Cisco White Paper confirms OM5 offers no reach advantage for most Cisco multimode transceivers

By Gary Bernstein

  Filed under: Data Center, Fiber
  Comments: Comments Off on Cisco White Paper confirms OM5 offers no reach advantage for most Cisco multimode transceivers

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Designing fiber optic networks and finding the right tools to optimize it is always a challenge. We need to find the right balance between demands of the network, cable performance and cost effectiveness. While fiber cable selection between singlemode and multimode networks is self-selecting, there is an array of options for multimode networks. The latest of which is OM5, which is designated as Wideband Multimode fiber (WBMMF) in the ISO/IEC 11801, 3rd edition Standard.

OM5 fiber is specified at 850 nm and 953 nm wavelengths. It was created to support Shortwave Wavelength Division Multiplexing (SWDM), which is used to transmit 400GBASE-SR4.2 over eight fibers. It can potentially be used to handle high-speed data center applications using two fibers to transmit from 40 Gb/s up to 100 Gb/s. However, this challenge can also be resolved with existing singlemode solutions.

All the current and future IEEE standards in development for 100/200/400/800 Gb/s data rates will work with either singlemode (OS2) or multimode (OM4). Some of these next-generation speeds, especially those operating at longer distances, will require singlemode. In addition, OM5 cabling costs about 20-30%  more than OM4. If you look at the cost of a full 100 Gb/s channel, including BiDi transceivers, the amount per channel is still 30-40% more than 100GBASE-SR4 supported by OM4.

A recent white paper published by Cisco “Understanding the Differences Between OM4 and OM5 Multimode Fiber”, discusses whether OM5 is an appropriate choice when OM4 will work just fine. There have been many claims that OM5 has better reach than OM4, although this is only true for a small handful of applications. For example, multi-wavelength transceivers with operating wavelengths that include longer wavelengths like 940 nm can leverage the reach advantage of OM5.

The TIA standard for OM4 only mandates a bandwidth of 4,700 MHz∙km at the 850 nm measurement wavelength. In contrast, OM5 has a requirement of 4,700 MHz∙km at 850 nm, but also has a requirement for 2,470 MHzkm at 953 nm. Does that mean OM5 is the better option? Not necessarily. Most of Cisco’s multimode transceivers are single-wavelength devices operating at 850 nm; therefore, there is no difference in reach for these transceivers whether OM5 or OM4 is used. BiDi uses two wavelengths and similarly the wavelength range does not present an opportunity to realize significant benefits from OM5.

The white paper concludes by stating that, “It’s an engineering truism that there’s no perfect solution, just the best solution for the application at hand. OM5 cable is not intrinsically better than OM4 cable. OM5 only delivers increased reach for transceivers with lanes operating at 940 nm. For conventional multimode transceivers operating at 850 nm alone, OM4 provides a cost-effective solution.”

Read the full Cisco White Paper

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