Tertiary Fiber Optics: Single-Mode, Multimode, Capacities
OM4 at 80%, OS2 on campus: what ISO/IEC 11801 recommends in tertiary buildings
80% of R+5 to R+10 tertiary buildings are over-specified with OS2 single-mode: OM4 is sufficient up to 100 Gb/s over 100 m (ISO/IEC 11801-1:2017), for a transceiver CAPEX 3 to 5 times lower. Which enterprise fiber optic category should you really choose for an 8-storey building with 90 workstations per floor? On this profile, aggregate IP traffic can reach several gigabits per second at peak, which saturates Cat 6A and requires a 10 Gb/s fiber backbone minimum. The ISO/IEC 11801-1:2017 standard offers four references: OM3, OM4, OM5 and OS2 single-mode. Kytom, founded in 2006, calibrates the optical category, the LC or MPO connectivity and the link length with the client IT department and the low-current systems integrator.
Kytom’s contrarian position: industry conventional wisdom over-specifies OS2. Our reading differs from the common practice of low-current systems integrators: below 200 m of link length and without a 400 Gb/s projection over 10 years, single-mode generates an LR4 transceiver overcost (1500 to 2500 EUR per port, from Kytom 2024 supplier figures) three to five times higher than multimode SR4 modules, with no real gain. On a single-block R+5 to R+8 building with a central server room, pure OM4 remains the technical and economic optimum: switching to OS2 as a precaution increases CAPEX by 25 to 35% for zero benefit as long as the IT department does not migrate to 100 Gb/s LR4. Conversely, on multi-building campuses exceeding 300 m or high environmental performance and BREEAM buildings with a constrained 2030 target, OM4 becomes undersized.
For the IT department: 5 phases, 12 weeks, OTDR acceptance testing on 100% of links (and 99.95% SLA)
This section reframes site execution from the IT department’s perspective: fiber is not a low-current systems topic, it is an IT infrastructure topic with a measurable SLA and audit traceability. The sequencing brings together the client IT department, the low-current systems integrator and the architectural project management around five phases over 12 weeks.
- IT audit and bandwidth calibration: the IT department sets the target per floor (10, 25 or 40 Gb/s) according to workstation density, business applications and the SLA contracted with the business units.
- Server room location: an intermediate level (R+3 to R+5 on 8 floors) to minimize lengths and balance distribution toward the VDI cabinets.
- Optical category choice: OM4 for short inter-floor links, OS2 for campus links, back-offices and 100 to 400 Gb/s bandwidths.
- Connectivity: LC duplex for SFP/SFP+ at 1, 10 and 25 Gb/s; MPO/MTP 12 or 24 fibers for QSFP+ 40 Gb/s and QSFP28 100 Gb/s, compliant with IEC 61754.
- Acceptance testing via standardized measurements: OTDR reflectometry, attenuation measurement IEC 61280-4-1, BER (Bit Error Rate) test on 100% of links.
For the IT audit: the as-built drawings and TIA-606-C labeling delivered at the end of the works bring the delivery into compliance with requirements on testing installed optical links, feed the DOE and the CMMS for operations, and provide the evidence base in the event of a network incident or an information security audit on the associated physical assets.
When the 5-phase method is oversized: a contrarian view. Contrary to the widespread practice in tertiary tenders that copy-paste a full fiber specification, for a single floor of fewer than 30 workstations or a project under 500 m², phase 2 falls away: a wall-mounted VDI enclosure fed with Cat 6A copper is sufficient, and the return on investment of a full fiber infrastructure proves hard to justify at this scale. Likewise, for a building leased under a 3-6-9 lease closed at 3 years, a full OM4 deployment is difficult to justify: a reinforced Cat 6A, faster to remove, is preferable.
Latency 0.5 ms, 25-year service life, 99.95% availability: the measured gains for the IT department
Across a portfolio of more than 1200 projects delivered since 2006, switching to a 10/40 Gb/s fiber backbone generates three benefits that the IT department can include in its SLA and its continuity plan.
- Network latency divided by 4 to 10: 0.5 ms in internal fiber transit versus 2 to 5 ms on Cat 6A at the end of range, with a direct impact on G.711 VoIP (RFC 3550 target below 150 ms end-to-end) and Microsoft 365 cloud sessions.
- 20 to 25-year service life on passive OM4 or OS2 fiber (FOTAG / IEC 60793-2-10 manufacturer data), versus 10 to 15 years for Cat 6A copper. The initial installation overcost is generally amortized over two network asset renewal cycles, especially since passive fiber costs 3 to 5 EUR per linear meter and the upgrade from 10 to 40 Gb/s only requires replacing the QSFP+ transceivers.
- Strengthened network availability after fiber deployment, thanks to the electromagnetic immunity of glass (IEC 61000-4-3) on dense floors combining LED, VRV and simultaneous workstations.
The electrical environment conditions these results: a 25 to 35% TGBT reserve on a 50 to 100 kVA subscription, a dedicated 6 to 10 kVA UPS for the server room (15 min autonomy per EN 62040-3), 30 cm separation between high- and low-current cables with perpendicular crossings in accordance with NF C 15-100.
Frequently asked questions
OM4 or OS2 for an 8 to 10-storey tertiary building?
Kytom’s contrarian position: integrator conventional wisdom pushes toward OS2 as a precaution, but our field experience shows this is rarely justified in single-block buildings. The choice depends on physical distance and target bandwidth. On an 8 to 10-storey building with a central server room and floor VDI cabinets, links are typically 50 to 150 m, which places OM4 in the optimal zone: 40 Gb/s over 150 m, 100 Gb/s over 100 m, with an SR4 transceiver cost three to five times lower than single-mode LR4 modules. OS2 becomes relevant beyond 300 m or with a 400 Gb/s projection over 10 years.
