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OSI
Model and
TCP/IP Model
The OSI model by IBM is the biggest
boon that systems developer can have had for their karma in the past
life.
What exactly is the OSI model?
As I had mentioned it is actually a
layered stack, with 7 layers. Each of these 7 layers are implemented by
different protocols for each layer. But keep in mind, its not necessary
that all these layers have to be used, when trying to device a system. A
system can also make away with any of the 7 layers like say the Session
Layer or the Application Layer.
Consider a Router for example. A Router is a device, which works in the
Network Layer of the OSI Model. Its main function is to route packets
that arrive at its ports. Router takes a routing decision for each of
the packet that arrives at its port, on the basis of Routing Tables.
Routing Tables are somewhat similar to the Time Table we follow while
attending our courses in college. Depending on the subject scheduled in
the timetable for a particular day, we decide whether to attend the
lecture or not. In a similar way the Router looks at the destination
address (which is the IP address) of the packet that is to be routed.
Now remember that an IP address is made up of a Host ID and a Network ID
(I will cover IP addresses in depth in the future articles). The Router
looks at the Network ID and compares it with the other Network ID’s
present in its Routing Tables. If the particular Network ID is found to
be present in the Router’s Table, the packet is routed to the
appropriate destination successfully. If the Network ID of the packet is
not present in the Router’s Routing Table, then the router forwards the
packet to the next nearest router and this process continues till the
packet reaches its intended destination.
This is very similar to what you do when you are asked where someone
stays, while walking on the road. You are shown the address by the
person, who wants to reach a place (Consider this Address as the
Destination IP address). Now what you do is directly look at the name of
the street. This is because the name of the building is not something
you will be immediately familiar with, as a building with the same name
will surely exist in thousand different streets.
If you are very well aware of the geography of the place, then by
looking at the Street (Network ID) you tell the person where the street
is. ( similar to a Router finding that the Network ID field of the IP
address of the packet is present in its Routing Table). If you don’t
know the street or the address, you tell the person to ask some one else
( that is you forward him to the next nearest Router).
OSI model doesn’t say you do things this way. It just tells you “ Hey
keep in mind that these things should be covered while designing a
feasible network solution “.
TCP/IP Model
The TCP/ IP model superseded the
OSI model. This is the model that is currently most widely used. The
various layers in the TCP/IP model are :
5) Application
4) Transport
3) Internet work
2) Data Link
1) Physical
Note that the Sessions and
Application Layer present in the OSI Model are absent in the TCP/IP
Model.
In the 1990’s, TCP/IP has become firmly established as the dominant
commercial architecture and as the protocol suite upon which the bulk of
new protocol development is to be done.
There are a number of reasons for the success of the TCP/IP model over
the OSI model:
- Internet is built on the foundation of
the TCP/IP suite. The tentacles of the Internet and the World Wide Web
have spread throughout the world and that is the main reason for the
success of TCP/IP model over the OSI model.
- TCP/IP protocols were initially
researched under a project in the Department of Defense (DOD). DOD was
committed to international standards and most of its operational
requirements couldn’t be met by the OSI model. So it started to
develop the TCP/IP. Since the DOD is the largest consumer of software
products in the world, the vendors were encouraged to develop TCP/IP
based products.
In the next article I will cover
more about the various layers in the OSI Model and the TCP/IP Model.
–
Deepak Chandrasekaran
August 25, 2002
Articles on
Computer Networking
Introduction
Computer Network - Basics
OSI Model and TCP/IP Model
Layer One of OSI Model - Physical Layer
Are you Safe on Network?
Bluetooth
GPS : Tension Free Driving
Robots in Human Life
Router : An Introduction
Secure Socket Layer : Online Data Security
Smart Wi-Fi
VoIP : Technology to Cut Phone Bills
VPN : An Introduction
Wireless Technologies: Voice and Messaging
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