4 QSI下三层的功能与服务如前所述,OSI各层是单向依赖的,每一层都为相邻的上一层提供服务。如果不想改变或不需要改变第N层刘第(N-1)层所提供的能力,那么就把它再提供给第(N+1)层。这样,任何一层就仅直接依赖相邻下一层,而与其它层无关。为了向相邻上一层提供服务,OSI每一层的实体本身必须执行一定的功能。服务与 4 QSI next three functions and services As mentioned earlier, OSI layers are unidirectionally dependent, each layer for the adjacent upper layer to provide services. If you do not want to change or do not need to change the capabilities provided by Level N-1 (N-1) of Nth level, then provide it to Level (N + 1). In this way, any one layer only depends directly on the adjacent next layer, but has nothing to do with other layers. In order to provide services to the next higher level, the OSI entities at each level must perform certain functions themselves. Service and