The International Standards Organization (ISO) has determined a general architecture of network specifications in their DIS 7498 model (applicable to most any digital network). These network specifications were developed as part of the Open Systems Interconnection (OSI) initiative to define the layering of communications in network protocol design. The OSI protocol stack is split into seven layers for modularity. All of the software layers work together, building and distributing the communications. Not all network protocols use every layer.
Digital fieldbus systems (FOUNDATION fieldbus and PROFIBUS PA) use three of the seven layers: 1, 2, and 7. This is because physical and logical layers are the same and there is no direct communication from one node to another across different segments.
The Layers in Detail
■ Level 1 – Physical Layer: Defines the electrical and physical specifications for devices. In particular, the physical layer defines the relationship between a device and a physical medium. This relationship includes the layout of pins, voltages, and cable specifications.
■ Level 2 – Data Link Layer: Is the protocol layer that transfers data between adjacent network nodes in a wide area network or between nodes on the same segment. The Data Link Layer provides the functional and procedural means to transfer data between network entities and provides the means for error detection and correcting events that occur in the physical layer.
■ Level 3 – Network Layer: Establishes procedures for the encapsulation of data into packets for transmission and reception.
■ Level 4 – Transport Layer: Defines how complete data files are handled over a network.
■ Level 5 – Session Layer: Organizes data transfer in terms of the start and end of a specific transmission.
■ Level 6 – Presentation Layer: Defines the character sets, terminal control, and graphics commands so that abstract data can be readily encoded and decoded between communicating devices.
■ Level 7 – Application Layer: Refers to standards for generating and/or interpreting communicated data in its final form. In other words, the actual software programs used to communicate data.
Also Read: HART vs Foundation Fieldbus