Voice quality is a well-known hot button in the world of telephony. Public telephone networks traditionally strove to provide a uniform level of voice quality that, while surprisingly limited in the dimension of bandwidth (the nominal 4 kHz being less than 25 percent of a normal young adult’s perceptible frequency range) was overall quite good. By the 1960s, the standard for toll (long-distance) connections in the United States was that 95 percent of users should rate them good or better on a subjective poor-fair-good-excellent scale with respect to key impairments such as circuit noise. With the rapid deployment of long-haul digital fiber systems in the 1980s, users became accustomed to essentially noiseless telephone calls, even to overseas points. Echo cancelers employing digital signal processing cleaned up echoes on long circuits without introducing the other impairments characteristic of cruder echo suppression techniques. After bad user reaction to the 500-ms round trip delay, satellite circuits were banished to those parts of the international network too lightly loaded to justify undersea fiber cables. Most fundamentally, the basic technologies of circuit switching combined with pulse-code modulation (PCM) encoding essentially guarantee that whatever waveform enters one end of the network comes out the other side with very little modification.
Users have shown a definite willingness to tolerate lower voice quality in exchange for convenience—hands-free operation with speakerphones, mobility with cordless phones and cellular services. However, as long as wireline telephone companies manage to continue delivering standard quality in combination with attractive calling plans, there does not seem to be much margin for services with lower quality and no compensating convenience factors. It remains to be seen whether the ability to talk into your computer will turn out to be viewed as a convenience factor.
So, the best advice is to be very cautious about specifying or allowing lower voice quality, except perhaps in specific enterprise network situations where there is a well-quantified economic benefit and the business function being supported by the service does not involve, for example, customer contact.
Vendors may state that they support toll quality voice or make some quantitative statements about quality. However, given the crucial role of voice quality in voice networks, rather than trying to interpret these statements by themselves, you should also do the following:
1. Be sure you understand the voice coding scheme being employed. If it includes compression, you are making a trade-off against quality; and, depending on how aggressive a compression technique is employed, the quality difference may be perceptible under certain conditions.
2. Review one of the many published reports of bake-off events, in which voice quality is always one of the key system properties tested.
Quality of service in telecommunications networks in fact has many dimensions other than voice quality per se. Other commonly specified parameters include dial tone delay, postdial call setup time, and percent blocking. Standard objectives for these parameters for public networks are published by ITU and by Telcordia (formerly Bellcore, and at one time the joint R&D arm of the U.S. RBOCs). The ITU standards tend to be on the liberal side to allow for the significant variation in public network standards among various countries of the world.
Other quality of service parameters apply to traditional data networks. These include the probability of dropped, duplicated, or misrouted packets, as well as the statistics of packet delay. In an integrated system employing Internet technology to deliver telecommunications applications, these parameters may not be directly observable by the users, but will instead influence other quality parameters such as voice quality, call setup time, and blocking.
An important fact to keep in mind about quality is that in general there is a trade-off against cost. If you want a higher-quality system, you may expect to pay for that. If you are looking for the ultimate in low cost, you may need to compromise on various quality parameters.
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