Question 2. Consider the state-transition-diagram for the Stop-and-Wait ARQ in Fig. 5.12. Assume that the transmission times of a frame and an ACK are t_f and t_a sec. respectively and processing and propagation delays are negligible. Let P_f denote the probability that an information frame will be received in error in a transmission and P_a probability that an ACK will be received in error. Determine, Probability distribution of the number of times that the frame will need to be transmitted for the system to make the transition from state (0, 0) to (0, 1). Probability distribution of the number of times that ACK will need to be transmitted for the system to make the transition from state (0, 0) to (1, 1). Average time that it will take for the system to make the transition from state (0, 0) to state (1, 1).

296 CHAPTER 5 Peer-to-Peer Protocols and Data Link Layer 0,1,0,1,0,1,0,1, 0,1,0,1,0,1,0,1 Rnext Slast Timer Slast Transmitter Receiver Rnext Error-free frame 0 arrives at receiver (0,0) (0, 1) Global state: (Slast, Rnext) ACK for frame 1 arrives at transmitter ACK for frame 0 arrives at transmitter (1,0) (1,1) Error-free frame 1 arrives at receiver FIGURE 5.12 System state information in Stop-and-Wait ARQ. by process B, and an acknowledgment with Rnext = 1 is resent. Eventually process A receives an acknowledgment with Rnext = 1 and then begins transmitting the next frame, using sequence number Slast = 1. The system is now in state (1,1). The transmitter and receiver are again synchronized, and they now proceed to work together on the transfer of frame 1. Therefore, a protocol that implements this mechanism (that follow the well-defined sequence of states shown in Figure 5.12) can ensure the correct orderly delivery of frames to the destination. THE PROTOCOL Let us review the operation of the Stop-and-Wait protocol. The protocol is initially with the transmitter in the ready state, the transmitter send-sequence number Slast to 0, and the receiver sequence number Rnext set to 0. protocol is initialized Transmitter: transmitter in the ready state waits for a request for service from its higher request occurs, a packet is received from the higher laver and the trans its higher layer. 11