PHY
Physical Layer
OSI Layer 1 — the layer responsible for converting digital bits into physical signals (electrical, optical, or radio) and back again. In Wi-Fi, "PHY rate" refers to the raw link speed negotiated at this layer before protocol overhead is accounted for.
The PHY (pronounced "fie") is the bottom of the networking stack. Every other layer — IP, TCP, HTTP — ultimately relies on the PHY to get bits from one place to another. The PHY chip on an Ethernet NIC encodes outgoing bits into voltage levels on copper wire using standards like PAM4 (for 25 Gbps+) or NRZ, and decodes incoming analog signals back into bits. On Wi-Fi hardware, the PHY modulates bits onto radio frequencies using OFDM subcarriers. The PHY has no concept of frames, addresses, or protocols — it is a pure bit pipe.
PHY by medium type
| Medium | PHY signal type | Examples |
|---|---|---|
| Copper (Ethernet) | Differential voltage (PAM2/PAM4) | 10BASE-T, 1000BASE-T, 10GBASE-T |
| Fibre optic | Light pulses (on/off or coherent) | 1000BASE-LX, 10GBASE-SR, 100G coherent |
| Wi-Fi (2.4/5/6 GHz) | Radio waves (OFDM modulated) | 802.11n/ac/ax |
| Coaxial (DOCSIS cable) | QAM-modulated RF | DOCSIS 3.0/3.1 |
| Phone line (DSL) | QAM over voiceband | ADSL2+, VDSL2, G.fast |
PHY rate vs actual throughput
In Wi-Fi, the PHY rate is the raw speed the link is operating at — visible in your OS as "Link Speed" (e.g., 1201 Mbps for Wi-Fi 5, 2402 Mbps for Wi-Fi 6 on 80 MHz). Actual application throughput is 50–70% of PHY rate because Wi-Fi is half-duplex, frames carry headers and interframe gaps, and retransmissions consume capacity. A higher PHY rate always means more headroom for real throughput. PHY rate drops when signal quality degrades — the radio falls back to a lower modulation scheme (e.g., from 1024-QAM to 64-QAM) to maintain a reliable link.
Frequently Asked Questions
What does the PHY layer actually do?
It converts bits to physical signals and back. For Ethernet, a PHY chip drives voltage onto copper pairs using line coding and recovers the clock from received signals. For Wi-Fi, the PHY modulates bits onto radio carriers using OFDM. The PHY has no awareness of frames or IP — it only sees a stream of bits.
What is PHY rate in Wi-Fi?
The raw bitrate negotiated between your device and the access point at Layer 1 — shown as "Link Speed" in Wi-Fi status. Actual throughput is 50–70% of PHY rate due to protocol overhead and half-duplex contention. PHY rate depends on channel width, MIMO streams, QAM order, and signal quality.
What causes a low PHY rate on Wi-Fi?
Poor signal quality: distance, walls, or interference force the radio to use a lower modulation scheme. Moving closer to the AP, using 5 GHz or 6 GHz, and reducing channel interference all raise PHY rate and throughput. Mismatched channel width (20 MHz vs 80 MHz) also significantly limits PHY rate.