UsingWi‑Fias an example, theMAC layeris the "network access manager" that sits above the PHY/Baseband and decides how a device joins a network, gains access to the medium, exchanges frames, retries lost packets, and maintains connectivity.
A simplified Wi‑Fi stack looks like:
Application TCP/IP ↓ 802.11 MAC ↓ 802.11 PHY/Baseband ↓ RF ↓ AntennaThe MAC layer performs:
Scanning
Authentication
Association
Security key exchange
Data transmission
Retransmission
Roaming
Power saving
Let's walk through what happens when an ESP32-S31 connects to a Wi‑Fi AP.
Step 1: Scanning
Before connecting, the STA (station, e.g., ESP32) must discover available APs.
Passive Scan
The STA simply listens.
APs periodically send:
Beacon Framecontaining:
SSID Channel BSSID Security Mode Supported Rates CapabilitiesExample:
SSID: HomeWiFi Channel: 6 Security: WPA2Active Scan
The STA sends:
Probe RequestExample:
Who has SSID "HomeWiFi"?AP replies:
Probe ResponseAt this stage, the MAC is handling management frames.
STA -------------------> Probe Request STA <------------------- Probe ResponseStep 2: Authentication
Historically, 802.11 defined an authentication procedure before association.
Open System Authentication
Today this is the most common.
STA sends:
Authentication Request `AP replies:
Authentication Response (Status = Success)STA AP Authentication Request -----> <------- Authentication ResponseThis doesnotyet verify the Wi‑Fi password.
It simply says:
You may proceed to associate.Step 3: Association
Now the station asks to join the AP.
Association Request
Contains:
SSID Supported Data Rates Capabilities QoS Features HT/VHT/HE CapabilitiesExample:
I support: 802.11n MCS0~MCS7 WPA2 QoSAssociation Response
AP replies:
Association ID (AID) Status CapabilitiesExample:
AID = 0x0021 Status = SuccessAt this point:
STA is associatedbut encrypted data is still unavailable.
STA AP Assoc Req -------------> <------- Assoc RespStep 4: WPA/WPA2/WPA3 Security
Now comes the real security process.
For WPA2-PSK the famous:
4-Way Handshakeoccurs.
Goal
Both sides derive:
PTK (Pairwise Transient Key)used to encrypt traffic.
Message 1
AP sends:
ANonceMessage 2
STA sends:
SNonce MICMessage 3
AP sends:
Install Key GTKMessage 4
STA acknowledges.
AP STA M1 ANonce ------------> <---- M2 SNonce M3 GTK -------------> <---- M4 ACKAfter this:
Encrypted traffic enabledNow IP traffic can start.
Step 5: DHCP
This is above the MAC layer but happens immediately afterward.
DHCP Discover DHCP Offer DHCP Request DHCP ACKNow the device obtains:
IP Address Gateway DNSExample:
192.168.1.105Step 6: Data Transmission
Once connected, the Wi‑Fi MAC handles frame delivery.
Example:
TCP Packetbecomes:
802.11 Data Framecontaining:
MAC Header Payload FCSMAC Addressing
Each device has:
48-bit MAC AddressExample:
ESP32: 84:FC:E6:12:34:56 AP: 18:E8:29:AA:BB:CCThe MAC layer inserts source and destination addresses into every frame.
CSMA/CA
Wi‑Fi MAC must share the air with other devices.
Unlike Ethernet:
Can't detect collisions reliablySo Wi‑Fi uses:
CSMA/CACarrier Sense Multiple Access with Collision Avoidance.
Example:
Device A wants to send.MAC first listens.
Channel Busy?If yes:
Wait Random Backoff RetryIf no:
TransmitListen ↓ Busy? ↓ Wait ↓ Backoff ↓ TransmitThis logic is entirely MAC-level.
ACK and Retransmission
After every unicast frame:
Receiver sends ACK.STA ---------- Data ----------> <------ ACK -----------If no ACK arrives:
Retransmit `The MAC maintains:
Retry Counter Sequence Number Duplicate DetectionThis happens below TCP/IP.
Power Save
For battery devices, the MAC supports sleeping.
AP periodically indicates:
Buffered packets waiting.through beacon information.
Station behavior:
Sleep Wake Check Beacon Retrieve Data Sleep AgainThis is one reason Wi‑Fi IoT devices can save power.
Roaming
When signal quality drops:
RSSI ↓the MAC scans for another AP.
Example:
AP1 -> AP2The MAC performs:
Re-authentication Re-association Key Updateswhile trying to keep connectivity.
Inside an ESP32-S31
Very roughly:
Application ↓ TCP/IP Stack ↓ Wi-Fi MAC ├─ Scanning ├─ Authentication ├─ Association ├─ Security ├─ ACK ├─ Retries ├─ Power Save └─ Roaming ↓ Wi-Fi Baseband ├─ OFDM ├─ Modulation ├─ Encoding └─ Synchronization ↓ RF ├─ 2.4 GHz LO ├─ PA ├─ LNA └─ AntennaA useful rule of thumb
MACdecideswho can talk, when to talk, and who we're talking to.
Basebandconverts packets into Wi‑Fi symbols and vice versa.
RFconverts those symbols into actual 2.4 GHz radio waves.
So when you see logs like:
wifi: scan start wifi: authentication success wifi: association success wifi: connectedthose events are primarily being driven by theWi‑Fi MAC layer, while the baseband and RF are the engines that physically move the bits through the air.
