Modems

Question 1

What does the word modem mean?

Answer 1

Modulator Demodulator

Question 2

What is a function of a modem?

Answer 2

Converts digital or analog signals into a form that can be transmitted over a transmission channel over a long distance.

Question 3

What situations require modems?

Answer 3

1) Limited bandwidth
2) Connection must be made by dialing a phone
3) Distance between DTEs is too far for null modem connection

Question 4

What are the three types of digital modulation?

Answer 4

1) Amplitude shift keying
2) Frequency shift keying
3) Phase shift keying

Question 5

What are the three types of analog modulation?

Answer 5

1) Frequency modulation
2) Amplitude modulation
3) Phase modulation

Question 6

What is the difference between AM and ASK?

Answer 6

AM: modifies the amplitude of the carrier signal based on audio signal

ASK: modifies the carrier signal based on the presence of 1s and 0s. 1s indicate a presence of a carrier signal and 0s represent an absence.

Question 7

What are the advantages and disadvantages of ASK?

Answer 7

Advantage:
- Simple to design

Disadvantage:
- Loss of connection is an issue since digital zero value is the same
- Noise can interfere with the signal

Question 8

What is the difference between FSK and FM?

Answer 8

FM: modifies the frequency of the carrier signal based on an audio signal

FSK: 1s are represented by higher frequency and 0s are represented by the original carrier signal

Question 9

What are the advantage and disadvantages of FSK?

Answer 9

Advantages:
- Immune to noise
- Signal is always present

Disadvantages:
- Requires 2 frequencies

Question 10

What is PSK?

Answer 10

Phase shift keying modifies the carrier signal’s phase. For 0s there is no phase shift but for 1s a 90 or 180 degree shift is present

Question 11

What are the advantages and disadvantages of PSK?

Answer 11

Advantages:
- Only needs 1 frequency
- Easy to detect loss of carrier

Disadvantages:
- Detecting phase shifting requires complex circuitry

Question 12

What is DPSK?

Answer 12

Di-bit Phase Shift Keying: data bits are combined to get various phase shifts. This system runs at 2 bits/baud which doubles the bit rate.

Question 13

What is QAM?

Answer 13

Quadrative Amplitude Modulation: amplitude and phase of the carrier signal are varied according to the possible combinations of digital signals. This system allows for 4 bits/baud quadrupling the bit rate.

Question 14

What is the purpose of a cavity filter?

Answer 14

A cavity filter prevents the modem from transmitting signals outside of it’s licensed frequency band

Question 15

How is half duplex communication achieved using a modem?

Answer 15

Use of XON/XOFF, X on character represents the start of a transmission and an X off character represents an end to the transmission.

Question 16

How is full duplex communication achieved using a modem?

Answer 16

Both modems divide the available bandwidth into 2 parts. This technique is called Frequency Division Multiplexing and allows for 2 carrier signals.

Question 17

What are the 3 main parts of a data radio system?

Answer 17

1) Radio set: re-modulates/demodulates (e.g. using FM)
these signals up/down to be transmitted/received using the
system’s assigned VHF/UHF radio channel.

2) Modem: Modulates/Demodulates digital data into audio tones that can be used by the radio set. Typically these are integrated in the radio.

3) Antenna: when transmitting it converts electric signals into electromagnetic waves. When receiving it converts the electromagnetic waves into electric signals.

Question 18

What affects how far a radio signal can be transmitted?

Answer 18

1) Power of the transmitter
2) Frequency of the radio signal
3) Physical obstruction between receiver and the transmitter
4) Atmospheric conditions
5) Daytime/Nighttime
6) Type of antenna
7) The height of the antenna