Marketing materials advertise WiFi routers with impressive numbers: "3,000 Mbps!", "WiFi 6E AX11000!". But what do these numbers actually mean? Why does your file transfer achieve only a fraction of the advertised speed? And how can you predict the actual throughput a deployment will deliver?

WiFi data rates are not straightforward. Multiple factors interact: channel bandwidth, modulation scheme, coding rate, spatial streams, guard intervals, and protocol overhead. Understanding these components is essential for capacity planning, troubleshooting, and setting realistic performance expectations.

This page dissects WiFi data rates from first principles. We'll build up from individual OFDM symbol calculations to complete MCS (Modulation and Coding Scheme) tables, then examine the overhead factors that reduce PHY rates to actual application throughput.

All modern WiFi (802.11a/g/n/ac/ax) uses OFDM (Orthogonal Frequency-Division Multiplexing). To understand data rates, we must understand OFDM's structure.

OFDM Symbol Structure:

An OFDM channel is divided into parallel subcarriers, each carrying an independent modulated signal. The key parameters:

802.11a/g/n/ac (20 MHz channel):

• Total subcarriers: 64
• Data subcarriers: 48 (carry user data)
• Pilot subcarriers: 4 (channel estimation)
• Null subcarriers: 12 (guard bands + DC)
• Symbol duration: 4 μs (3.2 μs symbol + 0.8 μs guard interval)

802.11ax (20 MHz channel):

• Total subcarriers: 256 (4× more dense spacing)
• Data subcarriers: 234 (4.875× more than legacy)
• Pilot subcarriers: 8
• Null subcarriers: 14
• Symbol duration: 13.6 μs (12.8 μs symbol + 0.8 μs GI)

Data Rate Formula:

The fundamental data rate calculation is:

Data Rate = (Data Subcarriers × Bits per Symbol × Coding Rate × Spatial Streams) / Symbol Duration

Example: 802.11ac, 80 MHz, 256-QAM, 5/6 coding, 2 spatial streams, short GI

• Data subcarriers (80 MHz): 234
• Bits per symbol (256-QAM): 8
• Coding rate: 5/6
• Spatial streams: 2
• Symbol duration (short GI): 3.6 μs

Data Rate = (234 × 8 × 5/6 × 2) / 3.6 μs
         = (234 × 8 × 0.833 × 2) / 0.0000036 s
         = 3120 / 0.0000036
         = 866.7 Mbps

This matches MCS 9 at 80 MHz with 2 spatial streams—a common configuration for mid-range 802.11ac devices.

Guard Interval (GI) Impact:

The guard interval is 'dead time' between OFDM symbols, preventing multipath-induced inter-symbol interference. Shorter GI increases throughput but requires cleaner RF environment:

Standard vs. Short Guard Interval:

Standard	Normal GI	Short GI	Throughput Increase
802.11n	800 ns	400 ns	+11%
802.11ac	800 ns	400 ns	+11%
802.11ax	800 ns	400 ns	+6%
802.11ax	1600 ns	800 ns	+11%
802.11ax	3200 ns	1600 ns	+6%

802.11ax introduces multiple GI options (0.8, 1.6, 3.2 μs) for different deployment scenarios. The longer symbol duration already provides multipath resilience, so normal GI (0.8 μs) is often practical.

Modulation determines how many bits each subcarrier carries per symbol. Higher-order modulation packs more bits but requires better signal-to-noise ratio (SNR).

WiFi Modulation Hierarchy:

BPSK (Binary Phase Shift Keying):

• Bits per symbol: 1
• Constellation points: 2 (separated by 180°)
• Required SNR: ~6 dB
• Use case: Maximum range, highly degraded conditions

QPSK (Quadrature Phase Shift Keying):

• Bits per symbol: 2
• Constellation points: 4
• Required SNR: ~9 dB
• Use case: Extended range with moderate performance

16-QAM:

• Bits per symbol: 4
• Constellation points: 16 (4×4 grid)
• Required SNR: ~15 dB
• Use case: Standard performance at medium range

64-QAM:

• Bits per symbol: 6
• Constellation points: 64 (8×8 grid)
• Required SNR: ~21 dB
• Use case: High performance at shorter range

256-QAM (802.11ac/ax):

• Bits per symbol: 8
• Constellation points: 256 (16×16 grid)
• Required SNR: ~27 dB
• Use case: Maximum performance, very close range

1024-QAM (802.11ax):

• Bits per symbol: 10
• Constellation points: 1024 (32×32 grid)
• Required SNR: ~33 dB
• Use case: Ideal conditions only, very short range

4096-QAM (802.11be/WiFi 7):

• Bits per symbol: 12
• Constellation points: 4096 (64×64 grid)
• Required SNR: ~39 dB
• Use case: Same-room, line-of-sight connections

*Range percentages are approximate, relative to BPSK range under identical conditions.

The SNR Reality:

Each 3 dB of additional SNR requirement approximately halves the usable range (due to inverse-square power law). Thus:

• 64-QAM (6 bits) works at ~45% of BPSK range
• 256-QAM (8 bits) works at ~25% of BPSK range
• 1024-QAM (10 bits) works at ~15% of BPSK range

This explains why maximum data rates are only achievable at close range, while devices automatically fall back to lower modulation as distance increases.

Coding Rate and Error Correction:

Forward Error Correction (FEC) adds redundancy to detect and correct transmission errors. The coding rate indicates the proportion of bits that carry actual data:

Coding Rate	Data Bits	Redundancy	Error Resilience
1/2	50%	50%	Highest
2/3	67%	33%	High
3/4	75%	25%	Medium
5/6	83%	17%	Low

Lower coding rates (more redundancy) are used with higher-order modulation to maintain reliability. For example:

• 64-QAM with 2/3 coding: Moderate performance, good reliability
• 64-QAM with 3/4 coding: Higher throughput, requires better conditions

The MCS index encodes the specific combination of modulation and coding rate.

The MCS (Modulation and Coding Scheme) index is WiFi's way of specifying the exact combination of modulation, coding rate, and spatial streams. Understanding MCS tables is essential for interpreting diagnostic data and planning capacity.

802.11n MCS Structure:

802.11n uses a linear MCS index (0-31) where:

• MCS 0-7: Single spatial stream
• MCS 8-15: Two spatial streams (2× single-stream rate)
• MCS 16-23: Three spatial streams (3× single-stream rate)
• MCS 24-31: Four spatial streams (4× single-stream rate)

802.11ac/ax MCS Structure:

802.11ac introduced a cleaner MCS structure where index is independent of spatial streams:

• MCS 0-9: Defined per spatial stream
• Multiply by number of spatial streams for total rate
• 802.11ax adds MCS 10 and 11 for 1024-QAM

802.11ax (WiFi 6) MCS Additions:

802.11ax extends the MCS table with 1024-QAM options and modified symbol structure:

MIMO (Multiple-Input Multiple-Output) enables multiple simultaneous data streams between AP and client. Each additional spatial stream multiplies the peak data rate.

Spatial Stream Requirements:

Both transmitter and receiver must support the same number of spatial streams:

• A 4×4 AP with a 2×2 client = 2 spatial streams maximum
• A 2×2 AP with a 4×4 client = 2 spatial streams maximum
• The limiting factor is always the lesser of the two

Device Classifications:

Client Type	Typical MIMO Config	Spatial Streams
Basic IoT/Sensors	1×1	1 SS
Budget smartphones	1×1 or 2×2	1-2 SS
Mid-range phones/laptops	2×2	2 SS
Premium laptops	2×2 or 3×3	2-3 SS
High-end workstations	4×4	4 SS
Enterprise APs	4×4 to 8×8	4-8 SS
High-density APs	8×8 or 12×12	8-12 SS (MU-MIMO)

The MIMO Reality Check:

Marketing materials advertise maximum rates assuming maximum spatial streams. Real-world observations:

1. Most clients are 2×2: Enterprise surveys show 80-90% of devices are 2-stream capable. Planning for 2 SS is reasonable for capacity.

2. MIMO degrades with distance: As SNR decreases, MIMO becomes less effective. A 4×4 system at range may perform no better than 2×2 at close range.

3. Antenna diversity vs. MIMO: Extra antennas can provide diversity gain (improved reliability) without additional spatial streams. A 3×3:2 configuration has 3 antennas but only 2 streams.

4. MIMO requires multipath: Spatial multiplexing depends on distinct radio paths. In line-of-sight outdoor environments, MIMO gains may be minimal.

The data rates in MCS tables are PHY rates—the raw bit rate at the physical layer. Actual application throughput is significantly lower due to multiple overhead sources:

Major Overhead Categories:

1. Frame Headers and Trailers (~5-10%)

   • MAC header: 34 bytes
   • LLC/SNAP: 8 bytes
   • FCS: 4 bytes
   • PHY preamble and training symbols

2. Medium Access Protocol (~15-25%)

   • DIFS/AIFS waiting time before transmission
   • Backoff procedure after collisions
   • ACK transmission after each frame
   • SIFS intervals between frame pairs

3. Encryption Overhead (~2-5%)

   • WPA2: 16 bytes per frame (CCMP)
   • WPA3: 16-28 bytes additional

4. IP/TCP Headers (~5-10%)

   • IP header: 20-60 bytes
   • TCP header: 20-60 bytes
   • TCP acknowledgments (reverse direction)

5. Contention and Collisions (~5-20%+)

   • Increases dramatically with more clients
   • Hidden node collisions
   • Retransmissions due to errors

Calculating Realistic Throughput:

A practical rule of thumb:

Actual TCP Throughput ≈ PHY Rate × 0.50 to 0.65 (ideal conditions)
Actual TCP Throughput ≈ PHY Rate × 0.30 to 0.45 (typical enterprise)
Actual TCP Throughput ≈ PHY Rate × 0.15 to 0.30 (congested/edge coverage)

Example:

• PHY Rate: MCS 9, 80 MHz, 2 SS = 866.7 Mbps
• Ideal throughput: 433-563 Mbps
• Typical throughput: 260-390 Mbps
• Congested/edge: 130-260 Mbps

802.11ax Efficiency Improvements:

802.11ax specifically targets overhead reduction:

1. OFDMA: Multiple clients share frames, reducing per-client overhead
2. Longer OFDM symbols: Better overhead ratio for small packets
3. Trigger-based access: Coordinated uplink reduces collisions
4. Multi-user transmission: Serves multiple clients simultaneously

802.11ax Efficiency Comparison:

Traffic Type	802.11ac Efficiency	802.11ax Efficiency	Improvement
Large file transfer	~60%	~70%	+17%
Web browsing	~40%	~55%	+38%
VoIP	~25%	~45%	+80%
IoT sensors	~15%	~35%	+133%

The gains are most dramatic for small-packet, bursty traffic where legacy WiFi overhead was most punishing.

Consumer router marketing uses aggregate numbers that can mislead buyers. Understanding these numbers prevents unrealistic expectations.

The "AX11000" Example:

A router advertised as "AX11000" might have:

• 2.4 GHz: 1148 Mbps (4×4, 40 MHz, 1024-QAM)
• 5 GHz Band 1: 4804 Mbps (4×4, 160 MHz, 1024-QAM)
• 5 GHz Band 2: 4804 Mbps (4×4, 160 MHz, 1024-QAM)
• Total: 10,756 Mbps ≈ "AX11000"

Reality Check:

1. No single device uses all bands simultaneously. A client connects to ONE band at a time.

2. 160 MHz channels are rare. Most deployments use 80 MHz due to interference.

3. 1024-QAM requires perfect conditions. Most clients operate at 256-QAM or lower.

4. 4×4 clients are uncommon. Most devices are 2×2, halving the maximum rate.

5. Actual throughput is 40-60% of PHY rate. Protocol overhead is unavoidable.

More Realistic Assessment:

A consumer connecting a 2×2 laptop to an "AX11000" router typically achieves:

• 802.11ax, 80 MHz, 2 SS, MCS 9: 961 Mbps PHY
• Actual TCP throughput: 400-550 Mbps

Still excellent, but far from "11 Gbps".

*Requires 160 MHz channel, 4×4 client, and ideal conditions.

Enterprise vs. Consumer Specifications:

Enterprise specifications tend to be more honest:

• Quote per-radio rates rather than combined totals
• Specify expected real-world throughput ranges
• Document capacity (number of clients) rather than just speed
• Provide detailed MCS distribution expectations