OSI Model·Lesson 3 of 12
7 Layers — Big Picture
All Seven, in Plain English
The OSI model — OSI stands for Open Systems Interconnection — splits network communication into seven layers. Each layer has one job and hands its work to the next. The top layers are close to you, the user; the bottom layers are close to the hardware. Let's meet all seven with a one-line summary of what each does, then see how they fit together.
Read the stack this way
Layer 7 (Application) is where your apps live. Layer 1 (Physical) is the actual wire or radio signal. Data starts at the top on the sender, travels down to the wire, crosses to the other device, then climbs back up to the top.
Here are the seven layers, top to bottom, each with its plain-English job:
The seven layers and their jobs
Application
Gives apps access to the network (web, email, file transfer)
Presentation
Translates, encrypts, and compresses the data
Session
Opens, keeps track of, and closes conversations
Transport
Delivers reliably between the two programs, end to end
Network
Adds addresses and chooses a route across networks
Data Link
Moves data between two directly connected devices
Physical
Sends the raw 1s and 0s as signals on the medium
A Trick to Remember the Order
Nobody remembers seven layers cold — everyone uses a mnemonic. Take the first letter of each layer and turn it into a sentence. Reading from the top (Layer 7) down to the bottom (Layer 1): Application, Presentation, Session, Transport, Network, Data Link, Physical.
Two popular mnemonics
- Top to bottom (7 to 1): "All People Seem To Need Data Processing" — Application, Presentation, Session, Transport, Network, Data Link, Physical
- Bottom to top (1 to 7): "Please Do Not Throw Sausage Pizza Away" — Physical, Data Link, Network, Transport, Session, Presentation, Application
How Data Actually Travels
When you send something — say, a chat message — it starts at the Application layer and moves down the stack. At each layer, that layer wraps the data with a little extra information of its own, called a header (the Data Link layer also adds a trailer at the end). This wrapping is called . By the time the message reaches the Physical layer, it's a stream of bits ready to travel over the wire.
Envelopes inside envelopes
Picture writing a note, sealing it in an envelope, putting that envelope inside a bigger one, and so on — each layer adds a wrapper with its own address. The receiver opens the wrappers one at a time, in reverse order, until they reach the original note. That nesting and un-nesting is exactly how data moves down and back up the OSI stack.
On the receiving device, the whole thing happens in reverse. The bits arrive at the Physical layer and climb the stack. Each layer removes and reads the header its peer added on the other side, then passes the rest up. This unwrapping is called decapsulation. When the data reaches the Application layer at the top, it's back to the original message — and the app shows you the chat.
The journey: down the sender, up the receiver
Sender: App → Physical
Each layer adds its header (encapsulation)
Across the medium
Bits travel as signals over cable or wireless
Receiver: Physical → App
Each layer removes its header (decapsulation)
What the Data Is Called at Each Layer
As the data gets wrapped, the chunk moving through each layer gets a specific name. The general term is PDU (Protocol Data Unit), but each layer has its own word for it. Knowing these names is a common interview point, so here they are side by side:
| Layer | Name of the data chunk (PDU) |
|---|---|
| Application, Presentation, Session (7–5) | Data |
| Transport (4) | Segment (TCP) / Datagram (UDP) |
| Network (3) | Packet |
| Data Link (2) | Frame |
| Physical (1) | Bit |
A memory hook for the chunks
From the top down: Data, then Segment, then Packet, then Frame, then Bits. Interviewers love asking "what's the PDU at the Network layer?" — the answer is a packet.
“"Data is physically handed from Layer 4 to Layer 4 on the other computer directly."”
Q:Walk me through what happens to data as it moves through the OSI layers.
A: On the sender, data starts at the Application layer and moves down. Each layer adds its own header (Data Link also adds a trailer) — this is encapsulation. At the Physical layer it becomes bits on the medium. On the receiver, the bits climb back up; each layer strips the header its peer added and passes the rest up — decapsulation — until the original data reaches the Application layer.
Quick Revision Cheat Sheet
Order (7→1): Application, Presentation, Session, Transport, Network, Data Link, Physical
Mnemonic (7→1): All People Seem To Need Data Processing
Encapsulation: Going down: each layer adds a header
Decapsulation: Going up: each layer removes its header
PDUs: Data → Segment → Packet → Frame → Bit
Peer layers: Talk logically; real data goes down to L1 and back up