explain type I hybrid an ACK is received, the next codeword is sent. If an ARQ protocols that combine error correction coding and NACK is received, the same codeword is retransmitted ARQ techniques. Finally, we overview type II until it is received correctly, as in Figure 3.4-16. hybrid -ARQ protocols where the transmitter answers Stop-and-wait ARQ has a simple implementation. In a retransmission request by sending additional parity particular, the codewords are not numbered. Its major symbols. drawback is the idle time spent by the transmitter waiting for an ACK. 3.4.5.1 Pure ARQ protocols 3.4.5.1.2 Go-back-N In a pure ARQ system, an information block of length k is In go-back-N ARQ, the transmitter sends the codewords encoded into a channel codeword of length n with an continuously without waiting for an acknowledgment. error-detecting code. The codeword is sent over the Suppose that the acknowledgment for codeword Ci channel and the received word is decoded. If no errors arrives after codewords Ci,. Ci+N-1 have sent. are detected, the transmitted codeword is assumed to be this acknowledgment is of the ACK type, the transmitter received correctly and need not be retransmitted. sends codeword Ci + N. Otherwise, the codewords Ci, Otherwise, the codeword must be sent again until it is ci+N-1 are sent again, as in Figure 3.4-17. On the receiver received correctly. To send feedback information to side, when an error is detected in a received word, this the transmitter, the receiver can use a positive word and the N - 1 subsequently received ones are acknowledgment (ACK) to indicate that the codeword ignored. Note that a buffer for N codewords is required was received correctly or a negative acknowledgment at the transmitter side. Retransmission Retransmission Transmitter Packet Packet 2 Packet 2 Packet 3 Packet 3 Packet Packet 2 Packet 2 Packet 3 Packet 3 Receiver Error detected Error detected Figure 3.4-16 Stop-and-wait ARQ. CHAPTER 3.4 Channel protection fundamentals Retransmission (Go-back-3) Transmitter Packet 1