Meet OM5 Multimode Fiber

In OM5, wavelengths increase each fiber’s capacity by at least a factor of four (there’s either a fourfold data-rate increase or a fourfold reduction in the fibers required to achieve a given data rate). The signals are sent down one fiber over four separate operating windows.

New Challenges with Multimode Fiber (MMF) Cabling Systems

Last year, 100G Ethernet became mainstream in cloud data centers as it’s deployed in core networks in enterprise and multi-tenant data centers. Due to the limited bandwidth of OM4 and other signal-transmission impairments, nominal transceiver reach dropped from 150 m with 40GBASE-SR4 (4×10G) to 100 m with 100GBASE-SR4 (4×25G).

Historically, high-volume, low-cost multimode transceivers employ 850 nm VCSEL as the light source. LC Duplex (one MMF pair) and MPO-12 (four MMF pairs) are the most popular structured cabling and connectivity solution.

Data center bandwidth requirements, however, increase much faster than technology development. The IEEE 802.3cd taskforce incorporated 200GBASE-SR4 using 50G-PAM4 technology; the IEEE 802.3bs taskforce incorporated 400GBASE-SR16 reusing 25G-NRZ technologies. Although 50G VCSEL technologies are still in development, 200G and 400G Ethernet deployment seems imminent.

 It’s becoming more costly for fiber optic cabling systems to support next-generation Ethernet speed migration. For example, 400GBASE-SR16 would need MPO-32 (16 MMF pairs) connectors, which employ a much higher fiber count as compared to MPO-12 connectors; it is also not backward compatible with currently installed MMF cabling.

OM5 Supports Transmission of Multiple Wavelengths

OM5 was developed to support shortwave wavelength division multiplexing (SWDM) from 840 nm to 953 nm. Lime (traditionally known as lime green) is the official OM5 jacket color, according to the TIA TR-42.12 Optical Fibers and Cables subcommittee; this will be released in the upcoming ANSI/TIA-598-D-2 document. The color definition for connectors and adaptors will be further studied and incorporated into the ANSI/TIA 568.3-D-1 document.

Key features of OM5 include:

• Clearly specified effective modal bandwidth (EMB) and fiber cable attenuation from 840 nm to 953 nm
• Support of 100GBASE-SR4 Ethernet and 32G Fibre Channel applications across the whole wavelength range
• Backward compatible with OM4 at 850 nm, with reduced chromatic dispersion 
  
  

Shortwave Wavelength Division Multiplexing in MMF

Using a single fiber to carry multiple wavelengths isn’t a new concept, and has been widely used in the telecomm world to reduce singlemode fiber numbers. In short-reach datacom applications, the Cisco 40G-BiDi (bidirectional optical technology) solution, which transmits two wavelengths (850 nm and 900 nm) over the same MMF, has proven to be a great market success.

Considering fiber infrastructure costs, parallel multimodeMPO cabling is significantly more costly than LC Duplex cabling. For direct port-to-port connections, it’s more desirable to use a single fiber pair instead of MPO trunk to keep costs low.

In 2015, the SWDM Alliance was formed by a group of transceiver, fiber and system vendors to develop a multi-source agreement (MSA) for SWDM transceivers. The wavelength grids are defined as 850 nm, 880 nm, 910 nm and 940 nm, with a spacing of 30 nm. New SWDM-based QSFP multimode transceivers, including 40G-SWDM4, 100G-SWDM4 and 100G-BiDi (850 nm and 900 nm), are already available.

In January 2017, a new study group was founded under IEEE 802.3 to build consensus for next-generation 200G and 400G Ethernet with fewer MMF pairs, with a goal of incorporating SWDM technology in IEEE 802.3 standard specifications for broader market acceptance.

OM5 Advantages over OM3 and OM4

As a new MMF type, OM5 offers improved performance over popular OM3 and OM4, especially when paired with SWDM and BiDi transceivers. Both OM3 and OM4 can also support SWDM and BiDi physical-media-dependent (PMD) sublayers with reduced reach.

When paired with single wavelength (i.e. λ = 850 nm) transceivers, e.g. 40GBASE-SR4 and 100GBASE-SR4 with an MPO interface, OM5 supports the same reach as OM4. In other words, OM5 parallel cabling does not have a clear advantage with these types of transceivers.

To sustain the appeal of multimode optics, some vendors are beginning to offer 100G multimode transceivers with extended reach of up to 300 m using digital signal processing technique; this, combined with SWDM and OM5, will pave the way for next-generation multimode optics.

If you opt for SWDM transceivers in your next data center deployment project, Belden recommends taking a close look at OM5 to support desired reach and link performance standards.