Do You Hear What I Hear?�Part I: Defining Quality of Service
In previous tutorials in this series, we have looked in detail at the key protocol families, such as H.323, developed by the International Telecommunications UnionTelecommunications Standard Sector (ITU-T), and the Session Initiation Protocol (SIP), developed by the Internet Engineering Task Force (IETF). This tutorial begins a series of discussions on a topic that is relevant to either architecture: the quality of service provided for that VoIP connection. To make sure that everyone is on the same page, let's begin with a foundation of history and a definition or two.
The reliability factor
If we roll back the clock to the pre- Divestiture dayssay the early 1980s or beforewe would find that the telephone industry was dominated by one central player, AT&T, and its subsidiary operating companies, such as Southwestern Bell, New York Telephone, Mountain Bell, and so on. This monopoly arrangement was sanctioned by the U.S. Government, and regulated by the Federal Communications Commission (FCC), because it was believed at that time that such an arrangement was in the best interest of the American public. But with that monopoly status came a fair amount of government oversight, whereby certain quality standards were defined and periodically measured. Those results could then be factored into rates and tariffs filings, hearings, and so on, that became an integral part of the telephone business at that time. In other words, the U.S. Government wielded a two-edged sword: granting monopoly privileges on one hand, but requiring strict adherence to quality standards on the other. And while some (namely AT&T's competitors) would argue that the arrangement at that time was unfair, few can argue that it produced one of the most reliable public systems, known as the Public Switched Telephone Network, or PSTN, that has ever been developed.
One of those performance benchmarks has come to be known as the Five Nines of Reliability, which, as history buffs recall, was the reliability objective for central office switching systems. That reliability benchmark specified two hours of downtime in forty years of operation. If you work out the math, you eventually arrive at the 99.999% reliability (or the five nines) factor that is so commonly quoted.
Say what you will about AT&T and the former Bell System, you have to admit that they developed a very reliable network infrastructure, that set a high standard for the performance of today's VoIP networks. Putting it another way, end users have experienced first hand the reliability and quality of the PSTN, and are not likely to be satisfied if the new VoIP system that you are installing (and/or asking them to pay for) provides a lower level of performance than they have grown accustomed to.
A different animal: Quality
But where network reliability can be concretely defined in terms of hours of downtime per month, year, or decade, defining network service quality is much more subjective (and therefore more difficult). The ITU-T, in their E.800 Standard, Terms and Definitions Related to Quality of Service and Network Performance Including Dependability, Quality of Service (QoS) is defined as follows:
"The collective effect of service performance, which determines the degree of satisfaction of a user of the service."
Note that these few words carry a strong message, as the ultimate arbiter of service performance is the end user, and the ultimate measurement is whether that end user is satisfied with how the network is operating. So all of your planning, implementation flowcharts, late nights and weekends, lost brain cells, and sleep will be for naught if your VoIP network implementation does not meet the performance criteria of the end users. And keep in mind that these end users may not care about the new technology, how much money it will save or cost, or how much it will increase your business efficiency. If it does not meet with their satisfaction, (which, by the way, is something the end users get to define), you are likely to have a difficult time with the conversion from a traditional voice network to an integrated VoIP solution.
Fortunately, a great deal of research has gone into the entire discipline of QoS, and there are a number of algorithms, protocols and other solutions that address these needs. But before we can discuss the solutions, we must first define the challengesthe factors that affect the quality of a voice connection. These issues are called transmission impairments, which will be the subject of our next tutorial.
Copyright Acknowledgement: © 2005 DigiNet ® Corporation, All Rights Reserved
Mark A. Miller, P.E. is President of DigiNet ® Corporation, a Denver-based consulting engineering firm. He is the author of many books on networking technologies, including Voice over IP Technologies, and Internet Technologies Handbook, both published by John Wiley & Sons.