User's Manual

TS4000 Radio Modem User’s Manual AirNet Packet Protocol 31
Tx Attack Time = 20 ms
Additional Transmit Attack Time = 0 ms
Over air channel rate = 9600 bps
Number of async chars in packet = 50
Number of data bits per async char = 8
Coding = Enabled
Packet Length = ((50 x 8) + 112) x 1.1 x 1.5 = 845 bits
Packet Transmit Time = 845 / 9600 = 88 ms
Total Packet Delay Time = 20 + 88 = 108 ms
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.
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 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 relays. Note that more
detail on the parameters and equations can be found later in this section.
Slot Time = Attack Time + Maximum Carrier Detect Time Variation
= 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 configuration menu or button. The
Additional Transmit Attack Time is the value set on the radio tab of
the modem configuration.
Example:
CSMA System
Setup
Basic System - Setup
Summary
Slot Time