4. A mobile phone service allows the user to enter a text message to be transmitted when the user presses a send button. If the cellular network accepts the message within a specified acceptable delay period, it sends an ACK message to the mobile. If the ACK message does not arrive on time, the mobile tries again. If it does not get an ACK message after n = 7 tries, the phone stops transmitting the message, and generates a busy signal. (a) Assume that all transmissions are independent, and the probability that a trans- mitted message will result in an ACK message returned on time is p. Find the PMF (probability mass function) of K, the number of messages transmitted in a call attempt? Explain (b) Find the probability that phone will generate a busy signal. Explain (c) As a designer, you want the probability of a busy signal to be less than 0.02. If p = 0.7, what is the minimum value of n necessary to achieve this goal. Explain