OSI LAYERS MODEL

Introduction
During the early years of our modern computer era, very few standards
and protocols existed between various manufacturers. However, as time
went on and computer technology continued to improve and become more
widespread, it became apparent that standards would be necessary to
ensure compatibility.


This was especially true with regard to networks, and networking
technology. Since the main purpose of a network is to share
information, a standard that governs how this information is formatted,
transmitted, received and verified would make it possible for
information to be shared openly, even when dealing with dissimilar
networks.


This need for a standard means of implementing open communications led
the ISO and ANSI to develop the seven-layer network communications
model known as Open Systems Interconnect. By providing guidelines
regarding the way network equipment should be manufactured and how
network operating systems communicate on a network, the OSI model
became the common link that allows data to be transmitted and exchanged
reliably. Although it does not actually perform any functions or do any
of the actual work, the OSI model defines the way things should be done
by the software and hardware on a network so that communications can
take place between two computers or nodes.


In this way, the OSI model provides a universal set of rules that make
it possible for various manufacturers and developers to create software
and hardware that is compatible with each other. This makes for
organized communications. As I thought about this, I related it to the
freeways that connect the various states of the mainland U.S. Because
all of these freeways were constructed with the same set of standards
regarding the width of each lane, the proper side that a person should
drive on, the speed at which they should travel, and so on, people can
comfortably drive across the country in an organized and efficient
manner and car manufacturers are able to design cars within these
guidelines as well.


On the other hand, if each state had devised its own set of rules, each
differing from the other, not only would there be a lot more chaos on
the roads, but also car manufacturers would have a hard time designing
vehicles that would be compatible with each state's roads. To me, this
illustrates the importance of the OSI model with respect to network
communications. Not only is it the foundation for all network
communications today, but also because it is such a fundamental part of
these communications, it becomes very apparent to me that it is very
important for a network technician to understand the OSI model in full
detail.


The OSI model is made up of the following layers: the physical, data
link, network, transport, session, presentation and application.
Together, these seven layers are collectively referred to as a stack.
As a node receives data, each layer starting with the physical layer
extracts the various portions of the packet and this process works its
way up to the application layer. When data is sent, it begins at the
application layer and travels down to the physical layer. The
information is pushed to the next layer of the stack by means of
commands called primitives. Each layer uses a peer protocol to encode
the information, which ensures that the same layer on the receiving
node will be able to understand the information.


Physical Layer

Beginning at the bottom, the first layer is the physical layer. It
governs the actual voltages, type of electrical signals, mechanical
connections and other items relating to the actual data transmission
medium. This includes cabling types, distances and connectors, as well
as protocols like CSMA/CD.


Data Link Layer

The next layer is the data link layer. This is the layer that actually
constructs the frames, and it also performs error checking using CRC.
It ensures that the frames are sent up to the next layer in the same
order that they were received, providing an error free virtual path to
the network layer. The data link layer consists of two sub layers; the
logical link control (LLC) and the media access control (MAC), which
provide reliable communications by ensuring the data link is not broken
and also by examining packet address information. A bridge is an
example of a device that works at this layer. A bridge learns, forwards
and filters traffic by examining the layer 2 MAC address.


This helps segment network traffic. More recently, bridges have been
replaced by switches, which performs the same functions as a bridge,
but can do so on each port. To find out more about switches, visit the
Products link on the left.


Network Layer
Moving up to the next layer in the stack we come to the network layer.
This layer actually routes packets of data, finding a path (both
physical and logical) to the receiving or destination computer. It
provides a unique address for each node through address resolution. One
of the most common protocols for routing information at this layer is
the Internet Protocol (IP). An example of hardware that can operate at
this layer is a router. Although routers are often used to allow a LAN
to access a WAN, layer 3 switches can also provide routing
capabilities, but often at full wire-speed.


Transport Layer

The transport layer makes sure that the data arrives without errors, in
the proper sequence and in a reliable condition. It uses flow control
to make sure that information is sent at the proper speed for the
receiving device to be able to handle it, and it repackages large data
into smaller messages and then back again at the receiving node. An
example protocol at this layer is the Transmission Control Protocol
(TCP). Layer 4 switches can use the port information found in the TCP
header to provide QoS (Quality of Service) and load balancing. To learn
more about multi-layer switches, visit the Products link.


Session Layer

The session layer establishes the link between two nodes and ensures
that the link is maintained and then disconnected. This is referred to
as the session. It also makes sure the session is orderly, establishing
which node transmits first, how long it can transmit, and what to do in
case of an error. It also handles the security of the session.


Presentation Layer

The presentation layer deals with the actual formatting of the data. It
handles compression, encryption, as well as translation to make sure
differences in formatting can be read by the receiving node. For
example, data might be converted from EBCDIC to ASCII formatting so
that the receiving node can understand it.


Application Layer

This brings us to the seventh and final layer, the application layer.
It allows applications access to network services, such as file and
printer sharing, as well as file transfer and management services. This
would be the layer that a programmer uses to allow his application to
access a network service, such as linking into a database.


Although this explains the flow of data and what processes are
performed by each layer starting with the physical layer and working to
the top, or application, layer, the process would be the same, only
reversed, for data flowing from the application layer and down to the
bottom, or the physical layer.