Teledesign Systems Inc TS4000EH Manuel D’Utilisation

1/2

 

 
To finally calculate the transmission index we need to estimate the number of 
backlogged nodes (the number of nodes that may want to transmit at the same 
time).  The difficulty in estimating this value is that for most systems this number 
is dynamic and can change dramatically depending on what is occurring in the 
system.   
 
For systems where the backlog can vary, estimate the average number of 
backlogged nodes for the most common scenario and also estimate the 
maximum number of backlogged nodes that will ever occur.  If the average 
number of backlogged nodes is more than 1/4 of the maximum, then use the 
average as the backlog number.  Otherwise use 1/4 of the maximum as the 
backlog number.  The reason for this is that the system must operate under the 
worst case conditions.  If the backlog is set too low then under worst case 
conditions, there will be an excessive number of collisions and the system will be 
very slow.   
 
In general it is a good idea to set the transmission index higher than expected as 
opposed to lower.  This allows the system to more gracefully handle peak traffic.  
However, this also causes average efficiency to drop and packet delay time to 
increase. 
 
 

B) 1/4 Maximum Possible Number of Backlogged Nodes 

 
Calculation of the transmission index. 

Tx Attack Time = 20 ms 
Tx Decay Time = 12 ms 
Additional Transmit Attack Time = 0 ms 

 

Over air channel rate = 9600 bps 

 

Coding = Disabled 

 

Average Packet Size = 400 bits 

 

Average Backlogged Nodes = 10 

 

Maximum Backlogged Nodes = 100 

 

Slot Time = 30 ms 

 
 

Packet Length = (Data Bits + 112) x 1.1 = (400 + 112) x 1.1 = 564 

 

Packet Transmit Time = Packet Length / Channel Rate 

 

 

 

 

 

 

   = 564 / 9600 = 59 ms   

 

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 = (Packet Detection Ratio)

1/2

 = (0.33)

1/2

 = 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