Hearing tests

Anyway you look at it, 3 billion of anything is an impressive amount. On a microscopic scale, we know there are 3 billion letters of DNA code in every cell of the human body. Amazing, but meaningless and difficult to grasp for most of us.

But time on an earthly scale, we can grasp. For example, 3 billion minutes amounts to more than 2 million days, or approximately 5708 years. And money we grasp even better: At a nickel per minute, 3 billion minutes comes to about $150,000,000.

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Three billion is the number of voice-over-IP (VoIP) minutes CommWorks claims it has carried over its systems in the last 17 months. For a nascent industry such as VoIP, the number is purely symbolic, especially when referring to minutes transported over a single vendor's systems. Impressive as the number is, it is only the beginning.

To ensure that end users are spending “quality time” on new and still-developing packet-voice networks, providers must keep an eye on voice quality. So must equipment providers. And so must the test equipment companies that helped develop the standards used in that equipment and across the network.

“The most important thing in making sure you have good voice quality is to make sure you can quantify the characteristics or the performance of the underlying IP network,” said Houman Modarres, director of softswitch product management for CommWorks. Many of those characteristics pertain to circuit-switched and packet networks. They include latency, jitter — which is a variation in delay — and lost packets. “The human ear is much more sensitive or much more annoyed by jitter than it is by delay,” Modarres said.

And VoIP is much more sensitive than data is to all these potential impairments. For VoIP — as it is for traditional circuit-switched voice — that performance is quantified at several stages, from the lab during development through network design and capacity planning to the end user sending live, revenue-generating traffic.

Agilent Technologies, as well as other test equipment vendors such as Abacus, Empirix and U.K.-based Malden Electronics (all of which CommWorks has worked with) often develop their test tools in concert with transport equipment companies in a lab environment. They also work within various forums and consortiums to test and develop standards. Agilent developed its Voice Quality Tester in labs across Europe, Japan and the U.S. The company has now evolved the VQT into a distributed product that can be deployed in live networks. Service providers also have their own labs. Agilent has worked with major interexchange carriers such as AT&T, WorldCom, NTT DoCoMo and Global Crossing as they developed their IP offerings.

However, testing companies must take their products a step further. In order to prolong the shelf life of their technology, test products must be made for mass deployment and provide measurement capabilities that go beyond what already can be had in transport equipment.

“Being able to test voice quality across the network is of limited value. But being able to identify the problems that are causing poor voice quality is very valuable,” said Steve Witt, vice president and general manager of Agilent's Network Systems Test Division.

To be able to determine root causes of voice quality impairments, Agilent tests for voice clarity, echo, silence suppression and signal loss, in addition to those previously listed. “You can use VQT to measure end user voice quality by decoding packets or measure the voice as it has been decoded by a network element where you can measure parameters like distortion,” said John Anderson, IP telephony manager for Agilent.

Testing in the lab is one thing, but evolving a test box for the field is more challenging: “When you test in the lab, you are the source and the destination of the call — that's easy,” Anderson said.

The trick is turning an expensive test box into a rack-mountable and portable unit cheap enough for mass deployment in the network. Different form factors could range from $4900 to $37,000.

“Every service provider sends us a very consistent message, and that is to eliminate truck rolls… so our model is shifting to have the same troubleshooting capability but being able to distribute by [making the test product portable] or having the instrument deployed in the network,” Anderson said.

How and where that equipment is distributed and how the network itself is engineered also play major roles in ensuring quality of service (QOS). An IP network for voice cannot be engineered in the same way as an IP network for data: “You have to handle voice traffic differently. It usually has a higher QOS requirement than standard data traffic,” said Ike Elliott, senior vice president of Global Softswitch Services for Level 3 Communications. “Most Internet data traffic is ‘best effort,’ whereas voice traffic has QOS parameters that require low latency and low packet loss.”

There is also a different price point for voice traffic, Elliott added. For the record, Level 3 saw a fourfold increase in voice traffic on its network last year. “The perception out there that the VoIP business is going in the wrong direction is incorrect. It is the opposite and has been consistently growing.”

Handling that growth with proper engineering is as important as test and measurement, Modarres said. Still, the same test tools are often used in both processes. “There is often scarcity of bandwidth, so you want to save as much of it as you can if you are a service provider. But trying to manage lower bandwidth often affects voice quality, so there is a major trade-off in network design right there,” Modarres said.

Once service providers' bandwidth is properly engineered, it is up to equipment companies such as CommWorks to make sure their equipment does not cause delays or other impairments. “Test supplier manufacturers make equipment to help us test our subsystems to make sure that our components are within delay budgets or so we can quantify delay,” Modarres said.

And once in place, it is up to test equipment companies to step back in and ensure quality on the network. If VoIP traffic continues to grow at its current pace and possibly faster, vigilance will be the order of the day.

With 27 different interfaces for its VQT, including 10/100 Ethernet — the predominant interface for VoIP — and a full line speed capture rate up to OC-12, Agilent believes it has the product to stand guard. However, even with its impressive growth rate, it may be some time before traffic volume causes much of a problem.