I’ve added a simple OSI diagram image at the top of this post for visual grounding.
Refer to it while reading — it makes the flow much easier to follow.
These notes are written for moments when the OSI model feels confusing.
They intentionally avoid jargon overload and focus on clear mental models.
1️⃣ What Is a PDU (Protocol Data Unit)?
A PDU is simply the form data takes at a given OSI layer.
👉 The key idea: the PDU name changes at each layer
| OSI Layer | PDU Name |
|---|---|
| Application / Presentation / Session | Data |
| Transport | Segment (TCP) / Datagram (UDP) |
| Network | Packet |
| Data Link | Frame |
| Physical | Bits (signals) |
At the top layers, the PDU is just Data.
Example:
"Hello"
2️⃣ Is Application Data Already 0s and 1s?
This is a very common confusion.
-
Logically: ❌ No
- It’s strings, bytes, JSON, text, objects
-
Physically (later): ✅ Yes
- It will eventually become signals
Important distinction:
- Higher layers think in structure and meaning
- Lower layers think in signals
Both are correct — just at different layers.
3️⃣ Transport Layer (TCP / UDP)
What the Transport Layer Adds
- Source Port
- Destination Port
- Transport control information
This is called the TCP header or UDP header.
✔️ Yes — the TCP header includes source + destination ports (and more).
TCP vs UDP (Quick Comparison)
| TCP | UDP |
|---|---|
| Reliable | Unreliable |
| Ordered | Unordered |
| Slower | Faster |
| Retransmits lost data | No retransmission |
- ✅ UDP is faster
- ✅ TCP is more widely used because it’s reliable
4️⃣ Network Layer
What the Network Layer Adds
- Source IP address
- Destination IP address
This is the IP header.
After this step:
Segment → Packet
IP answers one question:
“Where should this data go globally?”
5️⃣ Data Link Layer
What the Data Link Layer Adds
- Source MAC address
- Destination MAC address
- Error-checking trailer (CRC)
This is the MAC header + trailer.
After this step:
Packet → Frame
MAC answers:
“Which exact device on this local network should receive this?”
Critical Rule (Very Important)
- IP stays the same end-to-end
- MAC changes at every hop
6️⃣ Physical Layer
The physical layer:
- ❌ Does NOT understand headers or addresses
- ✅ Converts frames into physical signals
Examples:
- Wi-Fi → Radio waves
- Ethernet → Electrical signals
- Fiber → Light pulses
So yes:
- Wi-Fi = radio
- Ethernet = electricity
7️⃣ Encapsulation Flow (One Line)
Data → Segment → Packet → Frame → Signals
Each layer adds only its own information.
8️⃣ Header Mapping (Common Doubt)
| Header | Contains |
|---|---|
| TCP / UDP Header | Source Port, Destination Port, control info |
| IP Header | Source IP, Destination IP |
| MAC Header | Source MAC, Destination MAC |
✔️ If this was your doubt — your understanding is correct.
9️⃣ One-Sentence Mental Model
Application creates meaning → Transport manages conversation → Network finds destination → Data Link finds device → Physical moves signals
Final Note
These notes are:
- Lightweight
- Mental-model focused
- Safe to revisit anytime you’re confused
If this helped, feel free to bookmark or share.
Happy networking 🚀
Top comments (1)
nice post