OM1, OM2, OM3, and OM4 Fiber Cable

The demand for increased network bandwidth is driving the migration towards 40- and 100-GbE networks. This demand is being fueled by multiple factors, including ever-growing global IP traffic; greater switching, routing, virtualization, and data center connections; higher bandwidth applications; video-on-demand; convergence; and more.

When planning your 40-/100-GbE migration, consider your cabling infrastructure and how it will meet your current and future data requirements. What you install today needs to give you the scalability to accommodate the need for higher bandwidth for the next 15 to 20 years. The cables of choice for data center connectivity and what is recommended by the TIA are OM3 and OM4 laser-optimized multimode fiber.

There are different categories of graded-index multimode fiber optic cable. The ISO/IEC 11801 Ed 2.1:2009 standard specifies categories OM1, OM2, and OM3. The TIA/EIA recognizes OM1, OM2, OM3, and OM4. The TIA/EIA ratified OM4 in August 2009 (TIA/EIA 492-AAAD). The IEEE ratified OM4 (802.ba) in June 2010.

OM1 and OM2

OM1 specifies 62.5-micron cable and OM2 specifies 50-micron cable. These are commonly used in premises applications supporting Ethernet rates of 10 Mbps to 1 Gbps. They are also typically used with LED transmitters. OM1 and OM2 cable are not suitable though for today's higher-speed networks.

OM3 and OM4

OM3 is specified in ISO 11801. OM4 was ratified by the TIA in August 2009 (TIA/EIA 492-AAAD). The IEEE ratified OM4 (802.3ba 40G/100G Ethernet) in June 2010. It was amended in 2012 to IEEE 802.3-2012. The 802.3-bm Task Force is currently working on updates. The standard provides detailed guidelines for 40-/100-GbE transmission over OM3 and OM4 multimode cable and single-mode fiber optic cable. OM1, OM2, and copper are not included.

Laser optimized

OM3 and OM4 are both 50-micron laser-optimized multimode fiber (LOMMF) and were developed to accommodate faster networks such as 10-, 40-, and 100-GbE. They also support existing networks. Laser-optimized multimode fiber cable differs from standard multimode cable because it has graded refractive index profile fiber optic cable in each assembly. This means that the refractive index of the core glass decreases toward the outer cladding, so the paths of light towards the outer edge of the fiber travel more quickly than the other paths. This increase in speed equalizes the travel time for both short and long light paths, ensuring accurate information transmission and receipt over much greater distances, up to 300 meters at 10 Gbps. Laser-optimized cable is aqua colored.

Both OM3 and OM4 are designed for use with 850-nm vertical-cavity surface-emitting lasers (VCSELS) and have aqua sheaths.

OM3 specifies an 850-nm laser-optimized 50-micron cable with an effective modal bandwidth (EMB) of 2000 MHz/km. It can support 100-Gbps link distances up to 100 meters.

OM4 specifies a high-bandwidth 850-nm laser-optimized 50-micron cable with an EMB of 4700 MHz/km. It can support 100-Gbps link distances of 150 meters.

OM3 allows for 1.5 dB of connector loss at 100 meters at all speeds; OM4 allows for 1.0 dB of loss at 150 meters for 40-100-GbE. Both OM3 and OM4 rival single-mode fiber in performance while being significantly less expensive to implement. In addition, single-mode electronics are also expensive.

Manufacturing process

Laser-optimized OM3 and OM4 cable are made with a different process than OM1 and OM2, which are made with a small defect in the core called an index depression. These cables are used with LED light sources. OM3 and OM4 are manufactured without the center defect.

As networks migrated to higher speeds, VCSELs became more commonly used rather than LEDs, which have a maximum modulation rate of 622 Mbps and can’t be turned on and off fast enough to support higher-speed applications. Thus manufacturers changed the production process to eliminate the center defect and enable OM3 and OM4 cables to be used directly with the VCSELS.

Parallel transmission

40- and 100-GbE over OM3 and OM4 uses parallel optics where data is simultaneously transmitted and received over multiple fibers. 40-GbE consists of (4) 10-Gbps fiber channels each way, for a total of 8 fibers. 100-GbE consists of 10 fiber channels each way, for a total of 20 fibers. The signals are then aggregated at each end in an arrayed transceiver (connector) containing 4 or 10 VCSELs and detectors. For multimode fiber, the Media Dependent Interface (MDI) is the MPO adapter (IEC 61754-7).