OSI Model Textbook

OSI Model

Open Systems Interconnection (OSI) Reference Model developed by ISO.

Purpose of OSI Model

Standardize how network devices communicate and allow interoperability between different vendors.

Advantage of OSI Model

Isolate problems by testing layer by layer instead of the entire system at once.

Layer 1 - Physical

Transmits raw binary data (0s and 1s) over physical media.

Key Functions of Layer 1

Defines voltage, light, or radio wave patterns; controls cable types, signal strength, and data rates.

Devices/Tech in Layer 1

Ethernet cables (copper/fiber), hubs, repeaters, network interface cards (NICs).

Data Unit (PDU) of Layer 1

Bits.

Example of Layer 1

Sending electrical pulses down a copper wire to represent 1s and 0s.

Layer 2 - Data Link

Provides reliable node-to-node delivery within the same local area network (LAN).

Key Functions of Layer 2

Uses MAC addresses for communication; frames data for transmission; error detection with CRC.

Devices in Layer 2

Switches, bridges.

Data Unit of Layer 2

Frames.

Example of Layer 2

A switch sending a frame only to the intended device instead of broadcasting everywhere.

Layer 3 - Network

Handles logical addressing and routing.

Key Functions of Layer 3

Assigns IP addresses; determines the best path (routing); manages fragmentation and reassembly of packets.

Devices in Layer 3

Routers, Layer 3 switches.

Data Unit of Layer 3

Packets.

Example of Layer 3

A router forwarding a packet across multiple networks using IP addresses.

Layer 4 - Transport

Ensures end-to-end delivery of data.

Key Functions of Layer 4

Splits large data into segments; uses ports to direct data to correct applications.

Protocols in Layer 4

TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

Data Unit of Layer 4

Segments (TCP) / Datagrams (UDP).

Example of Layer 4

TCP ensuring that a web page loads completely and in the right order.

Layer 5 - Session

Establishes, maintains, and terminates sessions between devices.

Key Functions of Layer 5

Coordinates communication between applications; manages dialog.

Example of Layer 5

Keeping a user logged into a website while browsing multiple pages.

Layer 6 - Presentation

Translates data into a usable form for the application.

Key Functions of Layer 6

Data translation, compression, encryption/decryption.

Example of Layer 6

Converting an image file into a displayable format on a browser.

Layer 7 - Application

Closest to the end user — provides network services directly.

Common Protocols in Layer 7

HTTP/HTTPS, FTP, SMTP/POP3/IMAP, DNS, DHCP, SSH/Telnet.

Example of Layer 7

A user typing a web address in Chrome (HTTP request).

Encapsulation

Wrapping data with headers as it moves down the OSI layers.

Decapsulation

Reverse process at the receiving end — stripping headers at each layer until raw data is delivered to the application.