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
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
The Transport Layer
Examples of Transport Layer troublshooting are:
Do you have a firewall configured on your computer
The Network Layer
Examples of Network Layer troubleshooting are:
Can you ping your default gateway
The Data Link Layer
Examples of Data Link Layer troubleshooting are
Is the light lit on your network interface card (NIC)
The Physical Layer
Examples of Physical Layer troubleshooting are:
Is your network cable connected properly and secure
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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