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Gateways can be engineered to minimize impairments to QoS. Those impairments are echo, end-to-end delay, buffering delay, and silence suppression. Echo is a phenomenon where a transmitted voice signal gets reflected due to an unavoidable impedance mismatch and a four-wire/twowire conversion between the telephone handset and the communication network. Echo can disrupt the normal flow of conversation and its severity depends on the round-trip time delay. If a round-trip time delay is more
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Figure 8-1 Delay across a network including delay in a gateway (coding and packetizing)
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Mier Communications. Lab Report QoS Solutions. February 2001, pg. 2. www.sitaranetworks.com/solutions/pdfs/mier_report.pdf. 3 McCullough, John and Daniel Walker. Interested in VoIP How to Proceed. Business Communications Review. April 1999, pp. 16 22.
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Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
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Quality of Service (QoS)
8
than 30 milliseconds, the echo becomes significant, making normal conversation difficult. A gateway should use an echo canceller so that when delay reaches above 30 milliseconds, the echo canceller circuits can control the echo. Sometimes, due to a delay in transit, some cells might arrive late. In order to ensure that no underruns occur, the buffer size should exceed the maximum predicted delay. The size of the buffer translates into delay, as each packet must progress through the buffer at the receiving gateway at the emulated circuit s line rate. Voice communication is half-duplex, which means that one person is silent while the other speaks. A gateway can save bandwidth by halting the transmission of cells at the gateway during these silent periods. This is known as silence suppression or voice activation detection (VAD). Gateways can also offer comfort noise generation that approximates the buzz found on the PSTN that lets users duplicate the PSTN experience with which they are most comfortable. Some users find this silence disorienting. Many gateways offer VAD and comfort noise generators.4 Recent research performed by the Institute for Telecommunications Sciences in Boulder, Colorado, compared the voice quality of traffic routed through VoIP gateways with the PSTN. Researchers were fed a variety of voice samples and were asked to determine if the sample originated with the PSTN or from the VoIP gateway traffic. The result of the test was that the voice quality of the VoIP-gateway-routed traffic was indistinguishable from the PSTN. 5 It should be noted that the IP network used in this test was a closed network and not the public Internet or another long-distance IP network. This report indicates that quality media gateways can deliver QoS on the same level as the PSTN. The challenge then shifts to ensuring the IP network can deliver similar QoS (see Figure 8-2).
Perceptual Speech Quality Measurement (PSQM)
Another means of testing voice quality is known as PSQM. It is based on ITU-T Recommendation P.861 that specifies a model to map actual audio
Telica. Accelerating the Deployment of Voice over IP (VoIP) and Voice over ATM (VoATM). A white paper posted by International Engineering Consortium, www.iec.org. 5 Craig, Andrew. Qualms of Quality Dog Growth of IP Telephony. Network News. November 11, 1999, pg. 3.
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Quality of Service (QoS)
Quality of Service (QoS)
VoIP gateways or simple codecs PSQM comparison of the two signals PSQM to MOS mapping
Figure 8-2 How Perceptual Speech Quality Measurement (PSQM) scores are determined
Stored file speech or real conversation
signals to their representations inside the head of a human. Voice quality consists of a mix of objective and subjective parts, and it varies widely among the different coding schemes and the types of network topologies used for transport. In PSQM, a measurement of processed (compressed, encoded, and so on) signals derived from a speech sample is collected, and an objective analysis is performed, comparing the original and the processed version of the speech sample. From that, an opinion is rendered as to the quality of the signal-processing functions that processed the original signal. Unlike MOS scores, PSQM scores result in an absolute number, not a relative comparison between the two signals.6
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