What is QoS? : QoS is the ability for a network to be able to prioritise certain data in order to attempt to guarantee a level of service for a particular type of network traffic. Enterprises that switch to a low-cost VoIP service are often surprised to discover that they have entered a world where service quality can be elusive and hard to achieve. The shortcomings are manifold. Latency (a delay in packet delivery), packet loss (missing data), network jitter (data arriving out of order) and numerous other factors can all degrade a voice connection's quality, leading to distorted audio, disconnects and even total VoIP system collapse.

Why is it important? : Since few enterprises wish to subject their employees and clients to poor phone call quality (or QOS), it's critical to pay close attention to service quality issues when designing a VoIP system. Despite a widespread perception to the contrary, VoIP actually has the potential to deliver a higher QOS than the public switched telephone network (PSTN). In fact, if bandwidth and system costs weren't an issue, everybody would be enjoying phone calls with CD quality audio.

In the real world, however, enterprises and end users would be happy to settle for low-bandwidth, yet clear, glitch-free calls. Fortunately, such a goal is attainable for the enterprise that's willing to take QOS seriously and not cut corners.

Where does it take place? : If you plan to use a VoIP service provider to handle your site's inbound and outbound calls, a little research can pay big QOS dividends over the long term. Most VoIP providers publish QOS statistics on error probability, packet delays and other leading parameters. While these statistics often trend toward the highly optimistic, they make a good starting point for comparing providers' QOS levels.

Regardless of the service provider you ultimately select, you may decide to specify a service level agreement (SLA) that guarantees basic performance benchmarks, usually in exchange for a commitment to spend a fixed amount with the provider over a time period.

Many VoIP service SLAs are calculated on the basis of a Mean Opinion Score (MOS) that includes such factors as call completion rates and the length of time required for a user to hear a dial tone or to connect to the dialed party. Various measurement techniques are used in association with SLAs, including active network tests made at regular time intervals as well as passive measurements that are based on actual calls placed across network. Each approach has its benefits and drawbacks, yet both can be equally accurate tools for measuring QOS. The technique your SLA should specify depends on various factors, including the system's size, the service provider's preference, your preference and the exact type of MOS measurements planned.

QOS doesn't stop at the point the VoIP service enters and leaves your premises. Your enterprise's internal network also plays a major role in overall QOS.

Perhaps the most important factor affecting internal QOS is whether enterprise local-area networks (LANs) and wide-area networks (WANs) use switches, routers and other devices that support QOS standards, such as those defined in RFCs published by the Internet Engineering Task Force (IETF) and other working groups.

Network performance monitoring tools can also help you determine the health of your enterprise's LANs and WANs and the impact that any existing problems may have on QOS. Solutions made by companies such as Avaya and Cisco can help network managers spot bottlenecks and other problem areas and take any necessary corrective action.

Finally, in networks that share data and voice services, QOS can be enhanced by giving voice packets priority over other types of network traffic. Such special "white glove" service will make VoIP services less susceptible to latency, jitter and other undesirable conditions. Intelligent routing protocols, such as MPLS (multi-protocol label switching) and OSPF (open shortest path first), are available to prioritize network traffic. These protocols intelligently optimize the network traffic, in accordance with predetermined parameters.

How does it affect your service overall ? : Like many technology expressions, QOS means different things to different people. For most business customers, the term signifies getting the best possible VoIP service quality at the lowest possible cost. For service providers, QOS often means the exact opposite: delivering minimally acceptable VoIP quality at the highest price the market will bear. In order to get the most out of your network consider this list: