Troubleshooting Internet / Network Connectivity Using the Osi Model

We all hate the dreaded “This page cannot be displayed” or similar errors but how do we determine if the problem is network, cabling or just browser related. And what can we do to rectify the situation?

Using the Open Systems Interconnect model (OSI) we can troubleshoot our internet connectivity from our browser down to the physical cabling or from the cabling up to the browser.

The OSI model consists of 7 layers as shown:

7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Datalink
1 Physical

When a user tries to use a network service such as http (viewing a web page), ftp (file transfer), or email etc they are starting at the Application layer. Then the request or data has to pass down the remaining layers of the OSI model (a process called encapsulation) and then up the layers on the target computer (a process called decapsulation). It is in the passing down these layers that our internet connection has become corrupted or misconfigured, we just need to identify where.

In order to troubleshoot an internet connection we can either start at the bottom of the OSI model (bottom up troubleshooting) or the top (top down troubleshooting). Or we can take a guess and use “divide and conquer” troubleshooting, which will identify problems at the Network Layer.

For this troubleshooting example we will start at the top or Application (layer 7) layer of the OSI model and work our way down to the Physical (layer 1) layer.

Before we begin troubleshooting it is very important to understand what happens at each layer of the OSI model. Here are some brief descriptions of what happens at each layer.

7 Application – Deals with network services that interact with the user such as http, ftp, email, DNS etc. Problems related to browsers, ftp programs, email and network / internet programs can start here.

6 Presentation – Deals with data representation (data formatting) and encryption. Examples of technologies at this layer are ASCII, EBCDIC.

5 Session – Deals with interhost communication and is responsible for opening, closing and managing a session.

4 Transport – Deals with end to end connections, delivery of data and reliability. Examples of technologies at this layer are TCP / UDP and port numbers.

3 Network – Deals with logical address and routing (path determination) which includes IP addressing.

2 Datalink – Deals with physical addressing (MAC / LLC) and is responsible for getting data to other locations (LAN/WAN).

1 Physical – Media, signal and binary transmission. Putting the data on to the physical media.

For troubleshooting purposes we can group certain layers of the OSI model together. These are:

The Upper Layers – Layers 7 – 5 (Application, Presentation, Session)

Examples of Upper Layer trouble shooting are:

Can you view a web page in your browser
Can you send / recieve email
Are your username and / or password correct
Are your Internet Explorer connection settings correct

The Transport Layer

Examples of Transport Layer troublshooting are:

Do you have a firewall configured on your computer
Is your firewall blocking ports such as port 80, 21, 53 etc
Does turning you firewall off resolve the problem

The Network Layer

Examples of Network Layer troubleshooting are:

Can you ping your default gateway
Are you IP settings correct
Can you tracert a well know IP/DNS address / URL
Has your network adaptor been assigned an APIPA address
Are your DNS / DHCP settings correct
What out do you get from ipconfig, ping, tracert

The Data Link Layer

Examples of Data Link Layer troubleshooting are

Is the light lit on your network interface card (NIC)
Is your NIC inserted / installed correctly
Is the NIC disabled in Device Manager
Are your wireless settings correct
Is your wireless AP or wirless NICs functioning properly
Are the LEDs on your equipment on / blinking properly

The Physical Layer

Examples of Physical Layer troubleshooting are:

Is your network cable connected properly and secure
Are you using the correct cable type
Is your cable damaged or obstructed in any way

So as you can see. Using the OSI model we can easily identify or troubleshoot problems with our internet / network connection.

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