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Data Communication [CS 601]
Spring 2007
Assignment No. 4
Total Marks: 20
Dead line: May 28, 2007
Do your own work. If two solutions are found similar, no credit will be allocated to any
of the students. Write to the point and brief answer of the following question. No
Irrelevant material will be entertained.
Solution file
Q-1: Differentiate between Pulse code and pulse amplitude modulation? Please
concise and brief explanation. (10)
Solution:
Actually pulse amplitude modulation is the first step of pulse code modulation. We called
analog to digital conversion as pulse code modulation.
The complete Pulse code modulation has four steps.
1. Pulse amplitude modulation.
2. Quantization.
3. Binary conversion or encoding.
4. Digital to digital encoding.
1. Pulse amplitude modulation.
The first step in PCM. In this process the signal is sampled it mean it is divided into
specific and equal part or samples and then we create a series of pulses, based on the
result of sampling. Sampling means measuring amplitude of signal at regular interval
.we use sample and hold mean signal level is read and then held briefly. The pulses
are of any amplitude and still analog and to convert them to digital we jump to the
next step of quantization.
And also according to Nyquest theorem sampling rate must be at least twice the highest
frequency of the carrier.
2. Quantization:
Quantization is the assigning of integral value in a specific range to sampled instances.
This assigning process is according to well defined prepared scale on y-axis and plotting
the signal sampled on x-axis. Each instance has now a value in decimal either positive or
negative.
2. Binary encoding or conversion:In this step we convert the decimal digits into their equivalent binary digits simply
nothing more then this. Such as shown in the diagram below.
3. Digital to digital Encoding:And then we can use any type of d to d encoding scheme uni polar, polar and
bipolar….
In uni polar we use one voltage level (+) to represent 1 or 0 and zero voltage to
represent to represent 0….
In polar two voltage levels to ----In bipolar we use (+), (-) and zero voltage to represent binary digits.
Q2:-Whether null modem is a device or technology? Explain briefly?
Solution:
Null modem is a technique or technology it is not a physical device. We apply this
technique whenever there is a direct connection between two DTE’s. In this technique we
cross the sending and receiving pins in order to stop any data collision. Just as EIA 232
DTE-DCE cable does. The solution is a NULL Modem. A null modem provide DTA –
DTE interface w/o DCEs .But why use a Null Modem. If all you need is the interface,
why not just a standard EIA 232 cable. a null modem has female connectors at both ends
to allow it to connect to the EIA 232 DTE ports which a male.
The diagram show clearer picture. See the crossing sending and receiving pins.
(Note: Answer may not be more then 7 lines)