PDU at Each Layer - buaamer81/IT130-Networking-Wiki GitHub Wiki
PDU at Each Layer
π What is a PDU?
A Protocol Data Unit (PDU) is the name given to the unit of data used at each layer of the OSI or TCP/IP model.
As data is passed through the layers during encapsulation and decapsulation, it takes on different forms β each with its own structure and terminology.
Think of it as:
βData wearing a different uniform at each layer, tailored for the task itβs about to do.β
𧱠PDU Names in OSI Model (Top to Bottom)
| OSI Layer | PDU Name | Description |
|---|---|---|
| 7. Application | Data | User data (email, video, file) |
| 6. Presentation | Data | Same as above |
| 5. Session | Data | Session control info |
| 4. Transport | Segment (TCP) / Datagram (UDP) | Adds port info, reliability |
| 3. Network | Packet | Adds IP addresses |
| 2. Data Link | Frame | Adds MAC addresses and FCS |
| 1. Physical | Bits | Raw 1s and 0s sent across media |
π PDU Names in TCP/IP Model
| TCP/IP Layer | PDU |
|---|---|
| Application | Data |
| Transport | Segment (TCP) / Datagram (UDP) |
| Internet | Packet |
| Network Interface | Frame / Bits |
π¦ Encapsulation Review (Flow from Top to Bottom)
[User Data] β Segment (TCP Header + Data) β Packet (IP Header + Segment) β Frame (MAC Header + Packet + FCS) β Bits (Transmitted on wire/wireless)
π Real-Life Examples
- TCP Segment β Has port info (e.g., HTTP uses port 80)
- IP Packet β Contains source and destination IPs
- Ethernet Frame β Contains MAC addresses, used on LAN
- Bits β The actual electrical/light signals on a cable or fiber
π§ Why PDUs Matter
- Help identify what to troubleshoot (e.g., bad IP = Layer 3 issue)
- Understand how protocols operate per layer
- Learn what tools analyze which layer (e.g., Wireshark captures Frames & Packets)
π Visual Aids
π Related Pages
- Encapsulation Explained
- OSI Model Overview
- TCP/IP Model Overview
- IP Packet Structure
- Ethernet Frame Structure
- Segment vs Packet vs Frame
Return to: Encapsulation Explained