This Recommendation defines a limited set of classes that provide a basis for interworking between the different traffic class aggregates of different service providers@ while preserving the original intent of the service (although packet markings might change). The scope includes interworking between layers and between networks using similar technologies. Each QoS class is defined by its qualitative performance characteristics and the suggested groups of applications whose performance requirements match the characteristics of one or more classes. The limit on the number of classes is imposed by the space available in the widely-used three-bit codepoint space. End-to-end QoS requires a common understanding of the QoS class properties in all involved domains. While many similarities are expected between the deployed or planned QoS architectures of different providers@ these are often not obvious when starting interconnection negotiations. This specification aims on enabling end-to-end QoS (by preserving the intended service properties) across different packet networks and on simplification of the technical QoS interconnection negotiations between providers. Informational examples in the appendices provide: ? Codepoint to class association@ with an interworking example ? ITU-T Y.1541 QoS class mapping to interworking classes The allocation of numerical performance budgets among interworking networks in the user network interface to user network interface (UNI-UNI) path is out of the scope of this Recommendation. The issue of matching [b-3GPP TS 23.203] standardized QoS class identifier (QCI) characteristics to interworking classes is for further study.