RTA's Blog
A Refresher Course on OSI & TCP/IP
Jan
We spend so much time and energy talking about protocols, I thought now might be a great time to go back to basics. Let’s review the two models that most of those industrial protocols use to communicate with devices all over the factory floor: the OSI model and TCP/IP model.
A Brief History of OSI and TCP/IP Networking Standards
Networking standards is to define widely accepted ways of setting up networks and connecting them together.
The Origins of the TCP/IP Model
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 Origins of the OSI Model
The Open Systems Interconnection model (OSI) began when the International Organization of Standardization (ISO) 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.
OSI Model vs. TCP/IP Model
Let’s take a look at these models side by side.
Key Differences between the OSI and TCP/IP Models
The key difference between the two models is the number of layers. The TCP/IP model simplifies things by consolidating several OSI layers.
- Application Layer: TCP/IP combines OSI layers 5, 6 and 7 (upper layer) into a single Application layer
- Network Layer: TCP/IP combines OSI layers 1 and 2 (lower layer) into a single Link or Network Interface layer
Other Differences between the OSI Model and TCP/IP Model
- 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.
| OSI Model | TCP/IP Model |
|---|---|
| Generic model based on layer functionalities | Protocol-oriented model designed around a specific set of working protocols |
| Strict separation of services, interfaces and protocols | Blurred boundaries between services, interfaces and protocols |
| Theoretical; describes how communication should be done | Practical; describes standards on which internet was developed |
| Model-first design | Protocol-first design |
Similarities between OSI Model and TCP/IP
In both the OSI and TCP/IP model:
- The reference models are based upon layered architecture
- Protocols are defined in a layer-wise manner
- Data is divided into packets and each packet may take the individual route from the source to the destination
Pros and Cons of the OSI Model
Advantages of the OSI Model
- Standardize routers, switches, motherboards and other hardware
- Reduces complexity and standardizes interfaces
- Facilitates modular engineering
- Ensures interoperable technology
- Facilitates rapid innovation
- Allows protocols to be replaced by new protocols when technology changes
- Supports connection-oriented services as well as connectionless service
- Provides a standard model in computer networking
- It offers flexibility to adapt to various types of protocols
- Requires a tedious process to fit protocols into the 7-layer structure
- Functions as a theoretical reference rather than a practical tool
- Fails to define any specific protocol
- Duplicates some services in many layers such as the transport and data link layers
- Requires each layer to wait to obtain data from the previous layer
Disadvantages of OSI Model
Pros and Cons of the TCP/IP Model
Advantages of the TCP/IP Model
- Establishes a connection between different types of computers
- Operates independently of the operating system
- Supports many routing-protocols
- Enables inter-networking between organizations
- Features a highly scalable client-server architecture
- Operates effectively in decentralized environments
- Provides a framework for direct computer-to-computer connections
Disadvantages of the TCP/IP Model
- Presents complexities in initial set up and management
- Carries higher protocol overhead compared to legacy protocols like IPX (Internetwork Packet eXchange)
- Does not guarantee the delivery of packets in the transport layer
- Makes protocol replacement difficult
- Lacks a clear separation between services, interfaces and protocols
A New Perspective on Industrial Automation
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.
