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Unit 3-1


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1.(401) Modulation enables signals to be transmitted at frequencies
Higher than original
2.(401) What purpose of modulation involves modulationg low frequency signals for transmission over long distances?
Ease of Radiation
3.(402) What factor determines the spacing of the sidebands in an amplitude modulated signal?
Frequency of the modulating signal
4.(402) The relationship of the bandwidth required for DSBEC AM to the bandwidth of the modulating signal of the AM bandwidth alone is
twice the modulating bandwidth
5.(403) What type of device is used to recover the intelligence signal in DSBSC modulation?
Heterodyne detector.
6.(403) In double sideband suppressed carrier modulation, the intelligence is reconstrcuted by modulating the received signal at the exact frequency and phase relation as the
carrier frequency
7.(404) What is the relationship of the SSBSCs bandwidth compared to DSBEC or DSBSC signals?
SSBSC requires half the bandwidth of the other two.
8.(405) The ouptut of the oscillator in a FM modulator increases in frequency with each
positive half cycle of the modulating signal
9.(405) In FM, the amount of oscillator frequency change is
directly proportional to the amplitude of the modulating signal
10.(405) A 10kHz modulating signal has enough peak voltage to cause a deviation of 30 kHz. What is the modulation index?
11.(405) How many sidebands does FM produce?
an unlimited number
12.(406) In PM, the carrier's
phase is shifted at the rate of the modulating signal
13.(406) By using 8-PSK instead of binary phase shift keying, information capacity in increased
three times
14.(407) What is the first step in the pulse code modulation process?
The analog signal is band-limited
15.(407) Non-uniform quantizing and companding decreases the lengh of the code word from
11 to 7 bits
16.(408) How fast can an asychronous transfer mode network potentialy move data?
600 Mbps
17.(408) How does sychrounous transmission reduce the overhead costs of data transmission?
blocks many characters together for transmission
18.(409) Which RS-232C signal is the data from the data communications equipment to the data terminal equipment?
recieve data
19.(409) Which RS-232C siganl indicates the modem is ready to transfer data?
Data set ready
20.(409) The minimum pins required to establish a two-way data communications path with the RS-232 standard are
Pins 2,3 7
21.(410) Which standard designates the electrical standards for a balanced voltage digital interface circuit?
22.(410) What is the transition region voltage in an RS-422 line driver?
-2.0 to +2.0
23.(410) Which standard uses many of the same pins and funcions as RS-232?
24.(411) Approximately what percentage of all nework problems are cable related according to the National Assocition of Communications Contractors?
70 percent
25.(412) When using vertical redundancy check, what signaificance does the amount of ones have in a data bit pattern?
Determines parity
26.(412) Using the longitudianl redundancy check method of error correction, what does the reciever compare with the transmitter to ensure an accurate transmission of data?
The checksum
27.(412) What two error detection methods, when used together,are 98 percent effective in detecting errors?
Vertical redundancy check and longitudianl redundancy check
28.(413) What is the most common error correction technique?
Automatic retransmit on request
29.(413) When using forward error control as a method error correction, where does eror correction take place?
Recieving end
1.[401] What are the 2 primary purposes to modulate a signal?
Ease of radiation and channel allocation
2.[401] What first must be done to radiate a signal over long distances?
The signal must first be converted to analog format
1.[402] Name the part of the AM carrier that varies according to the modulating signal.
The amplitude
2.[402] What frequencies are present at the output of an AM modulator?
Carrier frequency, carrier plus the modulating frequency, and carrier minus the modulating frequency
3.[402] What would be the frequencies at the output of the modulator when you modulate a 500 kHz carrier with a 2 kHz tone?
500kHz 498kHz and 502kHz
4.[402] In conventional AM(DSBEC), what is the relation between the bandwidth required to transmit the signal and the bandwidth of the modulating signal?
the bandwidth required is twice the modulating signal
5.[402] What part of the bandwidth of a DSBEC signal carries the information signals?
the sidebands
6.[402] State the main disadvantage of DSBEC AM.
it wastes power
1.[403] Briefly describe the DSBSC process.
An AM process that transmits both sidebands minus the carrier
2.[403] In DSBSC operation, what parts of the transmitted signal carries information?
Both the amplitude and phase of the output signal carry information.
3.[403] In DSBSC, what must we do to recover the original intelligence signal?
Reinsert the carrier
1.[404] Briefly describe the SSBSC process.
An AM process that transmits one sideband without a carrier
2.[404] Why is SSBSC far more efficient than DSBEC and DSBSC?
It makes better use of power and bandwidth.
1.[405] What determines the amount of deviation of a FM carrier?
the amplitude of the modulating signal
2.[405] How does the rate of deviatoin relate to the frequency of the modulating signal?
They are directly porportional
3.[405] Who determines the maximum amount of deviation in FM?
The FCC establishes the maximum amount of deviation.
4.[405] How do you determine the modulation index in FM?
by dividing the amount of frequency deviation by the frequency of the modulating signal
5.[405] What is a significant sideband?
a sideband that contains at least 1 percent of the total trnasmitted power.
6.[405] Where do FM sidebands get their power?
from the unmodulated carrier
7.[405] What is the relationship between modulation index and sideband power?
A higher modulation index means more power in the sidebands. It is even possible to have all the power in the sidebands and none in the carrier. At this point,any further increase in modulation would start taking power from the sidebands and placing it back in the carrier resulting in a redistribution of power.
1.[406] What effect does the change in carrier frequency have in PM?
None. The frequecy change in PM is incidental
2.[406] Describe the effect the positive and negative alteration of a modulating signal has on the phase of the carrier in PM.
During the positive alternation of the modulating signal, the phase of the carrier lags behind the unmodulated carrier. During the negative alternation it leads the unmodulated carrier.
3.[406] When is the carrier at its rest frequency in PM?
During the constant amplitude part of the modulating frequency.
4.[406] What part of the modulating signal controls the amount of phase shift in PM?
5.[406] What part of the modulating signal controls the rate of phase shift?
6.[406] How can you increase effieciency in digital modulation?
by using multilevel encoding
7.[406] Describe the QPSK process.
The input NRZ signal is split into 2 directions (I-rail and Q-rail), each at 1/2 the data rate. The Q-rail signal is phase shifted 90 degrees and both are modulated in biphase modulators. The signals are recombined to form the QPSK signal
8.[406] What are the 2 reasons for using higher levels of PSK?
To increase capacity or decrease modulation rate
9.[406] How many bits are grouped in 8 psk? 16 psk?
3 bit combination; 4 bit combination
10.[406] What is the main disadvantage of higher level PSK?
You need a higher S/N ratio.
1.[407] Define the term
The process of converting analog signals into digital signals
2.[407] Name the four steps of PCM
Band limiting,sampling,quantizing, and encoding
3.[407] State the main purpose of the band-limiting filter.
It ensures the input to the sampler never exceeds a maximum frequency.
4.[407] Define sampling
converting a continuous time signal into a discrete time signal
5.[407] What part of the pulse train is varied using PAM,PWM, and PPM?
PAM=amplitude; PWM= width or duration; PPM= position
6.[407] What is sthe Nyquist samping rate?
Sampling the input at twice its highest frequency
7.[407] Which step of PCM assigns discrete amplitude values to the sampled amplitude values?
8.[407] Name the two methods of quantization.
Uniform quantizing, non-uniform quantizing
9.[407] Which method assigns amplitude values bases on an equal amplitude range?
Uniform quantizing
1.[408] At what speed does ATM operate?
600 Mbps or higher
2.[408] Which ATM, what is the length of each transmission unit?
One character in length
3.[408] What part of an ATM transmission tells the receiving device that a character is coming and that the character has been sent?
The start and stop bits
4.[408] How does the receiving device determine whether it has received a correct character?
By summing the 1 bits
5.[408] What is an advantage of synchronous transmission over asynchronous transmission?
To reduce the overhead costs of data transmission
6.[408] How does synchronous transmission differ from asychrounous transmisison?
Sychronous transmission blocks many characters together for transmission
1.[409] What capability does the RS-232C standard provide?
A standard for compatibility between communications devices
2.[409] What is data communicatins equipment?
the modem
3.[409] What is data terminal equipment?
The terminal or computer
4.[409] What is the maximum lenth of the RS-232C cable at 56 Kbps?
50 feet
5.[409] What voltages represent the logic levels for data in the RS-232C standard?
Logic 0 equals a voltage between +5 and +15. Logic 1 equals a voltage between
-5 and -15 volts.
6.[409] What indicates the start of serial data transmission?
The start bit
7.[409] How many stop bits are allowed by the RS-232C standard?
1,1.5,or 2 stop bits
8.[409]Which RS-232C pins carry data?
Pin 2(transmits data) and pin 3 (receives data)
9.[409] What is the purpose of the Clear to Send signal on pin 5?
To indicate the modem is ready to recieve data from the computer.
10.[409] What does the Data Terminal Ready signal on pin 20 indicate?
The data terminal is powered up and operational.
11.[409] Which lines are used for handshaking between the computer and peripheral equipment?
DTE, DCE, Data Terminal Ready, and Data Set Ready.
1.[410] What are the two ways to connect equipment to transmit serial data signals?
By single-ended and differential transmission.
2.[410] What are the distance and data rates for the RS-422 standard?
For data rates up to 10Mbps, up to 40 feet; for data rates between 100 Kbps and 10 Mbps, up to 4,000 feet
3.[410] What components make up the RS-422 digital interface?
Line drivers(also called generators) and line recievers interconnected by a balanced cable.
5.[410] What peak-to-peak voltage does the RS-422 driver produce?
4.0 to 12.0 volts peak-to-peak
6.[410] What are the distance and data rates for the RS-423?
100 Kbps for up to 300 feet; if the data rate is less than 1 Kbps, up to 4,000 feet
7.[410]What is the peak-to-peak voltage and the impedance for the Rs-423 line driver?
4.0 to 12.0 volts peak-to-peak (maximum operating range); impedance is 100 ohms or less
8.[410] What is the RS-530 physical specification?
A 25 pin D-type connector
9.[410] Why was the RS-530 standard developed?
In response to the disadvantages of the RS-449 standard.
10[410]. With which electrical interfaces was the RS-530 standard designed to be used?
With the electrical interfaces of RS-422 adn RS-423 for synchronous or asynchronous transmission.
11.[410] What are the data rates for the RS-530 standard with balanced circuits?
20Kbps to 2Mbps, with balanced circuits
12[410]. For what applications is V.35 most used currently?
Internodal trunks between x.25 exchanges.
1.[411] According to the National Association of Communications Contractors, what percentage of network problems can be traced to cabling problems?
70 percent
2.[411] In what year did the EIA develop the first building telecommucications cabling systems standards?
3.[411]What is the useful life of a building distribution system using TIA/EIA 568 standards?
10 years
4.[411] How often are TIA/EIA standards reviewed for currency?
five years
5.[411]How many levels are contained in the TIA/EIA category system?
1.[412] What does VRC check each incoming character for?
odd or even parity
2.[412] What is even partity?
There is and even number of ones in the bit pattern for each character
3.[412]What is the difference between VRC and LRC?
VRC checks each character for odd parity; LRC checks and entire horizontal line within a block for odd or even parity
4.[412] What transmitted character does the receiver use to determine if a transmission was error-free?
5.[412]When using the checksum method of error detection, what binary number is used to divide the sum of all the characters in order to derive the checksum?
6.[412] How does the CRC method of error detection determine the dividend when computing the BCC?
CRC treats the binary ones and zeros in the frame address,control, and information fields as one long binary number
7.[412]How effective is CRC at detecting errors in most applications?
99 percent
1.[413] How does ARQ work?
The reciever automatically sends a retransmittal request to the sender if an error is found in a recieved frame.
2.[413] What is the most common method of error correction?
3.[413] On what type of circuits is forward error control most useful?
On circuits with extremely high error rates or on 1-way broadcast circuits
4.[413]Using forward error control, at which end of the transmission link are errors corrected?
recieving end
5.[413] Using forward error control, what is the transmitter's function in error correction?
to transmit multiple copies of the same message to the distant end.
6.[413]Using forward error control, what is the receiver's function in error correction?
To compare all copies of transmitted message, then reconstruct the message using the good portions of the message copies received.

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