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Unit 2: Network Software

2.4 Reference Models Notes

2.4.1 Open Systems Interconnection (OSI) Reference Model

Now you will study the concept of OSI Reference Model.
The International Standardization Organization (ISO) developed the OSI model of data
communications in 1984. OSI specifies a seven-layer model as shown in Figure 2.4. In addition to
forming the basis of the ongoing development of OSI’s own protocols, it is used by the industry
as the frame of reference when describing protocol architectures and functional characteristics.
The ISO, in an effort to encourage open networks, developed an open systems interconnect
reference model. The model logically groups the functions and sets rules, called protocols,
necessary to establish and conduct communication between two or more parties. The model
consists of seven functions, often referred to as layers as shown in Figure 2.4.

Figure 2.4: OSI Model

Application
Layer (7)
Presentation
Layer (6)
Session
Layer (5)
Transport
Layer (4)
Network
Layer (3)
Data Link
tes Layer (2)
Physical
Layer (1)

The last three layers are mainly concerned with the organization of terminal software and are
not directly the concern of communications engineers. The transport layer is the one, which
links the communication processes to this software oriented protocols. The transmitting device
uses the top layer, at which point the data is placed into a packet, prepended by a header. The
data and header, known collectively as a Protocol Data Unit (PDU), are handled by each
successively lower layer as the data works its way across the network to the receiving node. At
the receiving node, the data works its way up the layered model, successively higher layers strip
off the header information.

Notes The basic philosophy of the 7-layer model is that each layer may be defined
independently of every other layer. Thus from the user point of view, interchange takes
effect across each operation passes down through the layers of the model until data
interchange is affected through the physical connection.

The underlying principles and guidelines that were applied to arrive at the seven layers are
given below:
1. A layer is created at different level of abstraction.

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