Question: Consider a GBN-ARQ scheme. Packet data units arrive at the transmission buffer of the sender according to a Poisson process with rate λ packet/s. The transmission of a PDU takes one time unit and the corresponding acknowledgment is received N - 1 time units upon the completion of the PDU transmission. Packet data units can be retransmitted for two reasons:
1. A given PDU i can be rejected a the receiver due to errors in the forward channel. In this case the transmitter will send PDUs i + 1, i + 2,...,i + N - 1 (if these are available) and, after that, it will go back, retransmitting PDU i, i + 1, i + 2,... . We assume that the PDU error rate over the forward channel is pfwd and the error process is iid.
2. The acknowledgment message for any given PDU, transmitted over the feedback channel, can be erroneous as well. The acknowledgment error rate over the feedback channel is indicated as pfdb and the error process over this channel is iid.
Calculate the average service time for a given PDU entering the ARQ system. (Hint: the transmitter's queue can be modeled using the M/G/1 theory.)