Comparison of the Layers of the OSI and TCP/IP Models

By Stephen Bucaro

The TCP/IP (Transmission Control Protocol/Internet Protocol) protocol suite was developed by the U.S. Defense department's Defense Advanced Research Projects Agency (DARPA) in the early 1970s. It's purpose was to create a communications network that would still operate in the event of an attack on the U.S. which destroyed large parts of the communications network.

The development of the TCP/IP protocol suite created the concept of a world-wide network of interconnected computers and networks. In 1982 DARPA's network was released for public use, and the Internet was born. At first the Internet was used by academic institutions to communicate research data. In 1990 Tim Berners-Lee proposed the the standards for HTML and HTTP.

The first graphical Internet browser, Mosaic, was released in 1993. But the didn't really take off until late 1995 when Microsoft released Windows 95, the first Windows operating system capable of dealing with a 32-bit Internet.

Development of the OSI (Open Systems Interconnection) networking model began in Europe in 1986. The OSI networking model correlates more accurately to the actual process of communicating over a network, and for that reason when network engineers are talking to each other about network processes, they frequently describe them using the OSI model. But the TCP/IP Internet was growing fast, and an actual OSI protocol suite was never developed.

Looking at the diagram shown above you'll note that the OSI model has 7 layers. From the top layer, the application layer featuring processes that enable applications to use network services, to the bottom layer, the physical layer which is the type of cable and connector, and the topology of the network such as ring or bus.

Again, looking at the diagram shown above you'll note that the TCP/IP model consists of four layers instead of seven layers. The functions of all the processes involved in the top 3 OSI model layers (Application, Presentation, Session) are combined into the TCP/IP model Application layer (layer 4). The OSI Presentation layer converts network communications formats into formats usable by applications, such as graphics formats .png, .jpg, and .gif. It converts network communications formats into text formats such as ASCII and EBCDIC. It converts network communications formats into sound and video formats such as MP3 and MPEG.

The function of the TCP/IP model Session layer, which is to synchronize data exchange between applications on different network devices, for example Sockets, is also combined into the TCP/IP model Application layer.

The functions of the OSI Data Link and Physical layers are combined into the Network Interface layer (layer 1) of the TCP/IP model. In the OSI model, the Data Link layer has two sublayers: The Logical Link Control (LLC) sublayer, which is responsible for error correction and flow control, and the Media Access Control (MAC) sublayer, which controls access to network media and uses the MAC address (the physical or hardware address) of the network interface circuit (NIC).

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