Mechanisms
Circuit switched networks, especially those intended for voice transmission, such as Asynchronous Transfer Mode (ATM) or GSM, have QoS in the core protocol and do not need additional procedures to achieve it. Shorter data units and built-in QoS were some of the unique selling points of ATM for applications such as video on demand.
When the expense of mechanisms to provide QoS is justified, network customers and providers can enter into a contractual agreement termed a service level agreement (SLA) which specifies guarantees for the ability of a network/protocol to give guaranteed performance/throughput/latency bounds based on mutually agreed measures, usually by prioritizing traffic. In other approaches, resources are reserved at each step on the network for the call as it is set up.
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