The Open Systems Interconnection (OSI) model consists of seven layers of network architecture. These layers are the Application, Presentation, Session, Transport, Network, Data-Link, and Physical layers. For the most part if you’re a network administrator, like me, most of your work will be concerned with the first three layers, Physical, Data-Link, and Network.

Now let’s break down the different OSI model layers and what they do:

Layer 7 – Application Layer

The Application layer is generally the closest to the end user. This is where the end user will interact with software in order to communicate across a network. General examples of this layer are telnet,  HTTP, FTP, SMTP.

Layer 6 – Presentation Layer

The Presentation layer is where the data from the Application layer is taken and encapsulated into a form of data that can be used for travel across a network. This layer converts data for use in the Application layer or the Session layer for use in the opposing layer.

Layer 5 – Session Layer

The Session Layer maintains communications or connections between nodes on a network. It establishes, maintains, and terminates. It can operate in full-duplex, half-duplex, or simplex operating modes. Any checkpointing or recovery operations in TCP usually happen on this layer. An example of this layer in action would be an application that uses RPC to execute actions in another address space remotely.

Layer 4 – Transport Layer

The Transport layer is where reliability is controlled through flow control, (de)segmentation, and error control. This is where all data is transfered between end users. Error checking happens on this layer and it can retransmit any failures. Examples of this implementations of this layer are TCP and UDP.

Layer 3 – Network Layer

The Network layer, my favorite, is where all routing happens on a network. This is where data is transferred to it’s destination across one or more networks. This is where routers operate. Data travels across this layer alot like traveling the highway system. Data flows between different routers (hops) to finally make it to it’s destination. The most known implementation of the Network layer is Internet Protocol or IP, as it is most called.

Layer 2 – Data-Link Layer

The Data-Link layer is where data is transfered between nodes on a network. This layer has the capability to detect and correct errors that might have occured on the Physical layer. One of the most well known implementations of this layer is Ethernet. You will find most network switches on this layer. The Data-Link layer provides a connection across the physical link by using vendor assigned hardware MAC address verses assigned IP addresses, like the Network layer does.

Layer 1 – Physical Layer

The Physical layer is where all electrical and physical connections are made. On this layer you will find cabling, hubs, and, network adapters to name a few. Where the Data-Link layer connects multiple nodes, the Physical layer is mostly concerned with connected a single device to the network medium.

That’s a rough introduction to the OSI model. There is a lot more theory of it, but I just wanted to give a quick overview.

All your troubleshooting efforts should start at layer 1, what I like to call ‘checking the basics’. Bypassing layer 1 can cost you alot of time in troubleshooting. I generally make sure cables are plugged in and all devices are powered on. A lot of times you may find a simple easy to correct issue here and save yourself alot of grief in troubleshooting the higher level layers. From there you can just work your way up the layers. This will provide good coverage of possible failures or misconfigurations across the network.

Some network administrators, including my self, talk of a ‘Layer 8′. This is the actual end user themself. I like to call this ‘operator head space and timing’. You should be able to judge and categorize your end users by level of knowledge if you’ve been working with them for some period of time. You can usually skip to ‘Layer 8′ if you’re able to identify the issue with the end user from their complaint. This of course is not part of the Cisco certification process and has more to do with experience in dealing with end users, which you will learn on the job.