Cisco CCENT / CCNA Certification Exam Training : An Introduction To The OSI Networking Model

As network admins, we're going to be primarily concerned with the first three layers of the OSI model - but for the CCENT and CCNA exam, we need to be concerned with all seven!

The OSI layers are often referred to by numbers, with the Application layer being Layer 7 ("L7"), the Presentation layer being Layer 6, and so forth all the way down to Layer 1, the Physical layer. To get you used to that, I'll refer to the layers by their names and numbers throughout these tutorials.

The Application Layer

This is the layer where the end users themselves interact with the network. Authentication services also run at Layer 7.

Protocols and services that run at L7 include:

- Email protocols SMTP and POP3
- Telnet
- HTTP
- File Transfer Protocol (FTP)
- Simple Network Management Protocol (SNMP)

The Presentation Layer

This layer answers one simple question: "How should this data be presented?" In addition to properly formatting data, encryption occurs at this layer.

The Session Layer

Layer 5 is the "manager" of the two-way communication between two remote hosts. This is the layer that handles the creation, maintenance, and teardown of communications between those two hosts.

The Transport Layer

TCP and UDP both run at the Transport layer, and we've got to know both of those protocols inside and out to pass the CCNA and CCENT exams. We'll look at those protocols in a future tutorial.

The Network Layer

It's at Layer 3 of the OSI model that you and I as network admins begin to have a great deal of interaction with the network. Internet Protocol (IP) runs at this layer, and since routers operate here at L3, this layer is often called "the routing layer".

In a nutshell, routing is a two-question process:

What valid paths exist from the local router to a given destination?
What is the best path to take to get there?

The Data Link Layer

Switches operate at Layer 2, as do Wireless Access Points (WAPs). We've got four major specifications that run here, some of which you may already be familiar with:

- Ethernet
- High Data Link Control (HDLC)
- Point-to-Point Protocol (PPP)
- Frame Relay

The Physical Layer

When things get a little complicated in networking, I like to remind myself that "it's all ones and zeroes!" Whatever data our end users are creating, it's going to eventually be "translated" into a series of 1s and 0s. Once that is done, it's the Physical layer that handles the actual data transmission. Anything to do with a physical cable - the pins, the connectors, the electrical current itself - is running at the Physical layer.