[July 26, 2001]

Packet Voice Testing Tools: Don't Deploy New Services Without Them

BY JOHN ANDERSON

For the past two years, global data traffic volumes have exceeded those for voice. Meanwhile, according to 2000 estimates, global voice services revenue is approximately $600 billion, while data service revenue stands at about $65 billion. Although this discrepancy can be partially attributed to the value of voice and its market demand, much of it can also be attributed to the low cost structure of transporting data. It is no wonder, then, that so many providers of voice services are investigating ways to transport voice on data networks.

The combination of real-time voice services and data packet networking results in new challenges that neither voice nor data networking encountered as separate technologies. Multi-vendor interoperability, and delivering acceptable quality for a time-sensitive service over a network designed for data, are just two of the daunting challenges in deploying packet voice networks.

The methods needed for testing and troubleshooting packet voice networks in order to address these challenges are not provided in traditional communications testing tools. This fact is evident in the proliferation of new testing tools designed specifically for VoIP networks. But sorting out the many different approaches, and determining precisely what needs to be done to get a network up and running, can be confusing.

So how can a network operator make VoIP work while meeting quality standards? The answer is to implement a comprehensive and carefully planned testing program.

Defining Test Parameters
The first step is to define what parameters need to be tested and what values need to be met. A packet voice network needs to be able to establish a service event (e.g., connect a call), deliver an acceptable level of voice quality, complete the service event (e.g., disconnect the call) and create a record for the event. This means testing both the operability and the quality performance of a network.

While basic call generators can test a network for overall operability in connecting and completing calls, it is critical to also test call signaling and control (S&C) in a network to determine if systems are interoperating and where call failures are occurring. Testing this is a necessary step for getting any packet voice network up and running. Ongoing testing capabilities are also needed for maintaining and troubleshooting the network.

Testing S&C protocols means testing the operation of individual systems (call agents, soft switches, gatekeepers, gateways, IP phones) and the operation of a network of systems, in accordance with the standards being used.

Testing the quality performance of a network is not as simple as testing its operability. A review of industry standards quickly exposes the fact that, while some Quality of Service (QoS) standards exist, this area is still lacking -- there are no encompassing standards to define what overall QoS needs to be. Furthermore, no standards specifically address QoS on a packet voice network nor specify how to balance QoS with bandwidth reduction techniques like voice compression and silence suppression.

However, a review of QoS testing methods indicates that there is a common understanding of key parameters for packet voice networks. These parameters include:

• Service establishment:
  • Service reliability (e.g., call completion rates over extended periods);
  • Service availability (e.g., call completion rates during busy hours); and
  • Call set-up time or post-dial delay.
    
    
• Voice quality:
  • Clarity;
  • Delay;
  • Echo (signal level and delay); and
  • Signal loss.
    
    
• Packet performance:
  • Packet loss, including erred packets and packets dropped from jitter buffer;
  • Packet latency; and
  • Packet jitter.

Overview Of Testing Tools
The next and most important step in a testing program is the selection of the right testing tools. The right testing tools can significantly reduce the time needed for testing, and the overall time needed to get the network up and running. In general, testing tools can be categorized into test domains and test methods.

Primary test domains are:

• Operational testing, which addresses the functional operation of the network in terms of interoperability among systems and the establishment of calls and services. In other words, does the network work?
• QoS testing, which addresses service performance and media quality. How well does the network work?

Primary test methods are:

• Active testing, in which an external test system stimulates the network (for example, by injecting traffic into the network) to capture and analyze the network's behavior.
• Passive testing, in which an external test system unobtrusively monitors the network's behavior in response to "natural" stimulation (e.g., real customer traffic).

A rigorous testing program will encompass all four of these primary test domain/method combinations for the purposes of network validation, certification, monitoring and, most importantly, troubleshooting when things go bad.

Types Of Testing Tools
Active QoS testing is performed with tools that can generate calls over a variety of interfaces, and then measure the quality of the media that is transmitted over bearer channels. Voice quality testers are available that will generate calls on a network and measure various parameters of voice quality. Call generators can also measure call completion rates and set-up times. A voice quality tester can mean the difference between months of trial-and-error attempts at meeting acceptable voice quality levels, and hours of targeted design and configuration changes that result in a consistently high level of voice quality.

Some tools exist to emulate VoIP packet streams without generating calls, but this method may not accurately measure voice quality, especially if QoS mechanisms are inherent in the call set-up process. Plus, this method cannot measure parameters such as call completion rates and call setup times.

To actively test Real-Time Transport Protocol (RTP) packet performance in a reliable manner, a tool needs to establish a "call" on the network. Tools that can generate VoIP calls over a data interface and measure loss and jitter are best suited for active testing.

Passive QoS testing is performed with tools that unobtrusively monitor traffic and measure key QoS parameters. Some protocol analyzers and network probes can monitor RTP streams and report packet throughput, loss, and jitter. It is important to use a tool that can provide real-time data capture and measurement at line rates. New tools are now emerging that apply a bit of intelligence to these measurements to predict subjective voice quality based on packet metrics.

While it is possible to get QoS metrics in a passive manner from network elements themselves, such as gateways and switches, using an external test system is by far the most reliable and accurate method. An external test system will not be susceptible to failures and performance degradations of the network under test. An external test system will also provide a common and consistent method for testing throughout a multi-vendor network.

Active operational testing is performed with tools that can generate signaling and bearer traffic, and test the response of the network in terms of functions performed. Signaling testers are available to test call agents, softswitches, media gateways, signaling gateways, and, in terms of signaling compliance, call control operations and even signaling performance. Network emulators, such as gateway emulators, can test the operation and performance of call agents and softswitches in handling calls for large-scale network deployments.

Tools that can generate calls are also available for feature testing -- this includes testing call processing agents for features such as call routing, call transfer and find me/follow me services. Feature testing is also useful in testing interactive voice response (IVR) systems.

Passive operational testing is performed with tools that unobtrusively monitor signaling, control, and media protocols; and capture data for testing the operation of a network in establishing services. Protocol analyzers rule this domain.

It is absolutely necessary, in both the deployment and operation of a packet voice network, to be able to troubleshoot call setup. This includes VoIP protocols such as SIP, H.323, and others. Often, calls fail due to problems in layers 2-5. Troubleshooting VoIP networks with a protocol analyzer can quickly uncover issues such as codec mismatches, DHCP problems, and IP addressing problems.

Protocol analyzers offer network protocol decodes, analysis and call trace capabilities that can expose within minutes a problem that otherwise could take days to uncover by examining network equipment logs.

Not included in this description of passive operational testing are operational support systems (OSS) designed for network management, billing, traffic reporting, and fraud management.

Selecting Testing Tools
Selection of testing tools should focus on an overall platform solution that comprises the different pieces needed. Testing solutions are now available that can be centrally managed and remotely operated, and that cover most types of testing. Distributed solutions are valuable for testing a national or global network from a central network engineering or operations center.

It is also important to identify the purpose for testing, and to get a solution that is matched to that purpose. Some tools are geared for development labs, other tools are best for troubleshooting networks, while others are designed for operational monitoring and SLA management.

Expert chefs claim the key to successful cooking is in the ingredients. Similarly, the key to successful testing is in the tools that are used. While there is an abundance of VoIP testing tools available today, an effective testing program needs only a few tools to perform the necessary testing operations. It is most effective for those tools to work cohesively, and those tool vendors who can offer a coordinated set of these tools provide the most value.

John Anderson is the IP telephony manager at Agilent Technologies, Network Systems Test Division. Agilent is a global technology leader in communications, electronics, life sciences and healthcare. With 48,000 employees serving customers in more than 120 countries, Agilent had net revenue of $10.8 billion in fiscal year 2000. Agilent's Network Systems Test Division provides telecom equipment manufacturers, service providers and enterprises with a suite of network testing solutions to test, monitor, measure, repair and troubleshoot PSTN, LAN, WAN ATM, IP and 3G networks and products. To find more information about Agilent's IP telephony testing solutions, visit www.agilent.com/comms/voicequality/.