Silver Peak Systems, Inc.

Is Voice over IP on a Collision Course with the Branch Office?

Tuesday, March 28, 2006 -- Mountain View, Calif. -- Voice over IP has been around for some time, but it is just starting to be deployed in volume as a serious business tool. While quality and reliability issues have been responsible for slowing the early adoption of VoIP within the enterprise, there are many solutions available today that overcome these challenges, preparing the industry for mass VoIP adoption. In fact, according to recent enterprise surveys conducted by Computerworld, Infoworld, and Ziff Davis Research, VoIP deployments will grow faster in the next 18 months than any other category of enterprise applications So prevalent has it become that most major enterprises have VoIP projects either underway or in the testing and evaluation stage.

As VoIP becomes more widespread throughout distributed enterprises, IT staffs are being forced to address the unique requirements of delivering voice to branch office and remote users. At the same time, to keep user performance acceptable for the newly centralized applications, they must also accelerate other business critical applications over the WAN. As data applications have very different delivery requirements from voice, video, and real-time applications, this can be a challenging task to accomplish.

Requirements for Supporting Voice

VoIP uses the User Datagram Protocol (UDP) for transport (as opposed to the Transport Control Protocol, or TCP, which is used by many other enterprise applications). As a result, companies considering VoIP and consolidation projects will require an acceleration platform that is specifically architected to support UDP.

It is not enough to simply reduce the bandwidth usage of other applications in order to make room for VoIP traffic over the WAN. Rather, it is a requirement to maintain LAN-like network characteristics over what will be an over-subscribed or congested WAN, without inducing added latency.

In addition, VoIP is highly sensitive to WAN traffic quality – jitter, latency and packet loss. The ITU (International Telecommunication Union), for example, recommends that the end-to-end delay associated with a voice call not exceed 150 milliseconds. Experience has shown that it is possible to exceed that goal by a small amount, but voice quality will degrade if latency becomes too large.

Jitter is a measure of how packet delay changes over time. While the vast majority of data applications are not impacted by jitter, voice is different. According to well accepted standards, if jitter exceeds 30 ms, voice quality will degrade noticeably.

Making the "Toll" Grade

IIn the VoIP world, voice quality is measured by a ‘Mean Opinion Score’, which is a number between 1 and 5 used to quantitatively express the subjective quality of speech in communications systems, especially digital networks that carry VoIP traffic. Anything above a 4.0 is considered toll grade.

The graph below shows that on a LAN it is easy to achieve a MOS of 4.4. On a standard WAN link with congestion, however, MOS drops to 3.0. This is well below toll quality.

Figure 1: On a standard WAN with congestion, MOS drops below toll quality

Maintaining Quality Over the WAN

A variety of tools and metrics can help overcome the challenges of delivering VoIP across a WAN, ensuring that MOS remains above toll grade:

• Quality of Service (QoS) enables enterprises to deploy VoIP in conjunction with other enterprise applications. In addition to honoring existing QoS markings, acceleration solutions provide native support for advanced QoS, including sophisticated classification logic, a variety of packet marking techniques, queuing, and traffic shaping. Also, they can ensure that specific applications, like VoIP, are guaranteed appropriate WAN bandwidth.
• Advanced header and payload compression is a tried and tested technique for reducing the amount of voice traffic traversing the WAN. This saves on WAN bandwidth utilization and improves perceived application response time, giving greater performance to end users.
• Adaptive Forward Error Correction (FEC) can help to improve VoIP performance during periods of high packet loss. By dynamically adding FEC packets when loss is detected, a WAN acceleration appliance can recreate lost packets on the far end of a WAN link with extremely low latency. The result is consistent voice quality, even during periods of extreme network congestion or across networks with high loss characteristics (e.g., the Internet).
• Packet coalescing is another useful tool that can be used to optimize VoIP performance across the WAN. With this technique, a number of smaller packets are repackaged into a single larger packet. By doing this in connection with header compression, acceleration tools can reduce the amount of bandwidth required for voice, improving the quality of voice service and optimizing network efficiency.
• Real-time data reduction techniques can be used to reduce the amount of repetitive information that crosses the WAN. This is done by fingerprinting all information destined for the WAN, and delivering duplicate information from local data stores whenever possible. This can be broadcast voice traffic, such as audio streaming or voice mail, as well as redundant bits of information that accompany normal voice calls (e.g., silence suppression). Data reduction can also be used to reduce the amount of non-voice traffic on the WAN, freeing up the total amount of available resources.

Figure 2: Data reduction is used to reduce the amount of repetitive information traversing the WAN. This can include broadcast voice traffic, redundant information that accompanies normal voice calls, and non-voice traffic

The above techniques must be performed with minimal impact on latency. Otherwise, they can actually do more harm than good when delivering time sensitive traffic, such as voice.

Virtually every study of the topic indicates that the percentage of companies that have deployed VoIP has steadily increased to the point where the majority of companies have already begun to deploy it. Market research also indicates that over time, most companies expect to increase their deployment of VoIP.

The increasing percentage of companies that are deploying VoIP introduces unique application delivery challenges for distributed enterprises. Application acceleration solutions can help significantly. By combining innovative data reduction techniques with advanced Quality of Service, compression, and loss mitigation, they can ensure LAN-like toll quality across most WAN links. From real-time conversations to audio streaming, WAN acceleration is the answer when enterprise VoIP is calling.

Jeff Aaron is the Director of Product Marketing at Silver Peak.