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日期:2021-10-11 07:57

EECS 3213 - Communication Networks (Fall 2021)
Assignment-1
Due Date: 19th Oct 2021
Maximum Points: 5 Points
1) If the probability of a bit error is p, what is the probability of a single, double, and triple
error in a 10-bit word? Express it in terms of p.
2) Why is delay-bandwidth important in data communication?
3) Circuit switching is often considered to be susceptible to substantial overhead, when the
channel is not used continuously. At the same time, a different kind of overheads exists in
packet-switched networks, in particular, when they are used for voice transmission. What
are the causes of that, and what are the ways to deal with such overheads?
4) Assume that the number of code and quantization levels used for PCM in some system
is 16. The following sequence of bits are the PCM encoded digital data received by a
destination device (assume no errors).
0101001100110010010001111010101010111100
Draw the output (i.e., recovered) audio signal at the destination device.
5) Assume the signal to noise ratio (SNR) at the input of an observed channel is 25 dB.
We know that the signal attenuation of the channel is 10 dB. At the same time, the noise
power at the output is twice as much as the noise power at the input. Determine the signal
to noise ratio in dB at the output. Note: The channel does not attenuate the noise!
6) If we are transmitting digital data at the rate 30 kbps by means of a digital signal that can
assume 8 possible levels, what is the pulse rate of the resultant digital signal in bauds?
7) What can you say about the frequency spectrums of a train of impulses and a train of
square pulses? Name one key similarity and one key difference?
8) How many bits are in transit during the propagation delay in the cases of a building (100 m),
a metropolitan area (100 km), a continent (5000 km), up and down to a geostationary satellite
(2 x 36000 km), if bits are entering the above networks at the following transmission
speeds: 10,000 bits/second; 1 megabit/second; 100 megabits/second; 10 gigabits/second?
2
9) What universal set of communication services is provided by TCP/IP?
10) What difference does it make to the network layer if the underlying data link layer provides
a connection-oriented service versus a connection-less service?
11) Suppose transmission channels become error-free. Is the data link layer still needed?
12) Why not put the transport layer inside the network and not just at the edges?
13) Suppose the city is covered by a large number of small antennas covering smaller areas.
Is the data link layer or network layer more appropriate for this situation? Or would you
use both?
14) Suppose an application layer entity wants to send an L-byte message to its peer process,
using an existing TCP connection. The TCP segment consists of the message plus 20 bytes
of header. The segment is encapsulated into an IP packet that has an additional 20 bytes
of header. The IP packet in turn goes inside an Ethernet frame that has 18 bytes of header
and trailer. What percentage of the transmitted bits in the physical layer correspond to
message information, if L = 100 bytes, 500 bytes, 1000 bytes?
15) Suppose that at a given time of the day, in a city with a population of 1 million, 1% of
the people are on the phone. What is the total bit rate generated by all these people if
each voice call is encoded using PCM? 30. Suppose that a signal has twice the power as
a noise signal that is added to it. Find the SNR in decibels.
16) Repeat if the signal has 10 times the noise power? 2n times the noise power? 10k times
the noise power?
17) What is the maximum reliable bit rate possible over a telephone channel with the following
parameters?
? W = 2.4 kHz SNR = 20 dB
? W = 2.4 kHz SNR = 40 dB
? W = 3.0 kHz SNR = 20 dB
? W = 3.0 kHz SNR = 40 dB
18) Suppose we wish to transmit at a rate of 64 kbps over a 3 kHz telephone channel. What
is the minimum SNR required to accomplish this?
19) Suppose that a low-pass communications system has a 1 MHz bandwidth. What bit rate is
attainable using 8-level pulses? What is the Shannon capacity of this channel if the SNR
is 20 dB? 40 dB?
3
20) Suppose that a paper letter is sent by fax. Is this mode of communications connectionless
or connection-oriented? Real-time or non-real time?
21) Repeat the previous question for a case of a voice mail message left at a given telephone.
22) Given that IP packets always carry the destination address, how is it possible that a packet
to a non-existing host can actually be transmitted several hops through the Internet?

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