1. An earth station transmits to a satellite 1000-bit information-carrying frames at speed of 1 Mbps through the uplink channel. The satellite transmits back to earth station on the downlink channel acknowledgement frames. The satellite acknowledges every frame it receives.Thepropagationdelayofeachchannel(uplink,downlink)is270msec. Thesize of acknowledgement frames is small, and you can assume transmission time of acknowledgement frames is zero. Processing time of received information frames by satellite and acknowledgement frames from earth station is assumed zero. Assume that all information frames and all acknowledgement frames arrive undamaged (no loss or damage occurs).
What is the maximum utilization for:
a. Stop-and-wait flow control?
b. Sliding-window flow control with window size of 127?
c. Sliding-window flow control with window size of 255? d. Sliding-window flow control with window size of 1023?
2. Multipoint topology should be used when (choose the one that you consider correct and explain briefly your choice) You have to explain your selection.
i.there are few ¡°busy¡± terminals.
ii.there are many terminals, each producing ¡°thin¡± traffic.
3. Station A sends information to station B. The communication channel, connecting the 2 stations, consists of 10 optical links. Neighboring links are connected with each other through repeater. The impairments present in each link produce a bit error rate (BER) equal to 10-10. What is the BER you expect to measure when comparing the bit sequence received by station B with the bit sequence transmitted by station A?
Provide clear explanation of your answer in order to receive marks.
4. Assume the use of the stop-and wait protocol. If an information frame is successfully received by the destination (it is not damaged or lost) with probability 0.8, what is the mean number of transmissions per frame? Assume that all acknowledgement frames (sent from the destination) are received and are decoded correctly by the sender.
Clearly present your solution/calculations to receive marks.
5. A sender is sending information bits in blocks of size K. The information goes through an error-prone channel to reach the receiver. An error correction/detection code is used at the transmitter and the receiver uses the corresponding decoder.
The encoder adds N redundancy bits to the K-bit long information , generating a codeword of length (K+N)-bit long.
The code can correct all single errors and detect up to 3 errors. The probability a bit is received in error due to channel impairments is PB. The occurring bit error events are independent from each other, i.e. the event {¡°bit i¡± is in error} is independent of the event {¡°bit j¡± is in error}, where ¡°bit i¡± and ¡°bit j¡± belong to the same
frame .
Calculate the probability:
(a) the decoder produces a correct output.
(b) The probability the decoder fails to correct errors or identify that errors have occurred.
6. For generator sequence G=110011 and information sequence M=11100011, write the Generator and Message polynomials and find the CRC.
7. A binary signal is transmitted using Angle modulation. You are located at the receiver and you observe the modulated signal using an oscilloscope. Can you identify if Frequency
Modulation (FM) or Phase modulation (PM) is used? Explain how. You have to provide clear explanation in order to receive marks.
8. Consider a geostationary satellite-based system. Earth stations are multiplexing digital information streams and they transmit them to the satellite. Due to the very long distance of the satellite is from the surface of Earth, you wish your earth station transmitters and the satellite transmitter to operate their amplifier at its maximum power output level. If you were the designer of the system, would you select to use Frequency Division Multiplexing (FDM) or Time Division Multiplexing (TDM)?
Clearly justify your selection in order to receive the marks.
9.
You want to build a low-cost baseband digital communication system. To do so, you want to use very low-cost clocking devices, which can experience significant shifts, thus run out of synchronization if they are not resynchronized frequently. The link connecting sender and receiver is wideband (you have no problem with bandwidth), however it has poor spectral characteristics at frequencies close to 0 Hz (very low frequencies). Your have to select among Non-Return to Zero-Invert (NRC-I), Bipolar Alternate Mark Inversion (AMI), and Differential Manchester, when it comes to digital signal format you will use. Make your selection and explain the logic behind your selection.
10. Provide all layers of the OSI and TCP/IP protocol architectures, starting from the lowest layer, proceeding to the higher one. Compare the 2 different architectures. Give advantages and disadvantages.
11. Explain what multipath propagation is, how it is created in mobile user environments and demonstrate how/why it damages the telecommunication signal. You can use figure(s) to support your answer.
12. A transmission channel between two communicating stations is made up of three sections. The first introduces an attenuation of 16 dB, the second an amplification of 20 dB and the third an attenuation of 10 dB. Assuming a mean transmitted power level of 400 mW, determine the output power level of the channel.
The overall loss is -16+20-10=6 db
Then, 6=10log10(400/P), P=400/(10^(6/10))=100.475 mW