Direct answer
Estimate fiber propagation delay from group index, actual fiber length and vacuum light speed. Jacket length alone is insufficient. Lane skew is relative delay between defined channels at defined boundaries, not total network latency. Specify length datums, channel differences and application limits separately.
Fiber propagation and network latency differ
Propagation delay is the transit time through a defined fiber segment. Group index is the relevant quantity for this estimate; an arbitrary material refractive index is not a substitute. Corning's SMF-28 Ultra information lists effective group index as a typical characterization, including 1.4682 at 1550 nm. Corning group-index information
Network latency also involves actual module/equipment processing, protocol and queuing behavior. This guide calculates only the stated fiber segment for trunk, harness and low-latency routing reviews; it makes no service-latency promise.
A reproducible one-way calculation
Use t = n_g × L / c, where n_g is group index, L is actual fiber length and c is vacuum light speed. NIST lists the exact value 299792458 m/s for c. NIST constants
Assume n_g = 1.4682 throughout the segment at 1550 nm. The table excludes equipment and queuing. It is an estimate using a cited typical value, not an ARRANEX assembly measurement.
| Assumed fiber length | Calculated one-way delay | Excluded |
|---|---|---|
| 10 m | Approximately 48.97 ns | Module/equipment processing |
| 100 m | Approximately 489.74 ns | Module/equipment processing |
| 1000 m | Approximately 4.897 µs | Module/equipment processing |
Label one-way and round-trip results separately. Define the return path and measurement process before interpreting a round trip; do not assume a different return route matches the outbound route.
Length matching does not complete skew verification
Skew is relative delay between the channels being compared. A 2013 parallel-SMF analysis hosted by the IEEE 802.3 working group separates optical-cable contribution, reference points and differential-length effects. Historical parallel-SMF analysis
This historical document supports the distinction; its old limits are not applied here to every present-day 400G/800G interface. Obtain allowable skew from the actual equipment, optics and applicable specification, and define measurement points and channel sets.
If fiber type, wavelength and conditions match, a 0.10 m length difference contributes approximately 0.490 ns under the stated assumption. Review actual fiber lengths, optical paths and conditions. Equal external branch lengths alone do not establish a skew pass.
Length and delay procurement fields
| Field | Record | Avoid |
|---|---|---|
| Length datum | Endpoints and mechanical datums | Equating housing distance with fiber length |
| Optical path | Fiber type, wavelength and length per channel | Overall assembly length alone |
| Delay object | One-way, round-trip or system end-to-end | Mixed measurement scopes |
| Skew definition | Channel set, reference points and source of limits | Rate-based universal numbers |
| Acceptance evidence | Configuration, method and channel results | Budget substituted for measurement |
Procurement FAQ
- Is delay per meter identical for every fiber? It depends on group index, wavelength and conditions; these values are assumed estimates.
- Do equal jacket lengths eliminate skew? Review actual optical paths and measurements.
- Is calculated fiber delay a Ping measurement? They have different measurement scopes.
- Is there one universal 800G skew limit? Obtain limits for the actual interface and application.
- What belongs in a length-matching RFQ? Channel datums, target differences, interfaces, fiber types and measurement reference points.
Related guides and configurations
Service slack and actual routing · Channel mapping · System and passive-test boundaries · Harness configuration · Trunk configuration
Contact an engineer. Share actual part references, drawings and test or acceptance needs for configuration review. Related links do not establish unverified supply, performance or qualification.
Sources and review date
- Corning SMF-28 Ultra typical group index
- NIST vacuum light-speed constant
- 2013 parallel-SMF analysis hosted by the IEEE working group
Sources reviewed on 2026-10-12. Manufacturer examples apply only to the cited configurations, and calculations depend on stated assumptions. Procurement tables do not replace full specifications or establish unverified ARRANEX performance or qualification.
Sources checked: 2026-10-12