Download Hello, I`m Brian Farrell, and welcome to PACE

Hello, I’m Brian Farrell, and welcome to PACE-IT's session on the OSI networking reference
model. Today, I'm going to begin by talking about a brief history of the reference models, and
then I'm going to talk about the network reference models themselves, and then I'm going to
conclude with a comparison between OSI and TCP/IP. There's a fair amount of ground to cover,
so let's go ahead and begin this session.
Let's begin with a brief history.
The Open System Interconnection (OSI) reference model is a conceptual model with two major
components. The first main component of the OSI reference networking model is an abstract
model of networking. It's a seven-layer model. The second major component is a set of specific
protocols, which allow differing computing systems to communicate with one another, despite
their different architecture.
Why was a networking model required? Early networks communicated using proprietary
languages. Because of those proprietary languages, early networks could only communicate
with like systems, so an IBM network could only communicate with another IBM network. In
addition to that, the U.S. government desired a robust computer communications system that
could survive disaster.
The first networking reference model that was developed was the TCP/IP reference model. The
Transmission Control Protocol/Internet Protocol reference model was published as the United
States Department of Defense's standard in 1982. All of the major system manufacturers
adopted the TCP/IP reference model beginning in 1984. AT&T moved the UNIX implementation
of TCP/IP to open source in 1989, further cementing TCP/IP's place in networking.
The OSI reference model came later than the TCP/IP reference model. The OSI model was
published in 1983 and it defines the relationships between differing protocols and hardware.
It's time to move on to a discussion about the networking reference models in more detail.
We're going to begin with the OSI reference model. It is a seven-layer reference model. Layer 1
is also called the physical layer. It standardizes the electrical signals that networks use. It also
defines cable standards and how the bits of data are placed on the physical media. Network
cables and hubs are part of Layer 1 of the OSI model.
Then there's Layer 2, which is also called the data link layer. It's responsible for identifying the
individual nodes, both the sending node and the receiving node. It also introduces an error
correction method known as the frame check sequence, or FCS. Layer 2 is composed of two
sublayers. The first of those is the logical link control layer. It is mainly responsible for flow
control and error correction. Then there is the media access control layer. It is mainly
responsible for node addressing. Switches and bridges are Layer 2 devices.
Then there's Layer 3, the network layer. It's responsible for routing functions between networks.
It also identifies networks and nodes on the network. Routers are Layer 3 devices.
Then there's Layer 4, the transport layer. It's responsible for breaking the data into smaller
pieces for the lower layers and for the actual data transport protocols, two of which are TCP and
UDP. The transport layer may be required to confirm the actual delivery of the data stream and
it may be required to offer error correction, and it does this through the use of TCP or
Transmission Control Protocol.
Then there's Layer 5, the session layer. This is the layer that is responsible for establishing the
initial parameters between two systems. It sets up and tears down the communication channel.
Layer 6, or the presentation layer, is responsible for taking data and converting it from a
machine-dependent language to a machine-independent language. This is also the layer that
has the main responsibility for encryption between networks.
Finally, we have Layer 7, the application layer. This is the layer that is responsible for the
protocols that request services or functions from other systems. These protocols may not be the
actual application. For instance, Internet Explorer is an application that uses HTTP at Layer 7 to
request Web pages.
Now let's talk about the TCP/IP reference model. This is a four-layer reference model. The
lowest layer is the network interface layer, which can also be known as the link layer. It handles
electrical signaling, flow control, error detection, and node addressing.
Then there's the Internet layer. This layer handles routing functions and identifies network
systems and nodes on those networks. Then there is the transport layer, which handles
breaking the data into more manageable pieces for the lower layers. It is also the layer that is
responsible for the delivery method, which can be either reliable or unreliable, and error
correction for when reliable delivery is used.
Finally, there is the application layer. This layer handles requests for services from applications.
It also handles translation to machine-independent languages and encryption. It also sets up
and tears down communication sessions between systems.
Now, let's do a comparison between the OSI and TCP/IP reference models. While TCP/IP is the
dominant model, most technicians communicate issues using the OSI model because it is more
specific. When a problem occurs, and believe me they will, it is easier to resolve them with the
more highly defined set of specifications of the OSI model versus the specifications of the
TCP/IP model. It's easier to resolve an issue at the session layer of the OSI model than it is to
track down what went wrong in the application layer of the TCP/IP model.
Both the OSI and TCP/IP models are reference models only. It is not mandatory that they be
followed. Each developer and manufacturer determines its own method of implementing the
reference models. While in theory there will never be a problem in communicating between
devices and systems, remember, it is only a theory.
Now, that concludes this session on the OSI networking reference model. I began with a brief
history, then I moved on to the networking reference models themselves, and then I did a very
brief OSI and TCP/IP comparison. On behalf of PACE-IT, thank you for watching this session
and I hope to do another one soon.