Belden : Achieving Solid Link Performance and Desired Link Distances with Singlemode Fiber

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With all the new technologies and products available in the data center market, it's beneficial to plan in advance for potential changes and upgrades. No matter which option you choose, low-loss, high-bandwidth fiber cable used in conjunction with low-loss fiber connectors will always provide solid link performance and desired link distances with the number of connections you need.

As we've mentioned in earlier blogs, it's important to understand the power budget of new data center architecture, as well as the desired number of connections in each link. The power budget indicates the amount of loss that a link (from the transmitter to the receiver) can tolerate while maintaining an acceptable level of operation.

This blog provides you with singlemode fiber (SMF) link specifications so your fiber connections will have sufficient power and reach and desired link distances. Unlike multimode fiber (MMF), SMF has virtually unlimited modal bandwidth, especially operating at the zero-dispersion wavelength 1300 nm range, where material dispersion and waveguide dispersion cancel each other out.

Typically, a singlemode laser has a much finer spectral width; the actual reach limit isn't bound by the differential modal dispersion (DMD) like it is in multimode fiber.

SMF Ethernet Link Model

In SMF, channel insertion loss includes fiber attenuation loss and connection/splice loss. There are also other transceiver penalties, which are included in the allocation for penalties shown below:

• Jitters: unpredictable electronic timing noise variations
• Laser noise: fluctuations of various parameters of laser light
• Intersymbol interference (ISI): one symbol interfering with subsequent symbols, causing signal distortion
• Reflection: light changing direction in a fiber and returning to the source
• Chromatic dispersion: different wavelengths arriving at their destination at slightly different times

Unlike MMF, impairment is mainly caused by insertion loss in SMF data transmission up to 10 km in data center environments.

Channel insertion loss(dB) = Total fiber loss + Total connection loss(dB)

IEEE 802.3 Ethernet SMF link power budget, * in progress (LR - Long Reach 10 km, FR, CLR, CWDM - Far Reach 2 km, PSM4 & DR4 - DC Reach 500 m)

Historically, for the 10 km reach in the Ethernet SMF link model, channel insertion loss includes fiber cable loss (derived from ITU-T G.695) plus 2.0 dB allocated for connection and splice loss at the 1300 nm wavelength range; however, since singlemode optics have entered data center applications, where shorter reach and more connection points are required, there are new variants of singlemode transceivers, such as 100G-CWDM4 (2 km reach), 100G-CLR4 (2 km reach) and 100G-PSM4 (500 m reach).

In the IEEE 802.3bs and 802.3cd taskforces, such variants have already been included in objectives for next-generation applications ranging from 50G to 400G.

In the IEEE 802.3 specification, the loss of a single connection shall not exceed 0.75 dB, and the initial connection at the transceiver is not considered as part of the channel link loss.

SMF Fibre Channel Link Model

In the Fibre Channel standard, the maximum link distances for singlemode fiber cable plant are calculated based on an allocation of 2.0 dB total connections and splice loss, except that the latest 128GFC variants PSM4 and CWDM4 are specially designed for data center SAN applications, with reduced reach and increased connection loss budget.

Fibre Channel SMF link power budget, (-M 1.4 or 2 km reach, -L 10km reach, -CWDM 2 km reach, -PSM4 500 m reach)

In the SMF power budget specification for 32GFC and the 128GFC, the allowed channel insertion loss budget can also change as a function of link distance and total connector loss. This is mainly due to the broadened optical bandwidth that is subject to more chromatic dispersion and wavelength-dependent impairments.

Low-loss connectors can also enable engineered links to extend the nominal reach, or to support more connection points in a shorter reach.

Channel link distance (m) vs. insertion loss budget (dB)

For more guidance on optical fiber cabling standards, download our new application note: Get Ready: New Fiber Infrastructure for Data Centers. To learn more about partnering with Belden for your data center project, visit info.belden.com/data-center.