TS4000 Radio Modem User’s Manual
AirNet Packet Protocol
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Min Idle Slots - Relay #N
= N
Min Idle Slots - Data Packets
= Highest Relay # + 1 = N + 1
Tx Index
Tx Index - Relays (All)
= 1 (Always transmit in their assigned slot)
Tx Index - ACK Packets
= 1 (Always transmit in the first slot)
Tx Index - Data Packets
= Attempt Rate (see Setting Tx Index)
The transmission index (TI) is the inverse of the probability of transmitting in an
idle slot. A TI of 10 corresponds to a 1/10 = 10% chance of transmitting in an
idle slot. The goal of setting TI is to maximize efficiency on the channel. If TI is
set too low then transmissions collide too often. If TI is set too high then there
are an excessive number of unused slots.
Setting Tx Index
AirNet allows TI to be set differently for each packet type (data, ACK or relay).
For most systems, TI is set to 1 for ACK and relay packets (see Setting Min Idle
Slots). The setting of 1 corresponds to always transmitting (100% probability) in
a particular slot.
To set TI, the user must make some practical estimates and then do some
calculations based on these estimates. First it is necessary to estimate the
average data packet length. To do this, estimate the average number of data
bits in a packet using the following formulas.
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 }
With this average packet length number, calculate the packet transmit time.
Note that the formulas require the configuration values for transmit attack and
decay time.
Packet Transmit Time = Packet Length / Channel Rate
Total Packet Time = Attack Time + Packet Transmit Time + Decay Time
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.
Calculate the packet detection ratio, which is the slot time normalized to the total
packet time. Then, using packet detection ratio, calculate the attempt rate as its
square root.
Packet Detection Ratio = Slot Time / Total Packet Time
