January 2005

Proactive Voice Quality Management: Can You Hear Me? BY Tony Rybczynski

Your network is 100 percent up, but an executive calls in a trouble report saying that an IP telephony call with an important client was unintelligible and was dropped. What tools do you have to diagnose the problem beyond the description from your executive? Was the problem due to packet loss, buffer overflow or excessive delay, variations? Very likely, you’re unable to do anything since any attempt to duplicate the problem will probably fail. In fact, according to the 2003 VoIP State of the Market Report by Steven Taylor of Distributed Network Associates, the second most common IP telephony deployment impediment was the lack of systems for managing and troubleshooting VoIP quality (foremost was lack of budget). A big part of the answer is proactive voice quality management, an approach that can be extended to multimedia as well.

Operationalizing IP Telephony Service Quality Management
Proactive voice quality management is a life-cycle approach, starting with a network assessment to identify systemic network impairments, followed by a pre-deployment phase to enable the network QoS and potentially run a small pilot project. Once the network is deployed, ongoing real-time monitoring of end user voice quality and overall system health is required. The last phase is voice and network quality reporting for SLA management and planning. Since the network is ever evolving, this process needs to be institutionalized as part of the operational environment.

The linchpin in linking network performance and end user voice quality is to be able to map how network impairments (e.g., delay, jitter, packet loss, and echo), media endpoint impairments, and analog effects impact the user perception of voice quality. On traditional telecom networks, a Mean Opinion Score from 1 to 5 (5 being ‘very satisfied’) is a widely adopted standard approach for assessing voice quality, which usually requires intrusive testing equipment to be measured accurately. The international standards bodies have developed what is known as the E.Model, which provides the basis to quantitatively measure network, media endpoint, and analog effects in real-time to accurately produce a call quality score. This approach enables real-time monitoring for actual calls on a dynamic packet network. The E.Model estimates user satisfaction associated with a number of technical impairments and the result is an R-value from 0-100. Toll quality is usually considered as a MOS of 4 and R-value of 80. There is a well-defined relationship between R-value and MOS, resulting in an accurate way to determine MOS in real-time for actual VoIP calls.

To date, the networking toolkit for IP telephony management has been limited to a protocol called Real Time Control Protocol (RTCP), a control relative of the Real Time Protocol (RTP), which is used to monitor the effectiveness of delivering a multimedia stream of data across the network measuring round-trip delay, packet loss, and jitter. Unfortunately, these metrics alone are too coarse and are insufficient to calculate the R-value or how the end-user actually perceived the quality of a given call. Moreover, these statistics are often averaged out and do not capture the transitory nature of network impairments. What is needed is to unobtrusively monitor the right set of metrics, which determine end-user Quality of Experience and do it in real-time on an end-to-end basis.

The New Language for Voice Monitoring
The IETF has issued a new VoIP management protocol, RFC3611RTP Control Protocol Extended Reports (RTCP XR). It defines a set of key call-quality-related metrics that contain information for assessing IP telephony call quality and diagnosing problems. RTCP XR can be implemented as software in IP phones, soft clients and gateways. Metrics such as echo, packet loss rate, jitter buffer discard rate and the distribution of lost and discarded packets can be captured by a service management system. The loss/discard distribution describes the call in terms of bursts (periods during which the loss/discard rate is high enough to cause noticeable quality degradation) and gaps (periods during which lost or discarded packets occur infrequently and hence quality is generally good). It also reports end system delay, which represents the delay that the IP telephony endpoint adds (because of encoding, decoding, and the jitter buffer), as well as the signal and noise level associated with the received signal, making it easier to identify signal- and noise-level problems.

Making Your Life Easier
Packet loss and jitter affect voice call quality. Delay and echo can make conversations difficult and make the effects of echo obvious. The jitter buffer removes the jitter in the receiving IP phone or gateway, but this process adds delay and causes packets that arrive late to be discarded. If a signal is too loud, too quiet or too noisy, call quality suffers. The VoIP performance metrics defined in RFC 3611 RTCP XR take the big picture into consideration when calculating a voice quality score, resulting in a better match to the real end user Quality of Experience for a given call.

Proactive voice quality management helps you determine quickly and easily how well IP telephony will work on a network prior to deployment. It helps you configure the end to end system for optimal performance. It measures voice quality in real-time from and end-user perspective using standards based technology and explains why you are experiencing reduced call quality. Reports allow you to monitor service levels, call quality, overall performance, usage trends and capacity planning. Most importantly, it will allow you to know about and fix problems before they become service impacting.

Tony Rybczynski is Director of Strategic Enterprise Technologies at Nortel. He has over 30 years experience in the application of packet network technology. Paul Relf, VoIP QoS Management PLM, is responsible for the definition and delivery of an Industry leading approach to manage IP Communications service quality for Nortel solutions. For more information, please visit www.nortel.com.

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