Which of the following are functions that are often provided by an ITSP but are not typically provided by the PSTN? (Choose two.)
- call setup and teardown
- audio signal compression
- call supervision
- call routing
Quality of Service (QoS) and audio signal compression are functions that are often provided by an Internet telephony service provider (ITSP) but are not typically provided by the public switched telephone network (PSTN). ITSPs enable customers to use Voice over IP (VoIP) to make phone calls over the Internet.
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.
Call setup and teardown, call supervision, and call routing are all functions that are provided by ITSPs and the PSTN. Call setup involves the series of events between a phone going off-hook and establishing a connection; these events include dial-tone 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.