Designing QoS for your network can be complicated. It is best to take your time and start with the basics. This article from Informit discusses the first steps in designing a QoS deployment.
A good place to begin is to decide which comes first: the cart or the horse. The horse, in this context, serves to pull the cart and is the enabler for this objective. Similarly, QoS technologies are simply the enablers to organizational objectives. Therefore, the way to begin a QoS deployment is not by glossing over the QoS toolset and picking a la carte tools to deploy. In other words, do not enable QoS features simply because they exist. Instead, start from a high level and clearly define the organizational objectives.
Some questions for high-level consideration include the following:
All traffic classes specified in the QoS Baseline model except one—the Locally-Defined, Mission-Critical Data application class—are determined by objective networking characteristics. These applications, a subset of the Transactional Data class, are selected for a dedicated, preferential class of service because of their significant impact on the organization's main business objectives.
This is usually a highly subjective evaluation that can excite considerable controversy and dispute. An important principle to remember when assigning applications to the Mission-Critical Data class is that as few applications as possible should be assigned to the Locally-Defined Mission-Critical class.
If too many applications are assigned to it, the Mission-Critical Data class will dampen, and possibly even negate, the value of having a separate class (from Transactional Data). For example, if 10 applications are assigned as Transactional Data (because of their interactive, foreground networking characteristics) and all 10 are determined to be classified as Mission-Critical Data, the whole point of a separate class for these applications becomes m
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oot. However, if only one or two of the Transactional Data applications are assigned to the Mission-Critical Data class, the class will prove highly effective.
Related to this point, it is recommended always to seek executive endorsement of the QoS objectives before design and deployment. By its very nature, QoS is a system of managed unfairness and, as such, almost always creates political and organizational repercussions when implemented. To minimize the effects of such non-technical obstacles to deployment, which could prevent the QoS implementation altogether, it is recommended to address these political and organizational issues as early as possible and to solicit executive endorsement whenever possible.
Classes of the QoS Baseline model
It is not mandated that enterprises deploy all 11 classes of the QoS Baseline model; this model is designed to be a forward-looking guide for consideration of the many classes of traffic that have unique QoS requirements. Being aware of this model can help bring about a smooth expansion of QoS policies to support additional applications as future requirements arise. However, at the time of QoS deployment, the organization needs to clearly define how many classes of traffic are required to meet the organizational objectives.
This consideration should be tempered with the consideration of how many classes of applications the networking administration team feels comfortable with deploying and supporting. Platform-specific constraints or service-provider constraints also might come into play when arriving at the number of classes of service. At this point, it also would be good to consider a migration strategy to allow the number of classes to be expanded smoothly as future needs arise.
When the number of classes of service has been determined, the details of the required marking, policing, and queuing policies can be addressed. When deciding where to enable such policies, keep in mind that QoS policies always should be performed in hardware instead of software whenever a choice exists.
Read more about QoS for different classes at Informit.
