What is Airtime

Airtime is a critical metric in wireless design. It determines how much time a device occupies the channel to transmit or receive data. When multiple clients share the same radio, excessive airtime consumption leads to poor performance.

This simplified calculation helps estimate how many AP radios are needed to support a given number of clients with specific application requirements.

Key Assumptions

  • Application throughput per client: 1 Mbps (e.g., basic web/email)
  • 1 AP with 2 radios (2.4 & 5 GHz)
  • Wi-Fi 6 (802.11ax) clients using typical MCS data rates
  • Goal: keep total airtime per radio under 80% utilization

Airtime Calculation Steps

  1. Determine Data Rate
    Based on MCS index, spatial streams (SS), modulation, and channel width.
    Data rate = MCS 5 + 2SS + 80 MHz

  2. Estimate Real Throughput
    Divide raw data rate by 2 to account for protocol overhead.
    Max STA Throughput = Data Rate / 2

  3. Calculate Airtime per Client Divide the APP throughput by the actual data throughput Airtime per STA (%) = (App Throughput / Max STA Throughput) × 100

  4. Calculate Total Airtime Multiply the total number of clients by the airtime per client Total Airtime (%) = Airtime per STA × Number of STAs

  5. Determine Number of Radios Needed Divide the total airtime by 80% of utilization as a best practice Number of Radios = Total Airtime / 80

  6. Calculate Number of APs
    Divide the number of radios by the number of AP radios Number of APs Needed = Number of Radios / Radios per AP

Example Calculation

Goal: Support 100 clients using 1 Mbps for web/email

  • Client Data Rate = MCS 5 + 2 spatial streams = 260 Mbps raw → 130 Mbps usable
  • Airtime per STA = (1 / 130) × 100 = 0.77%
  • Total Airtime = 0.77 × 100 = 77%
  • Radios Needed = 77 / 80 = 0.96 → 1 radio
  • APs Needed = 1 / 2 = 0.5 → 1 AP

Result: 1 AP with 2 radios can support 100 Wi-Fi 6 clients at 1 Mbps each.