QoS: Deciding who gets what

Many discussions of QoS focus on WHAT is being delivered, for example; bandwidth, latency, jitter, etc. However, customers also need to consider very carefully WHO can access different QoS profiles. Although it's common to assign QoS definitions based upon such things as physical port or VLAN ID number, this doesn't allow much flexibility in network configuration. A better approach would provide for automatic policy assignments, and even allow for the assignment of different QoS profiles to different applications.

In order for automatic QoS assignment to work well, switches must be able to detect protocol and application information embedded in the traffic flowing through them. For example, that information might take the form of a network address (OSI Layer 3) or socket number (Layer 4); in which case, the switch must be Layer 3- or 4-aware to make such an assignment. Likewise, a Layer 2 switch will not likely support QoS definitions based upon Layer 3 information. This means that applying QoS based upon higher-layer protocol information can require switches that identify more of the embedded protocol information.

If you're considering buying a switch because the vendor claims that it supports "Quality of Service," you really must decide how you want to assign QoS. If you're planning to differentiate QoS by more sophisticated information such as protocol or application, oftentimes a standard Layer 2 switch won't do the trick.

Next week, we'll look at how all of this may affect your switch product selection.

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