At Layer 1 of the OSI model, the term 'Bits' is used to describe the data. At Layer 2, the data is referred to as 'Frames'. At Layer 4, the data is termed 'Segments'. These terms are standard across networking and accurately describe the different units of data at their respective layers.
In the OSI model, the layers that deal with application-level data handling and management are the Session (Layer 5), Presentation (Layer 6), and Application (Layer 7) layers. When these are mapped to the Internet protocol suite (TCP/IP model), they are all combined into the Application layer. Therefore, the two OSI layers that are combined in the Internet protocol suite application layer are the Session layer (Layer 5) and the Application layer (Layer 7).
In the Internet protocol suite, also known as TCP/IP, the network access layer is responsible for the functions of both the Physical layer (Layer 1) and the Data Link layer (Layer 2) of the OSI model. These layers handle the hardware and protocols necessary for data transmission over a physical network, including modulation, signal generation, and framing of data packets.