Lecture 2(a): OSI (Open System Interconnection) Reference Model
Reference:
The word reference always between two things, there is no reference of only a single thing. The eye’s reference is with eye and the leg’s reference with leg. This reference means there is any two devices is involved in this reference not one device. OSI Reference Model is not for a single machine, this model is for networks.
Don’t Get Confused.
ISO - International Organization for Standardization
OSI - Open System Interconnection
IOS - Internetwork Operating System
The ISO created the OSI to make the IOS more efficient. The “ISO” acronym is correct as shown.
To avoid confusion, some people say “International Standard Organization.”
Why do we need the OSI Model?
To address the problem of networks increasing in size and in number, the International Organization for Standardization (ISO) researched many network schemes and recognized that there was a need to create a network model that would help network builders implement networks that could communicate and work together and therefore, released the OSI reference model in 1984.
Using the OSI model to discuss networking concepts has the following advantages:
• Provides a common language or reference point between network professionals
• Divides networking tasks into logical layers for easier comprehension
• Allows specialization of features at different levels
• Aids in troubleshooting
• Promotes standards interoperability between networks and devices
• Provides modularity in networking features (developers can change features without changing the
entire approach)
Communication between two clients in a network When a computer A communicates to computer between the both computers'peer layers. Every peer layer in the other comPuter
You can say this model as “Data Flow Chart”. The purpose of this model to understand the flow of data on the network, understanding the steps during moving the data in network.
It is a layered architecture (consists of seven layers). Each layer defines a set of functions in data communication.
7. Application Layer
Computer applications like MS Word, MS Excel etc. If we want to chat on Internet then we run MS Messenger (which is Application), without opening of any application on your computer you cannot make any work.
The first step for doing any work, to run application.
You send HELLO to other computer there is compulsory that same applications are running on both computers that want to communicate. If there are not running same applications on both computers then this HELLO not to be seen on computer screen.
6. Presentation Layer
Our language is English and computer learns that language is machine. This layer converts the data from one form to another form. It converts the data into machine language.
5. Session Layer
This layer is most critical and most conceptual. This layer creates virtual session. Virtual Session means to check the availability of other side. Sessions are of two kinds:
i. Physical Session (First we go and the checking)
ii. Virtual Session (First we checking then going)
Remember the word HELLO is not for our machine but it is another machine.
Example (Physical Session)
If you want to go to Lahore from Islamabad, then there are two ways. First is that you reached at Lahore without informing that person. When you asked about that person, you find that person is not available. You will come back by spending patrol, time without any progress.
Example (Virtual Session)
If you call that person before go to Lahore, if that person is available then I will move, otherwise I will not move. The advantage of this virtual session was, you have saved their resources.
Virtual Session provides the information about the other party available or not and also saving the resources.
Example
This layer normally to be said as G.M. This G.M. wants to move another G.M. of Company for meeting. There are seven gates for going, but only one gate from which G.M can go out. When two computers connected with each other then this is physical layer.
Example
Using MSN Messenger, you want to chat another person, if that person is available then the other window is opened otherwise a message will display as “User is not available”
4. Transport Layer
If other machine is available then this layer is responsible to transport the data on network. For the transport of data fragmentation is made. “The conversion of data into small pieces is called fragmentation or permutation”. E.g. HELLO is converted as:
| H | E | L | L | O |
There is MAC address exist, MAC address have used many devices like switches and LAN card.
As you know that data travel in form of 0 & 1 (these are the voltage level not mathematical and other number ), every device may contain different voltage, its means there is the involvement of voltage. If you have 3 or 5 voltage and its parallel there is line of any high voltage, then it may harm your data.
e.g.
During watching TV, you some time see lining on your TV set, this is why with any high voltage.
Same like above, if there is any data cable contains 3 volts but its parallel a high 5 volts is passing then it is very possible that any bit of your data changes (i.e. 0 to 1).
Example
During the chatting on messenger a person asked what is your age, he writes answer 25 and sends. During the data traveling its only one bit changes and your information will be changed as 65.
In reality no any system is so week, so to resolve this problem a layer is introduced called Data Link Layer. Its main function is the deduction of error.
CRC: (Cyclic Redundancy Code)
This algorithm is very important and famous. There may were mistakes take place, so an error deduction algorithm is made. It attaches an algorithm for sending data to the other side as it is.
Example
If we have data 25 (5 5) and generators of link side are 5. That generator will divide the 5 / 5 and remainder will 0. When you send 25 on Internet then not only 25 is passing but the footers (tailors) also. Due to some error the word 26 (5 5 1) instead of 25 is changed, on the other machine there will be attached a CRC Algorithm again to check the both checksum. If there is any different then there is any error in that data which you sent. If there is an error then your system (Receiver) will not correct that error but it will say to the sender to correct.
Retransmission is faster as compared to error correction.
There are two sources for errors:
a) Errors within the Machine or System (i.e. Computers, Routers, Modems etc)
b) Errors from outside the Machine (i.e. in media when data is sent)
According to the research there is 99% probability of error when data travels on Media. The error in system is very rare, if may occur then system recover or restore from its own cache.
1. Physical Layer:
After all the layers exist on a cable is attached to your computer, this cable works on which level is called physical layer.
Example
If you make a file and give to a peon for send to another place. The peon will move physically for the delivery of that file. The work of peon is to carry the data as it is.
Above you sent the word HELLO to other machine, this word travels on all layers by sequence, but if there is an error between the layers then the chances of data to be corrupted.
Q: Where that corrupted data is checked?
Ans: There is another Data Link Layer to the other side of Computer. From sender, the algorithm is attached with the data and will re verify by other side.
Switch, NIC Card (LAN Card) work on Data Link Layer.
•In order for data to travel from the source to the destination, each layer of the OSI model at the source must communicate with its peer layer at the destination.This form of communication is referred to as peer-to-peer.During this process, the protocols of each layer exchange information, called protocol data units (PDUs).
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