TS4000 Radio Modem User’s Manual
AirNet Packet Protocol
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The CSMA MAC (Medium Access Control) is used for systems in which multiple
modems will attempt to access the radio channel simultaneously (multi-access
systems). If two modems attempt to transmit simultaneously, a collision results
which prevents both transmissions from being successfully sent. The AirNet
protocol uses CSMA/CA (Carrier Sense Multiple Access with Collision
Avoidance) to provide multi-access capability. The CSMA refers to monitoring
the channel to ensure that it is unused before transmitting a packet.
CSMA System
Setup
Collision Avoidance
For multi-access radio systems CSMA alone is typically not enough to prevent
excessive collisions. The problem occurs when one modem is transmitting and
multiple other modems receive data for their hosts and become ready to
transmit. These other modems will wait until the first modem finishes its
transmission and then they will all attempt to transmit simultaneously, resulting in
a collision. This creates the need for collision avoidance. The AirNet protocol
provides this by having modems randomize their transmissions once they detect
an idle channel. In each slot after a modem detects an idle channel, it will decide
with some probability (based on the Transmission Index) whether or not to
transmit. This does not eliminate collisions, but, if the probability is set correctly,
minimizes the collisions to allow for efficient multi-access use of the radio
channel.
Slot Time
The AirNet protocol uses timing slots to determine when to attempt
transmissions. These slots are slightly different from the slots used in
conventional multi-access slotted MACs. The AirNet slots are the minimum
channel detection times or the minimum time from when one modem begins
transmission to when all other modems will detect that transmission. This size
slot guarantees that modems waiting to transmit in consecutive slots will not
collide and allows for very efficient use of the radio channel.
The following is a summary of the suggested settings for a basic CSMA system.
A basic system does not have any store and forward repeaters. Note that more
detail on the parameters and equations can be found later in this section.
Basic System - Setup
Summary
Slot Time = Attack Time + Maximum Carrier Detect Time Variation
Slot Time
= 1.5 x Attack Time
Where:
Attack Time = Radio 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.
Min Idle Slots - ACK Packets
= 0
Min Idle Slots
Min Idle Slots - Data Packets
= 1
Tx Index - ACK Packets = 1
Tx Index
Tx Index - Data Packets = Estimated Backlogged Nodes / Attempt Rate
Where:
Attempt Rate = (Packet Detection Ratio)
1/2
Packet Detection Ratio = Slot Time / Total Packet Time
Total Packet Time = Attack Time + Packet Transmit Time + Decay Time
