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The concept of Quality of Service (QoS) refers to a way of prioritizing certain types of network traffic. A good comparison is to think of how different groups are treated on airlines. First-class passengers are typically given a full three-course meal, large plush seats with their own television monitor, and free alcoholic beverages. They even receive a menu to choose their meal. You can?t find linen and fancy glassware in coach seats. These customers are the first onboard the plane and they are the first to leave the plane. While the rest of the coach passengers are scrambling by to find their seats, the steward may stop the stampede so he can serve drinks to the first-class passengers while they calmly wait for their journey to begin. Contrast this to the frantic rush for the coach passengers to claim overhead compartment space before someone else uses it. It?s clear that the airlines are prioritizing the first-class customers. While the coach passengers may think it?s unfair, the first-class customers paid a lot more money for their tickets and are receiving the level of service that the ticket differential deserves. The airline does its best to manage this unfairness. As network engineers, you can apply these same principles to the communications network. While there are only seven layers to the OSI reference model, traffic prioritization decisions must originate at even higher layers. These decisions are made at "Layer 8" (the business layer) and "Layer 9" (the politics layer) of the OSI model. Without such leadership direction, it will be impossible for a network engineer to roll out a policy that properly reflects the business environment.
To achieve this grandiose goal of prioritizing vital network traffic, applications need to be identified in a way that grants network resources to applications based on application requirements and user expectations. Traffic that is prioritized should receive better treatment than traffic that has not been prioritized in any way. Peeling the onion back, we can also deduce that there will be a hierarchy of prioritizations in the network. This is no different than rank or stature in government or enterprise environments. Quality of Service (QoS) tools will help an engineer create hierarchical network prioritization schemes.
QoS policies in service provider networks allow the service provider to differentiate their offerings and meet customer expectations for real-time traffic. Most service providers offer customers levels of service with a contracted rate for each service level. Although the many markings in some QoS technologies allow more granular treatment, most service providers offer five levels of service to their customers: premium, gold, silver, bronze, and best effort. Premium level services guarantee delay, jitter (delay variation), and packet loss. This level of service is normally used for real-time traffic (voice, video, and time sensitive data like streaming stock quotes). A gold level of service is protected from dropping, but usually does not have the delay and delay variation guarantees that the premium level of service has. Gold service levels are normally used to transport important data traffic like Enterprise Resource Planning (ERP) applications and other mission critical data traffic. The silver service level is used to transport data that is important but not mission critical to the organization. An example of this might be time reporting and accounting. The employees would not be very happy if this function stopped, but it would not put the company out of business. The bronze service level is used to carry data traffic that has been identified and should be given better treatment than best effort traffic. An example of this service level might be human resource intranet web resources. The last and final service level is that of best effort. While the Internet was built on best effort services, this model can break down in enterprise organizations that tax their network infrastructure. Even if the network infrastructure is not currently being heavily utilized, what will happen the next time a major threat like Nimbda or the Code Red worm strikes? The disaster caused by such a worm can be mitigated by restricting the amount of bandwidth that the best effort class can use across the WAN resources. Although this discussion has been from a service provider perspective, the same service levels could be used throughout the organization. After such implementation, the LAN resources would have the same protection level as the WAN resources.
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