Network Troubleshooting via the OSI Model
Should you intend to be involved in the computer networking field at any level, then an understanding of the Open Systems Interconnect (OSI) model is a must. There is NO way round this! Anyway, the better you understand this model and how it relates to "real world" computer networking, the better your network design and troubleshooting skills will be.
If you intend to sit IT industry professional exams such as CompTIA's N+ or Cisco's CCNA you WILL need to have at least a basic grasp of what the OSI model is, does and how to apply it in real world scenarios.
In this article we won't explain the OSI model in full or what it does. For full description on the OSI model click here
This article describes basic steps on how to use the OSI model when troubleshooting network connectivity issues. So let's begin!
We all hate the dreaded "This page cannot be displayed" or similar network / internet connection 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 begin to troubleshoot our internet connectivity issues from our browser down to the physical cabling (top down) or from the cabling up to the browser (bottom up) or somewhere in between (divide and conquer). This provides a structured approached to network troubleshooting rather than just guessing.
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. Please DO NOT confuse the Application layer with applications installed on a PC! 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. Known as "Top town" troubleshooting.
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 troubleshooting 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 (LAN / Proxy) 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 - 169.254*.*?
- 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 use a structured approach to easily identify or troubleshoot problems with our internet / network connection.
Last Updated (Wednesday, 11 September 2013 23:49)