Hello Juan
Yes, you got it. Remember the structure of the DS field:
Classes range from 1 to 4 with the following possible values of the class bits:
- Class 1: 001 (1 in decimal)
- Class 2: 010 (2 in decimal)
- Class 3: 011 (3 in decimal)
- Class 4: 100 (4 in decimal)
Drop probability ranges from 1 to 3 with the three drop probability bits having these possible values:
- Low: 010 (2 in decimal)
- Medium: 100 (4 in decimal)
- High: 110 (6 in decimal)
Now when the AF notation is used, we have AFXY, where X is the class and Y is the drop probability. However, the value used for X and Y is not intuitive. The values for X range from 1 to 4 which are the decimal values of the three-bit class field. However, the values for Y range from 1 to 3, where we use 1 for Low, 2 for Medium, and 3 for High. But these values are not the actual decimal values represented by the bits which we see to be 2, 4, and 6. And this is the reasoning behind the use of the number “2” in the formula.
As for the number 8, as you said, we use it because we use the three bits, and the minimum first value is indeed 8.
So the 8x + 2y formula works because the “y” value is not the value in decimal, but the value that corresponds to Low, Medium, and High in the AF notation. That’s why we need the “2” there. The final bit is always 0 as you can see from the table in the lesson.
I hope this has been helpful!
Laz