Teledesign Systems Inc TS4000EH Manuale Utente

 

 

Using the values from the previous example, calculate the average delay for 
various backlogs. 
 

Tslot = Slot Time = 30 ms = 0.03 sec 

 

PDR = Packet Detection Ratio = 0.33 (from previous example) 

 

TI = Transmission Index = 44 (from previous example) 

 
 

PR = (TI - 1)/TI = (44 - 1)/ 44 = 0.977 

 

 

Average Delay = Tslot x (1 + PDR - PRN) = 0.03(1 + 0.33 - 0.977N)  

 

 

 

 

 

 

PDR x ln(1/PR)  

       0.33 ln(1/0.977) 

 

 

 

 

     = 0.03(1.33 - 0.977N) = 3.91(1.33 - 0.977N) 

      0.00768 
 
 Backlogged 

Nodes 

(N) 

10 

 25 

 50 

 75 

 100 

 Average 

Delay 

(sec) 2.1 

 3.0 

 4.0 

 4.5 

 4.8 

 

 

 
The probable delay calculation allows the user to calculate the expected delay 
given some probability that the transmission actually occurs.  The probable delay 
value can be used for calculating a packet timeout value for a system where the 
ACK packets do not use an immediate ACK and have a transmission index the 
same as the data packets.  It can also be used to calculate timeouts for layers of 
the protocol stack above the modem on the host system.  Note that the probable 
delay value does not include any transmission times due to repeaters and 
acknowledgement packets. 
 
The basis of the probable delay is the average delay calculated above.  As noted 
before, the average delay will vary based on the actual number of backlogged 
nodes in a system. 
 

 
Calculate the probable delay for various probabilities of sending in terms of the 
average delay. 
 
Probability 

of 

Sending 

(%) 

25 50 75 90 95 99 99.9 

Probable Delay (Avg. Delays) 

0.29 0.69 1.38 2.30 3.00 4.61 6.91 

 
Note that the 50% probability of sending value is not equal to the average delay.  
This is because the delay spread is a statistical distribution where the mean and 
median delays are not the same.