TS4000 Radio Modem User’s Manual
AirNet Packet Protocol
40
A master-slave system is one where the host application is designed so that only
one node will ever attempt to transmit at a given time. An example of this type of
system is a polled system with a base station that sequentially poles a number of
remote nodes. In this case the base always initiates a pole and the remotes
respond with the desired data.
Master-Slave
System Setup
To set up AirNet for this type of system, select the Master-Slave selection in the
Packet General tab of the modem configuration. With this selection, the modem
transmits waiting packets as soon as it detects an idle channel. The master-
slave setting should not be used with systems that use store and forward
repeaters.
The packet timeout timer is used for only for individually addressed packets that
expect an acknowledgment (ACK). The packet timeout timer is started after a
data packet is sent. If an ACK is not received before the timer expires, then a
retry transmission of the data packet is sent. This timer should be set longer
than the worst case time it takes to receive an ACK packet.
Setting Packet
Timeout
For a master-slave system, an ACK packet is sent as soon as the data packet is
received and the channel is idle. This can start as soon as the decay time of the
originating modem is finished.
Packet Timeout Time = Decay Time + Attack Time
+ ACK Packet Transmit Time
Where:
Decay Time = Tx Decay Time + Additional Transmit Attack Time
Attack Time = Tx Attack Time + Additional Transmit Attack Time
Tx Decay Time and Tx Attack Time are fixed values that are preset
for the radio in the TS4000. These values can be read out of the
TS4000 using the Retrieve Radio Hardware menu or button. The
Additional Transmit Attack Time is the value set on the radio tab of
the modem configuration.
ACK Packet Transmit Time = ACK Packet Length / Modulation Rate
An ACK packet fits in one data frame (16 bytes) of data. If coding is
used then 50% coding overhead is added to this.
ACK Packet Length
-Uncoded = 16 bytes x 8 bits per byte = 128 bits
-Coded
=
128
bits
x
1.5
=
192
bits
Tx Attack Time = 20 ms
Example
Tx Decay Time = 12 ms
Additional Transmit Attack Time = 0 ms
Over air channel rate = 9600 bps
Coding = Enabled
ACK Packet Transmit Time = 192 / 9600 = 20 ms
Packet Timeout Time = 12ms + 20 ms + 20 ms = 52 ms