Osi Reference Model And Layers

By Author: Surya
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OSI Reference Model – OSI was developed by the International Organization for Standardization (ISO) and introduced around 1980. It is a layered architecture (consists of seven layers), which defines and explains how communication happens in between two or more network devices within the organization or the Internet. Each layer defines a set of functions in data communication.

Application Layer (Layer 7)
Application Layer is responsible for providing an interface for the users to interact with application services or Networking Services.
Ex: Web browser etc.
Identification of Services is done using Port Numbers.
Port is a logical communication Channel
Port number is a 16-bit identifier.
Total Ports 0 – 65535
Reserved Ports 1 – 1023
Unreserved Ports 1024 – 65535

Service Port #
HTTP 80
FTP 21
SMTP 25
Telnet 23
TFTP 69
Presentation Layer (Layer 6)
Presentation Layer is responsible for defining a standard format for the data.
It deals with data presentation.
...
... The major functions described at this layer are:
Encoding – Decoding
Ex: ASCII, EBCDIC (Text)
JPEG, GIF, TIFF (Graphics)
MIDI, WAV (Voice)
MPEG, DAT, AVI (Video)
Encryption – Decryption
Ex: DES, 3-DES, AES
Compression – Decompression
Ex: Predictor, Stacker, MPPC
So, in the OSI Reference Model, the other Layer is as Follows:-

Session Layer (Layer 5)
Session Layer is responsible for establishing, maintaining and terminating the sessions.
It deals with sessions or Interactions between the applications.
A session ID is used to identify a session or interaction
Ex: RPC, SQL, NFS
Transport Layer (Layer 4)
Transport Layer is responsible for end-to-end transportation of data between the applications
The major functions described at the Transport Layer are:
Identifying Service
Multiplexing & De-multiplexing
Segmentation
Sequencing & Reassembling
Error Correction
Flow Control
Identifying a Service: Services are identified at this layer with the help of port numbers. The major protocols which take care of data transportation at the Transport layer are…TCP, UDP

TCP
UDP
Transmission Control Protocol
User Datagram Protocol
Connection-Oriented Connection Less
Reliable communication (With ACK) Unreliable communication (No ACK)
Slower data transportation Faster data transportation
Protocol number is 6 Protocol number is 17
Ex. HTTP, FTP, SMTP Ex. DNS, DHCP, TFTP
So, in the OSI Reference Model, the other3rd Layer is as Follows:-

Network Layer (Layer 3)
OSI Reference Model - Network Layer -

Network Layer is responsible for end-to-end transportation of data across multiple networks.
Logical addressing & path determination (Routing) are described at this layer.
The protocols works at Network layer are
Routed Protocols:
Routed protocols act as data carriers and define logical addressing
IP, IPX, AppleTalk… Etc
Routing Protocols:
Routing protocols perform path determination (Routing).
RIP, IGRP, EIGRP, OSPF & others
Devices works at Network Layer are Router, Multilayer switch etc.
Data-Link Layer (Layer 2)
Data-Link Layer is responsible for end-to-end delivery of data between the devices on a LAN network segment.
Data link layer comprises two sub-layers.
MAC (Media Access Control)
It deals with hardware addresses (MAC addresses).
MAC addresses are 12 digit Hexa-decimal identifiers used to identify the devices uniquely on the network segment.
It also provides ERROR DETECTION using CRC (Cyclic Redundancy Check) and FRAMING (Encapsulation).
Ex: Ethernet, Token ring…etc
LLC (Logical Link Control)
It deals with Layer 3 (Network layer)
Devices works at Data-Link layer are Switch, Bridge, NIC card.
Physical Layer (Layer 1)
Physical Layer deals with the physical transmission of binary data on the given media (copper, fiber, wireless…).
It also deals with electrical, mechanical and functional specifications of the devices, media. Etc
The major functions described at this layer are.
Encoding/decoding: It is the process of converting binary data into signals based on the type of media.
Copper media: Electrical signals of different voltages
Fiber media: Light pulses of different wavelengths
Wireless media: Radiofrequency waves
Mode of transmissions of signals: Signal Communication happens in three different modes
Simplex
Half-duplex
Full-duplex
Devices works at the physical layer are Hub, Modems, Repeater, and Transmission Media.