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Pagina 1 di 10 Fundamentals of Communication Networks Prof. G. Maier Exam – 16th July 2019 Surname Name Student ID Total time 2:15 Ex1 (7 pt) Ex2 (6 pt) Ex3 (5 pt) Ex4 (6 pt) Ques (8 pt) 1 - Exercise (7 points) Consider the two alternative fixed-wireless systems represented in the figure, connecting two stations A and B located on the top of two mountains. The system at the bottom is a direct link, while the system on the top has an intermediate station X located exactly halfway that retransmits frames (operating in store-and-forward mode). The distance between A and B is d = 30 km. All transmitters operate at bitrate C = 320 Mbit/s. All data frames have length L = 1 kbyte, with negligible headers. ACKs are transmitted on frames of negligible length. A data block of size F = 7 kbyte is transferred from A to B (only ACKs are sent in the opposite direction). In both systems, frames are transmitted using an ARQ go-back-n protocol, with the following features: • In the single-link case, GBN-ARQ is active between A and B, with the minimum window size W that potentially allows continuous transmission • In the two-link case, there is a GBN -ARQ protocol active between A and X and another GBN -ARQ protocol between X an d B. On both links, the window size is W ’ = 4 frames. [hint: note that, as a consequence of the GBN-ARQ protocol, X will not retransmit frames arrived in X out-of-sequence. For both the systems: 1) In absence of transmission errors, draw the time diagram and compute the transfer time TGBN of the data block (from the beginning of the transmission of the first frame from A to the end of reception of the last frame in B); 2) Repeat 1) in case the second frame gets lost during the transmission between A and B (in the single -link case) and between A and X (in the…