Ethernet Frame Structure (IEEE 802.3) - Automotive Ethernet

IEEE 802.3 Ethernet Frame Structure

Ethernet Frame Layout

  ┌──────────┬──────────┬───────────┬──────────┬────────┬─────┬──────────┐
  │ Preamble │  SFD     │  Dst MAC  │  Src MAC │VLAN tag│ Len │  Payload  │  FCS   │
  │  7 bytes │  1 byte  │  6 bytes  │  6 bytes │4 bytes │EType│ 46–1500 B │  4 B   │
  │0xAA...AA │  0xAB    │           │          │(opt.)  │2 B  │           │CRC-32  │
  └──────────┴──────────┴───────────┴──────────┴────────┴─────┴──────────┘
  Total (untagged): 64–1518 bytes (including FCS)
  Total (VLAN tagged): 64–1522 bytes

  Destination MAC:
  ├── Unicast:   bit0 of byte0 = 0; e.g., 00:1A:2B:3C:4D:5E
  ├── Multicast: bit0 of byte0 = 1; e.g., 01:80:C2:00:00:0E (LLDP)
  └── Broadcast: FF:FF:FF:FF:FF:FF (all stations)

  Source MAC: always unicast (bit0 = 0); globally unique OUI prefix

  EtherType (≥ 0x0600):
  ├── 0x0800: IPv4
  ├── 0x0806: ARP
  ├── 0x8100: VLAN tag (802.1Q)
  ├── 0x8892: PROFINET
  ├── 0x88A4: EtherCAT
  └── 0x88F7: PTP (IEEE 1588 / gPTP)

  Minimum payload: 46 bytes (padded with zeros if shorter)
  Maximum payload: 1500 bytes (standard MTU); Jumbo frames: 9000 bytes

IEEE 802.1Q VLAN Tag

VLAN Tag Field Layout (4 bytes)

  ┌───────────────┬───────────────────────────────────────────────┐
  │  TPID         │  TCI (Tag Control Information)                │
  │  2 bytes      │  2 bytes                                       │
  │  0x8100       ├──────┬─────┬─────────────────────────────────┤
  │               │ PCP  │ DEI │ VID                              │
  │               │ 3 b  │ 1 b │ 12 bits (VLAN ID: 0–4095)       │
  └───────────────┴──────┴─────┴─────────────────────────────────┘

  PCP (Priority Code Point): CoS (Class of Service) — 8 levels (0–7)
  ├── 7: highest — safety-critical control (e.g., brake actuation)
  ├── 6: real-time audio/video (camera streams)
  ├── 5: signalling (SOME/IP-SD)
  ├── 4: standard-priority traffic
  ├── 3–1: background, diagnostics, bulk OTA
  └── 0: best-effort default

  DEI (Drop Eligible Indicator): 1 = frame may be dropped under congestion
  VID (VLAN ID): 0 = priority-only (no VLAN separation); 1–4094 = VLAN groups;
                 4095 = reserved

  Double tagging (QinQ, 802.1ad): outer tag 0x88A8 for service provider VLANs

EtherType	Protocol	Automotive Use
0x0800	IPv4	SOME/IP, DoIP, OTA, video streaming
0x0806	ARP	IP address resolution in flat L2 networks
0x8100	IEEE 802.1Q VLAN	VLAN separation; PCP priority marking
0x86DD	IPv6	Adaptive AUTOSAR (ara::com); future-proofing
0x88F7	IEEE 1588 / PTP	gPTP sync messages (not carried in IP)
0x8892	PROFINET	Industrial automation (rare in automotive)
0x893A	IEEE 802.1CB (FRER)	Frame Replication for TSN redundancy

Summary

The Ethernet frame format is fixed and hardware-stamped — the PHY adds the preamble and SFD; the MAC controller computes and appends the FCS CRC-32. Software never touches these fields. What software controls: destination/source MAC, EtherType, VLAN tag (PCP priority class + VID), and payload. PCP is the primary traffic class selector in automotive switches — a frame with PCP=7 is always forwarded ahead of PCP=0 frames in the same egress queue. All safety-critical TSN streams must be tagged with the correct PCP or the Time-Aware Shaper cannot prioritise them correctly.

🔬 Deep Dive — Core Concepts Expanded

This section builds on the foundational concepts covered above with additional technical depth, edge cases, and configuration nuances that separate competent engineers from experts. When working on production ECU projects, the details covered here are the ones most commonly responsible for integration delays and late-phase defects.

Key principles to reinforce:

Configuration over coding: In AUTOSAR and automotive middleware environments, correctness is largely determined by ARXML configuration, not application code. A correctly implemented algorithm can produce wrong results due to a single misconfigured parameter.
Traceability as a first-class concern: Every configuration decision should be traceable to a requirement, safety goal, or architecture decision. Undocumented configuration choices are a common source of regression defects when ECUs are updated.
Cross-module dependencies: In tightly integrated automotive software stacks, changing one module's configuration often requires corresponding updates in dependent modules. Always perform a dependency impact analysis before submitting configuration changes.

🏭 How This Topic Appears in Production Projects

Project integration phase: The concepts covered in this lesson are most commonly encountered during ECU integration testing — when multiple software components from different teams are combined for the first time. Issues that were invisible in unit tests frequently surface at this stage.
Supplier/OEM interface: This is a topic that frequently appears in technical discussions between Tier-1 ECU suppliers and OEM system integrators. Engineers who can speak fluently about these details earn credibility and are often brought into critical design review meetings.
Automotive tool ecosystem: Vector CANoe/CANalyzer, dSPACE tools, and ETAS INCA are the standard tools used to validate and measure the correct behaviour of the systems described in this lesson. Familiarity with these tools alongside the conceptual knowledge dramatically accelerates debugging in real projects.

⚠️ Common Mistakes and How to Avoid Them

Assuming default configuration is correct: Automotive software tools ship with default configurations that are designed to compile and link, not to meet project-specific requirements. Every configuration parameter needs to be consciously set. 'It compiled' is not the same as 'it is correctly configured'.
Skipping documentation of configuration rationale: In a 3-year ECU project with team turnover, undocumented configuration choices become tribal knowledge that disappears when engineers leave. Document why a parameter is set to a specific value, not just what it is set to.
Testing only the happy path: Automotive ECUs must behave correctly under fault conditions, voltage variations, and communication errors. Always test the error handling paths as rigorously as the nominal operation. Many production escapes originate in untested error branches.
Version mismatches between teams: In a multi-team project, the BSW team, SWC team, and system integration team may use different versions of the same ARXML file. Version management of all ARXML files in a shared repository is mandatory, not optional.

📊 Industry Note

Engineers who master both the theoretical concepts and the practical toolchain skills covered in this course are among the most sought-after professionals in the automotive software industry. The combination of AUTOSAR standards knowledge, safety engineering understanding, and hands-on configuration experience commands premium salaries at OEMs and Tier-1 suppliers globally.