OSI Model (Open Systems Interconnection model)

These mnemonics may
help you to remember the
order of the OSI Reference
Model layers

OSI Model Layers

v  (Please Do Not Throw Sausage Pizza Away) These are cute phrases where each word starts with the first letter of an OSI model layer, arranged in the correct order.

v  Reference model for how applications can communicate over a network

v  The International Standards Organization (ISO) developed the Open Systems Interconnection (OSI) model.

v  conceptual framework for understanding relationships

     1. Application Layer:

v  PDU (Protocol Data Unit): DATA

v  Humor – Machine Interface

v  Protocols: HTTP, FTP, SMTP, SNMP, DNS, TELNET, DHCP

v  Ex: Opening Browser and using HTTP opening the website and transferring money from one bank to another

     2. Presentation Layer:

v  PDU: DATA

v  Encryption –Decryption

v  Data will be given to Receiver as understandable format

v  Protocols: SSL, TLS

v  Ex: After typing Password in bank website. Password has been encrypted and travel threw network and decrypted in server

     3. Session layer:

v  PDU: Data

v  Establishment, Maintain, Termination of Sessions

v  Protocols: PPTP, SIP, SAP, Net-BIOS

v  Ex: User will be logged out in bank website. If the user inactive for long time.

     4. Transport Layer:

v  PDU: Segment

v  Reliable Transfer of data

v  Ensuring that data arrives to its destination with error free and in-order

v  Main aim of this layer is to be delivered the entire message from source to destination.

v  Segment and re-assembles data

v  Segmentation, Acknowledgement, Sequencing

v  It ensures complete data transfer.

v  The sender side breaks application messages into segments and passes them on to the network layer. The receiving side then reassembles segments into messages and passes them to the application layer.

v  Protocols: TCP, UDP

v  Ex: After Transferring Money to Other User. If I’m using multiple Tab in Browser like YouTube, Facebook etc. The acknowledge will be sent to bank tab only.

     5. Network Layer:

v  PDU: Packet

v  It Divides the outgoing messages into packets and assembles the incoming packets into messages

v  Addressing & Routing

v  Find the Shortest path to reach the destination by using routing protocol

v  Packets Forward Based on Source IP and Destination IP

v  Creating logical paths, for transmitting data from node to node.

v  Protocols: IPV4, IPV6, IPSec, ICMP, IGP, EGP

v  Device: Router

    6. Data Link Layer:

v  PDU: Frames (Covert 0,1’s to Frames)

v  Make Sure Data Transfer is error free from one node to another

v  By using MAC address forwards data

v  Error Detection: It will Expects acknowledgements for frames. If ACK not received means informed to sender that, frames not delivered.

v  Flow Control: It Establishes a Logical Layer Between sender and receiver, It Signals the sender to stop, when the frame buffers are full. Once the buffer is free, informed sender to send frames

v  Protocols: ATM, SLIP, Frame Relay, PPP

v  The data link layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control(LLC) layer. The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.

v  Devices: Switch, Bridge

     7. Physical layer:

v  PDU: Bits (0 and 1’s)

v  Binary Transmission

v  The relationship between a device and a physical transmission medium 

v  It Converts the digital/analog bits into electrical signal or optical signals.