Question 1.
The sampling rate chosen for telephony networks is

a) 6600 samples per second
b) 6800 samples per second
c) 7800 samples per second
d) 8000 samples per second

Answer

Question 2.
Why is 250 msec the maximum delay standard you must meet for proper voice service?

a) With more than 250 msec of delay, the circuit is likely to be released inadvertently at a random switch along the path.
b) 260 msec is the round trip delay for satellite links; terrestrial voice must be better than that.
c) A pause of 250 msec is culturally interpreted as the end of a speaker's "turn" in the conversation.
d) All of these are correct.

Answer

Question 3.
Which of the following is true of telephone loops and trunks?

a) Loops are 2-wire full-duplex circuits.
b) Trunks are 4-wire half-duplex circuits.
c) Loops are DC-powered, while trunks are AC-powered.
d) All of these are correct.

Answer

Answers

Question 1.
The sampling rate chosen for telephony networks is

a) 6600 samples per second
b) 6800 samples per second
c) 7800 samples per second
d) 8000 samples per second

Answer
d) 8000 samples per second

Explanation
The rate actually chosen for PAM sample taking was 8000 samples per second. The Nyquist rate would be 6800 samples per second. By taking 8000, oversampling provides some room for samples to be lost en route without the quality of the reconstruction being compromised at the receiving end.
[4135]

Question 2.
Why is 250 msec the maximum delay standard you must meet for proper voice service?

a) With more than 250 msec of delay, the circuit is likely to be released inadvertently at a random switch along the path.
b) 260 msec is the round trip delay for satellite links; terrestrial voice must be better than that.
c) A pause of 250 msec is culturally interpreted as the end of a speaker's "turn" in the conversation.
d) All of these are correct.

Answer
c) A pause of 250 msec is culturally interpreted as the end of a speaker's "turn" in the conversation.

Explanation
Across human cultures, after approximately 250 msec (a quarter of a second) of silence, listeners assume that the other person has finished, and it is now their turn to speak. If the delay on a circuit exceeds 250 msec, the speaker may finish, wait, and when the other party does not seem to begin to speak, begin speaking again. The other party, meanwhile, has begun speaking, and the delayed arrival of that speech "collides" with the first party's resumption of speech.

Switches do not drop a call after any amount of silence; they release the call when signaled due to an "on hook" condition or a line failure message.

Satellite delay for geosynchronous orbit is actually 240 msec each way; add in 10 msec for delay in the satellite itself, and you have 490 msec delay for the round trip -- over twice what people "expect" for gaps in the conversation. This is one reason satellite telephony has been less accepted by the market than undersea cables. Newer satellite networks, using low Earth orbit (LEO), have call hand-off issues similar to cellular, but at higher speeds.
[4145]

Question 3.
Which of the following is true of telephone loops and trunks?

a) Loops are 2-wire full-duplex circuits.
b) Trunks are 4-wire half-duplex circuits.
c) Loops are DC-powered, while trunks are AC-powered.
d) All of these are correct.

Answer
d) All of these are correct.

Explanation
Loops consist of 2-wire full-duplex twisted pair, powered by DC from the hybrid to the local instrument (which is why a building power loss does not cut off telephone access). Trunks, however, must travel longer distances, and must multiplex many calls; they are therefore 4-wire half-duplex twisted pair, powered by AC (which requires less frequent boosting).
[4149]