Principles and technologies of local area networks - Intelligent Buildings and Building Automation - page 51

Developers and vendors can each produce component parts of the whole,
yet provided that the interface between layers remains constant, there will be
assured interoperability. Figure 4.8 shows the OSI Reference Model, and the
layers on the basis of which many of the standards are developed.
The OSI Model partitions networking functions into seven layers, as
shown in Figure 4.8. Once the OSI Reference Model was adopted, standards
defining each of the OSI layers could be developed. For example, if a stand-
ard is established for the Data Link (DL) Layer, specifying its functions and
upward and downward interfaces, any product that implements the standard
in any way is compatible with other products that obey the same standards.
Usually, a LAN protocol refers to the specifications of the first two layers,
i.e. Physical Layer and Data Link Layer, which are technology-dependent
layers. The upper layers, i.e. Application, Presentation, Session, Transport
and Network Layers, are technology-independent layers.
Physical Layer: The Physical Layer specifies the electrical (and/or optical, or
even radio) signalling, and the mechanical or physical connections applicable

OSI Model

Application Layer
Provides the means by which the
application process may access the
communication environment

Presentation Layer
Provides for the common representation
of data while in transit
Session Layer
Provides a means for organized and
synchronized data exchange

Transport Layer
Provides a guaranteed quality of service
in terms of reliability and throughput LAN application

Network Layer
Provides a means of establishing a
connection between networks

Logical Link Control (LLC)
Provides consistent level of services to
the Network Layer

Data Link Layer
Controls the flow of information
between a system and the next adjacent
system on the path

Media Access Control (MAC)
Encapsulates/decapsulates data to/from
the LLC and monitor medium; provides
basic error detection and low-level
addressing

Physical Layer
Provides a direct connection to the
physical medium

Physical Layer
Converts information to/from a medium-
independent bit stream, plus provides
the electrical/optical connection

Figure 4.8 The ISO Reference Model.