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.
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of free Cisco CCNP, BSCI, and BCMSN training and CCNA 640-802 tutorials, The Ultimate CCNA Study Package, and Ultimate CCNP Study Packages.
You can also visit his blog, which is updated several times daily with new Cisco certification articles, free tutorials, and daily CCNA / CCNP exam questions!
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