Each layer of the OSI model can COMMUNICATE only with the layer above it, below it, and parallel to it (a PEER layer). For example, the presentation layer can communicate with only the APPLICATION layer, session layer, and presentation layer on the machine it is communicating with. These layers communicate with each other using protocol data units (PDUs). These PDUs control INFORMATION that is added to the user data at each layer of the model. This information resides in fields CALLED headers (the front of the data field) and trailers (the end of the data field).

Each layer of the OSI model can communicate only with the layer above it, below it, and parallel to it (a peer layer). For example, the presentation layer can communicate with only the application layer, session layer, and presentation layer on the machine it is communicating with. These layers communicate with each other using protocol data units (PDUs). These PDUs control information that is added to the user data at each layer of the model. This information resides in fields called headers (the front of the data field) and trailers (the end of the data field).