Principles and technologies of local area networks - Intelligent Buildings and Building Automation - page 50

Further benefits result from the fact that network models used in the BA
profession follows the principle of layered architecture.
The benefit of layered architecture results from the fact that higher lay-
ers can use services provided by lower layers and do not have to deal with
the details of lower layer operations. For example, a top-layer AP creates a
message, places it in the A/C interface, and assumes that the message will be
delivered error-free to the AP in a destination node. The AP can also assume
that received messages placed in the A/C interface by the middle-layer CP are
error-free. Eventually, from the viewpoint of the top-layer application proces-
sor, the network appears to be operating error-free. Of course, transmission
errors do happen on the network, but they are handled by the middle or
lowest layer, and top-layer APs are not involved. A commonly used approach
to partition the tasks and functions into layers is to use the ISO Reference
Model as described below.
4.2.2 OSI Reference Model
Development of hardware and software to allow many computers and stations
to communicate on a network is a very complex task. It requires a lot of effort
even if all elements of the network are developed by a single manufacturer.
Manufacturers optimize their designs to make them cost-effective and thus
more competitive. As a result, products developed by different manufacturers
tend to be incompatible. If we attempt to create a network using computer
and control stations from several vendors, the problem of integration becomes
serious. The need to allow processor-based products offered by different man-
ufacturers to communicate with each other on a network led the International
Organization for Standardization (ISO) to propose a network architecture
model referred to as the Open Systems Interconnection (OSI) Model. The
goal of the ISO subcommittee developing the OSI Model was to provide a
framework for networking standards, acceptable to all manufacturers, which
would help them design unique and thus competitive products that could still
communicate with compatible products offered by other vendors. The OSI
Model is generic and applies to both WANs and LANs.
As explained in the previous section, the problem of complexity in com-
puter network communication is best handled by using a layered-architecture
approach, in which all networking functions are partitioned into several
groups (called layers), in such a way that upper layers use services provided
(functions performed) by lower layers. The OSI Model implements the
layered architecture concept and defines a number of layers, the functions
performed by each layer, and interlayer interfaces. Partition of all network-
ing functions into layers is guided by two contradictory constraints. When
more layers are used each becomes smaller and simpler. On the other hand,
the use of many layers creates many interlayer interfaces, and the processing
overhead necessary to handle additional interfaces may offset the benefits
gained by layer simplification.