Timing budget
Your roam-time requirement is what is left of the tightest connection timeout after the wired path and the endpoints take their share, and the worst-case gap has to fit inside it.
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Timing budget
Wi-Fi will never be deterministic, so prove the worst-case gap fits inside what the tightest timeout leaves for the radio.
Where each timeout comes from
| Connection | Timeout | Example |
|---|---|---|
| CIP Safety input connection | RPI x (timeout multiplier + network delay multiplier) | 40 ms at Input RPI 10 ms, Timeout multiplier 2, Network delay multiplier 200 % |
| CIP Safety output connection | safety task period x (timeout multiplier + network delay multiplier - 1) | 60 ms at Safety task period 20 ms, Timeout multiplier 2, Network delay multiplier 200 % |
| EtherNet/IP standard I/O | RPI x connection timeout multiplier, but never less than the minimum timeout | 100 ms at RPI 20 ms, Connection timeout multiplier 4 x, Minimum timeout 100 ms |
| PROFINET IO | update time x accepted update cycles without IO data | 12 ms at Update time 4 ms, Accepted update cycles without IO data 3 |
| PROFIsafe | F-monitoring time, entered directly | 100 ms at F-monitoring time (F_WD_Time) 100 ms |
How to use the budget
- List every connection on the link: safety, standard I/O, fleet traffic.
- Take the tightest timeout configured in the project, not the default.
- Example: CIP Safety input at defaults is 10 ms x (2 + 2) = 40 ms.
- Subtract what the wired path and endpoints spend. Measure it; do not guess.
- What remains is the outage budget: the longest gap Wi-Fi may ever cause.
- Measure the worst-case roam on the real vehicle, client, and security.
- Pass only if the worst gap fits. One gap over the budget faults the connection.
- Raising a timeout is the machine builder's call. It lengthens the worst case.
- Time is distance: at 30 mph (13.41 m/s), each 100 ms is about 1.3 m.
Published roam interruption ranges (typical, measure your own)
| Roam | Typical interruption |
|---|---|
| Full 802.1X reauthentication | No published range: measure yours |
| PSK four-way handshake (no fast roaming) | 55 to 520 ms |
| PMK caching and opportunistic key caching | No published range: measure yours |
| 802.11r fast BSS transition | 15 to 90 ms |
| Vendor fast-roam and make-before-break schemes (vendor claim) | 63 to 150 ms |
| Wi-Fi 7 multi-link operation | No published range: measure yours |
| 802.11bn seamless mobility domain | No published range: measure yours |
About this page
Built from 9 sources: 1 standards body or lab, 2 protocol owners and alliances, 2 research papers and theses and 4 vendor documents. Researched and drafted with AI assistance, then reviewed and approved by Ben Rutter on . How pages are made
- First published
- Last updated
Change history (1)
- First published
Cite this page
Plain
Ben Rutter. "Timing budget." OT Wireless, published October 6, 2026, updated October 5, 2026. https://otwireless.com/cards/timing-budget/
APA 7
Rutter, B. (2026, October 6). Timing budget. OT Wireless. https://otwireless.com/cards/timing-budget/
BibTeX
@misc{rutter2026timingbudget,
author = {Rutter, Ben},
title = {{Timing budget}},
year = {2026},
howpublished = {\url{https://otwireless.com/cards/timing-budget/}},
organization = {OT Wireless},
}