Wi-Fi MAC-PHY Interface

Advanced Wi-Fi Hardware and Verification · 2/21

Module 1: Hardware Partitioning

Wi-Fi MAC-PHY Interface

Learning objectives

  • Explain the core mental model behind Wi-Fi MAC-PHY Interface
  • Apply Wi-Fi MAC-PHY Interface within Hardware Partitioning
  • Identify important boundaries, trade-offs, and failure modes
  • Produce concrete evidence from the practice exercise

Related: WIFI Overview and Architecture | WIFI MAC Architecture | WIFI PHY Architecture | WIFI Timing IFS and NAV

Interface Purpose

The MAC-PHY boundary carries payload bytes plus timing and radio metadata. A weak interface definition causes integration bugs even when MAC and PHY are individually correct.

TX Vector

The MAC provides a TX vector describing how the PHY should transmit:

  • packet format/generation
  • channel bandwidth
  • MCS
  • number of spatial streams
  • guard interval
  • coding type
  • PSDU length
  • transmit power or power class
  • preamble/protection mode
  • aggregation indication

The TX vector must be stable before the PHY commits to transmission.

RX Vector

The PHY returns an RX vector describing what it received:

  • detected format
  • rate/MCS/NSS/bandwidth
  • PSDU length
  • RSSI per chain
  • EVM/SNR-like quality
  • frequency offset estimates
  • PHY error status
  • timestamp
  • channel/band indication

The MAC uses this to parse, filter, update statistics, compute NAV, and drive rate-control feedback.

CCA Interface

Clear Channel Assessment tells the MAC whether the medium is busy.

CCA may include:

  • preamble detect
  • energy detect
  • virtual/OBSS information in newer modes
  • primary/secondary channel busy indication

MAC backoff must react to CCA without firmware latency.

Turnaround Timing

Critical timing paths:

  • RX end to ACK/BA TX start after SIFS
  • CCA idle to backoff slot decrement
  • MAC TX request to PHY start
  • PHY abort/error to MAC retry update

These should be verified as cycle-accurate contracts.

Common Integration Bugs

  • MAC computes duration using different rate metadata than PHY used.
  • RX vector arrives after MAC needs ACK decision.
  • PHY reports length in bytes while MAC interprets symbols or words.
  • CCA crosses clock domains without clean pulse/level semantics.
  • Timestamp captured at different reference points in TX and RX.

Related

  • WIFI Timing IFS and NAV — Timing counters
  • WIFI Verification and Debug — Interface checkers
  • WIFI Common HW Pitfalls — CDC and metadata bugs

Practice lab

Draw or encode one legal transaction trace for Wi-Fi MAC-PHY Interface. Annotate fields, channel events, ordering points, and completion conditions; then construct one illegal or adversarial trace and define the checker that should catch it. Add an operational constraint such as concurrency, recovery, security, latency, or cost, and defend the resulting design trade-off.

Review questions

  1. What problem does Wi-Fi MAC-PHY Interface solve, and what assumptions does it rely on?
  2. Which boundary or failure case is easiest to miss, and how would you expose it?
  3. What alternative design would you consider, and what trade-off would change the decision?
  4. What artifact, trace, test, or metric proves that your implementation is correct?

Completion evidence

  • A working artifact, annotated trace, or reproducible experiment
  • At least one normal case and one deliberately failing or boundary case
  • A concise explanation of the design choice and its trade-offs
  • Saved output showing how correctness was evaluated