1. Understanding the OSI Reference Model

2. Layered Architecture of N/W: WHY?
It provides a logical way of understanding how in a network various devices communicate.
Like an OSI model provides us a logical view in terms of 7 different layers or TCP/IP describes the same in terms of 5 different layers.
Fixing the problem(bug) is easier.

3. Layered Architecture of N/W: WHY?
It reduces design complexity.
Purpose of a Layer: to provide services to its higher layer while hiding the implementation details.
What is a protocol?
agreement between communication parties on how communication is to proceed.
e.g. “namasakar” is a protocol whenever we meet anyone to initiate conversation.

4. Diagram 1:

5. Explanation: Entities at same layer are called peers.
(peers can be software processes, h/w devices, or even human beings)
Does direct communication happen between any two peers?
Between each layer there is an interface. An interface defines which primitive operations and services the lower layer makes available to the upper one.

6. Network Architecture A set of layers and protocols.
A list of protocol used by a certain system, one protocol per layer, is called a “Protocol Stack”.

8. Diagram 3: Information flow supporting virtual communication

9. Design issues for the Layers:
Reliability
1. The messages might have been damaged during transit (electrical noise, random wireless signals, hardware flaws, software bugs etc). How to detect them?
2. Finding a working path through a network.
Adapting to the evolution of networks.
To be able to identify the sender and the receiver.
Security

10. Connection-oriented vs. Connection-less services
Telephone system vs. Postal system.
Six different types of services.

11. Difference between services and protocols

12. The OSI Reference Model
Open Systems Interconnection
Has seven layers.
A layer should be created where a different abstraction is needed.
Each layer should perform a well-defined function.

14. The Physical Layer Retransmits raw bits over communication channel.
Design issue: when one side sends 1 bit it is received as bit 1 on the other side and not as bit 0.
Typical questions:
What electrical signals should be used to represent 1 and 0.
How many nanoseconds a bit lasts.
to summarize it is concerned with mechanical, electrical and timing interfaces as well as selection of physical transmission medium

15. The Data Link Layer
Main task is to transform the raw bits into frames( a frame might have a few hundred or thousand bytes).
Why frames?
It makes possible to mask errors in one or more bits from the network layer( the next higher layer)

16. Continued.. Design issues:
Flow control : the sender might be faster as compared to the receiver.
Medium access control: in case of broadcast networks.

17. The Network Layer
Key design issue is determining how packets are routed from source to destination.
Paths can be stored (static) or determined dynamically.
Handling congestion along with the help of higher layers.
Quality of Service(delay, transit time, jitter etc) are also concerned with network layer.

18. The Transport Layer Basic function:
Accept data from application layer.
Split it into smaller pieces if needed.
Pass them to network layer.
Ensure that the pieces arrive correctly at the other end.
*Transport layer is a true end-to-end layer*

19. The Session Layer:
allows users on different machines to establish sessions between them.
Offers various services:
Dialog control: keeping track of whose turn it is to transmit
Token management: preventing two parties from attempting the same operation simultaneously.
Synchronization: check-pointing long transmissions.

20. The Presentation Layer
Concerned with the syntax and semantics of information transmitted.
Eg.
1) converts the request into a more general format.
2) It also includes some encryption services.

21. The Application Layer Contains a variety of protocols.
One of the widely used is HTTP.
When a browser want a web page, it sends the name of the page it wants to the server hosting the page using HTTP. Server then sends the page back.
Other protocols are used for file transfer, electronic mail, etc.

22. World Wide Web is a “distributed system” that runs on the internet
World Wide Web is a “distributed system” that runs on the internet. (what’s the difference between a distributed system and computer network!)

23. “The way to really know if you know something is to teach it to somebody.” Thank You!