# Data link layer Sliding Window utilization formula query

In the data link layer, the Sliding Window is used for flow control.

There are two link utilization formula that can be use. They are :

U = 1, for N > 2a + 1

OR

U = N/(1+2a), for N < 2a + 1

In both formula, N denotes the window size and a = Propagation time / transmission time

I am not really understanding which formula to use in what situation although the condition is specified. What does 2a+1 actually mean? Can anyone help me to understand it in simple term. Thanks!

• What is $a$? And no, you can not use one or the other; they are one formula, defining $U$ in a piecewise fashion. – Raphael Nov 14 '16 at 19:47