# CS601-Data Communication midterm past papers

### CS601-Data Communication midterm past papers

Q. Write down the types of digital to analog modulation.
Types of digital to analog modulation
FSK
PSK
QAM

Q.Give methods of Analog to Analog conversion.
Analog Conversion: Representation of analog information by an analog signal is Analog –to- analog conversion.

Q.What is periodic signal?
Answer:  A signal is called Periodic if it completes a pattern within a measurable time frame called a Period and then repeats that pattern over identical subsequent Periods

Q .What do you understand by the term DCE? Explain DCE.
Any device that transmits or receives signal through network is DCE.
It is any functional unit that transmits or receives data in the form of an analog or digital signal through a network. At the physical layer DCE takes data generated by DTE, converts it to the signal.

Q.How many types of Bipolar encoding scheme exist?
Bipolar uses three voltage levels.
There are three types of bipolar encoding:
1. AMI (Alternate Mark Inversion)
2. B8ZS
3. HDB3

Q. What is frequency spectrum?
Frequency spectrum is the collection of a l l the components frequencies it contains.

Q. Define Sampling Rate.
The accuracy of any digital reproduction of an analog signal depends upon the o. of samples taken.

Q. How bandwidth of a Modem is measured?
Bandwidth of a modem is not measured in bytes, it is measured in bits. Modem Speed –ASK (full-duplex)

Effective bandwidth of a telephone line is 2400 Hz, the maximum baud rate is also 2400. Baud rate and bit rate are equal for ASK, so maximum bit rate is also 2400 bps.

Q. Describe different types of NRZ encoding scheme?
In NRZ encoding the level of signal is either positive or negative. NRZ is
classified into 2 types:
NRZ-L, NRZ-I
NRZ-L:
Level of the signal depends on the type of bit it represents.
Positive voltage usually means the bit is a 1 and a negative voltage usually means the bit is a 0.
NRZ-I:
The inversion of the level represents a 1 bit. A bit 0 is
represented by no change.
NRZ-I is superior to NRZ-L.(PAGE 73)

Q.How bandwidth of a Modem is measured?
Bandwidth of a modem is not measured in bytes, it is measured in bits.
Effective bandwidth of a telephone line is 2400 Hz, the maximum buad rate is
also 2400.
Baud rate and bit rate are equal for ASK, so maximum bit rate is also 2400 bps.

Q. What is digital signal? Give an example
Answer:  A digital signal is discrete. It can have only a limited number of defined values, often as simple as 1s and 0s. The transition of a digital signal from value to value is instantaneous like a light being switched ON and OFF.

– Any time, data contains long strings of 1’s or 0’s, Rx can loose its timing
In unipolar, we have seen a good solution is to send a separate timing signal but this solution is both expensive and full of error
– A better solution is to somehow include synch in encoded signal somewhat similar to what we did in NRZ-I but it should work for both strings of 0 & 1
– One solution is RZ encoding which uses 3 values : Positive, Negative and Zero
– Signal changes not b/w bits but during each bit
– Like NRZ-L , +ve voltage means 1 and a –ve voltage means 0, but unlike NRZ- L, half way through each bit interval, the signal returns to zero
– A 1 bit is represented by positive to zero and a 0 is represented by negative to zero transition
– The only problem with RZ encoding is that it requires two signal changes to encode one bit and therefore occupies more BANDWIDTH
– But of the 3 alternatives we have discussed, it is most effective value.

Q. Compare Phase Modulation (PM) with Frequency Modulation (FM)?
Phase modulation (PM)
– Simpler hardware requirements
– Phase is modulated with the amplitude
– Amplitude & Frequency of the carrier signal remain constant
Frequency Modulation (FM)
– Frequency of carrier signal is changed according to the amplitude of modulating signal
– Amplitude and Phase of the carrier signal remain constant
FM Bandwidth
Bandwidth of FM signal (modulated signal) = 10 * bandwidth of modulating signal Significant spectrum of FM
audio = 15 KHz
Minimum 150 KHz bandwidth

Q.Write down three characteristics of Sine Wave.
Amplitude
Period/Frequency
Phase

Q. A Sine wave has a frequency of 6 Hz. What is its period?
F= 6 , t= ?
T = 1/f = 1/6 = 0.17 sec

Q. Give similarities between 4 QAM & 8 QAM.
Answer: In both case no. of amplitude shifts is more than the no. of phase shifts
Because amplitude changes are susceptible to Noise , number of phase shifts used by QAM is always larger than the amplitude shifts.

Q. How does noise affect FSK?
Answer:  The duration of the noise is normally longer than the duration of a bit which means that when noise affects data, it affects a set of bits.

Q. Write down the names of layers present in TCP/IP Protocol Suite?
Physical (physical standards)
Network (Interconnectivity)
Transport (Transport Functions)
Application (Session, Pres, app of OSI)

Q. What is simplex mode of transmission?
Communication is Unidirectional
Only one of the two stations can transmit