Exercise 7.1.
A receiver that implements the bit destuffing procedure of the HDLC protocol receives the 78-bit sequence (a point is inserted after each group of 10 bits): 0111001111.1100111111.0010111110.1110111110.1101011110.1111011111.1011111100.11111100. Si chiede di Identify the occurrence of flags, any stuffing bits entered by the transmitter and the possible occurrence of errors in the received string.

Exercise 7.5.
On a link between stations A and B, station B receives a frame whose content, after the removal of the flags and downstream of the bit destuffing operation, is the 15 bit string 101000110101010 (it is assumed that the fields address, control, information of a frame can have arbitrary length). Knowing that the divisor polynomial is D(x) = x^5 + x^4 + x^2 + 1, determine whether the received string indicates the occurrence of transmission errors or not.

Exercise 7.7.
Two stations A and B are connected by a bidirectional data transmission system. The line protocol, which controls the frame transmission on this link, is ARQ of type go-back-n. The exchange of frames between the two stations takes place with these hypotheses:
• the two stations operate regularly and the data connection has already been established, when station A sends the first frame at time t = 0,
• only station A sends frames to station B and the transmission buffer is never empty; therefore station B sends only acknowledgment frames (positive and negative),
• the transmission time of a frame and an acknowledgment is, respectively, T_f = 1 ms and T_a = 0.5 ms, the propagation time between the two stations is \tau = 0.6 ms, the time-out for frame retransmission is given by T_o = 4T_f,
• the transmission medium is subject to errors which can affect any type of frame; it is assumed that only one error occurs when exchanging frames,
• the sender window has the maximum possible opening based on the ARQ protocol features,
• the frame processing time Tp is considered null. Knowing that:
• station A sends 10 consecutive frames starting from t = 0,
• station B sends acknowledgments on the reception of all the frames except the third, it is requested to:
• draw the space-time diagram showing the exchange of frames by associating the type of IU to each frame/ acknowledgment followed by the numbering related to that type of IU (for example, the sequence I,2,3 indicates an information frame with N(S) = 2 and N(R) = 3), assuming that I,0,0 is the first frame sent by station A,
• determine the minimum opening W_{s-min} of the sender window and W_{r-min} of the receiver window,
• find the minimum number b of bits for the frame numbering consistent with the frames exchanged.

Exercise 7.15.
Determine the block code which provides the same coding as the AMI code, also providing the corresponding correspondence table.