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The OSI Model — 7 Layers, PDUs & Data Encapsulation

Every layer, its purpose, the devices that operate on it, the PDU it uses, the protocols that live there, and the animated journey of data flowing down and back up the stack.

7 Layers PDUs Protocols Encapsulation

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Section 01

The OSI Model at a Glance

🎓

Mnemonic — Top to Bottom

All People Seem To Need Data Processing
Application · Presentation · Session · Transport · Network · Data Link · Physical

Section 01 · Purpose

Why the OSI Model Exists

🏛️ HISTORY

A universal reference for how networks talk

The Open Systems Interconnection (OSI) model was published by the ISO in 1984 as a 7-layer reference for how networked systems communicate. Each layer has a single, well-defined job. A change in one layer never ripples into the others.

You will hardly ever run "pure OSI" — the internet uses TCP/IP. But every network engineer thinks in OSI layers because it maps neatly onto everything that exists.

7Layers

1984Published by ISO

1 jobPer layer

LayeredIndependent + swappable

Section 01 · Reference

All 7 Layers — Quick Reference

#	Layer	PDU	Key job	Devices	Sample protocols
7	Application	Data	User services	Host	HTTP · HTTPS · DNS · SMTP · FTP
6	Presentation	Data	Encode / encrypt / compress	Host	TLS · SSL · JPEG · ASCII
5	Session	Data	Start / manage / end sessions	Host	NetBIOS · RPC · PPTP
4	Transport	Segment / Datagram	End-to-end delivery	Firewall (L4)	TCP · UDP · SCTP
3	Network	Packet	Logical addressing & routing	Router · L3 switch	IP · ICMP · OSPF · BGP
2	Data Link	Frame	MAC addressing, framing	Switch · Bridge · NIC	Ethernet · PPP · ARP · Wi-Fi (802.11)
1	Physical	Bits	Signals on the wire	Hub · Repeater · Cable	Ethernet PHY · DSL · fibre optics

Section 02 · Layer 1

Layer 1 — Physical Layer · What & Why

1

Physical Layer

Bits on the wire — voltages, light pulses and radio waves

⚡

Job — Move Raw Bits Between Two Nodes

The physical layer converts 1s and 0s into electrical signals (copper), light pulses (fibre) or radio waves (Wi-Fi/5G), and vice versa. It knows nothing about addresses, routing or reliability — only signals and timing.

🔌

Signaling

encoding

Turn bit sequences into voltage levels, light on/off, or modulated radio signals.

⏱️

Timing & Clocks

synchronisation

Sender and receiver must agree on bit-rate and clock boundaries — otherwise noise.

📏

Media Standards

connectors

RJ-45 / RJ-11 / SFP / SC-LC connectors, cable categories (Cat 5e/6/6a), impedance.

Section 02 · Layer 1

Layer 1 — Devices, PDU & Protocols

Devices	PDU	Protocols / standards
Hub · Repeater · Cable (copper / fibre) · Modem · Antenna	BITS	Ethernet PHY (100BASE-T · 1000BASE-T) DSL · DOCSIS (cable) Fibre (SONET / SDH / DWDM) Bluetooth PHY · Wi-Fi PHY · 4G / 5G radio

🔎

Hub vs Repeater — Both Are Pure L1

A hub forwards every incoming signal to every other port with zero intelligence. A repeater amplifies a signal so it can travel further. Neither understands MAC or IP addresses — that's what makes them L1 devices.

💡

Modern Reality

Hubs are effectively extinct today (replaced by L2 switches). "Modem" (modulator/demodulator) still lives at L1 — converting digital bits to analog signals over DSL / cable / fibre.

Section 03 · Layer 2

Layer 2 — Data Link · What & Why

2

Data Link Layer

Node-to-node delivery on the same physical network using MAC addresses

🏷️

Job — Deliver Frames Between Directly-Connected Devices

Layer 2 wraps the network packet into a frame with a source MAC and destination MAC — a hardware address burned into every NIC. It handles local delivery on the same LAN; getting between LANs is Layer 3's job.

🔖

MAC Addressing

48-bit hardware ID

Every NIC has a unique MAC like A4:B1:C1:D2:E3:F4. First 3 bytes = vendor, last 3 = serial.

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Framing

wrap the packet

Adds a header (src+dst MAC, EtherType) and a trailer (CRC checksum) around the L3 payload.

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Error Detection + MAC control

CRC · CSMA/CD

CRC catches corrupted bits; media-access control decides who gets to talk on shared media.

Section 03 · Layer 2

Layer 2 — Devices, PDU & Protocols

Devices	PDU	Protocols
Switch · Bridge NIC (Network Interface Card) Wireless Access Point (AP)	FRAME	Ethernet (IEEE 802.3) Wi-Fi (IEEE 802.11) PPP · HDLC ARP · VLAN (802.1Q) STP (Spanning Tree)

🔀

The Switch — the Star of Layer 2

A switch reads the destination MAC and forwards the frame only to the correct port (unlike hubs which flood everywhere). It builds a MAC address table automatically by watching source addresses on incoming frames.

🔍

ARP Lives Here Too

ARP (Address Resolution Protocol) maps a known IP to the MAC on the same LAN — the bridge between L3 addressing and L2 delivery.

Section 04 · Layer 3

Layer 3 — Network · What & Why

3

Network Layer

Logical addressing + routing between different networks worldwide

🌍

Job — Get Packets Across the Internet

Layer 3 gives every device a logical (IP) address that works globally, and picks the best route across many networks to reach the destination. Where Layer 2 handles "next hop on this LAN", Layer 3 handles "how to reach New York from Delhi".

📍

Logical Addressing

IPv4 · IPv6

Assigns 32-bit (v4) or 128-bit (v6) addresses that identify a device on the global network.

🗺️

Routing

path selection

Routers exchange reachability info via OSPF/BGP and pick the best next hop for each packet.

✂️

Fragmentation

MTU handling

Splits packets when the next hop's Maximum Transmission Unit is smaller than the packet.

Section 04 · Layer 3

Layer 3 — Devices, PDU & Protocols

Devices	PDU	Protocols
Router · Layer-3 Switch Multi-layer firewall Load balancer (L3/L4)	PACKET	IP (IPv4 & IPv6) — addressing ICMP — `ping` & error reporting OSPF · EIGRP — internal routing BGP — internet routing between ISPs IPsec — L3 encryption NAT — private↔public translation

🧭

Router = the Post-Office

A router looks at each packet's destination IP, checks its routing table, and forwards the packet toward the correct "next hop" — repeating until the packet reaches its network.

🛠️

Everyday Tools You've Already Used

ping uses ICMP. traceroute/tracert uses ICMP+TTL. ipconfig/ifconfig show your L3 address. All of these live at Layer 3.

Section 05 · Layer 4

Layer 4 — Transport · What & Why

4

Transport Layer

End-to-end delivery between processes on two hosts, using ports

📮

Job — Deliver Segments to the Right Application

Layer 3 gets a packet to the right machine. Layer 4 gets it to the right application on that machine — using port numbers (HTTP=80, HTTPS=443, DNS=53, SSH=22). It also decides between reliable (TCP) and fast (UDP) delivery.

🔢

Ports

0 – 65,535

Every packet has a source and destination port so the OS knows which app receives the data.

✂️

Segmentation

chop & number

Splits large data into segments, numbers them, and lets the receiver reassemble in order.

🛡️

Reliability

ACK · retransmit

TCP tracks acknowledgements and retransmits lost segments. UDP does not — it's "fire & forget".

Section 05 · Layer 4

Layer 4 — Devices, PDU & TCP vs UDP

Devices	PDU	Protocols
L4 firewall (port-based) L4 load balancer (mostly end-host software)	SEGMENT (TCP) DATAGRAM (UDP)	TCP — reliable, ordered, connection-based UDP — connectionless, low-overhead SCTP — TCP + UDP hybrid QUIC — modern encrypted UDP-based transport

Feature	TCP	UDP
Connection	3-way handshake first	None — just send
Reliability	ACK + retransmit	None — app must handle
Order	Guaranteed	Not guaranteed
Header size	20+ bytes	8 bytes (tiny)
Speed	Slower (overhead)	Very fast
Used by	HTTP · HTTPS · SSH · email · file transfer	DNS · VoIP · gaming · video streaming · DHCP

Section 06 · Layer 5

Layer 5 — Session Layer

5

Session Layer

Open, manage and close conversations between two hosts

🔓

Setup

start dialogue

Establishes a session, negotiates parameters, authenticates if needed.

🔁

Maintain

checkpoints

Adds sync points so a big transfer can resume from the last checkpoint if it drops.

🚪

Teardown

close cleanly

Ends the conversation and releases resources on both sides.

PDU	Protocols
DATA	NetBIOS · RPC · PPTP · SOCKS · SIP (session initiation)

🎯

Real-World Reality

In modern TCP/IP, session-layer functions are usually rolled into the application itself (cookies, JWTs, session IDs). L5 is more of a conceptual placeholder in real networks — but essential vocabulary in the OSI model.

Section 07 · Layer 6

Layer 6 — Presentation Layer

6

Presentation Layer

Translate, encrypt and compress data so both ends can read it

🔤

Translation

encoding

ASCII, UTF-8, EBCDIC, image formats (JPEG, PNG, GIF), video codecs (H.264, VP9).

🔐

Encryption

confidentiality

TLS/SSL encrypts application data before it hits L5–L1. This is where HTTPS lives.

🗜️

Compression

smaller pipes

Shrinks data (gzip, brotli) so it uses less bandwidth on the wire.

PDU	Standards / protocols
DATA	TLS · SSL · ASCII · UTF-8 · JPEG · PNG · MPEG · MIME · gzip · brotli

🔒

HTTPS = HTTP over TLS at Layer 6

When you visit an https:// site, the encryption is negotiated by TLS at Layer 6 before HTTP data (Layer 7) is sent. That's why the padlock icon protects your credentials.

Section 08 · Layer 7

Layer 7 — Application Layer · What & Why

7

Application Layer

Where users and applications interact with the network

🖥️

Job — Provide Network Services to Applications

The application layer is the interface between the network and the app you're using. When you type a URL, click "Send", or open Zoom — L7 protocols like HTTP, SMTP, and RTP take over. It's the only layer users ever see.

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Web

HTTP · HTTPS

Every browser request, every API call. HTTP/1.1, HTTP/2, HTTP/3 (over QUIC).

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Email

SMTP · IMAP · POP3

SMTP sends mail; IMAP/POP3 fetch it. All three are pure Layer 7.

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File Transfer

FTP · SFTP · SCP

Move files between hosts. SFTP/SCP add encryption over SSH.

Section 08 · Layer 7

Layer 7 — Devices, PDU & Protocols

Devices	PDU	Protocols
Application-layer firewall (WAF) API gateway Reverse proxy (Nginx, HAProxy) End hosts (browser, mail client, apps)	DATA	HTTP · HTTPS — web DNS — name resolution SMTP · IMAP · POP3 — email FTP · SFTP — files SSH · Telnet — remote shell DHCP — auto IP assignment SNMP — device monitoring RTP · SIP — voice / video

🎯

Why "Application Layer" ≠ "Your App"

The OSI Application Layer isn't your Word or Chrome program — it's the network-facing services those programs use (HTTP, DNS, SMTP…). Your app talks to L7 protocols; the protocols talk to L6 and below.

Section 09 · Devices

Which Device Lives on Which Layer?

🎯

The Rule

The higher the layer a device operates at, the smarter — and slower — it is. Hubs blindly forward bits; L7 firewalls understand entire HTTP requests.

Section 10

Data Encapsulation — the Idea

📮 ANALOGY

Wrapping a letter into envelopes

Imagine posting a letter overseas. You seal it in an envelope (the recipient's address goes on). That envelope goes into a courier bag (with the courier's routing label). The courier bag goes onto a plane (with the airline's manifest). Each layer wraps the previous one and adds its own address information — without opening what's inside.

Data encapsulation in networking works exactly the same way. As data flows down the OSI stack, each layer adds its own header (and sometimes a trailer). On the receiving side, the process is reversed — each layer strips its header on the way back up. This is called decapsulation.

📋 THE 5 STEPS ON THE SENDING SIDE

L7–5

Application creates raw DATA.

L4

Transport adds a TCP/UDP header (ports, sequence) → SEGMENT.

L3

Network adds an IP header (src IP, dst IP) → PACKET.

L2

Data Link adds a MAC header + CRC trailer → FRAME.

L1

Physical converts the frame into BITS on the wire.

Section 10 · Full Flow

Encapsulation & Decapsulation — Full Journey

wraps the previous PDU with its own header — reversed on the receiver

🎬

Watch the Packet Grow

Data starts small at L7, then each layer wraps it with its own header (and L2 also adds a CRC trailer). By the time it hits L1 it's a fully-labelled bit stream on the wire. The receiver undoes it one wrapper at a time.

Section 11 · TCP/IP

The TCP/IP Model — the Model the Internet Actually Uses

🌐 HISTORY

Born in the labs, before OSI existed

The TCP/IP model (also called the DoD model) was designed by Vint Cerf and Bob Kahn in the 1970s for the US Department of Defense's ARPANET — ten years before the OSI reference model was published.

It has just 4 layers because it was built for real, working networks, not as a theoretical reference. Today it runs literally every internet-connected device — while OSI stays the vocabulary we use to talk about them.

4Layers (vs OSI's 7)

1970sCerf & Kahn

ARPANETFirst deployment

100%Modern internet

🎯

Practical vs Theoretical

OSI is the textbook. TCP/IP is what actually runs. TCP/IP combines OSI's Session + Presentation + Application into one Application layer, and OSI's Physical + Data Link into one Network Access layer.

Section 11 · Stack

The TCP/IP 4-Layer Stack

🧩

Same Job, Fewer Boxes

The 4 TCP/IP layers do all the same work as OSI's 7 — the model just groups related functions together (Network Access = Physical + Data Link, Application = Session + Presentation + Application).

Section 11 · Protocols

TCP/IP — Protocols & Devices per Layer

Layer	PDU	Key protocols	Devices
L4 · Application	Data	HTTP · HTTPS · DNS · SMTP · IMAP · POP3 · FTP · SFTP · SSH · Telnet · DHCP · SNMP · RTP · SIP · TLS	WAF · API gateway · reverse proxy · end hosts
L3 · Transport	Segment / Datagram	TCP · UDP · SCTP · QUIC (encrypted UDP-based)	L4 firewall · L4 load balancer
L2 · Internet	Packet	IPv4 · IPv6 · ICMP · IGMP · OSPF · BGP · IPsec · NAT · ARP*	Router · L3 switch · L3 firewall
L1 · Network Access	Frame / Bits	Ethernet (802.3) · Wi-Fi (802.11) · PPP · HDLC · MAC · DSL · fibre PHY · 4G/5G radio	Switch · Bridge · NIC · AP · Hub · Cable · Modem

📝

*ARP's Weird Spot

ARP maps an IP (Internet layer) to a MAC (Network Access layer). Different books place it differently — some at L1 (Network Access), some at L2 (Internet). Both are defensible.

🎯

The Sample Journey

When you open https://google.com: TLS + HTTP run at L4 (Application) → TCP runs at L3 (Transport) → IPv4/IPv6 runs at L2 (Internet) → Ethernet/Wi-Fi runs at L1 (Network Access). Every packet you send touches all four.

Section 12 · Compare

OSI vs TCP/IP — Side by Side

🎯

Why Both Still Exist

The internet actually runs on TCP/IP — its 4 layers collapse several OSI layers into one. But engineers still describe networks using OSI's 7 layers because it's more granular and precise for teaching, troubleshooting and certification (CCNA, CompTIA Net+).

Section 12 · Differences

OSI vs TCP/IP — Detailed Differences

Property	OSI Model	TCP/IP Model
Layers	7 layers	4 layers
Developed by	ISO (1984)	DoD / Vint Cerf & Bob Kahn (1970s)
Purpose	Reference / teaching model	Practical / working implementation
Approach	Protocols came after the model	Model came after the protocols (retrofit)
Session & Presentation	Separate layers (5 & 6)	Rolled into Application (L4)
Physical & Data Link	Separate layers (1 & 2)	Rolled into Network Access (L1)
Reliability	Guaranteed at Transport (Layer 4)	Guaranteed at Transport by TCP
Real deployment	Rarely — reference only	Runs the entire modern internet
Used for	Teaching · troubleshooting vocabulary · exam certs	Actual network design & operation

💬

Everyday Reality

Network engineers say "that's a Layer 3 issue" (OSI-speak) but the fix lands in a TCP/IP Internet layer config file. Both languages point to the same code — you just need to be fluent in both.

Section 13 · Part 1

OSI & TCP/IP Golden Rules — 1 to 4

🏆 CORE RULES · 1–4

1

7 layers, top to bottom: All People Seem To Need Data Processing. Application → Presentation → Session → Transport → Network → Data Link → Physical.

2

Each layer has one job — and one PDU. L7–L5 = Data · L4 = Segment · L3 = Packet · L2 = Frame · L1 = Bits.

3

Devices reveal their layer. Hub = L1 · Switch = L2 · Router = L3 · Load-balancer/firewall = L4 or L7. Higher = smarter but slower.

4

MAC is L2, IP is L3, Ports are L4. Master this and you can debug 90% of network issues by asking "which layer is broken?"

Section 13 · Part 2

OSI & TCP/IP Golden Rules — 5 to 8

🏆 CORE RULES · 5–8

5

Encapsulation = wrap with a header on the way down. Every layer prepends its own metadata (L2 also appends a trailer). Decapsulation reverses this on the way up.

6

TCP vs UDP is a Layer 4 choice. Reliable + ordered = TCP (web, email, SSH). Fast + connectionless = UDP (DNS, VoIP, gaming).

7

TLS = Layer 6, HTTPS = L7 over L6. Encryption is a presentation-layer service. It's added around L7 protocols like HTTP.

8

Real internet uses TCP/IP's 4 layers — but OSI's 7 layers remain the industry vocabulary for troubleshooting, design and every certification exam.

FINAL

You Now Speak OSI & TCP/IP Fluently

7OSI layers

5PDU names

10+Device types by layer

30+Protocols mapped

WrapEncapsulation

UnwrapDecapsulation

🎯

You Can Now…

Identify any layer's role, name its PDU, list its devices and protocols, and trace data as it's encapsulated down the sender's stack and decapsulated up the receiver's stack. This is the vocabulary the entire industry uses for designing and debugging networks.

📚

Where To Practise

Fire up Wireshark and watch real packets — every layer is right there in the capture. Do the Cisco Packet Tracer OSI labs. Read the classic Tanenbaum "Computer Networks" for the theory.

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