A Refresher Course on OSI & TCP/IP

EtherNet/IP. PROFINET. Modbus. MQTT. OPC UA. We spend so much time and energy talking about protocol after protocol, which ones work, which ones don’t, I thought now might be a great time to go back to basics and review the two models that most of those industrial protocols use to communicate with devices all over the factory floor.

The idea behind the creation of networking standards is to define widely accepted ways of setting up networks and connecting them together. The Transmission Control Protocol/Internet Protocol model (TCP/IP) is a result of the research and development by the Defense Advanced Research Projects Agency (or DARPA) during the 1960s. Vinton Cerf and Robert Kahn submitted a paper titled A Protocol for Packet Network Intercommunication at a conference in 1974 and immediately began receiving edit suggestions. The two made multiple revisions and in 1981 they published the ‘v4’ specification of TCP/IP.

The Open Systems Interconnection model (OSI) began when the International Organization of Standardization needed a program for methods of networking in the 1970s. The OSI model came out in 1983 intending to provide detailed specifications of actual interfaces, but it wasn’t until 1984 that OSI was announced as an international standard. Large IT companies and organizations got behind this model and started using it instead of TCP/IP.

Let’s take a look at these models side by side.

    The key difference between the two models is the number of layers. The TCP/IP model simplifies things by collapsing several OSI layers.

  • OSI layers 5, 6, 7 are combined into one Application Layer in TCP/IP.
  • OSI layers 1, 2 are combined into one Network Access Layer in TCP/IP.
    Other Differences between the OSI Model and TCP/IP

  • The OSI model is a generic model that is based upon functionalities of each layer. The TCP/IP model is a protocol-oriented standard.
  • The OSI model distinguishes the three concepts (services, interfaces, and protocols) while the TCP/IP does not have a clear distinction between these three.
  • The OSI model gives guidelines on how communication needs to be done, while the TCP/IP protocols lay out standards on which the internet was developed, making TCP/IP a more practical model.
  • The OSI model was developed first and then the protocols in each layer were developed. In the TCP/IP model, the protocols were developed first and then the model was developed.
    Similarities between OSI Model and TCP/IP

  • Both the reference models are based upon layered architecture.
  • In both the models, protocols are defined in a layer-wise manner.
  • In both models, data is divided into packets and each packet may take the individual route from the source to the destination.
    Advantages of the OSI Model

  • It helps standardize routers, switches, motherboards and other hardware.
  • It reduces complexity and standardizes interfaces.
  • OSI facilitates modular engineering.
  • It helps you to ensure interoperable technology.
  • OSI helps you to accelerate the evolution.
  • It allows protocols to be replaced by new protocols when technology changes.
  • It provides support for connection-oriented services as well as connectionless service.
  • OSI is a standard model in computer networking.
  • It supports connectionless and connection-oriented services.
  • It offers flexibility to adapt to various types of protocols.
    Advantages of TCP/IP

  • It helps you to establish/set up a connection between different types of computers.
  • TCP/IP operates independently of the operating system.
  • It supports many routing-protocols.
  • It enables the internetworking between the organizations.
  • TCP/IP model has a highly scalable client-server architecture.
  • It can be operated independently.
  • TCP/IP supports several routing protocols.
  • It can be used to establish a connection between two computers.
    Disadvantages of OSI Model

  • It makes fitting of protocols a tedious task.
  • It can only be used as a reference model.
  • OSI doesn’t define any specific protocol.
  • It duplicates some services in many layers such as the transport and data link layers.
  • It doesn’t allow layers to work in parallel; each layer needs to wait to obtain data from the previous layer.
    Disadvantages of TCP/IP

  • TCP/IP is a complicated model to set up and manage.
  • The shallow/overhead of TCP/IP is higher-than IPX (Internetwork Packet eXchange).
  • It doesn’t guarantee the delivery of packets in the transport layer.
  • TCP/IP makes replacing protocols difficult.
  • It has no clear separation from its services, interfaces, and protocols.

For me, getting back to basics and reevaluating these two models offers a new perspective on the way industrial automation protocols function within them. If you’re looking for more information, our website offers tech pages highlighting all the major protocols. Or you can call 800-249-1612 or email us at solutions@rtautomation.com for whatever else you might need.