Quality of Service for Voice Managers

Every voice manager knows that Quality of Service (QoS) is important. It can be complicated, however, and a bit scary. The complexity of the situation is compounded by terms like Class of Service and Quality of Service and the relationships between some key mechanisms used to provide it. Adding more confusion are carriers who mix conventional terms and acronyms with their own marketing terms. To start with, you will need a basic understanding of the technology.

You need to know where in the network the packets are marked or re-marked. You also need to know what scheduling or queuing techniques are used at each point in the network. Ask specifically about QoS in the LAN. In addition, find out what is happening on the WAN backbone. You can expect quite different answers to these questions as it is generally appropriate to use different techniques in each location. Some technologies, such as ATM have very robust QoS mechanisms built in. Others, like Ethernet and 802.11 wireless do not.

Of course, marking and queuing are quite different. Marking identifies the packet and queuing determines how each device in the network acts, based on the markings. Keep in mind that it is possible to mark packets and not configure scheduling or queuing, or configure scheduling and not mark packets. Neither one would be effective.

In your discussions with carriers and equipment providers make sure you hear them refer to "Priority Queuing" and/or "Low Latency Queuing" often. These are widely accepted and best-practice for providing QoS for VoIP.

As you get further into these discussions, you will need to familiarize yourself with the difference between congestion avoidance and congestion management. Obviously, one attempts to minimize congestion before it happens and the other deals with unavoidable congestion.

Becoming familiar with some of these technologies and doing a little homework will pay huge dividends in terms of being prepared for meeting with vendors and your own management.