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.
|OSI Model||TCP/IP Suite|
|Layer 7||Application||Application||Layer 4|
|Layer 4||Transport||Transport||Layer 3|
|Layer 3||Network||Internet||Layer 2|
|Layer 2||Data Link||Network Interface||Layer 1|
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).