OPEN SYSTEMS INTERCONNECTION MODEL

This model was created by the International Organization for Standardisation(ISO). This provides a framework for communication between any two networks. There are 7 layers in this model. The lowest is the physical layer and the highest one is the application layer.The purpose and design issues of each of the 7 layers are different. Each layer performs a particular activity concerned with the task of communication between devices. Although I will be giving you a concise explanation for the design issues of each layer, go through the following diagram well. It explains a lot about the different layers of OSI.

Following image taken from google images.

Following are the different layers of the OSI model and their purpose.

7.APPLICATION LAYER
This layer provides network services to user applications. It consists of a set of protocols that define specific user-oriented applications such as e-mails, file transfers and virtual terminals. Some of the protocols used in this layer are TELNET(Virtual terminal),FTP(File transfer),E-mail(SMTP and POP), and also HTTP.

6.PRESENTATION LAYER
This layer is responsible for how the data looks or is formatted. It is concerned with the syntax and semantics of data getting transmitted. Data compression, encryption and decryption happen at this layer. In practice, this layer is usually incorporated with the application layer.

5.SESSION LAYER
Provides for how two devices establish, maintain and manage a connection. Also responsible for enforcing the rules of dialog,synchronising the flow of data and reestablishing a connection in the event a failure occurs.

4.TRANSPORT LAYER
Ensures error-free delivery of data. Accepts data from the session layer, partitions the data into smaller packets, passes them to the network layer and ensures that the data arrives correctly at the destination.

3. NETWORK LAYER
Responsible for end-to-end routing or switching of data using intermediate routers.

2. DATA LINK LAYER
This layer groups the bits that are received(or to be transmitted)into what are called as frames. This layer provides for error detection, framing and flow control. Also handles lost, damaged or duplicate frames.

1. PHYSICAL LAYER
This is the lowest layer of the OSI reference model. This layer is responsible for the actual transmission of bits from one end to another.

The message to be transmitted is referred by different terms in different layers. Let us go through them.
7. Application layer - Message
6. Presentation layer - Message
5. Session layer - Message
4. Transport layer - Message
3. Network layer - packets
2. Data link layer - Frames
1. Physical layer - Bits

PREVIOUS EXAM QUESTIONS AND ANSWERS

1. Match the following:
(a) Physical layer (i) Allow resources to network access
(b) Data link layer (ii) Move packets from one destination to another
(c) Network layer (iii) Process to process message delivery
(d) Transport layer (iv) Transmission of bit stream
(e) Application layer (v) Formation of frames

Codes:	a	b	c	d	e
(A)	iv	v	ii	iii	i
(B)	v	iv	i	ii	iii
(C)	i	iii	ii	v	iv
(D)	i	ii	iv	iii	v

June 2014 - Paper II Q.No 5
Ans:- A
Explanation:- If you find many options confusing choose one which you are very confident and about and then compare the rest of the options. Here, Transmission of bit streams is the work of the physical layer is something which one can be very sure about. So, a is matched to iv. There is only one option which does that and it is A. So just compare the rest of the codes and confirm that they are right. Data link layer is formation of frames. Network layer move packets from one destination to another. Transport layer is process to process message delivery and finally Application layer is allow resources to network access. So the correct answer is A.

2. Which layers of the OSI reference model are host-to-host layers?
(A) Transport, session, presentation, application
(B) Network, transport, session, presentation
(C) Data link, network, transport, session
(D) Physical, data link, network, transport
June 2014-Paper III
Ans:- A
Explanation:- The host-to-host layers are the four from the upper layers. Application, presentation, session, transport. The last three are referred to as media layers.

3. Encryption and decryption is the responsibility of _____layer.
(A) Physical
(B) Network
(C) Application
(D) Data link
December 2013 - Paper II
Ans:- C
Explanation:- Since the presentation layer is not available as an option, the right answer is Application. In the explanation above for Presentation layer, there is a mention that in practice it is combined with Application. So the right answer is C.

4. Which layer of OSI reference model is responsible for decomposition of messages and generation of sequence numbers to ensure correct re-composition from end to end of the network?
(A) Physical
(B) Data link
(C) Transport
(D) Application
December 2013 - Paper III
Ans:-C
Explanation:- Actually physical, data link and application are completely ruled out for the above question and so the right answer is C.

There are many more questions from the same topic which will be covered later.