User guide

Global Positioning System (GPS)
Rev 2.3 Feb.12 211
Store and Forward Reliable Mode
The Store and Forward Reliable Mode allows the AirLink device to ensure all
messages are received by the server even if the connection between them goes
down for a period of time (such when a vehicle passes through a location where
the cellular signal is weak or non-existent).
With Reliable Mode, *PPSNFR, enabled, the AirLink device will transmit a
sequence number (1 to 127) as part of a packet of messages (may contain one or
more reports). To reduce overhead, the server only acknowledges receipt of
every eighth packet. The AirLink device considers that 8 a “window” of
outstanding packets.
If the AirLink device doesn’t receive acknowledgement for a “window”, the modem
will PING the server with a message containing the sequence numbers of the first
and last packets that haven’t been acknowledged. The AirLink device will
continue until the server acknowledges receipt. When the AirLink device receives
the acknowledgement, it will advance its “window” to the next group.
When the AirLink device is first powered on (or reset), it will send a Set Window
message to sync up with the server for the current “window”.
On the other side, if the server receives an out of sequence packet, it will send a
message to the modem noting the missing sequence and the AirLink device will
retransmit.
Simple Reliable Mode will ‘give up’ after a configured number,
*PPMAXRETRIES, of attempts and discard messages that cannot be transmitted
or received after that number of tries.
Sending Reports Based on an Interval
You can configure the AirLink device to send reports based on a time interval and/
or on the movement of a vehicle (based on it’s position from one time to the next).
Figure D-1: ACEmanager: *PPTIME, *PPDIST, *PPTSV, *PPMINTIME
• *PPTIME - Location report sent every set time interval (seconds).
• *PPDIST - Location report sent only if the position is more than the set
distance (x 100 meters)
• *PPTSV - Location report sent if the vehicle has been in one location
(stationary) for more than a set time interval (minutes).
• *PPMINTIME - Location report sent at no less than this time interval
(seconds).