THE OSI PROTOCOL ARCHITECTURE

The Open System Interconnection (OSI) model includes a set of protocols that
attempt to define and standardize the data communications process. The OSI
protocols were defined by the International Organization for Standardization
(ISO).

The OSI model is not a single definition of how data communications actually
takes place in the real world. Numerous protocols may exist at each layer. The
OSI model states how the process should be divided and what protocols should be
used at each layer. If a network vendor implements one of the protocols at each
layer, its network components should work with other vendors’ offerings. The OSI
model has seven layers.

Open System Interconnection Reference Model

The Open System Interconnection (OSI) reference model describes how
information from a software in another computer. The OSI reference model is a
conceptual model composed of seven layers, each specifying particular network
functions. The model was developed by the International Organization for
Standardization (ISO) in 1984, and it is now considered the primary
architectural model for intercomputer communications. The OSI model divides the
tasks involved with moving information between networked computers into seven
smaller, more manageable task groups. A task or group of tasks is then assigned
to each of the seven OSI layers. Each layer is reasonably self-contained so that
the tasks assigned to each layer can be implemented independently. This enables
the solutions offered by one layer to be updated without adversely affecting the
other layers.

The following list details the seven layers of the Open System
Interconnection (OSI) reference model:

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

Characteristics of the OSI layers

The seven layers of the OSI reference model can be divided into two
categories:
Upper layers and lower layers.

The upper layers of the OSI model deal with application issues and generally
are implemented only in software. The highest layer, the application layer, is
closest to the end user. Both users and application layer processes interact
with software applications that contain a communications component. The term
upper layer is sometimes used to refer to any layer above another layer in the
OSI model.

The lower layers of the OSI model handle data transport issues. The physical
layer and data link layer are implemented in hardware and software. The lowest
layer, the physical layer, is closest to the physical network medium (the
network cabling, for example) and is responsible for actually placing
information on the medium.

Protocols

The OSI model provides a conceptual framework for communication between
computers, but the model itself is not a method of communication. Actual
communication is made possible by using communication protocols. In the context
of data networking a protocol a formal set of rules and conventions that governs
how computers exchange information over a network medium. A protocol implements
the functions of one or more of the OSI layers.

A wide variety of communication protocols exist. Some of these protocols
include LAN protocols. WAN protocols, network protocols, and routing protocols.
LAN protocols operate at the physical and data link layers of the OSI model and
define communication over the various LAN media. WAN protocols operate at the
lowest three layers of the OSI model and define communication over the various
wide-area media. Routing protocols are network layer protocols that are
responsible for exchanging information between routers so that the routers can
select the proper path for network traffic. Finally, network protocols are the
various upper-layer protocols that exist in a given protocols suite. Many
protocols rely on others for operation. For example, many routing protocols use
network protocols to exchange information between routers. This concept of
building upon the layers already in existence is the foundation of the OSI
model.

OSI Model and Communication Between Systems

Information being transferred from a software application in one computer
system to a software application in another must pass through the OSI layers.
For example, if a software application in System A has information to transmit
to a software application in System B, the application program in System A will
pass its information to the application layer (layer 7) of System A. The
application layer then passes the information to the presentation layer (layer
6), which relays the data to the session layer (layer 5), and so on down to the
physical layer (layer 1). At the physical layer, the information is placed on
the physical network medium and is sent across the medium to System B. The
physical layer of System B removes the information from the physical medium, and
then its physical layer passes the information up to the data link layer (layer
2), which passes it to the network layer (Layer 3), and so on, until it reaches
the application layer (layer 7) of System B. Finally, the application layer of
System B passes the information to the recipient application program to complete
the communication process.

Interaction Between OSI Model Layers

A given in the OSI model generally communicates with three other OSI layers:
the layer directly above it, the layer directly below it, and its peer layer in
other networked computer systems. The data link layer ins System A, for example,
communicates with the network layer of System A, the physical layer of System A,
and the data link layer in System B.

OSI Layer Services

One OSI Layer communicates with another layer to make user of the services
provided by the second layer. The services provided by adjacent layers help a
given OSI layer communicate with its peer layer in other compute systems. Three
basic elements arte involved in layer services: the service user, the service
provider, and the service access point (SAP). In this context, the service user
is the OSI layer that requests services from an adjacent OSI layer. The service
provider is the OSI layer that provides services to service users. OSI layers
can provide services to multiple service users. The SAP is a conceptual location
at which one OSI layer can request the services of another OSI layer.

OSI Model Layers and Information Exchange

The seven OSI layers user various forms of control information to
communicate with their peer layers in other computer systems. This control
information consists of specific requests and instructions that are exchanged
between peer OSI layers.

Control information typically takes one of two forms: headers and trailers.
Headers are prepended to data that has been passes down from upper layers.
Trailers are appended to data that has been passed down from upper layers. An
OSI layer is not required to attach a header or a trailer to data from upper
layers.

Header, trailers, and data are relative concepts, depending on the layer
that analyzes the information unit. At the network layer, for example, an
information unit consists of a Layer 3 header and data. At the data link layer,
however, all the information passed down by the network layer ( the layer 3
header and the data ) is treated as data. In other words, the data portion of an
information unit at a given OSI layer potentially can contain headers, trailers,
and data from all the higher layers. This is known as encapsulation. The picture
shows how the header and data from one layer are encapsulated into the header of
the next lowest layer.

Information Exchange Process

The information exchange process occurs between peer OSI layers. Each layer
in the source system adds control information to data, and each layer in the
destination system analyzes and removes the control information from the data.

If System A has data from software application to send to System B, the data
is passed to the application layer. The application layer in System A then
communicates any control information required b the application layer in System
B by prepending a header to the data. The resulting information unit (a header
and the data ) is passed to the presentation layer, which prepends its won
header containing control information intended for the presentation layer in
System B. The information unit grows in size as each layer prepends its won
header (and, in some cases, a trailer) that contains control information to be
used by its peer layer in System B. At the physical layer, the entire
information unit is placed onto the network medium.

The physical layer in System B receives the information unit and passes it
to the data link layer. The data link layer in System B then reads the control
information contained in the header prepended by the data link layer in System
A. The header is then removed, and the remainder of the information unit is
passed to the network layer. Each layer performs the same actions: The layer
reads the header from its peer layer, strips it off, and passes the remaining
information unit to the next highest layer. After the application layer performs
these actions, the data is passed to the recipient software application in
System B, in exactly the form in which it was transmitted by the application in
System A.