Does QOS Matter for Voice over IP?

For anyone over the age of about 35, the first memory of Sprint long-distance service may very well have been a commercial in which the company demonstrated the quality of its network by dropping a pin on a table and claiming the other end of the connection could hear it. More recently, Verizon Wireless's airwave saturation of the “Can You Hear Me Now” commercials moved the concept beyond brand awareness and into a cultural phenomenon.

Time will tell if the same idea ever pervades the voice-over-IP (VoIP) industry, but based on early deployments, quality isn't exactly what the VoIP market is targeting. And for most consumers paying $19.95 or less per month for unlimited service, quality may not be the highest priority. However, for business users for whom VoIP provides the greatest potential — in both cost savings and productivity-gaining applications — quality is almost always at the top of the list, particularly as you move up in the scale of the business.

“You run the gamut to some extent. The larger and more sophisticated the organization, the more they're aware of voice over IP, and simply being aware of VoIP will make them ask quality questions,” said Jerry Knight, chief technology officer of AccessLine Communications, which provides hosted and managed voice service to Fortune 1000 firms and acts as a wholesaler for other carriers. “For the smaller market and residential users, I have a feeling people don't want to ask the question. They want to simply rely on the provider to have done the work.”

As it stands now, most providers targeting the largest enterprise users have done the work if only because they wouldn't be around long if they hadn't. At the same time, though, the industry is divided on how to ensure that quality of service (QOS) is a priority in a VoIP world, how it's impacted by the a potential exponential rise in the number of voice-enabled devices and even the ultimate goal. One thing most everyone agrees on, though, is the fact that ensuring QOS for voice in a converged, IP-centric world is significantly more difficult than in a traditional TDM environment.

Broken into specific elements, carriers and vendors tend to look at three key metrics in measuring VoIP QOS. Like traditional IP traffic, they look at both packet loss and latency (or delay). In addition, most also look at jitter or ensuring that packets arrive in the correct order, which, if unwatched, leads to inaudible voice.

“QOS is a kind of broadly used term, and you need to reduce it down to specific measurable items,” said Dave Schaeffer, Cogent CEO. “If you look at anyone's [service level agreements], it comes down to three metrics — availability, latency and dropped packets. Jitter is packet success rate, and for many of the real-time applications, like voice and video-on-demand, this is the most critical measure.”

For carriers, simply monitoring traffic is just one part of the equation. Just as important is determining exactly what can and can't be monitored. Enterprise customers, regardless of size, don't care if problems with a voice call can potentially be linked to three or four networks or the end device itself. They want it fixed. However, monitoring all parts of the VoIP traffic flow becomes increasingly difficult in a truly open environment without fixed paths, said Michael Burrell, senior product manager for Equant's voice and video solutions group.

“In 2004, it was just QOS from router to router,” he said. “Now customers are looking for QOS from end to end.”

In most cases, though, that simply isn't possible if the carrier doesn't own all portions of the network. Just as difficult is controlling QOS inside the enterprise network where the introduction of softphones and emerging Wi-Fi and WiMAX handsets will mean a significant increase in the number of IP devices acting as “phones.”

“You can still detect quality,” said Ken Hallibut, chief marketing officer of AccessLine. “And the instrumentation is monitoring, for all sessions, the QOS conditions, which will trigger, on thresholds, alarms. But outside of your domain, there's little ability to influence control.”

However, not being able to exert power over the entire path doesn't mean carriers can't offer QOS assurances.

Equant, for instance, has about 150 large enterprises using its VoIP service, which travels over the carrier's multi-protocol label switching backbone (MPLS). The first step in the process of adding a customer, though, starts at the customer's locations, where the carrier will do a network readiness assessment.

“If [the LAN] is over 75% utilization, and it's not at least a Layer 2 switch, we'll make upgrade recommendations,” Burrell said. “The added complexity is when you have a user using a laptop with a softphone at home over a cable modem or DSL. The phone can actually change the IP precedence [and impact overall QOS].”

At the same time, the carrier is seeing a lot of users adopt session initiation protocol (SIP)-based end devices, which not only offer greater functionality and lower costs but helps manage QOS within the network.

“SIP offers a lot of promise, and it unlocks a lot of potential,” Burrell said. “The main things that attract enterprise customers, though, are the ability to use third-party end points. The handset usually represents 40% of the cost.”

In the campus environment, which may or not include carrier network equipment, there also can be issues that impact quality, according to John Borusheski, vice president of emerging technologies at NEC Unified Solutions, which provides professional services to both carriers and enterprises. In doing network assessments, Borusheski said, the company often finds QOS tapering in areas that are in between the enterprise and carrier networks.

“This is where we'll find the warts,” he said. “If [the enterprise or campus developer] was trying to skimp and use Cat 3 instead of Cat 5 cabling, we'll find it in the network assessment. But what we find more often is that people have grown more than they expected.”

It's in the carrier backbone, though, where most carriers are concentrating their QOS efforts. In most, but certainly not all, instances, carriers are migrating VoIP traffic to MPLS backbones because it has so many hooks inherent in the technology that deal with QOS, said Mike O'Malley, group manager of portfolio marketing for Tellabs. In fact, MPLS is almost becoming a de facto panacea for many portions of the network as long as IP is the underlying transport.

“It's still relatively early in the market, but there's growing interest in taking MPLS as a technology and pushing it further out into the network,” O'Malley said. “It's moved into intelligent edge devices. Looking into the future, what we're seeing is MPLS being pushed into the access network.”

The MPLS momentum is not universal, though. Cogent's solution to QOS in the VoIP environment is to throw more bandwidth at the issue by overprovisioning capacity on those routes. Equant also overprovisions but has opted to use MPLS on many segments.

“On those paths, we overprovision in two dimensions,” Cogent's Schaeffer said. “In the transport layer, we guarantee that the bandwidth is not oversubscribed. The second place is in the routing layer. You have to take the worst of all possible worlds, and that's every bit that is routed into a 64-bit frame.”

Schaeffer, in fact, views MPLS as a solution for fiber-starved carriers.

“It is our firm belief that the price of bandwidth will continue to fall at probably 50% per year,” he said. “That's driven by silicon economics, and that trend is infinite as far as we can see into the future. There are a number of solutions designed to say some packets are more valuable. Those technologies are actually Band-Aids for a resource-starved network. The second interesting ploy has been selling MPLS as a panacea. MPLS is just a way of taking a Layer 3 port, which is 10 times more expensive than a Layer 2 port, and making it emulate a Layer 2 port.”

Naturally not everyone — particularly vendors with stakes in MPLS — agrees with that argument.

“This is the ongoing religious argument in the IP community,” said Jason Collins, senior marketing manager of IP strategy for Spirent. “The fact is, we've always needed ways to do gross engineering of our backbone networks. overprovisioning eventually runs out.”

Ultimately, like beauty being in the eye of the beholder, VoIP quality rests in the ear of the user. AccessLine's Knight said it's actually more important for carriers to be consistent in their quality level than to have superior quality calls on an inconsistent basis.

“Quality is all about human perception,” he said. “It isn't enough for a service provider to chase absolute quality. It's actually more important to strive for determinism. If the quality is indeterministic, and an enterprise isn't sure every time they pick up the handset, that is a much worse scenario.”