The OSI Reference Model is really just a guideline. Actual protocol stacks often combine one or more of the OSI layers into a single layer.
A protocol stack is a group of protocols that all work together to allow software or hardware to perform a function. The TCP/IP protocol stack is a good example. It uses four layers that map to the OSI model as follows:
- Layer 1: Network Interface– This layer combines the Physical and Data layers and routes the data between devices on the same network. It also manages the exchange of data between the network and other devices.
- Layer 2: Internet– This layer corresponds to the Network layer. The Internet Protocol (IP) uses the IP address, consisting of a Network Identifier and a Host Identifier, to determine the address of the device it is communicating with.
- Layer 3: Transport– Corresponding to the OSI Transport layer, this is the part of the protocol stack where the Transport Control Protocol (TCP) can be found. TCP works by asking another device on the network if it is willing to accept information from the local device.
- Layer 4: Application– Layer 4 combines the Session, Presentation and Application layers of the OSI model. Protocols for specific functions such as e-mail (Simple Mail Transfer Protocol, SMTP) and file transfer (File Transfer Protocol, FTP) reside at this level.
As you can see, it is not necessary to develop a separate layer for each and every function outlined in the OSI Reference Model. But developers are able to ensure that a certain level of compatibility is maintained by following the general guidelines provided by the model.