow, enable and use hardware flow control. Hardware flow control is
enabled with the ATCH 1 command. Note that the FireLine modem will
always indicate the status of its internal buffer using the CTS signal on the
DB-9 serial connector. When CTS is negated, the internal buffers are more
than 80% full. When it is asserted and it is “Clear to Send”, the buffers are
less than 80% full.
Packet Size
The over-the-air packet size may be set with the ATTT xx command. Once
the modem receives one full packet of data into via the serial port, it will
automatically key the transmitter and send the data. Factory default is 80
bytes.
Key-On_Data
When serial data is entering the FireLine’s RS-232 port, the FireLine looks for
pauses in the data as indication that it is time to send a packet of data over
the air. The factory default duration of the pause it looks for is 20mS, but the
user may change this to over values using the ATR3 xxx command, where
xxx is in milliseconds.
Busy-Channel Lock Out
If your system operation require the FireLine modem to monitor-before-
transmit, of if you do not want the FireLine to transmit on a channel that is
busy, you can enable “Busy-Channel-Lockout”, using the ATBC 1 command.
ATBC 0 disables BCL, and thus the modem will transmit whenever it has data
to send out. The factory-default is BCL disabled. Use caution when enabling
it, as a CW interferer, PC with poor shielding, or some other source of RF can