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OSI Model

Introduction

  • OSI - Open Systems Interconnection.
  • OSI Model has 7 layers.
    • Application
    • Presentation
    • Session
    • Transport
    • Network
    • Data Link
    • Physical

Physical Layer

  • Physical components of the hardware in networking.
  • For example: Ethernet cables
  • Data link layer deals with the physical addressing of the transmission.
  • Every networking device comes with the hardware called the Network Interface Card(NIC) which contains the unique MAC address to identify it.
  • Data link layer receives the IP address from the networking layer and adds the MAC address.

Network Layer

  • The most optimal path for the data to be transmitted will be determined in this layer.
  • OSPF - Open Shortest Path First.
  • RIP - Routing Information Protocol.
  • The optimal path would be determined by the considering the following factors.
    • Shortest path.
    • Reliable path.
    • Fastest path.
  • In network layer, everything is dealt with IP addresses.
  • Routing and reassembly happens at this layer.
  • Routers are called layer 3 devices.

Transport Layer

  • Transport layer plays a vital role is transmitting data across the network.
  • There are two protocols for transmission of data.
    • Transmission Control Protocol (TCP).
    • User Datagram Protocol (UDP).

Transmission Control Protocol (TCP)

  • Characteristics of TCP
    • Reliable
    • Accurate
    • Slower
    • Needs reliable connection
  • TCP is best suitable for the following
    • File transfers
    • Internet browsing
    • Sending emails

User Datagram Protocol (UDP)

  • Characteristics of UDP
    • Faster
    • Does not need reliable connection.
    • No error checking
    • Not reliable or accurate.
  • UDP is best suitable for the following
    • Video streaming

Session Layer

  • Session layer creates and maintains a connection after the data is translated from the presentation layer.
  • Session will be active as long as the connection is active.
  • Session layer is also responsible for closing the connection when it is not used for a while.
  • Sessions are unique and data cannot travel across different sessions.

Presentation Layer

  • Standardisation happens in this layer.
  • This layer acts as a translator for data to and from the application layer.
  • For example: Two users using different email clients, but the data should be handled in the same way.
  • Security features such as data encryption happens in this layer.

Application Layer

  • Most familiar layer. User interaction happens in this layer.
  • Protocols and rules are present in this layer.
  • Email clients, browsers, etc are part of this layer and these application provide a user friendly Graphical User Interface (GUI) for the user to interact with.

Summary

  • To remember all layers in proper order, use the phrase "Please Do Not Throw The Spinach Pizza Away".
  • Physical Layer
    • 1st Layer
    • Physical Data Transmission Media.
    • Protocols: Electrical, optical, and wireless signals
  • Data Link Layer
    • 2nd Layer
    • Reliable Data Transfer within the network between adjacent nodes.
    • Protocols: Ethernet (802.3), WiFi (802.11)
  • Network Layer
    • 3rd Layer
    • Logical addressing and Routing between networks.
    • Protocols: IP, ICMP, IPSec
  • Transport Layer
    • 4th Layer
    • End to End Communication and Data Segmentation.
    • Protocols: UDP, TCP
  • Session Layer
    • 5th Layer
    • Establishing, maintaining, and synchronising sessions.
    • Protocols: NFS, RPC
  • Presentation Layer
    • 6th Layer
    • Encoding, Encryption and Compression happens in this layer.
    • Protocols: Unicode, MIME, JPEG, PNG, MPEG
  • Application Layer
    • 7th Layer
    • Provides services and interfaces to applications.
    • Protocols: HTTP, FTP, DNS, POP3, SMTP, IMAP
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