Layer 2 - Data Link

Ref: https://learn.cantrill.io/courses/2022818/lectures/45636947 and https://learn.cantrill.io/courses/2022818/lectures/45636954

YouTube: https://www.youtube.com/watch?v=c5lV995dSAA and https://www.youtube.com/watch?v=Q_55Lryu9zc

💡 In OSI-model, higher layers add features and capabilities to lower layers

L2 runs over L1
- L2 devices understand both L2 and L1
Anything below a layer is abstracted away
- e.g. L2 doesn't care whether we use cable or WiFi

OSI L2 (Data Link) - Key Concepts

🔧 Protocols for reliable device-to-device communication in the SAME network segment
- Adds features to L1 like media access control, collision detection…
Introduces:
- L2 data containers (e.g. Ethernet frames) that encapsulate data
- MAC addresses that identify devices
  - ‼️ HW address, NOT assigned by any SW!!
  - Format: 48 bits in hexadecimal (e.g. 5C:0A:1D:22:F3:D2)
  - First 24 bits: Organizationally Unique Identifier (OUI) → identifies manufacturer
  - Last 24 bits: Network Interface Controller (NIC)
  - 💡 OUI + NIC makes a MAC address globally unique… in theory. There are some edge cases where two devices in the world could potentially have same MAC address.
L2 protocol examples
- Ethernet → for cables in local NWs
- IEEE 802.11 → for wireless local NWs (WiFi)
- Point-to-Point (PPP) → for direct communication between routers
- …

❗ When L1 transmits L2 frame, L1 doesn't understand the frame (L1 simply transmits data)

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MAC Header
1. Preamble = start frame delimiter → receiver device knows a frame is incoming
2. Destination (DST) & Source (SRC) MAC addresses
  - If DST MAC = FF:FF:FF:FF:FF:FF (All Fs) → broadcast frame to all devices in NW
3. Ether Type (ET) → specifies which L3 protocol (e.g. IP) puts data (payload) inside frame
Payload = Data (provided by L3)
Frame Check Sequence (FCS) → identifies frame errors (simple CRC, checks corruption)
- 💡 CRC = Cyclic Redundancy Check

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