Page 34 - Intelligent Communication Systems
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I 8 INTELLIGENT COMMUNICATION SYSTEMS
FIGURE 3.3 Structure of the data unit.
FIGURE 3.4 Protocol structure over the X.25 packet switching network.
3.2.4 Specific Structure of the OSI Reference Model
In the OSI reference model, a seven-layer model is specified. Each layer has its
own specific function, is independent of any other layer, and has an interface with
adjacent layers. The seven-layer model is shown in Figure 3.4, which portrays a
node and the peer node. Each node, which may be a computer, a terminal, or a
workstation, has seven layers: a physical layer, a data link layer, a network layer,
a transport layer, a session layer, a presentation layer, and an application layer.
The functions of each layer are as follows. The first layer, the physical layer,
defines the rales and interfaces of the bit streams transmitted between adjacent
nodes. The electrical, mechanical, or physical conditions, such as the electrical cur-
rent, voltage, or pin size or its layout, are defined in this layer. Specifications such
as RS-232C, RS-422/423, RS-449, and X.21 are examples of this layer.
