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I've been trying to make a graph of the estimated RTT in the TCP protocol, and the formula to get the estimated RTT is:

EstimatedRTT1 = (1- α)*EstimatedRTT0 + α*RTT

where EstimatedRTT1 is the value of the current estimation of the RTT and EstimatedRTT0 is the value of the previous estimation, RTT is the round trip time for a determined packet in a precise moment and α is just an tipical value that equals 0.125 . We assume that the first value for EstimatedRTT0 is equals α*RTT, and afterwards EstimatedRTT0 is EstimatedRTT1 of the previous RTT.

Example:

RTT = 23ms
α = 0.125
EstimatedRTT0 = RTT*α = 23 * 0.125 = 2.875

EstimatedRTT1 = (1- α)*EstimatedRTT0 + α*RTT = (1-0.125)*2.875 + 0.125*23
EstimatedRTT1 = 5.390

then the next estimation goes like this:

RTT = 30ms
EstimatedRTT0 = 5.390 (last EstimatedRTT1)

EstimatedRTT1 = (1-0.125)* 5.390 + 0.125*30 = 8,466

and so on..

The graphic should look like this:

Example: estimated RTT graph

The problem I'm having is that I can't do the graph with Minitab, because I can't find the way to put in a column the result of this formula applied to a column with RTT values.

If I were to put the result of the first operation with EstimatedRTT0 = α*RTT, then every cell in the column would need the value of the previous cell, in order to calculate its own value.

I've been wondering if it is even possible to do such a thing with Minitab..

So, can it be done? Is there any software capable of doing this, or should I try to make a little program to output the result of the formula with a little function?

I'm adding the minitab file with the RTT samples. MPJ file

Thanks,

and sorry if I didn't explain myself very clearly.

TheLaser
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2 Answers2

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Found the way to do it with Excel.

You just have to input the formula in a cell, and then copy that cell in the lower cells, and it copies the formula but it replaces the objective cells with the ones relative to the new cell.

In this cell I input the formula

Then I copy the contents of the cell, and we can see how the parameters of the formula have changed

It was pretty easy after all.. It was a matter of trying things out.

TheLaser
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you can do it with a DO ... ENDDO and using constants. You have to switch the constant values before the ENDDO and you store your result into a cell (like C1[k1]=EstimatedRTT; where k1 is the constant of your DO ENDDO) ex:

let k2 = 23 'RTT=23ms
let k3=0.125 'α = 0.125
let k4= k2*k3 ' EstimatedRTT0 = 23 * 0.125 = 2.875
DO k1=1:10 ' 10 iterations
let k2 = C1[k1] ' k2=RTT and RTT values are stored into column C1
let k5=(1-k3)*k4+k3*k2 'EstimatedRTT1 = (1- α)*EstimatedRTT0 + α*RTT
let C2[k1]=k5 'you store your result in C2
let k4=k5 'you switch the constant value to use it in the next iteration
ENDDO
Patinber
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