TS4000 Radio Modem User’s Manual
AirNet Packet Protocol
41
For a master-slave system, the data packet transmit time is constant for a given
packet size. As long as the channel is not busy, a data packet will be sent
immediately upon becoming available for transmission.
Data Packet Transmit
Time
Calculating the delay is very similar to the calculation for the packet timeout time
above.
Total Packet Delay Time = Attack Time + Packet Transmit Time
Where:
Attack Time = Tx Attack Time + Additional Transmit Attack Time
Note that the packet delay time does not include the transmit decay time. This is
because the packet is available at the receiving modem as soon as all the data is
transmitted.
Packet Transmit Time = Packet Length / Channel Rate
Packet Length = (Data Bits + Overhead Bits)
x Framing Overhead x Coding Overhead
Overhead Bits = 14 bytes x 8 bits per byte = 112 bits
Framing Overhead = 1.1
Coding Overhead (optional) = 1.5
Packet Length = (Data Bits + 112) x 1.1 { x 1.5 }
Tx Attack Time = 20 ms
Example
Additional Transmit Attack Time = 0 ms
Over air channel rate = 9600 bps
Number of async chars in packet = 50
Number of data bits per async char = 8
Coding = Enabled
Packet Length = ((50 x 8) + 112) x 1.1 x 1.5 = 845 bits
Packet Transmit Time = 845 / 9600 = 88 ms
Total Packet Delay Time = 20 + 88 = 108 ms
