Calculate the total time to send data to n selected secondaries. We initially assume there are NO errors so ACK is used only to control the flow of data being sent. If there are errors then ACK is used only when data is received without error. Calculate total time to transmit n frames. Distances traveled are smaller in a local network, so propagation delays are not as high.
Data rates are also lower, so overall utilization is higher.
However, it is quite possible that a satellite link with a higher maximum data rate may still best suit one's needs. As a decreases, utilization increases. Each of these has disadvantages. Looking at figure 5.5, as the normalized a approaches 0, or as the propagation time is decreased, we have higher utilization. For example with 3 bits, frames are numbered 0,1,2, upto 7 i.e.