Teledesign Systems Inc TS4000EH Manuel D’Utilisation

 

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 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. 
 
 
Slot Time   = Attack Time + Maximum Carrier Detect Time Variation 
 

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. 

 

 
Tx Index - ACK Packets = 1 
Tx Index - Data Packets = Estimated Backlogged Nodes / Attempt Rate 
 Where: 
 

1/2