hat it is unwieldly. The modular structure of layering provides a great deal of flexibility by enabling implemen- tations to change over time with new technology, by allowing tailoring of the invoked functions to optimise a specific operating EXECUTION configuration, and by satisfying the broadest range of applications. A layered architecture, as shown in Figure 12.3, can be referred to as an "onion skin structure" to describe a communications environ- Figure 12.3 Onion skin structure ment. The users on either side represent the corresponding APs that Layered architecture 12/5 SYSTEM A SYSTEM B PEER PROTOCOLS PHYSICAL MEDIA Figure 12.4 OSI environment resides in the LSE of a system and binds to the OSIE for communi- which are not part of the OSIE, provide the interconnecting trans- cating with another AP in a remotely-located system. Each system mission paths between the systems. At the destination system, each has a component of each of the layers that are referred to as layer layer in turn executes its functions and passes the action to the next entities. An instance of a layer entity represents the set of functions layer above, in ascending order. of the specific layer that are active in a system to support the instance of communication. The functions are invoked in the layer 12.3.2 Layer Concept entities in the originating system and executed by the respective layer entity in the destination system. Specifically, the functions invoked by the entity in layer 7 of system A are executed by the The concept of a layer is illustrated in Figure 12.5. OSI characterises entity in layer 7 of system B. These functions are logically conveyed an (N)-layer as a subdivision of the OSI architecture that interacts between systems using peer protocols, which are defined for each with elements in the next layer above, the (N+1)-layer, and the next of the layers to perform particular tasks. Each of the descending layer below, the (N-1)-layer. The (N)-entity is an active element layers in the originating system adds its functions