A group called the International Standards
Organization (ISO) has put together the Open
Systems Interconnect (OSI) Reference Model,
which is a model that describes seven layers of
protocols for computer communications. These
layers don't know or care what is on adjacent
layers. Each layer, essentially, only sees the
reciprocal layer on the other side. The sending
application layer sees and talks to the
application layer on the destination side. That
conversation takes place irrespective of, for
example, what structure exists at the physical
layer, such as Ethernet or Token Ring. TCP
combines the OSI model's application,
presentation, and session layers into one which
is also called the application layer.
- The application layer
refers to application interfaces, not
programs like word processing. MHS (Message
Handling Service) is such an interface and it
operates at this level of the OSI model.
Again, this segmentation and interface
approach means that a variety of email
programs can be used on an intranet so long as
they conform to the MHS standard at this
application interface level.
- The presentation layer
typically simply provides a standard interface
between the application layer and the network
layers. This type of segmentation allows for
the great flexibility of the OSI model since
applications can vary endlessly, but, as long
as the results conform to this standard
interface, the applications need not be
concerned with any of the other layers.
- The session layer allows
for the communication between sender and
destination. These conversations avoid
confusion by speaking in turn. A token is
passed to control and to indicate which side
is allowed to speak. This layer executes
transactions, like saving a file. If something
prevents it from completing the save, the
session layer, which has a record of the
original state, returns to the original state
rather than allowing a corrupt or incomplete
transaction to occur.
- The transport layer
segments the data into acceptable packet sizes
and is responsible for data integrity of
packet segments. There are several levels of
service that can be implemented at this layer,
including segmenting and reassembly, error
recovery, flow control, and others.
- The IP wrapper is put
around the packet at the network or Internet
layer. The header includes the source and
destination addresses, the sequence order, and
other data necessary for correct routing and
rebuilding at the destination.
- The data-link layer frames
the packets-for example, for use with the PPP
(Point to Point). It also includes the logical
link portion of the MAC sublayer of the IEEE
802.2, 802.3 and other standards.
- Ethernet and Token Ring are
the two most common physical layer protocols.
They function at the MAC (Media Access
Control) level and move the data over the
cables based on the physical address on each
NIC (Network Interface Card). The physical
layer includes the physical components of the
IEEE 802.3 and other specifications.