User's Manual

4. Using the FireLine Modem – Packet Mode
This section describes the operation of the when it is in the Packet Mode of
operation. Packet Mode is the factory-default operating mode. It is the
easiest and most reliable mode of operation for a modem. Note: The
configuration of the FireLine is done when the FireLine is in the “Command
Mode”. Refer to Section 6 on page 25 for details on all of the available
commands and programmable features.
In Packet Mode, all transmissions are sent in bursts or packets, and contain
address, error detection, and error correction information. Date enters the
FireLine modem’s serial I/O port, and is stored in a buffer within the modem
until it is ready to be transmitted. Packetized operation has these advantages
over non-packet modems:
Packet Mode Advantages
1. Error Detection The modem uses a 16-bit CRC at the end of every
packet of data. The CRC is used to check the data for errors, and if there
are any errors, the data will not be passed onto the user.
2. Error Correction Automatic error correction may be used. FireLine
modems incorporate an optional ARQ method to re-transmit packets with
error, to ensure the user’s data is delivered error-free.
3. Addressing Packetized operation allows for a more versatile network
architecture, with source, destination, and network addresses. FireLine
uses a 16-bit address to identify data packets.
4. No Dribble Data Even in the presence of noise, the FireLine modem will
not output extra data or have random bit errors. Modems without packet
operation generally do not work well with weak noisy signals.
5. Transparent Operation Because of the high-reliability and error-free
operation the Packet Mode offers the user, most user applications will
seamlessly work using the FireLine in its Packet Mode.
Streaming Mode Advantages
1. Low Latency The transmitter will key-up immediately upon the user’s first
byte of data entering the modem. Packetized operation waits until a
packet has been loaded before keying. (Although high serial-port data
rates can minimize this packet latency to a negligible level).
2. Data with Errors The FireLine will continue to receive data, down into the
noise-floor of the radio. If the channel is noisy or the signal is weak, there
may be bit-errors in the data, or the FireLine may output additional noise
data. User applications must (and often do), take this into account, and
thus can operate with weaker signals and have longer communication
range.