ECE 457 Fall 2009 Midterm 1 Solutions

ECE 457 Fall 2009 Midterm 1 Solutions - CS/ECE 457 Fall...

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Page 1 of 4 CS/ECE 457 Fall 2009 Mid-term examination 1 solution Instructions: •N u m b e r o f p r o b l e m s : 9 •T o t a l p o i n t s : 1 5 p o i n t s Problem 1. (1 point) [Check one answer in each of these multiple-choice questions] 1. Which of these tasks is not performed in a shared single-link network? a. Transmission and reception of data bits b. Multiplexing Xc .S w i t c h i n g d. Error control 2. Which of these tasks is not included in the data-link layer grouping of tasks? Xa .M o d u l a t i o n b. Error control c. Flow control d. Multiplexing 3. The MAC sub-layer of the data-link layer consists of tasks related to a. transmitting bits on a channel Xb .s h a r i n g a c o m m u n i c a t i o n l i n k c. correcting errors d. detecting errors 4. A bidirectional (full-duplex) link is served by a. a transmitter at one end and a receiver at the other end of the link .a t r a n s m i t t e r a n d r e c e i v e r a t b o t h e n d s Problem 2. (1 point) The SNR of a received signal is 30dB, and the bandwidth of the communication channel is 1GHz. What is Shannon’s chan- nel capacity? [Do not forget the unit.] Answer: or ~10Gbps SNR is 30dB. Since , is 1000, since . Given that W=1GHz, we com- pute Shannon’s channel capacity as Problem 3. (3 points) The physical-layer delay to send a file of size 10KB across an optical fiber link is . The speed of light in fiber is . The transmission (emission) delay is . a. What is the dielectric constant of fiber? Answer: 1.75 9.97 Gbps SNR dB  10 SN 10 log = 1000 10 log 3 = CH 1 S N --- +   2 log 1 1 1000 + 2 log 1 1001 10 log 2 10 log ----------------------------- 9 . 9 7 == 9.51 s 2.27 10 8 ms 8.192 s
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Page 2 of 4 ; and . Therefore: b. What is the transmission rate? Do not forget the unit in your answer. Answer: 10 Gbps ; Therefore c. What is the length of the link? Do not forget the unit in your answer. Answer: 300m The physical-layer delay is emission delay + propagation delay; Therefore, propagation delay is . As propagation delay is , and , or ~300m Problem 4. (2 points) [The two parts are unrelated.] a. An information bit stream 10011101 is encoded with a CRC scheme using a generator polynomial prior to transmission. What is the transmitted bit string (codeword)?
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ECE 457 Fall 2009 Midterm 1 Solutions - CS/ECE 457 Fall...

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