I'm studying the OSI architecture model and I have doubts about the role of the SAP (Service Access Point). From what I'm getting it's an interface between the (N+1)-PDU and the (N)-SDU but does it also offer some sort of "virtual" connection between the same layers of two systems? Can you clarify this aspect?
Each layer adds header during "send" operation. The corresponding layer in the destination (or next hop) reads the contents of that header during "receive" operation. We can see this as a virtual connection between layers of those two hosts.
Here is an image from Wikipedia illustrating this concept: https://en.m.wikipedia.org/wiki/File:OSI-model-Communication.svg