Link Layer
Framing, error detection, medium access (CSMA family), Ethernet, Wi-Fi, switches, VLANs.
The link layer is the last layer between you and the wire. Ethernet is the dominant wired link; Wi-Fi is the dominant wireless link; both share the same medium-access ancestry (CSMA family) even though they handle collisions differently (CD for Ethernet, CA for Wi-Fi). The medium-access story has two parents — static allocation (TDMA / FDMA / CDMA, still the right answer for cellular and satellite) and stochastic access (ALOHA → slotted ALOHA → CSMA), which the rest of the topic walks in order.
Most backend engineers don't write link-layer code, but they should know enough to read tcpdump output, understand how a switch differs from a router, and reason about VLANs when the ops team mentions them.
Key concepts
- The link layer frames bits into packets and detects errors with CRC / FCS
- Static MAC (TDMA / FDMA / CDMA) pre-allocates the channel — right for cellular and satellite
- Stochastic MAC (ALOHA / slotted ALOHA / CSMA family) takes turns by collision detection
- Medium access matters when the link is shared (Wi-Fi, classic Ethernet hubs)
- Switches forward based on MAC addresses; routers forward based on IP
- VLANs let one physical switch host multiple logical broadcast domains
- Wi-Fi has different security generations (WEP / WPA / WPA2 / WPA3) — pick the latest
Reference template
// Reading an Ethernet frame
1. Preamble + SFD (clock sync, ignored by upper layers)
2. Destination MAC (who's this for?)
3. Source MAC (who sent it?)
4. EtherType / Length (what protocol is the payload?)
5. Payload (often an IP datagram)
6. FCS (CRC-32; if mismatched, drop)Adapt to your problem; the structure is the load-bearing part.
Common pitfalls
- Confusing a switch (L2, forwards by MAC) with a router (L3, forwards by IP)
- Treating Wi-Fi as 'wireless Ethernet' — the MAC layer differs significantly
- Forgetting VLAN tag (802.1Q) when capturing on a trunk port
- Assuming a working physical link guarantees a working IP path
Related topics
Items (7)
- The Data Link Layer
Framing, error detection, and medium access — what the link layer adds on top of raw signal.
Concept Foundational - Static Medium Access — TDMA, FDMA, CDMA
Pre-allocated time, frequency, or code slots. The schemes mobile and satellite links rely on, and why static beats stochastic for them.
Concept Intermediate - ALOHA and Slotted ALOHA
The original stochastic medium-access protocol from 1971 Hawaii, the throughput math (18% and 37%), and what slotting bought.
Concept Foundational - Medium Access Control — CSMA, CSMA/CD, CSMA/CA
Stochastic medium access: listen, defer, transmit, detect collisions. The mechanism behind classic Ethernet and Wi-Fi.
Building Block Intermediate - Ethernet — Frame Format, Switches, VLANs
Preamble + dst + src + EtherType + payload + FCS; the switch fabric; VLAN tagging (802.1Q); the spanning tree.
Building Block Foundational - Spanning Tree Protocol (STP)
How a switched LAN avoids loops — BPDUs, root-bridge election, port roles, blocked links — and why RSTP and MSTP exist.
Building Block Intermediate - Wi-Fi (802.11) Basics
PHY layers (a/b/g/n/ac/ax/be), CSMA/CA, association, hidden terminals, RTS/CTS, the security generations (WEP→WPA→WPA3).
Building Block Intermediate