Engi 4557: Data and Digital Communications - Fall 2017
- Midterm exam solutions.
- Practice Problems - Part
2. Answers to selected problems:
1-a) , 1-b) bit 4, 1-c) 2.1e-11
2-a) H has 5 rows and 9 columns, 2-b) there are 16 codewords, min
weight is 3 for data bits = 0100, 2-c) can FEC 1 error, same as
Hamming code because dmin=3.
3-a) s =  error detected, 3-b) dmin=4 (after listing
codewords), 3-c) 1.4e-23
4-a) k=3, n-k=4, 4-b) b6 = b1+b2, 4-c) dmin=4 (after listing
codewords) so it can be used to FEC up to 1 error, 4-d) 7e-20
5-a) 15.6 Mbps, 5-b) max 5.06 Mbps, min 4.94 Mbps
6-a) 0.71 bits, 6-b) max 10.65 Mbps, min 9.64 Mbps
11-a) 169 channels, 11-b) for a>100, due to mux. gain only 20%
higher number of trunks is needed for Pb less than 1%. Therefore
169 is plenty for low Pb, 11-c) 20 calls/min
14-a) 2.21 Mbps, 14-b) Since (1-Pf) is close to 1, the effective
bit rate increases with nf. For example with nf=5000 bytes the
efficiency increases to 83% (from 74% in (a)), 14-c) Ws=2 is
enough, Reff=2.9 Mbps
16-a) Ws=42, 16-b) 23.8 Mbps
18-a) Ws = 8, 25.1 Mbps, 18-b) at least 5 bits
19-a) huge frame 200e3 bytes, 19-b) Ws=50, 57.8 Mbps
20) nf=160e3 bits
21) 225 bits
22-a) 1183 frames/s, 22-b) 427.9 frames/s, 22-c) polling is
23-a) CSMA-CD is better, 23-b) use 45000 bytes, 89 Mbps
25-a) need a huge frame with 105e3 bytes for rho=0.95, 25-b)
Slotted Aloha is better 36.8 Mbps
27-a) 0.984, 27-b) 0.833, 27-c) 0.996
29) 7.128 ms
- Final project presentation guidelines: Each student is expected
to present for approximately 7 minutes. Total presentation time is
at most 24 minutes for groups of 3 students and at most 16 minutes
for groups of 2 students. The rest of the time is allocated for
- Tuesday, November 28
- 4:00 pm Group 1: GFSK modulation
- 4:25 pm Group 2: Deflate Compression
- 5:00 pm Group 3: Digital Filters
- Thursday, November 30
- 4:00 pm Group 4: JPEG decoding
- 4:35 pm Group 5: Error control
- Project progress presentations will be on Thursday, November
16. Please see Tutorials and
Assignments for details. Monday, November 13 will be a regular
- Next quiz is Monday, November 6th.
- Practice Problems - Part
1 for midterm. Answers for some problems: 1-a) Use PCM, SNR=38
dB, 1-b) 240 kbps; 2) You have to assume some baseband encoding,
assuming NRZ: baseband bandwidth is 64 kHz (per channel), after
multiplier (and same for QAM signal) is 128 kHz centred at 850 MHz;
3-a) 1110010 (CRC is 10), 3-b) yes because g(1)=0 can not be a
factor of an e(x) with odd number of terms since e(1) must be 1;
4-a) need 7 bits, 4-b) 240 kbps; 8) Shannon's channel capacity is
151 kbps, so not possible to achieve 1 Mbps; 10-a) each channel
needs 250 kpulses/s, using raised-cos pulses needs 250*1.25/2 kHz
baseband bandwidth. QAM signal bandwidth is 312 kHz, 10-b) 15 dB.
- Midterm Date: Tuesday, October 24th
- Quiz 2 date: Monday, October 2nd
- Quiz 1 date: Monday, September 25th
- Form project groups and think about project topics by next
Tutorial session (September 18th).
- Office hours: Tuesdays/Thursdays from 2:30pm to 3:30 pm.
- Tutorial times: Mondays from 4:00pm to 5:30pm in AT 4019.
- Lecture times: Tuesdays/Thursdays from 4:00pm to 5:30pm in BB 2006.
Course Outline link
Tutorials and Assignments
Link to Tutorials and Assignments page.
Link to Class Notes Page
Link to Project information