7 Layers of OSI Model, Functions & Protocols

What is the OSI Model in Networking?

OSI stands for Open System Interconnection. OSI is a conceptual model of telecommunication networking where the whole process is divided into seven different layers of network.

What are the 7 Layers of OSI Model?

7 layers of OSI model
7 Layers of OSI Model and their PDUs

There are 7 logical layers of OSI model in Networking– 

  • Physical Layer
  • Data-link Layer
  • Network Layer
  • Transport Layer
  • Session Layer
  • Presentation Layer
  • Application Layer

7 Layers of OSI Model in Computer Network & their Functions:

Physical Layer: 

  • Synchronization of Bits- Raw bits are synchronized in this layer
  • Representation of Bits.
  • It provides physical characteristics of devices and media.
  • It determines the physical topologies- Star/Ring/Bush/Mesh etc.
  • It specifies the transmission mode
    b)Half Duplex
    c)Full Duplex
  • Line Config- a) P2P b) P2M

Devices Used in Physical layer- Hub, Repeater, Network Interface card, Connectors.

Data-link Layer:

  • Framing: In data-link layer bits are transmitted as frames by adding a special pattern in the beginning and end of the bits. 
  • Physical Addressing: While framing a physical address is attached with each of the header of the bits.
  • Error Checking: Data-Link layer has error checking ability. If error found, bits are fixed and re-transmitted.
    The two error checking mechanism used by data-link layer are-
    1) CRC- Cyclic Redundancy Check
    2) FCS- Frame Check Sequence
  • Access Control: Mac sublayer determines which device will have access to the data.

Note: There are two sublayers in Data-Link layer,

  • MAC- Media Access Control
  • LLC- logical link control: It identifies the running network layer protocols.

Devices Used in Data-link layer- Bridges, Switches(L2/L3)

Network layer: 

  • Logical addressing: network layer in OSI miodel provides logical addressing.
    1) It provides a source & destination post address.
    2) It provides a source & destination IP address.
    3) It provides a source & destination MAC address.
  • Routing: Routing is the process to identify the best possible way between source to destination. This process is done by network layer protocols.

Devices Used in Network layer in OSI model- Routers, L3 switches

Transport Layer:

  • Segmentation & Reassemble: At the sender’s end, the transport layer receives data packets and breaks them into small units. Each of the units get separate headers. This is segmentation.
    At the receiver’s end, these small units are re-assembled into data packets. 
  • Connection Creation: It’s a 3 way process also called as 3 way handshaking
    1) Sending sync signal
    2) a) negotiation for connection
        b) Sync signal received and connection built.
        c) Data transfer
    3)Connection termination.
  • Connection control
  • Windowing
  • Error checking and connection
  • Source Port addressing

Devices Used in Transport layer in OSI model- Gateways, Firewalls 

Session Layer:

  • Session starting, maintaining and ending: Session layer starts the process of connection, maintains the connection until data transmits and then terminates the connection.
  • Synchronization

Devices Used in Transport layer in OSI model- Gateways, Firewalls, PCs

Presentation Layer:

  • Data Translation: It is a process of converting the data type from one to another like ascii is converted to ebcdic.
  • Data Encryption: It translates the data to another form or code or you can say it encrypts the data.
    At sender’s end plain text is encrypted to cipher text and at receiver’s end cipher text is again decrypted to plain text. This is a part of Cryptography
  • Data Compression: Reduces the amount of bits which need to be transmitted.

Devices Used in the Presentation layer in OSI model- Gateways, Firewalls, PCs

Application Layer:

  • It is the layer where the user first interacts.

Devices Used in Application layer in OSI model- Gateways, Firewalls, PC

Important OSI Model Protocols and their Functions

There are lots of OSI model protocols that work in the background of OSI layers but I will mention only the most important ones. If you need details for a protocol that I am not mentioning here, please let us know in the comment section below. So let’s start,

TFTP: Trivial File Transfer Protocol. Port NO. 69

  • It’s an application layer protocol.
  • It is used to transfer files from TFTP server to TFTP clients.
  • It has no directory browsing ability
  • It is very fast.
  • It is mostly used to take backup of Cisco Router, Cisco Switch, Cisco firewall or any firewall, IOS (Inter Network Operating Systems), NVRAM (Non Volatile RAM)

FTP: File Transfer Protocol. Port No: 20 & 21

  • It’s an application layer protocol.
  • It is generally used to transfer files from FTP server to FTP clients.
  • It is slower that TFTP
  • It has directory browsing ability.
  • Port No: 20 for FTP Data & 21 for FTP Control

HTTP/HTTPS: Hyper Text Transfer Protocol / over Secure Socket Layer

  • It is also an application layer protocol
  • HTTP and HTTPS both are used to access the webpage normally or securely.
  • Port No. 80 (HTTP) & 443 (HTTPS)

POP3: Post Office Protocol version 3

  • It is a mail retrieval protocol. 
  • It is used to retrieve your mails from mail server.
  • Port No. 110 & 995 (SSL)

IMAP4: Internet Message Access Protocol Version 4

  • It is also a mail retrieval protocol like POP3
  • Port No. 143 & 993 for SSl

SMTP: Simple Media Transfer protocol

  • It is used to send files.
  • Port No. 25/ 465 for SSL/ 587 for TLS

DHCP: Dynamic Host Configuration Protocol

  • It is used to provide IP address, subnet mask, default gateways, preferred DNS, alternative DNS, DHCP server address, domain name etc.
  • Port No. 67 for DHCP server & 68 for DHCP client.

Learn DHCP & DHCP DORA Process

DNS: Domain Name System

  • It is used to resolve FQDN to IP and vise versa.
    FQDN- Fully Qualified Domain Name.
  • Port No. 53

LPD: Line Printer Domain

  • It is used to share a Printer over the network.
  • Port No. 515

Telnet: Tele Communication Network

  • It is used to access any remote device like PC, Cisco router, Cisco Switch, firewall etc through command line interface.
    In this process username and password is transferred as clear text so security level is very low.
  • Port No. 23

SNMP: Simple Network Management protocol

  • It is used to monitor the network activities. 
  • Port no. 161

TCP: Transmission Control protocol

  • TCP is a connection building protocol. It helps to create connection between devices to transmit data and terminate the connection after transmission.
  • Port No. 20 & 21

UDP: User Datagram Protocol

  • It is also a connection oriented protocol just like TCP.
  • Port No. 22 & 23

IP- Internet Protocol

  • It is a routing protocol.
  • It is used to transfer data from one interface to another interface or one device to another device.

ARP: Address Resolution Protocol

  • It resolves IT to MAC

RARP: Reverse Address Resolution Protocol

  • It is used to resolve MAC to IP.

ICMP: Internet Control Message Protocol

  • This protocol runs in background while checking the network connectivity using Ping or Trace route command.

Check all the PDUs of each layers of OSI layers

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