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The OSI Reference Model

Digital data communication may be described in many ways. For example, a connection formed between two computers to exchange a text document is a multi-layered activity, involving many steps to convert human language into electrical impulses for transmission, then re-convert those electrical impulses into human language again at the receiving end. Not surprisingly, there usually exist many different ways to perform this same task: different types of networks, different encodings, different communications and presentation software, etc.

To illustrate by analogy, think of all the actions and components necessary to transport items using an automobile. In order to move furniture from an apartment to a house, for example, you would require the following:

 • An appropriate vehicle

Addresses or directions for both locations

A driver’s license and knowledge of driving rules

Fuel for the vehicle

Knowledge of how to safely stack furniture for transport

Knowledge of how the furniture is to be placed in the house


These details may seem trivial to mention, as human beings familiar with the common task of moving personal belongings from one location to another, but imagine having to describe every single action and component to someone from another planet ignorant of vehicles, addresses, maps, driver’s licenses, fuel, etc. One way to help describe all this complexity would be to assign different people to different layers of detail. For example, an automotive engineer could discuss the details of how engines burn fuel to do mechanical work (propelling the vehicle) while a furniture loader could describe how furniture is to be loaded and taken off the vehicle. A driving instructor could then explain all the procedures of safely driving the vehicle, while a city planner could explain the organization of streets and addresses in relation to a city map. Finally, an interior decorator could wax eloquent on the proper placement of furniture in the house. Each person would be describing a different aspect of the furniture move, each one of those aspects being important to the overall goal of moving furniture from one location to another.

Moreover, for each one of the aspects described by a specialist, there may exist several different alternatives. For example, there are many different models and designs of vehicle one might use for the job, and there may be different driving rules depending on where the two locations are for the move. Addresses and directions will certainly vary from city to city, and even within one city there will be alternative routes between the two locations. Finally, there is virtually no end to arrangements for furniture at the destination house, each one with its own functional and aesthetic merits.

By the same token, we may divide the act of digitally communicating data into several distinct aspects, from the physical representation of 0 and 1 bits as electrical/optical/radio signals to the final presentation of data in a form meaningful to human beings. Each of those aspects is important to the overall goal of digital data communication, and there may very well be many alternative methods (standards) for each aspect. We may represent 0 and 1 bits using NRZ (Non-Return to Zero) encoding, Manchester encoding, FSK modulation, etc.; the signals may be electrical or they may be optical or they may even be radio waves; the options for electrical cables and connector types are many. Bits may be framed differently as they are packaged for transmission, and arbitration between devices on the network handled in a variety of different ways. How we address multiple devices on a network so messages get routed to their proper destinations is important as well.

A scheme originally intended as a formal standard, but now widely regarded as a general model to describe the portions of other standards, helps us clarify the complexity of digital communications by dividing communication functions into seven distinct “layers.” Developed by the ISO (International Organization for Standards)1 in 1983, the OSI Reference Model divides communication functions into the following categories, shown in this table with examples:


Layer 7


This is where digital data takes on practical meaning in the

context of some human or overall system function.

Examples: HTTP, FTP, Telnet, SSH

Layer 6


This is where data gets converted between different formats.


Layer 5


This is where "conversations" between digital devices are opened,

are also specified here.

Examples: Sockets, NetBIOS

Layer 4


This is where complete data transfer is handled, ensuring all data

gets put together and error-checked before use.

Examples: TCP, UDP

Layer 3


This is where the system determines network-wide addresses,

ensuring a means for data to get from one node to another.

Examples: IP, ARP

Layer 2

Data link

This is where basic data transfer methods and sequences (frames)

are defined within the smallest segment(s) of a network.

Examples: CSMA/CD, Token passing, Master/Slave

Layer 1


This is where data bits are equated to electrical, optical, or other

signals. Other physical details such as cable and connector types

are also specified here

Examples: EIA/TIA-232, 422, 485, Bell 202


The vast majority of digital networking standards in existence address mere portions of the 7-layer model. Any one of the various Ethernet standards, for example, applies to layers 1 and 2, but none of the higher-level layers. In other words, Ethernet is a means of encoding digital information in electronic form and packaging that data in a standard format understandable to other Ethernet devices, but it provides no functionality beyond that. Common industrial network standards such as EIA/TIA-232 and EIA/TIA-485 don’t even go that far, being limited mostly to layer 1 concerns (signal voltage levels, wiring, and in some cases types of electrical connectors). By contrast, other industrial networking standards specify nothing about lower-level layers, but focus instead on high-level concerns. Modbus, for example, is concerned only with layer 7, and not with any of the lower-level layers2. This means if two or more industrial devices on a network (such as programmable logic controllers, or PLCs) use “Modbus” to communicate with each other, it refers only to the high-level programming codes designed to poll and interpret data within those devices. The actual cable connections, electrical signals, and communication techniques used in that “Modbus” network may vary widely. Anything from EIA/TIA-232 to Ethernet to a wireless network such as WLAN may be used to actually communicate the high-level Modbus instructions between PLCs.

The following articles explore some common networking standards used for industrial instrumentation systems. The OSI Reference Model will be mentioned when and where appropriate.


1If you are thinking the acronym should be “IOS” instead of “ISO,” you are thinking in terms of English. “ISO” is a non-English acronym!

2It should be noted here that some network standards incorporating the name “Modbus” actually do specify lower-level concerns. Modbus Plus is a layer 2 standard, for example.


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Comments (1)Add Comment
written by K.sreeni, October 15, 2013
This is the best description i have read about OSI reference model

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