Voice Quality Metrics
There are three basic categories of quality for voice [2]:
■■ Listening quality—How users rate what they hear during a call. Instances
where only the listening quality is critical are presentation/lectures
over the network that are temporarily similar to other streaming media
applications (such as Internet radio).
■■ Conversational quality—How users rate the ease of conducting an inter-
active voice conversation. This includes echo and delay that we will
discuss here in more detail.
■■ Network quality—The impairments caused by the network are ordered
here by severity:- The network is unavailable.
- Voice dropouts caused by long packet loss bursts are experienced.
- High delay is irritating in interactive conversations.
- Delay variations (also called jitter) can induce the loss of voice sam-
ples in the receive buffer. - High packet loss that is sensed as low speech quality is experienced.
- Miscellaneous—Occasional interruption of the call or failed call
attempt because of dynamic IP address change (possibly once a day
for certain ISPs) or, less often, happens because of route flapping on
the Internet.
While these are basic considerations for voice quality of service, the argu-
ment is often made that different market segments and different customers
may have different requirements for quality. There is, for example, the percep-
tion that “business-quality” voice must be better than “consumer-quality”
voice. We will leave it to the reader to decide how such distinctions may or
may not apply if a consumer calls a business for some service.
Delay Limits for Voice
The ITU-T recommendation G.114 is generally accepted throughout the tele-
phone industry. Following are the values for one-way delay:
■■ Less than 150 ms for acceptable conversation quality.
■■ No more than 400 ms for tolerable conversational quality.
■■ Delay over 400 ms is deemed as unacceptable.Quality of Service for Real-Time Internet Communications 303