Measuring Voice Quality in VoIP Networks:
Going Beyond "Can You Hear Me Now?"
BY LAURA HOLLY
Enterprises everywhere are looking carefully at VoIP these days. And no
wonder: the economic advantages of sending voice and data over the same
low-cost, IP network can be attractive.
Of course, adopting VoIP means different things to different companies.
Some find that bridging their existing PBX systems via IP trunking is a
cost-effective way to leverage their IP network and preserve their
investment in traditional circuit-switched infrastructure. Others are
extending IP telephony throughout the enterprise with software-based call
servers and IP phones on the desktop. Additionally, many enterprises are
also taking advantage of IP-based audio conferencing and call centers.
Smaller companies, eager to offload voice infrastructure maintenance, are
turning to provider-hosted solutions, such as IP Centrex or unified
But whereas some IT professionals are leaping in with both feet, others are
hanging back. And while some are not yet convinced of the VoIP business
case, the vast majority is hesitant due to well-founded concerns about voice
quality -- especially for calls that will be routed over a WAN or the
Internet. Voice is a mission-critical application and many enterprises are
simply unwilling to make the leap to VoIP until they can be assured that
users will enjoy the same voice quality and call reliability they currently
have with their legacy PBX/PSTN infrastructure for each and every call.
ITï¿½S THE NETWORK
While a number of factors can degrade voice clarity, such as poor-quality
endpoint equipment or repeated voice signal compression and decompression,
by far the number one culprit is the underlying IP network itself. Packet
loss, either due to packets dropped by the network or by an endpoint jitter
buffer because of excessive network jitter and latency, can significantly
degrade the quality of voice transmission. Although voice clarity is an
essential indicator of quality, other factors must also be considered. For
example, irritating delays caused by excessive network latency can result in
parties talking over one another or adopting a walkie-talkie style of
conversation. In addition, the call connection experience also impacts
usersï¿½ overall perceptions of call quality. If users donï¿½t receive a prompt
dial tone when taking a phone off-hook, or if their calls take a long time
to connect, fail to connect, or disconnect prematurely, users will be
dissatisfied -- even if the voice quality is stellar.
Taken together or separately, any of these problems can make carrying on
a normal conversation an exercise in frustration. To make matters worse,
these performance factors are not static targets -- the fact that call
quality was acceptable yesterday or even ten minutes ago does not ensure
good quality on the next call, due to the dynamic nature of IP networks and
their ever-changing usage patterns.
Given these network realities, how can enterprises ensure both clear
voice quality and reliable call connections? While television ads imply that
call quality can be assured simply by roaming about with a phone and asking,
ï¿½Can you hear me now?,ï¿½ the reality is quite different. To meet usersï¿½
high-quality standards, they must have an effective approach to measuring
VoIP network and application performance. This approach must be:
ï¿½ Pervasive -- Measurement must encompass all IP network infrastructure and
services engaged in delivering the VoIP application.
ï¿½ Continuous -- The VoIP network must be measured continuously, in
real-time, and throughout the life cycle of the VoIP application.
ï¿½ Comprehensive -- Network measurement must include both active
performance verification and passive performance monitoring.
Letï¿½s review each of these critical network measurement characteristics in
The only meaningful measurement approach is one that accurately measures
the entire IP infrastructure engaged in delivering the VoIP service.
Anything less will result in incomplete data and a misleading analysis of
service performance. All of the components that support the voice call must
be exercised, including, of course, network, DNS and DHCP service, gateway,
gatekeeper, proxy server, or other call and application server performance.
Performing effective measurement requires an enterprise-wide approach, with
instrumentation at all service origination or delivery points in the
network, such as each branch office or data center. Enterprises deploying
VoIP to the desktop should also extend their measurements within locations
by measuring call performance directly to endpoint or IP phones at the site.
Each measurement point is instrumented with a verifier, which, depending on
the application and deployment location, may either be a purpose-built
hardware appliance or a lightweight software agent designed specifically to
perform VoIP testing. Verifiers measure VoIP transactions and communicate
raw transaction performance data to a centralized administration portal that
provides data analysis, aggregation, and real-time reporting. This pervasive
approach is the only way to precisely pinpoint the causes of poor voice and
Measuring the network intermittently will not tell you much about VoIP
quality. Thatï¿½s because IP networks -- unlike more static and predictable
circuit-switched networks -- are by their very nature dynamic. With
asymmetric routing, dynamic routing modifications, ever-changing traffic
loads, and bandwidth availability, network performance can vary from one
moment to the next. Continuous measurement is, therefore, essential.
When we say ï¿½continuousï¿½ we mean both around the clock and throughout the
VoIP application lifecycle, including pre-deployment testing right through
the deployment and production phases. Measuring application and network
performance before deployment is critical to avoid unpleasant surprises when
the application is launched. Many VoIP quality problems -- and unhappy users
-- could have been avoided had they been identified pre-launch. Incremental
testing before and during deployment also makes the task of validating the
network more manageable, accelerating time-to-launch, and avoiding costly
and time-consuming analysis and redesign after deployment.
The need for monitoring doesnï¿½t end once the VoIP application is
successfully rolled out. In fact, continuous monitoring is most important
once the service goes live and is subjected to real world, highly changeable
traffic loads and network conditions. Around-the-clock testing and
monitoring of network performance enables administrators to pinpoint trouble
areas rapidly to minimize or avoid impact on service quality. Continuous
testing also provides the data to perform meaningful trend analyses that
enable better capacity planning and network utilization decisions.
To provide that meaningful data, network measurement must be
comprehensive, employing both passive monitoring and active testing.
Active testing should simulate actual end-user usage patterns. This means
executing the same high-level transactions, such as a call, using the same
types of protocols, like SIP, H.323, MGCP, or SCCP, and involving the same
call components, such as gateways or proxy servers, that users do. This
testing should be flexible enough to allow precise control over every aspect
of configuration, scheduling, and measurement. And it should provide the
flexibility to test all aspects of the service from call initiation, media
transmission, and delivery through call completion as a single transaction
or by isolating specific portions. Most importantly, because active testing
can be done independently of users, administrators can proactively uncover
potential voice quality problems before users do.
However, it is also important to have the option to passively monitor the
quality of a particular call to identify user, location, or time-specific
complaints. For example, for a poorly performing on-net call between a user
in the New York-based headquarters and a colleague in the Paris sales
office, the network administrator must be able to identify whether the
impairments can be attributed to performance issues in the WAN provided by
the service provider, the LAN in New York, or the LAN in Paris in order to
fix the problem. By measuring a callï¿½s performance, passive testing provides
insight on the user experience -- as it happens.
A PER-CALL QUALITY METRIC
While measurement of packet loss and latency is required to diagnose
performance problems, the bottom line for network administrators is whether
theyï¿½re providing toll-quality service to their users. How do you achieve
this single measure of user satisfaction or a MOS (Mean Opinion Score)
without relying on the subjective opinions of human testers? Todayï¿½s most
sophisticated measurement suites deliver an objective measure of voice
quality based on the ITU-T G.107 (E-Model) standard.
PUTTING THE QUALITY IN QoS
As cost pressures continue to mount on enterprises, and legacy equipment
continues to age, more and more organizations will embrace the advantages of
VoIP. As they do so, however, enterprise IT managers must recognize the
importance of adopting a VoIP measurement strategy that is pervasive,
continuous, and comprehensive. Not doing so leaves mission-critical voice
applications at risk for crippling service degradations and outages -- and
carries with it its own steep price tag.
Planned and implemented correctly, VoIP network measurement can:
ï¿½ Provide a precise picture of call quality for each and every call,
using the same criteria that determines user satisfaction;
ï¿½ Pinpoint trouble areas, in time to prevent service interruption;
ï¿½ Enable enterprises to evaluate service provider responsibility for
performance issues; and
ï¿½ Provide the information needed to plan network and service enhancements
Using a sophisticated and flexible measurement approach gives enterprises
what they really need: insight into the myriad network factors that impact
voice quality, and finally giving them control over the user experience.
Can you hear me now?
Laura Holly is market development manager at Chelmsford,
Massachusetts-based Brix Networks, a provider of service assurance and
performance management solutions for advanced IP services, including VoIP,
VPNs, Web-hosted applications, and streaming media. For more information
visit the company online at www.brixnet.com.
To The April 2003 Table Of Contents ]