Quick Understanding of Core Concepts

OM1, OM2, and OM3: These are fiber grade standards defined by the International Organization for Standardization/International Electrotechnical Commission (ISO/IEC 11801) and the TIA (Telecommunications Industry Association). "OM" stands for "Optical Multi-mode." A higher number indicates better performance and supports higher transmission rates.

G.651: This is the standard designation for "50/125μm multimode fiber" established by the International Telecommunication Union (ITU-T). You can think of it as a generic term or predecessor to OM1, OM2, OM3, and other fibers within another standardization system.

Detailed Difference Analysis

1. OM1 Fiber (ISO/IEC Grade)

Appearance: Core diameter is 62.5μm, cladding diameter is 125μm. The outer jacket is typically orange.

Light Source and Bandwidth: Originally designed for LED (light-emitting diode) light sources. Its modal bandwidth is relatively low (typically 200MHz·km @850nm).

Application: Primarily used in early, low-speed networks such as 100M Ethernet (100BASE-FX) and Token Ring.

Current Status: This product is now obsolete and is largely no longer used in new deployments.

2. OM2 Fiber (ISO/IEC Grade)

Appearance: Core diameter is 50μm, cladding diameter is 125μm. The outer jacket is typically orange.

Light Source and Bandwidth: Also primarily designed for LED light sources, compared to OM1, it has optimized bandwidth (modal bandwidth up to 500MHz·km @850nm).

Application: Supports Gigabit Ethernet (1000BASE-SX), but with shorter transmission distances (up to approximately 550 meters).

Current Status: This fiber is gradually being replaced by higher-grade fibers, and its application is declining.

3. OM3 Fiber (ISO/IEC Grade) - Laser-Optimized Multimode Fiber (LOMMF)

Appearance: Core diameter is 50μm. To distinguish it, it typically uses an aqua outer jacket.

Core Technology: This is the key difference! OM3 fiber is specifically designed and optimized to support VCSEL (Vertical Cavity Surface Emitting Laser) laser sources. By improving the fiber's refractive index profile, it significantly increases the bandwidth in the 850nm window (typically achieving an effective modal bandwidth (EMB) of up to 2000MHz/km).

Applications: Primarily used for 10 Gigabit Ethernet (10GBASE-SR). Using OM3 fiber, 10G network transmission distances can reach up to 300 meters. It is also backward compatible with Gigabit and 100M networks.

Relationship with G.651: OM3 is an enhanced 50/125μm fiber with performance far exceeding the requirements of earlier G.651 standards.

4. G.651 Fiber (ITU-T Recommendation)

Definition: ITU-T G.651 is the general specification for "50/125μm gradient-mode multimode fiber," which was defined earlier. It primarily covers characteristics similar to OM1 (62.5/125μm) and OM2 (50/125μm) fibers.

Classification: G.651 actually has more detailed classifications (such as G.651.1), but generally speaking, OM1 and OM2 fibers can be considered specific products that comply with the G.651 specification.

Key Point: G.651 itself is not directly equivalent to OM3. OM3 is a new generation of 50/125μm fiber with significantly improved performance, and its standards far exceed the original G.651 definition. The ITU-T later released higher-level specifications corresponding to OM3/OM4 (such as subsequent versions of G.651.1).

Maintaining Legacy Systems: If you encounter fiber with an orange sheath, it's likely OM1 or OM2. Be aware that these fibers cannot support long-distance 10GbE transmission.

When you see G.651: It can be understood as referring to multimode fiber with performance similar to OM2 or earlier, lacking the high bandwidth characteristics of OM3 and later.

Simply put, the transition from OM1 to OM3 (and then to OM4/OM5) is an evolutionary process of continuous performance upgrades to meet the demands of higher-speed networks. G.651 is a broader, older standard name that typically corresponds to OM2 and earlier fiber grades.