User's Manual

TS4000 Radio Modem User’s Manual AirNet Packet Protocol 37
Total Packet Time = Attack Time + Packet Transmit Time + Decay Time
= 20 ms + 59 ms + 12 ms = 91 ms
Packet Detection Ratio = Slot Time / Total Packet Time
= 30 ms/ 91 ms = 0.33
Attempt Rate = sqrt(Packet Detection Ratio) = sqrt(0.33) = 0.57
Since: Max Backlogged Nodes / 4 > Average Backlogged Nodes
Estimated Backlogged Nodes = Max Backlogged Nodes / 4
= 100 / 4 = 25
Transmission Index = Estimated Backlogged Nodes / Attempt Rate
= 25 / 0.57 = 44
The packet timeout timer is used for individual packets that expect an
acknowledgment (ACK). This 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 typical
amount of time it takes to receive an ACK packet.
The following calculations are for systems that are setup so that ACK packets are
sent immediately after the data packet transmission is completed without
contending for the channel (see Setting Min Idle Slots). For this type of CSMA
system the packet timeout time is the same as for a Master/Slave system. The
ACK is sent as soon as the decay time of the sending modem is finished.
Packet Timeout Time = Decay Time + Attack Time
+ ACK Packet Transmit Time
Decay Time = Tx Decay Time + Additional Transmit Attack Time
Attack Time = Tx Attack Time + Additional Transmit Attack Time
ACK Packet Transmit Time = ACK Packet Length / Channel 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
The following calculations are for systems that are setup as described in the
Setting Min Idle Slots section. The packet timeout should be set to the amount of
time it takes to send the data packet and then the amount of time it takes to get
back an acknowledgement.
Packet Timeout = Relay Delays for Data Packet
+ Ack Packet Delay at Destination Node
+ Relay Delays for ACK Packet
The amount of time it takes to send a data packet is the sum of the amount of
time it takes each relay to send the data packet.
Relay Delays for Data Packet = Relay #1Data Packet Delay
+ Relay #2 Data Packet Delay
...
...
+ Relay #Y Data Packet Delay
Setting Packet
Timeout
Systems without Relays
Systems with Relays