THE OSI MODEL

THE OSI MODEL

The Open Systems Interconnection (OSI) reference model was developed by the
International Organization for Standardization (ISO)l as a model for a computer
protocol architecture and as a framework for developing protocol standards. The
OSI model consists of seven layers:
• Application
• Presentation

• Session
• Transport
• Network
• Data link
• Physical
Figure 4.3 illustrates the OSI model and provides a brief definition of the functions
performed at each layer. The intent of the OSI model is that protocols be developed
to perform the functions of each layer.
The designers of OSI assumed that this model and the protocols developed
within this model would come to dominate computer communications, eventually
replacing proprietary protocol implementations and rival multivendor models such


as TCP/IP. This has not happened. Although many useful protocols have been
developed in the context of OSI, the overall seven-layer model has not flourished.
Instead, the TCP/IP architecture has come to dominate. There are a number of reasons
for this outcome. Perhaps the most important is that the key TCP/IP protocols
were mature and well tested at a time when similar OSI protocols were in the
development stage. When businesses began to recognize the need for interoperability
across networks, only TCP/IP was available and ready to go. Another reason
is that the OSI model is unnecessarily complex, with seven layers to accomplish
what TCP/IP does with fewer layers.