{
  "$schema": "https://faxvio.com/api/v1/openapi.json",
  "endpoint": "https://faxvio.com/api/v1/transmission-time.json",
  "description": "The constants and the formula for estimating how long a fax takes to send: page geometry from ITU-T T.4, session overhead from T.30, and the published modem bit rates.",
  "unit": "seconds",
  "provenance": "An estimate computed from ITU-T T.4 page geometry, ITU-T T.30 session overhead and published modem bit rates. It is a model, not a measurement: real duration depends on the line and on the receiving machine.",
  "caveat": "An estimate with a ±20% band, not a measurement. Compression depends on what is on the page, and a VoIP leg caps the modem rate and adds retrains, which is why a fax over an internet line is slower than the number printed on the machine.",
  "model": {
    "line_pels": 1728,
    "lines_per_page": {
      "standard": 1145,
      "fine": 2290
    },
    "resolution_dpi": {
      "standard": "204 × 98",
      "fine": "204 × 196"
    },
    "compression_ratio": {
      "light": 20,
      "dense": 10,
      "form": 6,
      "photo": 2.5
    },
    "modem_bit_rates": {
      "V.27ter": 4800,
      "V.29": 9600,
      "V.17": 14400,
      "V.34": 33600
    },
    "setup_seconds": 20,
    "teardown_seconds": 3,
    "per_page_overhead_seconds": 4,
    "ecm_factor": 1.1,
    "voip_factor": 1.15,
    "voip_max_bit_rate": 14400,
    "uncertainty": 0.2,
    "formula": "seconds = setup + pages × (line_pels × lines_per_page ÷ compression_ratio ÷ bit_rate × ecm_factor × voip_factor + per_page_overhead) + teardown",
    "sources": [
      "ITU-T Recommendation T.4 — page geometry and coding",
      "ITU-T Recommendation T.30 — session handshake and per-page confirmation",
      "ITU-T Recommendations V.27ter, V.29, V.17, V.34 — modem bit rates"
    ]
  },
  "examples": [
    {
      "label": "One text page, standard resolution, 14.4k landline",
      "pages": 1,
      "resolution": "standard",
      "bit_rate": 14400,
      "content": "light",
      "voip": false,
      "seconds": 35,
      "seconds_low": 28,
      "seconds_high": 41
    },
    {
      "label": "Three-page form, fine resolution, 14.4k landline",
      "pages": 3,
      "resolution": "fine",
      "bit_rate": 14400,
      "content": "form",
      "voip": false,
      "seconds": 186,
      "seconds_low": 149,
      "seconds_high": 223
    },
    {
      "label": "Ten dense pages, standard resolution, 33.6k landline",
      "pages": 10,
      "resolution": "standard",
      "bit_rate": 33600,
      "content": "dense",
      "voip": false,
      "seconds": 128,
      "seconds_low": 102,
      "seconds_high": 153
    },
    {
      "label": "Three-page form, fine resolution, over VoIP",
      "pages": 3,
      "resolution": "fine",
      "bit_rate": 33600,
      "content": "form",
      "voip": true,
      "seconds": 209,
      "seconds_low": 167,
      "seconds_high": 251
    }
  ],
  "related": {
    "human_page": "https://faxvio.com/tools/fax-transmission-time/"
  },
  "licence": {
    "name": "CC BY 4.0",
    "url": "https://creativecommons.org/licenses/by/4.0/",
    "attribution": "Faxvio — https://faxvio.com",
    "note": "Free to reuse, including commercially, with a link back to the endpoint or the page it came from."
  }
}