Interfaces: Vertical (Adjacent Layer) Communication
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The seven layers of the OSI Reference Model are used to split up the various functions that are required to implement a networking system. On any given device in a network, different software and hardware routines and devices may be functioning on any or all of these layers simultaneously. Since in general, all of these are supposed to be working together to implement networking functions, there is a need for layers to communicate vertically between the layers within a particular host.
In OSI Reference Model parlance, the mechanism for communication between adjacent layers in the model is called an interface. Of course, the term interface is also used widely in other contexts in the computer and networking worlds, since its generic meaning refers to connecting just about anything together. However, when someone talks about an interface between OSI model layers, that person typically refers to the process by which data is passed between layer N of the model and layer N-1 or layer N+1.
These relationships are demonstrated in Figure 13. For example, the layer 2/3 interface is used by a layer two and layer three protocol to pass data and control information; the layer 3/4 interface connects layers 3 and 4 together.
Vertical communication is done up and down the protocol stack every time anything is sent across the network, and of course, whenever anything is received. This occurs because the higher levels are implemented as logical functions, in software; there is no actual physical connection. The higher layers package data and send it down to the lower layers for it to be sent across the network. At the very lowest level, the data is sent over the network. On the receiving end, the process is reversed, with the data traveling back up to the higher layers on the receiving device. The next topic dealing with horizontal communication explains more about this logical interaction between corresponding layers.
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