1. Layered Task, OSI Model, TCP/IP Model
Chapter 2
Layered Task, OSI Model, TCP/IP Model

2. Protocol Layers
In Chapter 1, we discussed that a protocol is required when two entities need to communicate. When communication is not simple, we may divide the complex task of communication into several layers. In this case, we may need several protocols, one for each layer.
Let us use a scenario in communication in which the role of protocol layering may be better understood. We use two examples. In the first example, communication is so simple that it can occur in only one layer.

3. Layered TASK
We use the concept of layers in our daily life. As an example, let us consider two friends who communicate through postal mail. The process of sending a letter to a friend would be complex if there were no services available from the post office.
Topics discussed in this section:
Sender, Receiver, and Carrier Hierarchy

4. Figure 2.1 Tasks involved in sending a letter

5. Why layered communication?
To reduce complexity of communication task by splitting it into several layered small tasks
Functionality of the layers can be changed as long as the service provided to the layer above stays unchanged
makes easier maintenance & updating
Each layer has its own task
Each layer has its own protocol

6. Reference Models
OSI reference model
TCP/IP

7. OSI Reference model Open System Interconnection 7 layers
Each layer performs a well define function
Functions of the layers chosen taking internationally standardized protocols
Number of layers – large enough to avoid complexity

8. Seven layers of the OSI model

9. The interaction between layers in the OSI model

10. OSI Layers

11. Physical layer Transporting bits from one end node to the next
type of the transmission media (twisted-pair, coax, optical fiber, air)
bit representation (voltage levels of logical values)
data rate (speed)
synchronization of bits (time synchronization)

12. Physical Layer
Physical layer is responsible for electrical, mechanical and procedural checks. Data will be converted into Binary(i.e) 0’s & 1’s. Data will be in the form of electrical pulses if it is Coaxial or twisted pair cable and in the form of light if it is fiber optic cable.
Devices working on physical layer are hubs, Repeaters, Cables, Modems etc.

13. Note The physical layer is responsible for movements of
individual bits from one hop (node) to the next.

14. Data Link layer Transporting frames from one end node to the next one
- framing physical addressing
- flow control error control
access control

15. Data Link Layer Data Link layer is divided into two sub layers:
LLC- Logical Link Control
It talks about WAN protocols E.G. PPP, HDLC, Frame-relay
MAC- Media Access Control
it talks about physical Address. It is a 48 bit address i.e. 12 digit hexadecimal Number.
It is also responsible for Error Detection.

16. Data Link layer - example-

17. Note
The data link layer is responsible for moving frames from one hop (node) to the next.

18. Figure 2.8 Network layer

19. Network Layer
Network Layer is responsible for providing best path for data to reach the destination. Logical Addressing works on this layer. Router is a network device layer device. It is divided into two parts
Routed Protocols
e.g. IP, IPX, Apple Talk
Routing Protocols
e.g. RIP, IGRP, OSPF, EIGRP

20. the source host to the destination host.
Note
The network layer is responsible for the delivery of individual packets from
the source host to the destination host.

21. Figure 2.9 Source-to-destination delivery

22. Figure 2.10 Transport layer

23. Note
The transport layer is responsible for the delivery of a message from one process to another.

24. Transport Layer
It is responsible for end to end conectivity. It is also known as the heart of OSI Layers. Following tasks are performed at transport layer:
Identifing service
Multiplexing & De-multiplexing
Segmentation
Sequencing & Reassembling
Error Correction
Flow Control

25. Identifying Servicing

26. Identifying Servicing
TCP
UDP
Transmission Control Protocol
Connection Oriented
Acknowledgement
Reliable
Slower
Port no. 6
E.g. HHTP, FTP, SMTP
User Datagram Protocol
Connection Less
No Acknowledgement
Unreliable
Faster
Port No. 17
E.g. DNS, DHCP, TFTP

27. Multiplexing and Demultiplexing

28. Segmentation
Hello! How are you?
Hello!
How
Are
You ?

29. Sequencing and Reassembling
Hello! How are you ?

30. Session Layer
Session Layer is responsible for establishing, maintaining and terminating session.
Session Id works at session layer.
Examples:
RPC – Remote procedure call
SQL- structured query Language
NFS- Network File System

31. Presentation Layer
Presentation Layer is responsible for converting data into standard format.
Examples: ASCII, EBCDIC, JPEG, MPEG, BMP, MIDI, WAV, MP3
Following tasks are perform at Presentation layer:
Encoding Decoding
Encryption -Decryption
Compression - Decompression

32. Application Layer
Application layer is responsible for providing Networking services to the user. It is also known as Desktop layer. Identification of services is done using Port Numbers.
Ports are Entry and Exit Points to the Layer
Total no. Ports
Reserved Ports
Open Client Ports