For a better understanding of what is happening when data is transmitted
via e-mail, from web pages, or even between machines on a computer network,
this month I will explain the Open System Interconnection (OSI) model.
The OSI reference model is an international standard serving as the
framework for protocols used for communications between differing hardware
and systems. The OSI model has seven layers and control is passed from
one layer to the next. (See figure 1.) The communication starts at the
application layer in one station moving through to the bottom layer, over
the communication mechanism channel, and then up the layers of the next
station. (See Figure 2 inside).
Designed in 1978, the OSI model was designed to establish data communications
standards that would promote multi-vendor interoperability. In short,
it was established so hardware designed by Company A could communicate
seamlessly with hardware from Companies B, C, D, etc.
The model consists of seven layers, with a specific set of network functions
allocated to each layer, and guidelines for implementation of the interfaces
between layers. These layers include:
Layer 7—Application (all 7)
This top layer defines the language and means by which programs will
communicate with other programs. The application layer provides the purpose
of communicating in the first place. For example, a program on a workstation
uses commands to request that a document on the server be opened.
Layer 6—Presentation
When data is transmitted between different types of computers, the presentation
layer translates the way in which the data is presented to each system.
Layer 5—Session
The session layer coordinates communication. It can be looked at as
a mediator. It ensures that requests are fulfilled and that they are sent
and responded to in the correct order. This layer also checks for connection
failure and provides for recovery if a failure occurs.
Layer 4—Transport
The transport layer does what its name suggests. It is responsible for
end-to-end validity and integrity of the transmission. If a packet gets
lost during transmission, the transport layer will detect it. It ensures
that the size of the file sent is the size of the file received.
Layer 3—Network
The network layer establishes the route between the sending and receiving
stations. If all stations are contained within a single network, then
this layer is not required. It is used to traverse different network segments.
Layer 2—Data Link
The data link is responsible for the node-to-node validity and integrity
of the transmission. It is a more detailed check of the data that takes
place in layer 4.
Layer 1—Physical
The physical layer is responsible for passing bits onto and receiving
them from the connecting medium. This layer has no understanding of the
meaning of the bits; it deals solely with the mechanics and hardware used
to pass information.
