Andy Huckridge

Assuring VoIP Quality for Triple Play: It’s a Whole New Ballgame

by Beth Wingerd and Andy Huckridge, M.Sc. (Telecoms)

5/19/2005

Guaranteeing VoIP quality across triple play deployments is a tricky process. Firstly, routers and other standard IP equipment were not originally designed to deliver voice or video services. Secondly, service providers, creating complex integration topologies / architectures are often using legacy equipment. Also problems such as latency, jitter and packet loss — while not of concern in a data-only network — can significantly affect the quality of both voice and video.

Is there a solution? Ensuring that voice, video and data applications run across a network in harmony are really a function of testing and diagnostics. Most at risk is voice quality, after all, if a file transfer is delayed a half of a second, no one will even notice. But a half-second delay on voice will often result in immediate complaints — and if it happens often enough will simply result in the loss of a customer.

But let’s back up a minute. Before testing even starts, it’s critical to know what is considered an acceptable level of voice quality. For a VoIP call to be considered POTS quality, quality measurements must meet the following generally accepted standards:

Call Setup/Post Dial Delay: < 3 s
Latency: < 150ms
Jitter: < 10ms
Loss: 10**-5 BER

If these QoS parameters are met, voice quality generally achieves a MOS score — the de-facto industry standard for measuring voice quality — of 4.0 or higher, which indicates toll quality voice. The factors affecting MOS include latency, jitter, loss, as well as codec, tandem encoding/decoding, background noise and echo. Maintaining a MOS score of 4.0 or higher requires controlling and testing these factors — as well as a host of other issues — in the equipment, within the network and in the service delivery.

Covering the Basics

On the equipment side, savvy network equipment manufacturers (NEMs) are testing the basic functionality of their equipment in the labs before it even reaches their customers. Such testing includes ensuring that voice is prioritized ahead of data.

NEMs are also relying on test equipment to determine the ROI of the equipment for their service provider customers. This means determining specific quality and capacity metrics such as how many voice channels the equipment can support as well as how many packets per second can be processed. They are also pre-testing the robustness of their equipment to withstand security attacks that are designed to kill or crash the systems.

On the network side — particularly with enterprise networks — it is critical to test a network prior to deployment to ensure that it can support high quality VoIP. If service providers try to deploy VoIP for an enterprise customer without first testing that customer’s network, they will be blamed when service quality is not acceptable — even if the problem lies within the customer’s network. And with an increasing number of VoIP providers available it’s easy for a customer to switch providers if quality does not meet expectations from day one.

When pre-testing a network, the test systems need to simulate the flooding of the network with high call volumes to determine the affect on call quality. When moving to trial a service during a First Office Application or bringing a new customer online during the provisioning verification stage, the ability to emulate the customer’s traffic is essential. Service providers need testing equipment that allows them to simulate the transmission of packets using the customer’s actual IP addresses and subnets.  Testing a network before service turn-up is also crucial to uncovering configuration mistakes that could lead to quality problems. For instance, if gateways are not configured for proper IP address to phone number translations, calls cannot be set up. Router configurations must be accurate to enable optimal forwarding and QoS for voice traffic, which can be accomplished by implementing DiffServ prioritization and queuing, and MPLS traffic engineering. In addition, queuing choice and configuration are also critical because incorrect queuing can cause inappropriate packet delays.

Post-Deployment: A Look at Services

The services side mainly involves assuring quality after the service is in the production phase. There are three aspects of a good VoIP service assurance solution:

* Fault management, or the identification of fault events (via alarms) in the network and the correlation of these events

* Performance management, or the collection and trending of performance-related data over time for customer reporting and proactive monitoring purposes

* Diagnostics, or the ability to identify the exact cause and required repair for a known performance or fault event

By far the most important aspect of the three listed above is diagnostics. Good diagnostic solutions can decrease a service provider’s mean time to repair by 50 percent — while also allowing less skilled technicians to easily diagnose network problems.

Uncovering the Various Layers

Having a good diagnostic solution is particularly critical when supporting triple play services, where the same transmission equipment is often used to support a variety of different applications. This makes it very difficult to uncover what equipment is causing a particular problem. That is why one of the most critical features of a good diagnostic system is the ability to diagnose problems across the various layers of the network using one integrated tool.

For instance, a quality issue in Layer 7 — the application layer — may not be caused by a problem in that layer. It could be caused by a Layer 1 problem — that is, a physical layer problem — that was created by a backhoe that brought down the connection to a DSLAM. Or it could have been caused by a Layer 2 problem — for instance, by a glitch in the Ethernet switch that was causing traffic to be sent to the wrong location. With the ability to run tests at different layers — as well as within layers — technicians can quickly isolate the problem to a specific protocol or equipment without having to be IP experts themselves. Even further diagnostics can also be conducted to isolate problems to a specific network segment.

Without the ability to diagnose services issues across layers, resolving them becomes much more complex. The inability to correctly diagnose network problems can actually cost service providers money. For instance, one Ethernet service provider was paying a customer rebates every month because the customer claimed that the service provider was not meeting the quality standards outlined in its SLA. Using diagnostic tools, the service provider monitored the customer’s traffic and discovered that the customer was exceeding the agreed upon bandwidth, which was in turn causing dropped packets. The service provider no longer had to pay rebates, the customer was able to reconfigure their equipment, and the problem was easily fixed

The bottom line is that offering triple play services — whether they are delivered to enterprises or residential customers — introduces complexity into a network, which in turn makes the need for network testing critical. As more and more VoIP providers enter the marketplace, ensuring high quality VoIP services from day one of deployment will be imperative to retaining customers. After all, isn’t creating sticky customers the goal of offering triple play services to begin with?

About the Authors

Beth Wingerd, Senior Director, IP Services and Andy Huckridge, Product Marketing Manager, IP Telephony, for Spirent Communications Systems

About Spirent Communications

Spirent Communications  is a global provider of integrated performance analysis and service assurance systems that enable the development and deployment of next-generation networking technology such as Internet Telephony, broadband services, 3G wireless, global navigation satellite systems, and network security equipment. Spirent’s solutions are used by more than 1,500 customers in 30 countries, including the world’s largest equipment manufacturers, service providers, enterprises and governments.