engleski

Analysis of echo in PSTN-to-VoIP transmigration

The growth of the Internet and its impact on other communication technologies is nothing new today. One of the issues is the migration of voice traffic from Public Switched Telephone Network (PSTN) to packet networks. In this paper we will show the impact of that migration on the occurrence of echo, and the use of solutions that are implemented in the VoIP network. An echo occurs when a part of the signal from the transmitting path leaks to the receiving path. When this happens the speaker hears his voice delayed, which can be annoying depending on the amplitude and the delay of the echo. Human hearing system has a minimum time interval between sound events that determines whether those events will be perceived as a single event or as two separate events. Because analog transmission is very fast, PSTN connections have a very short round-trip time delay generally lower than 20 ms, so even very loud echoes are imperceptible cause they are masked by the sidetone. Voice over Internet Protocol (VoIP) was introduced as an application of the packet networks for transporting voice. The main issue is satisfying the end users concerning the quality of transported speech on which they were used to when using PSTN. Our main quality consideration in this paper is echo. The question is: How will the migration from the PSTN to the packet networks (e.g. VoIP) affect the issue of echo, if we said that echo is only the problem in the analog networks? The answer is the fact that every packet network inserts extra delay and the amount of the delay depends on the network size (the number of the network elements that process the packets, thus inserting new delay). The use of the packet network transmission link imposes an extra delay, which makes the existing echoes that were imperceptible in "pure" PSTN network, perceptible. In our measurement we tried to eliminate echo from our test network environment. We did that by using echo cancelers that are implemented in the voice gateways, and we showed that in order to fully eliminate the occurring echo when migrating from the PSTN to the VoIP network, every voice gateway that terminates the call towards the PSTN must use the echo cancelling techniques.

echo; VoIP; ELR

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