A.2.1 The Seven Layers of the OSI Reference Model
Figure A.2 below shows the layers of the OSI Reference Model:
Figure A.2 The seven layers of the OSI Reference Model.
- The physical layer is concerned with the physical wiring used to connect
different systems together on the network. Examples include the serial and
parallel cables mentioned earlier, Ethernet and Token Ring cabling, telephone
cables, 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 as that they would
function with other vendor's implementations, which in turn would make it
impossible for any communications to occur whatsoever. Each of these wiring
systems therefore follows very strict standards, ensuring that the systems will at
least be able to communicate without having to worry about the underlying
- The data-link layer is used to define 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 system,
AM/FM radio and television - use analog sine-waves to transmit information,
while most computer networks use digital "square" 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 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
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 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 to either request
a retransmission or to return 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; indeed many
networks provide unreliable transport layers for non-essential services such as
- 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 as
- 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 data-
compression, allowing applications to take advantage of the performance gains
that compression provides without forcing the applications to develop these
- Finally, the application layer provides the network's interface to end-user
applications and services such as printing or file-sharing. This layer also
provides some management services to ensure that the interfaces are being
addressed and used correctly.
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 from
a word processor 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.
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