OSI Model

The Open Systems Interconnection (OSI) model is a conceptual framework that standardizes the functions of a telecommunication or computing system into seven abstraction layers. Each layer serves the one above it and is served by the one below.

The Seven Layers

flowchart TD
    L7[7 — Application<br/>HTTP, FTP, SMTP, DNS]
    L6[6 — Presentation<br/>TLS, SSL, JPEG, ASCII]
    L5[5 — Session<br/>NetBIOS, RPC, PPTP]
    L4[4 — Transport<br/>TCP, UDP]
    L3[3 — Network<br/>IP, ICMP, routers]
    L2[2 — Data Link<br/>Ethernet, MAC, switches]
    L1[1 — Physical<br/>Cables, hubs, signals]

    L7 --> L6 --> L5 --> L4 --> L3 --> L2 --> L1

Layer Summary

#	Layer	Unit (PDU)	Responsibility	Examples
7	Application	Data	Interface with end-user applications	HTTP, FTP, SMTP, DNS, SSH
6	Presentation	Data	Translation, encryption, compression	TLS/SSL, JPEG, MPEG, ASCII
5	Session	Data	Establish, manage, terminate sessions	NetBIOS, RPC, PPTP
4	Transport	Segment	Reliable delivery, flow control, segmentation	TCP, UDP
3	Network	Packet	Logical addressing and routing between networks	IP, ICMP, IPSec, routers
2	Data Link	Frame	Node-to-node delivery, MAC addressing, error det.	Ethernet, PPP, switches, NICs
1	Physical	Bit	Raw bit transmission over physical medium	Cables, fiber, hubs, repeaters

Encapsulation Flow

When data is sent, each layer adds its own header (and sometimes trailer) — a process called encapsulation. The receiver reverses it (decapsulation).

flowchart LR
    subgraph Sender
        A1[Application Data] --> A2[+ Transport Header<br/>= Segment]
        A2 --> A3[+ Network Header<br/>= Packet]
        A3 --> A4[+ Data Link Header/Trailer<br/>= Frame]
        A4 --> A5[Bits on the wire]
    end
    A5 ==> B5
    subgraph Receiver
        B5[Bits on the wire] --> B4[Frame]
        B4 --> B3[Packet]
        B3 --> B2[Segment]
        B2 --> B1[Application Data]
    end

OSI vs TCP/IP

The TCP/IP model is the one actually used on the Internet. It collapses several OSI layers.

flowchart LR
    subgraph OSI[OSI — 7 layers]
        O7[Application]
        O6[Presentation]
        O5[Session]
        O4[Transport]
        O3[Network]
        O2[Data Link]
        O1[Physical]
    end
    subgraph TCPIP[TCP/IP — 4 layers]
        T4[Application]
        T3[Transport]
        T2[Internet]
        T1[Network Access]
    end
    O7 --- T4
    O6 --- T4
    O5 --- T4
    O4 --- T3
    O3 --- T2
    O2 --- T1
    O1 --- T1

Mnemonics

To memorize the layers (top → bottom):

All People Seem To Need Data Processing
Or bottom → top: Please Do Not Throw Sausage Pizza Away

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