L4.2: Link Layer #1 (1)

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L4.2 — Link Layer #1 by Kirk Martinez. This lecture introduces the link layer — the hidden layer that actually moves IP packets across physical media. It covers where the link layer fits in the OSI/TCP-IP models, encapsulation into frames, the role of MAC addresses, physical media types (twisted pair, fibre, Wi-Fi, powerline, sound), bit encoding (Manchester encoding), link layer services (flow control, acknowledgements, error detection), CRC checksums, the Ethernet frame structure, the Media Access Control (MAC) sublayer, CSMA/CD for shared media, modern switched Ethernet vs Wi-Fi contention, and how multiple link layers chain together across the internet.

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📋 At a Glance

[ ] Know where the link layer sits: above the physical layer, below the network (IP) layer
[ ] Understand that the link layer insulates IP from the physical world — IP doesn't care how bits travel
[ ] Know the terminology: IP sends packets, the link layer sends frames
[ ] Understand encapsulation: data → TCP header → IP header → link layer header + trailer (frame)
[ ] Know that the link layer only moves data one hop (to the next device — AP, switch, router)
[ ] Explain how an IP packet traverses multiple different link layers end-to-end
[ ] Know the Ethernet frame structure: preamble, destination MAC, source MAC, type, payload, CRC
[ ] Explain what a MAC address is and why it's used at the link layer instead of IP
[ ] Know that MAC addresses are supposedly globally unique but only need to be locally unique in practice
[ ] List physical media types: twisted pair (Ethernet), fibre optic, Wi-Fi, powerline, satellite, sound
[ ] Understand bit encoding — physical layers encode bits (e.g. Manchester encoding) rather than sending raw 1s and 0s
[ ] Explain link layer flow control — the receiver can tell the sender to back off when buffers are full
[ ] Describe link layer acknowledgements: per-frame ACK, block ACK (Wi-Fi), or no ACK at all
[ ] Explain error detection using checksums and CRC (Cyclic Redundancy Check)
[ ] Know the difference between a simple checksum (add bytes, keep low bits) and CRC (more robust)
[ ] Use ip -s link to view link layer statistics (packets sent, errors, drops)
[ ] Explain the MAC sublayer and why it matters more for shared media (Wi-Fi) than switched Ethernet