"Quality of service" means different things to different people. At an architectural level, QoS is composed of two things:
• Finding a path through your network that can provide the service you offer
• Enforcing that service
Finding the path can be as simple as using your IGP metric to determine the best route to a destination. Enforcing that service can be as simple as throwing so much bandwidth at your network that there's no need to worry about any other sort of resource contention tools. This is sometimes called "quantity of service," but in the most generic sense, it is a method of providing good service quality, and therefore good quality of service.
Or you can make things complex. You can find a path through your network with an offline TE-LSP placement tool, much like ATM PVC placement. Enforcing that path can be done using DiffServ mechanisms such as policing, marking, queuing, and dropping. MPLS (specifically, MPLS TE) is only a tool you can use to help provide high-quality service.
There's a range of options in between these two choices. In general, the more time and money you spend on path layout, provisioning, and DiffServ mechanisms, the less money you need to spend on bandwidth and the associated networking equipment. Which direction you decide to go is up to you.
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