ELG5374 EECS, University of Ottawa Fall 2017 1 ASSIGNMENT 1 Problem 1 Two monitoring stations,

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ELG5374 EECS, University of Ottawa Fall 2017 1 ASSIGNMENT 1 Problem 1 Two monitoring stations, S1 and S3, are collecting and transmit data to station S2, which acts as the collecting and data processing centre. The relevant position of the three stations is as follows: S1 —————L meters ————— S2 —————L meters ————— S3 All three nodes have the same RF coverage capability, it being M meters, with L < M < 2 L. Upon arrival of a frame, and assuming that collision has not occurred, S2 sends an acknowledgement frame of size Fack to the corresponding station. Assume the transmission speed is the same for all three stations, and it is equal to Rt bps. The frame generation (traffic) rates produced by S1 and S3 are 1 and 3 frames per second respectively, both following Poisson distribution. The frame sizes produced by these stations are F1 for S1 and F3 for S3 (numbers refers to bits). Fack is of insignificant size (it is considerably smaller compared to F1 and F3). The signal propagation velocity through the medium is C meters/sec. Assume a reference frame size Fr. Determine the loss rate S2 experiences for each of the traffic streams coming from S1 and S3, when: 1) F1= F3= Fr; (11 marks) 2) F3 = 2 F1 and F1 = Fr; (3 marks) 3) F3 = F1/2 and F1= Fr; (3 marks) 4) F3 = F1 = Fr//2 (3 marks) Clarification: RF coverage refers to the distance the signal of a station is strong enough to allow communication. In our case, we assume that if “Station A” is at a distance from “Station B”, larger from the RF coverage area of Station B, the signal of Station B cannot produce collisions at Station A. Problem 2 A communications system operates on the principles of Asynchronous transmission. Each character is transmitted in the form of a frame consisting of 15 bits (including the start bit). ELG5374 EECS, University of Ottawa Fall 2017 2 Assume that the synchronization at the start of the first bit could be early or late by at most 10% of the actual bit period, TB. Calculate the value of the normalized error Abs {(TS – TB ) / TB } that allows correct detection of the frame. TS represents the symbol period estimate at the receiver. Abs{ x } represents the absolute value of x. Assume that errors could occur only due to synchronization offsets. Include all steps in your solution. Problem 3 Why is not necessary to have NACK0 and NACK1 for stop-and-wait ARQ? (NACK: Negative Acknowledgment) Problem 4 Consider a polling network that consists of 4 secondary stations spaced 300 Kms apart (see figures below). By assuming that: • the line speed is 4800 bps, • polling and acknowledgment messages are 48-bits long, • the time that a secondary station takes to process a polling message is 20 msec • the propagation delay is 2 msec/100 Kms, • the time to process an acknowledgement is negligible, • in the case that a secondary does not have data to send, the secondary sends an acknowledgment message to the primary to let it know of this situation, • secondary stations do not have any data to transmit. Calculate the time needed to complete a polling cycle for both roll-call and hub polling. Consider two cases, one with the primary at one end of the network, the other with the primary in the middle of the network (see figures below). For both cases: a) draw your timing diagrams. b) provide the expressions for computing the time needed to complete a polling cycle (this is known as the walking time). c) explain your finding by comparing the numerical values for the two polling schemes for the two cases under consideration. ELG5374 EECS, University of Ottawa Fall 2017 3 P S S S S 300 miles 1 2 3 4 300 miles Case 1 P S S S S 300 miles 1 2 3 4 300 miles Case 2 Problem 5 We are given a bit transmission facility that transmits each bit correctly with probability (1 – BER) and incorrectly with probability BER, where BER = 10-6. The bit errors are independent. We send bits in groups of 7, and we add a parity bit so that the groups of 8 bits that we send always contain an even number of 1’s. What is the probability that the group of 8 bits arrives with transmission errors not detected by the parity bit?

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