Lesson 5: Layer 1 (Physical) & Encapsulation
Layer 1: Physical Layer
- Purpose: Responsible for the physical transmission of raw bits over the communication medium (cables, wireless).
- Characteristics: Defines voltage levels, data rates, pin layouts, and cable specifications.
- Key Devices: Hubs (obsolete), Repeaters, Network Interface Cards (NICs), cabling.
- Data Unit: Bit.
Data Encapsulation and De-encapsulation
As data moves down the OSI model layers from the Application layer (L7) to the Physical layer (L1) on the sending host, headers are added at each layer. This process is called Encapsulation.
| Layer | PDU (Protocol Data Unit) | Action |
|---|---|---|
| 7, 6, 5 | Data | Preparation for transmission |
| 4 | Segment/Datagram | Adds TCP/UDP Header (Port Numbers) |
| 3 | Packet | Adds IP Header (Logical Addresses) |
| 2 | Frame | Adds MAC Header and Trailer (Physical Addresses) |
| 1 | Bits | Physical transmission |
When the data reaches the receiving host, the process is reversed (De-encapsulation), and headers are removed layer by layer until the original data is delivered to the application.