er it. Whilst each layer performs its functions independently of other The connectionless-mode simplifies operations where the over- layers, interaction between layers is essential to ensure full co-oper- head of error control and recovery is not needed. It also eliminates ation in support of the communication. The services are provided to the overhead of establishment and release of connections for a faster the (N+1)-layer (see Figure 12.5), which is characterised as the (N)-service-user in each system by the companion technical report, ISO TR 8509. This concept is illustrated in Figure 12.6, which applies to any of the layer boundaries. The (N+1)-entity in each system is the service user of the functions of the (N)-layer and any (N+1) ENTITY (N+1) ENTITY PROTOCOL lower layers, which are characterised collectively in Figure 12.6 as the (N)-service-provider. Therefore, each of the layer services can be defined from specific vantage points using this model. Four interaction primitives are defined to further characterise the semantics of the dynamics of operation within a system as follows: (N+1) LAYER (N) LAYER REQUEST is provided by the originating service user to acti- vate a particular service by the service provider. (N) CONNECTION 2. INDICATION is provided by the service provider to the desti- nation service user to advise that a particular service is acti- vated. 3. RESPONSE is provided by service user back to the service Figure 12.7 (N) connection provider in response to an incoming indication primitive. Layered architecture 12/7 END-SYS A END-SYS B RELAY SYSTEM Figure 12.8 Relay function reaction communication. Typically, the connectionless-mode has globally unique or may only be specific to a particular instance of been applied to routing data to destination systems through telecom- communication, depending on the layer involved. munication resources. In this case, some layers are operated in the A single SAP can support multiple instances of connections, connectionless-mode, whilst others are operated in