The Seven Layers of the OSI Network Model

The OSI Reference model is comprised of seven layers. This
network model is used to describe the different layers, or
"parts" of a network. Ranging from software related to
hardware related, they are: Application, Presentation,
Session, Transport, Network, Data-link, and Physical.
 
The Application layer refers to the software used on the
network, such as a web browser, and ftp client, or the
client's operating system, all of which are used to access
resources from the host. Even though all these applications
are considered part of the Application level, they may
develop their own methods of communicating between each
other, just as long as they are compatible with the level
below them. This means following the standard protocol for
tasks such as file transfer and email. FTP would be the
most common example of a file transfer protocol, and the
X.400 message protocol and the X.500 directory protocol are
worldwide accepted protocols for their two functions.
 
The second layer from the top is the presentation layer.
This layer follows a common protocol in communicating with
the Application layer, and of course the layer below it,
the Session layer. The presentation layer, though, is
concerned with taking the message contents and transforming
them appropriately for transmission across the network.
This means changing the Application contents into digital,
or binary, data. ASCII (American Standard Code for
Information Interchange) is an example of a component of
the presentation layer. This takes displayed characters and
converts them into binary values, so they can be
transmitted over the network.. In addition to external data
representation transformation, which has just been covered,
the presentation layer is also used for compression and/or
encryption of the data. However, the presentation layer on
the other end must also know how to decrypt or decompress
this data.
 
The third layer from the top is the Session layer. This
layer includes functions that establish a connection with
the host. After the presentation layer transforms the data
for transmission, the session layer makes it easy for the
data connection to be maintained between processes on
different systems. In other words, when a process requests
a connection with another process on another system, the
session layer helps establish the link. Therefore, this
layer is responsible for the following tasks:
 
 1. Starting and activating the session 
 2. Stopping and releasing the session 
 3. Synchronizing the two ends 
 4. Dialog control 
 5. Transfer of data 
The session layer is the last "upper" or software-related
layer, and provides the connection to the "lower", or
hardware-related layers.
 
The first of the two middle layers is the Transport layer.
This layer is for decomposing packets that are sent from
the session layer, so they may be transmitted over the
network. On the other end, the transport layer puts these
back together again. When Transport breaks up these layers,
it assigns a sequence number to each one, so if they are
received out of order on the other side, the remote
Transport layer can recompose them as if they had never
been broken up. Afterward, the transport layer puts the
correct destination address on the packets and does simple
routing of them between networks if it is in an
internetwork.
 
The next layer down is called the Network layer. This layer
accomplishes the task of telling the packet exactly which
hosts to go through to reach the final destination host. It
does detailed routing of the message so the message knows
how to get there. A simple analogy is comparing what
Network does to looking carefully at a roadmap and getting
explicit directions, rather than saying merely that you
want to go from location x to location y. A widely accepted
Network Layer protocol is X.25.
 
Now that the message is properly encoded, broken down, and
routed, the only thing left to with it is physically
transport it. The Data-link layer, the first of the two
physical layers, does data transport. It sends data from
one end of a line to the other, it activates and
deactivates the link, it detects errors in the data link,
it allows the link to be shared, it provides transparent
data flow, and allows for error recovery and modification.
Commonly accepted standards for the Data-link layer are
ANSI's ADCCP (Advanced Data Communications Control
Procedure), HDLC (High-level Data Link Control), SDLC
(SNA's Data Link Control), and IEEE's 802.2.
 
The final layer accomplishes the task of actually sending
the message over the physical link. This link is called the
Physical Link. There are different ways to transmit this
binary data, such as analog signals, amplitude or frequency
modulation (AM or FM), light signals, or radio signals.
Standards for transmission of binary data in this layer
are: CCITT's x.21 for digital transmission, and v.35 & v.24
for analog transmissions.
 
The whole object of the OSI Reference Model is in its name:
Open Systems Interconnect. This means that it has to be
open for different types of protocols or methods for each
layer, while still providing compatibility between the
other layers. This will allow flexibility for programmers
while still maintaining a strict standard for how these
layers must interact with the ones above and below it.