The OSI Model

OSI Model

Open Systems Interconnection Model

• A conceptual framework used for standardizing the functions of telecommunication and computing systems

OSI Layers

Please Do Not Throw Sausage Pizza Away

1. Physical Layer

2. Data Link Layer

3. Network Layer

4. Transport Layer

5. Session Layer

6. Presentation Layer

7. Application Layer

Application Layer (L7)

• handles the network services to end-user applications

• e.g. web browsers

Presentation Layer (L6)

• sometimes generically called the syntax / formatting layer

• Translates, formats, and encrypts data for the application layer

• Like putting a letter in an envelope; formatting it in a way the network can understand

• e.g. encrypting and compressing images for sending

Session Layer (L5)

• Manages sessions and dialogues between applications

• Like a conversation

  • how to initiate (standardization, communication settings)

  • maintaining communication

  • timing and synchronization

  • when to terminate (e.g. not terminating could lead to DDoS attacks)

Transport Layer (L4)

• Ensures complete and reliable data transfer

• TCP (reliable) and UDP (not as reliable)

Network Layer (L3)

• Responsible for data routing, addressing, and packet forwarding

• Determines best route to transmit the data on

• Like mail: now we have the mailing address and the return address

• IP addressing and routing protocols

Data Link Layer (L2)

• made of two sublayers

• Logical Link Control (LLC)

  • connection establishment, flow control, and error correction

• Media Access Control (MAC)

  • carrier sense, multiple access, collision detection / avoidance

  • manages access to a shared physical network medium, ensuring devices can transmit reliably without interfering with each other

  • operates on hardware-based addresses

Physical Layer (L1)

• physical, tangible assets of the network

• binary, cabling, radiated energy, analog signals

Encapsulation

• Wrapping data with protocol information as it descends OSI layers and is sent to the receiving device.

• Each OSI layer adds a header or footer with specific control information.

• Converts SDUs into PDUs, adding layer-specific control information as the data descends through OSI layers.

Decapsulation

• Occurs as data ascends OSI layers in the receiving device.

• Each layer removes and processes its corresponding header or footer.

• Ensures correct data interpretation and handling across network systems.

• Strips off headers or footers from PDUs, converting them back into SDUs as data ascends through OSI layers in the receiver.

Encapsulation Process: Application Layer

Encapsulates data with application-specific protocols like HTTP, SMTP, or FTP.

Encapsulation Process: Presentation Layer

Adds format and encryption information, ensuring data can be correctly interpreted.

Encapsulation Process: Session Layer

Includes control tokens and synchronization data, managing dialogue between systems.

Encapsulation Process: Transport Layer

Splits payload into segments or datagrams (chunks of the original data), adding source and destination port numbers for communication endpoints.

Encapsulation Process: Network Layer

Encapsulates segments into packets, adding logical addressing information with IP headers for routing.

Encapsulation Process: Data Link Layer

Frames the packets, adding MAC addresses and error-checking information like CRC (Cyclic Redundancy Check).

Encapsulation Process: Physical Layer

Converts frames into electrical, radio, or optical signals for transmission over physical media.