As network admins, we are going to be primarily concerned with the first three layers of the OSI model - but for the CCENT & 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, & so forth all the way down to Layer 1, the Physical layer. To get you used to that, I will refer to the layers by their names & 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 & services that run at L7 include:
Email protocols SMTP & 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, & teardown of communications between those two
hosts.
The Transport Layer
TCP & UDP both run at the Transport layer, & we have got to know both of those protocols inside & out to pass the
CCNA & CCENT exams. Right. We’ll look at those protocols in a future tutorial.
The Network Layer
It’s at Layer 3 of the OSI model that you & I as
network admins begin to have a great deal of interaction with the network. Internet Protocol (IP) runs at this layer, & 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 & zeroes!” Whatever data our end users are creating, it is going to eventually be “translated” into a series of 1s & 0s. Once that
is done, it is 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, & BCMSN training & CCNA 640-802 tutorials, The Ultimate CCNA
Study Package, & Ultimate CCNP Study Packages.
You can also visit his blog, which is updated many times daily with new Cisco certification articles,
free tutorials, & daily CCNA / CCNP exam questions!
For a FREE copy of his latest e-books, “How To Pass The CCNA” & “How To Pass The CCNP”, just
visit the website! You can also get FREE CCNA & CCNP exam questions every day!
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