ECE4603 Fall'00 Homework Assignment #3 (v.1.0)
Due before 8:00 p.m. Wednesday 10/11/00
Instructions. Copy this document from the Web Page, or save it from
your email program, as a "text" file. Edit it in a word processor to
add the answers into the square brackets after each question. Save it
as a "text" file, and email it back to me:
(john.copeland@ece.gatech.edu).
Email the completed document as the body of a message (not as an
attached document) with the Subject exactly as "HW-3";. Please send
questions or comments in a separate message, with a different Subject
(e.g., "Question on HW-3").
I will not accept paper returns. If you can not complete the assignment on
time, tell me why and turn it in as soon as possible for partial credit.
Remember, home work grades count as 10% of the final grade.
Your return will be graded by a computer program that looks for your
answers between square brackets. Please do not add or delete square
brackets (or the ***???*** flags). The format and units of answers
should be those indicated in the problem (e.g., a letter, a group of
letters, or a number). Each question counts equally. Each answer within
a question counts equally, but the value depends on the number of
answers within the question.
See Tips on
Submitting HW for more information.
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***START_HW*** (do not delete preceding flag, or the -XXXX flags
below).
[ ]-NAME Enter your name (form: last, first middle initial)
[ ]-PRISM Enter your Prism account code (form: "gtXNNNA" where "N"
is a number, "A" is a letter, and X is either. Do not include
"@prism.gatech.edu").
[ ]-EMAIL Enter the email address ("account@server") where you
would like to receive your graded homework. If on Prism, you may leave
completely blank).
#1. Edit the following ASCII character string so that shows the
transmitted characters to be framed by Flag sequences with the normal
DLE "escape-character" (or "quoting character") stuffing to protect the
flags (DLE STX, DLE ETX). Then add the Flags.
X Y DLE DLE ETX P D STX R . DLE STX - data (copied below for you to edit)
[ X Y DLE DLE DLE DLE ETX P D STX R . DLE DLE STX ] Add DLE quoting characters.
[ DLE STX X Y DLE DLE DLE DLE ETX P D STX R . DLE DLE STX DLE ETX ] Add framing
characters.
Now reverse the process like the receiving node would do.
[ X Y DLE DLE DLE DLE ETX P D STX R . DLE DLE STX ] Delete the framing characters.
[ X Y DLE DLE ETX P D STX R . DLE STX ] Delete the DLE quoting characters.
#2. Edit the following bit string so that it shows the transmitted bits
in the data field of a packet to be framed by Binary Flag sequences with
the normal bit stuffing to protect the flags. Then add the Flags.
01111110011110011111110 - data (copied below for you to edit)
[ 0111110100111100111110110 ] - Stuff 0's to eliminate false flags.
[01111110011111010011110011111011001111110] - Add the flags to make a "packet" or "frame."
#3. Match the answers and questions below (put the letter in the square
brackets). Not all answers are used, but none should be used twice.
A. Scrambler
B. Altenate Mark Inversion (AMI)
C. Full Duplex
D. RS-232
E. Congestion Control
F. Flow control
G. Coding rule violation.
H. Framing
[E|F] Used to tell a distant computer to slow down. (F IS MORE CORRECT)
[ G ] Framing Technique not mentioned in problem 1 and 2.
[ A ] Used to prevent long strings of zeroes or ones.
[ B ] Modulation technique that keeps the DC level near zero.
[ H ] Technique used to tell when a packet starts and stops.
[E|F] Way for a computer to tell a distant computer to pause sending.
(F IS MORE CORRECT)
[ D ] Electrical and mechanical specification for connecting nearby units.
#4. Sliding-Window Protocol (n = 8, W = 7)
Station A transmitted frames "6 7 0 1 2". B's last acknowledgement was "7".
[ 3 4 5 6 | 3 4 5 ] (2nd answer is if "7" was the number in the ACK)
What are the next frames that can be sent without another acknowledgement.
(separate frame numbers with spaces or commas)
[ 7 0 1 2 | 6 7 0 1 ] What additional frames can be sent if B acknowledges 3?
(2nd answer is if "7" was the number in the ACK, meaning "6 received")
#5. Go-back-n Protocol
Station A transmitted frames "6 7 0 1 2". B sends NACK 0 (negative ACK).
[ 0 1 2 ] What are the next three frames that A sends?
#6. Selective-Reject Protocol
Station A transmitted frames "6 7 0 1 2". B sends NACK 0 (negative ACK).
[ 0 3 4 ] What are the next three frames that A sends?
#7. Stop and Wait Flow Control Utilization (hint - see p.227 in text)
[ 1200 ] What is the time to send a frame of 1500 bytes at 10 Mbps (in microsec (us)).
(this is t-frame) = ( (8 * 1500) / 10 b/us)
[15,000] How long does it take for a bit to travel 3000 km (at 200 m/us) (in us).
(this is t-prop) = 3,000,000 m/ (200 m/us)
[ 0.0385 ] What is the utilization factor for a stop and wait protocol with these
parameters?
= 1200 us / (1200 + 2 *15,000) us = 0.0385
[ 384,615 ] What is the average throughput (rate x utilization) in bits/second?
10 b/us * 0.08 = 0.8 b/us = 800,000 b/s or = (8*1500 b)/(1200+2*15,000)
#8. Sliding-Window Utilization (use same parameters as 7, and W = 7) (p. 229).
[12.5] What is the "a" factor (see bottom p. 227).
a = t-prop / t-frame = 15000 / 1200 = 12.5
[ 7 ] How many frames can be sent without receiving an acknowledgement (ACK).
W = 7 (given)
[0.269] What is the utilization factor?
= W/( 1 + 2a) =7&/(1 + 2 * 12.5 ) = 7/26 = 0.269 (W * Stop & Wait's value)
[ 2,690,000 ] What is the average throughput (bits per second).
#9. An amplifier has a gain of 20dB.
[ 100 ] What is the ratio of ouput power to input power?
Rp = 10^(20dB/10) since Power dB = 10 * log( Rp )
[ 10 ] What is the ratio of output voltage to input voltage?
Rv = sqrt(100) or 10^( 20dB/20) since Voltage dB = 20 * log( Rv)
***END_HW*** (do not delete preceding flag) (1.0 - web)