User's Manual

TS4000 Radio Modem User’s Manual AirNet Packet Protocol 35
For systems with one or more relay nodes, the simplest and most efficient system
setup is where each relay is assigned a particular slot. This way the relays do not
collide or contend for the channel the way data packets do. The data packets are
set so that they will leave the necessary number of slots open for the relays and
ACK packets.
This type of setup has the advantage that the delay for sending data through the
relay(s) is consistent and predictable. This makes it much easier to set an
appropriate packet timeout (see Setting Packet Timeout).
Min Idle Slots - ACK Packets = 0
Min Idle Slots - Relay #1 = 1
Min Idle Slots - Relay #2 = 2
...
...
Min Idle Slots - Relay #N = N
Min Idle Slots - Data Packets = Highest Relay # + 1 = N + 1
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.
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
Systems with Relays
Min Idle Slots
Tx Index
Setting Tx Index