User's Manual

TS4000 Radio Modem User’s Manual AirNet Packet Protocol 34
Relay #Y ACK Packet Delay = Decay Time
+ (Y x Slot Time)
+ Attack Time
+ ACK Packet Transmit Time
ACK Packet Transmit Time = ACK Packet Length / Channel Rate
ACK Packet Length -Uncoded = 16 bytes x 8 bits per byte = 128 bits
-Coded = 128 bits x 1.5 = 192 bits
Decay Time = Tx Decay Time + Additional Transmit Attack Time
Attack Time = Tx Attack Time + Additional Transmit Attack Time
The slot time should be set to the attack time of the radio plus the maximum
variation (uncertainty) in the carrier detection circuit. The variation in the carrier
detection circuit is the difference in the carrier detect time between the radio with
the fastest carrier detect time and the radio with the slowest carrier detect time.
Note that the attack time is made up of the worst case transmitter power ramp up
time plus the worst case carrier detect time. Typically the maximum variation of
the carrier detect circuit is less than half (50%) of the attack time.
Slot Time = Attack Time + Maximum Carrier Detect Time Variation
= 1.5 x Attack Time
Attack Time = Tx Attack Time + Additional Transmit Attack Time
Tx Attack Time is a fixed value that is preset for the radio in the
TS4000. This value can be read out of the TS4000 using the retrieve
radio configuration menu or button. The Additional Transmit Attack
Time is the value set on the radio tab of the modem configuration.
The minimum idle slot setting defines the number of slots which a modem will
leave vacant after the modem detects an idle channel and before the modem
attempts to transmit. A setting of 0 means that the modem will begin attempting
transmission in the very first slot after the channel becomes available (idle). A
setting of 1 means that the modem will wait 1 slot after the channel is available
before attempting to transmit. The number of minimum idle slots can be set
differently for each packet type (data, ACK or relay).
The simplest and most efficient system setup is where ACK (acknowledgment)
packets are sent immediately after a valid data packet is received. With this
setup the ACK packets do not contend for the channel the way data packets do.
Correspondingly, the data packets are set so that they will leave the first slot open
for the ACK packets.
This type of setup has the advantage that the delay for receiving an ACK packet
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 - Data Packets = 1
Tx Index - ACK Packets = 1 (Always transmit in the first slot)
Tx Index - Data Packets = Attempt Rate (see Setting Tx Index)
Setting Slot Time
Setting Min Idle Slots
Systems without Relays