in network architecture,a group of services,functions and protocols that is complete from a conceptual point of view,that is one out of a set of hierarchically arranged groups,and that extends across all systems that conform to the network architecture

• 在计算机网络体系结构中，从概念的观点着眼，一组完整的服务程序、功能和协议，是分层的结构组之一，可扩展到遵循网络体系结构的所有系统