Interview Drills for Ethernet MAC / PHY Basics focuses on packet loss, CRC error count, line-rate efficiency. The goal is to connect the observable symptom to protocol mechanism, ownership, and regression risk.
PROMPT
You see packet loss, CRC error count, line-rate efficiency on Ethernet MAC / PHY Basics. Walk through root cause and fix.
STRONG ANSWER
1. Names the layer and transaction identity.
2. Explains Ethernet splits packet framing, MAC behavior, PCS, PHY training, and board/channel constraints.
3. Requests packet capture, MAC counter dump, PHY status register log.
4. Proposes one reduced sequence and one system regression.
WEAK ANSWER
Jumps to widening the interface, increasing FIFO depth, or blaming firmware without evidence.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.ROOT-CAUSE TREE — Ethernet MAC / PHY Basics
packet loss, CRC error count, line-rate efficiency looks wrong
|
reproducible?
/ \
no yes
| |
flaky env same first transaction every time?
/ seed / \
yes no
| |
protocol rule timing/reset/PVT
or config bug or load-dependentUSB/Ethernet/MIPI failures cross MAC counters, PCS framing, PHY adaptation, and channel SI.
HIGH-SPEED STACK
app -> MAC/framing -> PCS/encoding -> SerDes/PHY -> channel
CRC errors often mean PCS/PHY/channel, not TCP.BER vs EQ SETTING
BER
1e-3 |*
1e-6 | *
1e-9 | **** usable window
1e-12| *
+-----------------> EQ tapCRC error rate
retrain count
frame drop
lane error
BER
Ethernet link up at 100G but lossy: equalization margin on one lane narrow after package change. Digital counters were clean; PHY margin was not.
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.
Ask: what is the first transaction that deviates, and which spec rule does it test?
Connect every protocol claim to a transaction identity and measurable metric.
Store the artifact (waveform, log, counter) next to every signoff decision.
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.
Draw layers first, then place the failing transaction on the diagram.
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