The OSI Network Model
By Shaun Hummel
Enterprise Networking Defined
An enterprise network is a large network comprised of hundreds or thousands of
workstations all connected with WAN circuits across many states and⁄or countries. The
components are comprised of network devices, circuits, servers, applications, security and
network management that connect employees for the purpose of supporting business
processes. It is business processes that make companies profitable. The leveraging of new
technologies to reduce costs and increase revenue is a business strategy that many
companies have employed recently. That is contrasted with the Internet Service Provider
(ISP) market, which refers to companies whose business it is to sell infrastructure, web
hosting and application services to small, medium and enterprise companies.
Many enterprise companies now rely on ISP for some or all of the mentioned components
that comprise an enterprise network. Usually all circuits are provisioned with the ISP.
Some devices are ISP owned and the servers and applications can be all ISP owned if you
have a hosting agreement with them that provides those services. Management and security
of your network can be outsourced to an ISP as well, which is becoming popular with many
Fortune 500 companies. Examples of that are AT&T and IBM, who offer outsourcing agreements
with many services and otions.
OSI Model as a Phone Call
The OSI model was developed years ago as a reference for network protocol and
application designers to build their products with an open standard. That would promote a
standard for developing network devices, interfaces, applications and services that would
work together. It is a model rather than a specification since it defines layers and
services for those layers and how each layer works with the layer above and below it. The
model is a representation of how a network operates much the same as a phone call.
A phone call from your home to a particular destination requires a physical connection
that electrically connects those two locations. When you take your phone off hook to make a
call that is similar to layer 1 physical and layer 2 data link services. When you dial a
number that is a layer 3 network service or phone number routing. When someone answers the
phone call that is similar to layer 4 transport and layer 5 session services with talking
and listening. The specific language that you are speaking is layer 6 presentation and
layer 7 application services. Today's data networks use similar design principles.
OSI Layers and Your Business Network
Your desktop computer at work is connected to an application server over a physical
media such as unshielded twisted pair (UTP) cable. That layer 1 service is the electrical
signaling from your desktop to a campus switch and from a campus switch to a server. Your
desktop network interface card connects your desktop to the network cable and uses a layer
2 data link protocol such as Ethernet that establishes a connection with the campus switch
and defines when that desktop has access to the network.
Before sending data, the desktop must know the address of the server to which it is
sending data. That is a layer 3 network service. The layer 4 transport service establishes
a logical connection between server and desktop for coordinating traffic flow. As well
layer 4 does error correction and re-transmission of packets if there are problems with
data delivery. With the data connection established it is the layer 5 session service
that maintains a logical connection between application processes at the desktop and server.
The formatting of the data for a particular application is a layer 6 presentation
service. Many applications will utilize their own formatting which is something you learn
when you open files from different word processing vendors. The application interfaces
with the OSI model at layer 7, which is the application layer. This level defines an API
that applications use to develop their specific applications that will work with that layer.
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