Which of the following are functions that are provided by either an ITSP or the PSTN? (Select 3 choices.)

A.
QoS

B.
call setup and teardown

C.
audio signal compression

D.
call supervision

E.
call routing

Explanation:
Call setup and teardown, call supervision, and call routing are all functions that are provided by Internet
telephony service providers (ITSPs) and the public switched telephone network (PSTN). ITSPs enable
customers to use Voice over IP (VoIP) to make phone calls over the Internet. Call setup involves the series of
events between a phone going offhook and establishing a connection; these events include dialtone signals and
ring signals. Call supervision involves the change in the state of a line or trunk port, such as line seizure,
answer, or disconnect. Call routing involves selecting the path on which a call is transported from a source
endpoint to a destination endpoint. On a VoIP network, voice gateways are responsible for call routing.
Quality of Service (QoS) and audio signal compression are functions that are often provided by ITSP but are
not typically provided by the PSTN. QoS is a VoIP technique that ensures call quality and integrity by mitigating
delay and dropped packets, which can interrupt the flow of a VoIP call. Typical QoS techniques include buffer
management and the use of multiple transmission queues to separate types of multimedia packets. Because
voice traffic is sent in real time, quality is critical. Audio signal compression replaces consecutive repeating
audio signals with code that instructs an endpoint to play one specific signal a given number of times. The
bandwidth consumed by a call is reduced when compression is used.

https://www.packetizer.com/ipmc/papers/understanding_voip/how_voip_works.html
http://www.unc.edu/courses/2010fall/law/357c/001/VoIP/basics.html
https://www.cisco.com/c/en/us/support/docs/voice/digital-cas/14007-net-signal-control.html#Topic5B