Ethernet MAC / PHY Basics: Step-by-Step Walkthrough

Step-by-step analysis walkthrough

Follow this when you own Ethernet MAC / PHY Basics in a protocol review or bring-up war room.

1. State expected transaction in plain language (who initiates, what completes).

2. Draw layer stack and mark clock/reset boundaries.

3. List channels: request, data, response, snoop, credit, or lane.

4. Tag ID/address/endpoint on the failing run.

5. Find first cycle where progress stops or semantics change.

6. Check bridge: width, ID remap, burst, ordering attributes.

7. Check flow control: ready, credit, FIFO, link state.

8. Check firmware/register mode vs hardware capability.

9. Build minimal replay; confirm legal vs illegal per spec.

10. Estimate metric delta from proposed fix.

11. Run compliance + product traffic regression matrix.

12. Write signoff memo with owners and artifacts attached.

Artifacts to collect

• packet capture, MAC counter dump, PHY status register log

• VIP transaction log

• Waveform with annotations

• Spec clause reference

• Regression manifest

Decision memo template

PROTOCOL DECISION MEMO — Ethernet MAC / PHY Basics
metric:
transaction id:
layer:
hypothesis:
experiment:
fix:
validation:
owners: network IP owner, PHY owner, software owner

Reference visuals

MAC / PCS / PHY split

ETHERNET STACK

  [ software / TCP-IP ]
        |
  [ MAC ]  framing, CRC, addressing
        |  (MII/GMII/XGMII)
  [ PCS ]  encoding, alignment
        |  (SerDes)
  [ PMA/PMD / PHY ] electrical, channel

CRC errors -> usually PCS/PHY/channel, not MAC software.

Protocol deep dive

USB/Ethernet/MIPI failures cross MAC counters, PCS framing, PHY adaptation, and channel SI.

Concept diagram

HIGH-SPEED STACK

app -> MAC/framing -> PCS/encoding -> SerDes/PHY -> channel

CRC errors often mean PCS/PHY/channel, not TCP.

Metric graph

BER vs EQ SETTING

BER
1e-3 |*
1e-6 |  *
1e-9 |     **** usable window
1e-12|          *
     +-----------------> EQ tap

Metrics and artifacts to collect

• CRC error rate

• retrain count

• frame drop

• lane error

• BER

Mini case study

Ethernet link up at 100G but lossy: equalization margin on one lane narrow after package change. Digital counters were clean; PHY margin was not.

Debug branches

• If link up but lossy, PHY margin and retrain.

• If enumeration OK but throughput low, check packet size and DMA batching.

• If MIPI frame drops, blanking budget and lane polarity.

Senior review question

Ask: what is the first transaction that deviates, and which spec rule does it test?

Key takeaways

• Connect every protocol claim to a transaction identity and measurable metric.

• Store the artifact (waveform, log, counter) next to every signoff decision.

Common pitfalls

• Debugging timeouts without finding the first bad transaction.

• Quoting peak bus width without payload efficiency and retry overhead.

• Treating VIP compliance as a substitute for system integration replay.

Principal review addendum

Re-read Ethernet MAC / PHY Basics against one concrete product workload, not a synthetic directed test.

Ethernet splits packet framing, MAC behavior, PCS, PHY training, and board/channel constraints.