OSI Reference Model

The OSI Reference Model is a conceptual model that uses seven “layers” to identify
the various functions provided by a network, and these seven layers can be
used to compare different protocols using a common framework. Each layer
within the OSI Reference Model has a very specific function, and each layer
depends on the other layers in order for the entire model to function properly.
Each layer only communicates with the layers immediately above or below it. If
there is a problem at one layer, it is the responsibility of that specific layer to provide
feedback to the layers surrounding it.
The OSI Reference Model is extremely useful as a tool for discussing various network
services. For example, if we were to look at a simple network service such
as printing a document to a locally attached printer, we could use the OSI Reference
Model to determine how this simple task was being achieved. We could also
use the model to determine how printing over a Novell network was done, or
how printing over a TCP/IP network was accomplished. Because all three of these
examples use the same model, they can all be compared to each other even
though they all use extremely different technologies to achieve the same objective.
Not all networking technologies have seven layers, nor do they all match up to the
seven layers in the OSI Reference Model exactly. Most of them do not match it
except in small, specific ways, although all of them can be compared to the model
with a little bit of thought. This flexibility is what makes it such a popular tool.


The physical layer
The physical layer is concerned with the physical wiring used to connect different
systems together on the network. Examples include serial and parallel
cables, Ethernet and Token Ring cabling, telephone wiring, and even the specific
connectors and jacks used by these cabling systems. Without strictly standardized
definitions for the cabling and connectors, vendors might not
implement them in such a way that they would function with other implementations,
which in turn would make it impossible for any communication


whatsoever to occur. Each of these wiring systems therefore follows very strict
standards, ensuring that network devices will at least be able to communicate
without having to worry about issues such as voltage and impedance.
The data-link layer
The data-link layer defines how information is transmitted across the physical
layer, and is responsible for making sure that the physical layer is functioning
properly. Some networks—such as the public telephone network, radio, and
television—use analog sine-waves to transmit information, while most computer
networks use square-wave pulses to achieve this objective. If there are
any problems with transmitting the information on the physical cabling (perhaps
due to a damaged wire or circuit), then this layer must deal with those
errors, either attempting to retransmit the information or reporting the failure
to the network layer.
The network layer
The network layer is used to identify the addresses of systems on the network,
and for the actual transmission of data between the systems. The network
layer must be aware of the physical nature of the network, and package
the information in such a way that the data-link layer can deliver it to the
physical layer. For example, if a telephone line is the physical layer, then the
network layer must package the information in such a way that the data-link
layer can transmit it over an analog circuit. Likewise, if the physical layer is a
digital Ethernet LAN, then the network layer must encapsulate the information
into digital signals appropriate for Ethernet, and then pass it to the data-link
layer for transmission.
On many networks, the network layer does not provide any integrity checking.
It simply provides the packaging and delivery services, assuming that if
the data-link layer is not reporting any errors then the network is operational.
Broadcast television and radio work in this manner, assuming that if they can
transmit a signal, then it can also be received. Many digital networking technologies
also take this approach, leaving it up the higher-level protocols to
provide delivery tracking and reliability guarantees.

The transport layer

The transport layer provides the reliability services lacking from the network
layer, although only for basic transmission services, and not for any application-
or service-specific functions. The transport layer is responsible for verifying
that the network layer is operating efficiently, and if not, then the transport
layer either requests a retransmission or returns an error to the layer above it.
Since higher-level services have to go through the transport layer, all transport
services are guaranteed when this layer is designed into the network software
and used. Not all systems mandate that the transport layer provide reliability,

and many networks provide unreliable transport layers for nonessential services
such as broadcast messages.
The session layer
The session layer is responsible for establishing connections between systems,
applications, or users. The session layer may receive this request from
any higher layer, and then will negotiate a connection using the lower layers.
Once a connection is established, the session layer simply provides an interface
to the network for the higher layers to communicate with. Once the
higher layers are finished, the session layer is responsible for destroying the
connection.
The presentation layer
The presentation layer provides a consistent set of interfaces for applications
and services to utilize when establishing connections through the session
layer. Although these interfaces could also exist at the session layer, that
would burden it unnecessarily. It is better to have the session layer only manage
sessions and not worry about verifying data or providing other extended
services. An example of a service provided by the presentation layer is datacompression,
allowing applications to take advantage of the performance
gains that compression provides without forcing the applications to develop
these services themselves, and without forcing the transport layer to provide
this service when it may not always be needed.
The application layer
Finally, the application layer provides the network’s interface to end-user
application protocols such as HTTP and POP3. This layer should not be confused
with the part of the end-user application that displays data to the end
user. That function is an entirely separate service, and is outside the scope of
the OSI Reference Model.
Although every network must use all seven layers of the OSI Reference Model in
some form or another, not every network design provides distinct protocols or services
that match all seven layers precisely. TCP/IP is one such networking design,
with many layers that do not match up to each of the layers used by the OSI Reference
Model.