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The extent and working of a particular protocol layer can be further understood from the protocol layer termination point of view. The general working model of a protocol layer consists of functions and procedures that it requires to perform to facilitate the various services to be offered to the higher layer, as described later in Section 3.8.

Protocol layer termination refers to the making available of the various services by the concerned protocol layer to its adjacent layers at the same time facilitating a peer‐to‐peer communication between two network elements over a logical interface. To understand a protocol layer termination, start from the UE/MS end and proceed toward the radio access or CN. A protocol layer terminates at the destination or peer network element or domain. Find and look at the corresponding network element containing the terminated protocol layer. Next, look at its position, e.g. Layer #2, Layer #3, and so on, within the protocol layers’ organization supported by the concerned network element. The protocol stack and its particular layer termination also identify the network elements that exchange various messages using the concerned layer protocol specification. For example, as mentioned in Section 3.3, the AS protocols terminate at the UMTS UTRAN or LTE E‐UTRAN or 5G NG‐RAN, whereas the NAS protocols terminate at the respective CN end. For more examples of protocol layer terminations, refer to TS 25.301 [49].