A networking model describes how two computers communicate and transfer data in a network. There are two popular networking models: the OSI Reference model and the TCP/IP layers model. Both models divide the communication process into several layers. The OSI Reference model divides it into seven layers and the TCP/IP layers model divides it into five layers.
Each layer in both models provides specific functionalities in the communication process. To provide these functionalities, they interact with other layers on the same computer and layers on the remote computer. The interaction process describes how layers interact with each other on the same computer and between two computers. There are two types of interactions: the adjacent-layer interaction and the same-layer interaction.
The adjacent-layer interaction
The adjacent-layer interaction describes how layers on the same computer interacts. On the same computer, if an application or hardware implemented in a layer wants to use a service available in the next layer, it requests the next layer to provide that service. The software or hardware implemented in the lower layer provides the requested service. A layer can request a service only from the lower layer. It cannot request the higher layer to provide a service or a function. For example, the presentation layer can request the session, transport, network, data link, and physical layers but it cannot request the application layer for a service or a function. The application layer is the topmost layer. Protocols and devices running in this layer can request a service or a function from all other layers. The physical layer is the lowest layer. Protocols and services running in this layer cannot request any other layer for a service or a function.
Let us take an example. Web browsers use the HTTP protocol to get web content from the web servers. The HTTP protocol works in the application layer. The application layer does not provide error recovery but HTTP needs this functionality to work properly. The TCP protocol provides error recovery. The TCP protocol works in the transport layer. The transport layer is a lower layer. The application layer is a higher layer. So, the HTTP protocol requests the TCP protocol to provide error recovery. The TCP protocol provides error recovery to the HTTP protocol.
The same-layer interaction
The same-layer interaction describes how layers on different computers interact. Layers on different computers can communicate only with the same layers. They cannot communicate with the different layers. For example, the application layer of a computer can communicate only with the application layer of the other computer.
In the communication process, protocols and devices encrypt and encode data in such a way that it can be de-encrypted and decoded only by them. For example, on the sender device, the transport layer breaks the data stream into segments and attaches a header to each data segment. The header includes all the necessary information to join all segments. But it can be read only by the transport layer. On the receiver device, the transport layer reads header information from all segments and produces the original data stream back.
Differences between the adjacent-layer and same-layer interactions
| Same-layer interaction | Adjacent-layer interaction |
| It happens between different computers. | It happens on the same computer. |
| It happens between the same layers. | It happens between the different layers. |
| Services and devices use it to encode and decode data. | Services and devices use it to request other services to provide specific functions. |