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Practice Makes Perfect:

Best Practices for VoIP Readiness Assessment

By Dave Zwicker


IP telephony resellers are doing it. VoIP service providers are doing it. Enterprise IT organizations are beginning to do it. And, increasingly, IP PBX equipment manufacturers are requiring it. We are talking about performing a VoIP readiness assessment � a relatively simple network planning process that has become a revenue generator for VoIP suppliers and a cost saver for their customers.

VoIP assessments take the guesswork out of preparing networks for voice/data convergence and help to assure the quality of VoIP services delivered to end users. The best practices for a VoIP readiness assessment are easy to follow and often produce revealing results that save time, money, and aggravation when deploying a VoIP service. However, in spite of its many benefits, the VoIP readiness assessment has yet to become standard practice for every VoIP project. Why? It�s a puzzling question that is difficult to answer. Let�s walk through a classic VoIP assessment process and see if you don�t agree... it�s an ounce of prevention that is worth more than a pound of cure.

VoIP Assessment Defined
First, let�s review the elements of a typical VoIP assessment and define some terms. A VoIP assessment is an evaluation of the connectivity, capacity, and configuration of a production network that is destined for voice and data convergence. We�ll assume that the goal of this convergence is to eliminate the cost and complexity of operating a separate TDM voice network by creating an IP-based communications system that mixes voice and data traffic over shared network facilities. As a result, an interesting chicken and egg problem emerges. How can you predict the performance of the new VoIP service before the IP PBX is installed and the flood gates are opened? And how can you avoid introducing new performance problems for existing mission-critical data applications when prioritized voice traffic is added to the network?

To resolve the dilemma, specialized VoIP assessment tools are used to simulate voice traffic before IP telephony systems are installed and services are deployed. A common approach used by assessment systems to simulate VoIP traffic is to deploy software agents at various network endpoints to generate synthetic calls. These agents are used to measure the quality of the VoIP service while the traffic load is progressively increased. Traffic agents can be very intelligent, performing a variety of end-to-end troubleshooting tests during the assessment process. Sophisticated VoIP diagnostic tests are designed to ferret out problems before they can manifest themselves as service degradation and user complaints later, after the VoIP service has been deployed.

Detecting and correcting problems in a simulated VoIP environment as opposed to a production environment has obvious benefits. VoIP assessment traffic agents typically operate under the control of a centralized system that determines the frequency and duration of VoIP simulations and aggregates their measurement and diagnostic data onto a dashboard display or a database for data mining and reporting. There is a long list of features and functions available in VoIP assessment systems. The ability to replicate VoIP calls using a variety of voice compression codecs and signaling standards is one such important feature. Automated tests for diagnosing network pathways during the assessment to detect QoS problems or identify excessive packet impairments (delay, loss, and jitter) are also important features. If the assessment tool also has the ability to generate and diagnose data traffic, you can model a number of different scenarios to understand the impact of voice and data traffic peaks during busy hours.

Once you have acquired the right assessment tool for the job, there are six classic steps you will want to follow to perform a comprehensive network readiness assessment to insure success for your VoIP project in every way � from its quality of service to its cost effectiveness.

Step 1: Discover the network inventory�
The first step in the process is to document the network devices (routers, switches, and traffic agents) that are participating in the assessment. You will want a record of equipment model numbers, software revision levels, and network addresses so you can understand the structure of the network and its components as they are organized into subnets to provide a context for your assessment results. If you conduct multiple assessments for many remote sites or external customers, creating a record of the network inventory in service at the time of assessment is essential. Before you run your simulations, make sure the infrastructure that is under evaluation is well documented.

Step 2: Verify the network configuration
The next step to be taken before you begin to generate traffic and take measurements is to identify any configuration problems that can skew your assessment results. There are some common network problems that impact VoIP calls differently from traditional data applications. One of these is the duplex mismatch, which is a problem that occurs when there are two consecutive devices along the network path that are configured to work in full-duplex and half-duplex modes respectively. This configuration will create problems for one direction of a VoIP call while having little or no effect on the other. Another common problem is a misconfiguration of QoS settings in network devices and their failure to properly prioritize voice packet delivery over data. There is an automated end-to-end diagnostic test in VoIP assessment systems that detects duplex mismatches and other tests that verify proper QoS operation. Be sure to correct these problems before you run your VoIP traffic simulations so that call quality measurements are based on a clean network environment.

Step 3: Determine the call quantity/quality threshold
Now you are ready to start generating some voice traffic in order to determine the maximum number of VoIP calls conveyed over the network that will meet a predetermined call quality standard. Your assessment system should be configured to generate synthetic VoIP calls using an appropriate compression codec (e.g., G.711, G.723.1, G726, G.729a, and G.729b) and signaling protocol (e.g., SIP or H.323). As the system gradually increases the number of simulated calls over time, it will measure the change MOS values. Recall that MOS is the five-level industry standard for call quality metrics. A MOS score of 4.0 or above is generally considered to be toll grade in the VoIP world. You can set any minimum standard (MOS value) for call quality and use it to gauge the quality of your future VoIP service when performing this preliminary assessment.

Step 4: Make necessary network adjustments
Very often a preliminary assessment will produce unexpected results. This is a beneficial aspect of the VoIP assessment process because you are detecting problems in the network that might otherwise go undetected. If capacity planning was a simple paper and pencil exercise, then life as a network planner would be much easier. In reality, networks are complex and dynamic. So, if the initial VoIP assessment indicates a lower call volume than expected, then it�s time to troubleshoot the links and make the necessary network modifications. VoIP diagnostic tests are available to analyze the end-to-end route quality of a call path with poor results. A route quality test drills into every switch or router in the path to examine health and performance information. Excessive packet loss, delay, or jitter can be traced to individual switch/router metrics, such as the CPU, memory, or port utilization percentages, to quickly identify segments that are under-powered relative to the simulated load. There may also be configuration errors or intermittent equipment failures that are contributing factors. After the root cause is isolated and the necessary changes are made, repeat Step 3 and re-run the preliminary assessment until a satisfactory call volume is reached. Performing Steps 3 and 4 is often an iterative process that tunes the network for delivering maximum performance.

Step 5: Simulate a complete business cycle
After determining the maximum call volume and completing all network modifications, you are ready to perform a final comprehensive assessment. This step will serve as a dress rehearsal for your future VoIP service as it generates the maximum call volume over the production network during a complete business cycle. A business cycle can vary from company to company, but is typically three to five days in duration with eight hours of activity per day. The goal of this comprehensive assessment is to duplicate minimum and maximum utilization levels to ensure that consistent service quality can be delivered during the periods of peak demand, whenever that may be, throughout the course of a business day or week.

Step 6: Determine service level benchmarks
At the conclusion of your VoIP assessment, you will have made critical changes to the network, documented important facts about the network environment and gathered vital information about service levels for your VoIP project. This information provides a record of the assessment process and a deliverable for customers and business constituents. Equally important are the benchmarks that have now been established for monitoring the VoIP service going forward. Service level benchmarks can provide a reference point used for building service level agreements (SLA). They can also become the performance thresholds used for detecting service degradation when the VoIP service is fully operational.

These six basic steps represent the best practices for performing a VoIP assessment. It�s a �measure twice and cut once� approach for outfitting your organization with integrated voice and data services. Remember, it�s much easier to make changes to the network BEFORE your VoIP service is operational. Take it for granted that your end users will expect, if not demand, the quality and availability of a traditional telephone service. Follow these VoIP assessment best practices and you will be on track to meet those expectations while realizing the many business benefits of VoIP. IT

Dave Zwicker is vice president of marketing at Viola Networks. For more information, please visit

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