How Far Can Multimode Fiber Support 10G, 40G, and 100G Ethernet?

Multimode fiber reach is not a single number, and it is not determined by the fiber grade alone. The distance a multimode link can support depends on three interacting variables: the Ethernet rate and optical variant (10GBASE-SR, 40GBASE-SR4, 100GBASE-SR4, or a proprietary multi-wavelength scheme), the fiber’s effective modal bandwidth (EMB), and the link-loss budget available to the transceiver.

The headline figures are well established. For 10GBASE-SR, OM3 supports 300 m and OM4/OM5 support 400 m. For 40GBASE-SR4 and 100GBASE-SR4, OM3 supports 100 m and OM4/OM5 support 150 m[reference:0]. These standardized distances assume the specific 850 nm SR4 optics defined by IEEE 802.3ba and an engineered link within the specified loss budget[reference:1].

The critical nuance is that OM5 does not extend these conventional 850 nm SR4 distances beyond OM4. Both OM4 and OM5 share the same minimum EMB of approximately 4,700 MHz·km at 850 nm, so a 40GBASE-SR4 or 100GBASE-SR4 link specified at 150 m on OM4 is also specified at 150 m on OM5[reference:2]. OM5’s additional reach appears only when the optical architecture uses multiple wavelengths, such as shortwave wavelength-division multiplexing (SWDM), which operates across a band extending to 953 nm[reference:3].

1. Multimode Fiber Distance by Ethernet Rate: 10G, 40G, and 100G

The table below consolidates the commonly specified maximum distances for the mainstream 850 nm multimode Ethernet variants. These are standardized reach values tied to specific transceiver types; they are not universal guarantees for every possible link implementation.

The OM3-to-OM4 step is a genuine bandwidth-driven improvement. OM3 specifies a minimum EMB of approximately 2,000 MHz·km at 850 nm; OM4 raises that minimum to approximately 4,700 MHz·km[reference:4]. That bandwidth increase translates directly into longer standardized reach at the same data rate: 300 m to 400 m for 10GBASE-SR, and 100 m to 150 m for 40GBASE-SR4 and 100GBASE-SR4.

The OM4-to-OM5 step, for conventional 850 nm SR4 optics, is not a distance improvement. OM5 maintains the same 4,700 MHz·km minimum EMB at 850 nm as OM4 and adds a separate bandwidth specification at 953 nm for wideband applications[reference:5]. Unless the transceiver actually uses that wider band, OM5 behaves like OM4 on reach.

2. What Actually Sets the Distance Limit: Effective Modal Bandwidth, Not Core Size

OM3, OM4, and OM5 all use a 50 µm core and 125 µm cladding. The difference between them is not the core size—it is the effective modal bandwidth (EMB) and, for OM5, the addition of a second specified bandwidth at 953 nm.

Multimode fiber carries multiple optical modes simultaneously. Each mode travels a slightly different path and arrives at the receiver with a slightly different propagation delay. This difference—modal dispersion—spreads the optical pulse in time and limits how fast the fiber can carry data over a given distance[reference:7]. A higher EMB specification means the fiber is engineered to minimize the differential mode delay across the launch conditions used by VCSEL-based transceivers, allowing shorter pulses and higher data rates to survive over longer distances.

The practical consequence is that distance and data rate are traded against EMB. A fiber with 2,000 MHz·km of bandwidth (OM3) supports 10G over a longer distance than it supports 40G or 100G. A fiber with 4,700 MHz·km (OM4 or OM5) supports the same rates over longer standardized distances. Attenuation and connector loss also consume the link budget, but for the reaches relevant to data center multimode links (tens to hundreds of meters), modal dispersion is the primary rate-and-distance constraint[reference:8].

3. OM5’s Actual Role: SWDM and Multi-Wavelength Reach, Not 850 nm SR4

OM5 is often described as the “highest grade” of multimode fiber, which creates an expectation that it should deliver the longest reach for every application. That expectation is incorrect for conventional 850 nm SR4 Ethernet.

OM5 was standardized as wideband multimode fiber to support shortwave wavelength-division multiplexing (SWDM) and other multi-wavelength schemes across a band from roughly 850 nm to 953 nm[reference:9]. Its 953 nm EMB specification of at least 2,470 MHz·km is what makes that wider band usable with specified performance. When a transceiver actually uses multiple wavelengths—such as a 40G SWDM4 or 100G SWDM4 module—OM5 can support longer reaches than OM4 in the same architecture.

Published SWDM distances illustrate the difference. For 40G SWDM4 over a duplex LC pair, commonly cited reaches are 240 m on OM3, 350 m on OM4, and 440 m on OM5. For 100G SWDM4, the corresponding figures are approximately 75 m on OM3, 100 m on OM4, and 150 m on OM5[reference:10]. These are not 850 nm SR4 distances; they are architecture-specific reaches for multi-wavelength SWDM optics.

4. Beyond 100G: What the Same Multimode Fiber Supports at 400G and 800G

The 10G/40G/100G distances above are the mainstream reference points, but the same OM3, OM4, and OM5 fibers are also specified for higher-rate multimode Ethernet. These higher-rate distances are shorter, and they follow the same bandwidth-and-architecture logic.

The 400G-SR4.2 row is an example of an architecture where OM5’s wideband capability translates into a reach advantage: 150 m on OM5 versus 100 m on OM4 and 70 m on OM3 for that specific multi-wavelength transceiver type[reference:11]. The 800GBASE-VR8 specification is anchored at a minimum of 50 m across the three grades, reflecting the tighter loss and dispersion margins at 100G per lane over parallel multimode fiber[reference:12].

The pattern at higher rates is consistent with the lower-rate data: more lanes and higher per-lane symbol rates shorten reach, and OM5’s advantage remains tied to whether the transceiver uses the 953 nm band, not to the fiber grade label alone.

5. Selection Framework: Matching Fiber Grade to Rate, Reach, and Transceiver

For network design, the useful question is not “which multimode fiber is best” but “which fiber grade supports this Ethernet variant at the required distance within the link-loss budget.”

For new conventional multimode deployments at 40G and 100G, OM4 is often the practical default: it provides the 150 m reach that OM3 cannot guarantee for SR4, without paying for OM5’s SWDM capability that conventional optics will not use. OM3 remains rational where the existing infrastructure is OM3 and the link distances fit comfortably within its 100 m specification for 40G/100G SR4. OM5 becomes the appropriate choice when the optical architecture is genuinely SWDM-based or when a multi-wavelength transceiver’s specified reach on OM5 is required.

6. Conclusion: Bandwidth and Architecture Set the Distance, Not the Grade Name

Multimode fiber supports 10GBASE-SR to 300 m on OM3 and 400 m on OM4/OM5; 40GBASE-SR4 and 100GBASE-SR4 to 100 m on OM3 and 150 m on OM4/OM5. These distances are set by the fiber’s effective modal bandwidth and the Ethernet variant’s signaling architecture, not by the numerical order of the fiber grade names.

OM5 does not extend conventional 850 nm SR4 reach beyond OM4. Its value appears in SWDM and other multi-wavelength architectures that use the 953 nm band, where it can provide longer reach than OM4—for example, 440 m versus 350 m for 40G SWDM4, or 150 m versus 100 m for 100G SWDM4. At higher rates such as 400G and 800G, the same logic holds: reach shortens, and the fiber’s role is to provide the bandwidth margin the transceiver architecture requires.

The correct selection process begins with the Ethernet variant and the required distance, then matches the fiber grade whose EMB specification and standardized reach support that requirement within the measured link-loss budget. The fiber grade is a specification to be matched, not a hierarchy to be climbed.

7. Q&A