The OSI Layers

Background Information

The OSI architecture was set up by the ISO (International Standards
Organization) as the first formally defined way of connecting computers. The
OSI architecture divides network functionality up into seven layers, where
various protocols implement the functionality assigned to a given layer. 

The Layers

The Physical Layer 
This is the first layer of standards. It’s a set of rules regarding the hardware
used to transmit data. Among items covered at this layer are the voltages
used, the timings of transmission, and the rules used for the initial
handshaking connection. 

The Data Link Layer
The physical layer provides the data link layer with bits. Now this layer
provides the bits with some meaning. We no longer deal with bits but
instead with data frames - packets, containing data as well as control
information. The data link layer adds flags to indicate the start and end of

This layer’s standards perform two important tasks. It ensures that data is
not mistaken for flags, and that it checks for errors within the frame.

 The Network Layer
The network layer, is concerned with packet switching. It establishes virtual
circuits (Paths between terminals) for data communications. As the sending
end, the network layer repackages messages from the transport layer above
it into data packets, so the lower layers can transmit them.

The Transport Layer
The transport layer of the OSI model has many functions, including several
order f error recognitions and recoveries. As the highest order, the Transport
layer can detect errors, identify packets that have been sent in the incorrect
order, and then rearrange them. The transport layer also regulates the
information flow by controlling the messages movements.

The Session Layer
The session layer is concerned with the management of the network. The
user communicates directly with this layer. It can verify passwords entered
by the user. It can determine who uses the network, for how long, and for
what purpose. It controls data transfers and even handles recovery from
system crashes.

The Presentation Layer
This layer is concerned with the network security, file transfers and
formatting functions. At the bit level it is capable of encoding data in a
variety of different forms including ASCII and EBCDIC.

For true Communication, both communicating computers must contain the
same protocols. This level handles protocol conversions between different
computers using different formats.

The Application layer
The application layer handles messages, remote logons and the
responsibility of network management statistics. At this level are the
database management programs, electronic mail, file server and printer
server programs. The operating systems command and response language.


Unfortunately (for the many people and companies that spent so much time
and money on the effort), the TCP/IP suite of protocols has eclipsed OSI,
and you don't hear much about OSI anymore (except for a few applications,
such as the X.500 directory service).

When work began (in the late 1970s) on providing a standard method for
communications between different hardware platforms, TCP/IP was not
considered an option for serious commercial applications, since TCP/IP:

Required you to run UNIX (which, at the time, was not used for commercial
applications and had only a command-line user interface)

Had poor security and management features

Had too small an address size

Therefore the ISO promoted development of OSI.

Although all major (and many minor) computer vendors now have OSI
products, the OSI protocols were never widely implemented, and TCP/IP has
become the first choice for multi-vendor networking, because of its:

Lower-cost and more-efficient implementation (less CPU time required,
smaller programs)

Availability for most operating systems

Fast standardization and development cycle (usually using the Internet to
facilitate communications) when a new requirement is identified

Familiarity among college graduates (universities use TCP/IP, so once out of
school, a graduate's first choice when designing a system is to use TCP/IP)

Easier-to-access (and zero-cost) documentation and standards (they are all
available on the Internet)

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