Certifying MMF for 100G Ethernet transmission

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by Rick Pimpinella and Gaston Tudury

To support the changing and fast-growing bandwidth demands of business-critical data centers and Internet service providers, the IEEE is developing a standard (802.3ba) that will support data rates for next-generation Ethernet networks. The standard, scheduled for ratification by mid-2010, will address fiber- and copper-based networks and will cover both 40- and 100-Gbit/sec speeds.

To meet future high-speed demand for 100 Gbits/sec and above, data communications will likely employ multi-lane technologies with each lane carrying a portion of the aggregate channel data stream. The advantage of multi-lane systems is that each lane operates at a much reduced data rate than the aggregate stream.

The prospect of deploying multi-lane optical interconnection for next-generation Ethernet has gained much recent attention, but it has raised concerns about fiber channel "readiness" to support higher speeds as well. This article discusses the key performance parameters affecting multi-lane transmission over multimode fiber as proposed for 40- and 100-Gigabit Ethernet (GbE) and relates these parameters to channel performance.

There are two primary methods of implementing multi-lane optical interconnection: wavelength-division multiplexer (WDM), where discrete optical wavelengths are coupled into a single optical fiber; and parallel optics, where discrete transceivers communicate in parallel over multiple fibers. For communications requiring a 10- or 40-km reach, WDM over singlemode fiber (SMF) is the technology of choice for the new IEEE standard.

In short-reach applications such as data centers, multi-lane parallel optics over multimode fiber (MMF) will be specified (see Fig. 1). This approach is designed as a low-risk, cost-effective offering that leverages technology developed for 10GbE. At the time of writing, the MMF reach objective is 100 m; however, the reach will likely be extended to 150 m over OM3 fiber and 250 m for MMF having an effective modal bandwidth (EMB) of at least 4,700 MHz·km (to be specified as OM4).

Because 40- and 100GbE data traffic will be carried over parallel lanes of four or ten discrete fibers, respectively, differences in bit transport time over individual fibers must be kept to a minimum to ensure multi-lane data traffic can be resynchronized at the receiver. Ideally, bits transmitted through a fiber ribbon array arrive at the receivers at the same time. The difference between the fastest and slowest bit arrival time in a multi-lane link is called skew. More skew requires more electronic processing to correct for the skew, resulting in higher power dissipation and latency.

Inter-modal and chromatic dispersion, differences in fiber length, and deviations in refractive index can affect the total skew in a multi-lane optical channel. Some of these parameters are inherent to optical fibers and may be caused by process variation or stress in the individual fibers during their manufacture. The better the fiber manufacturing process and the tighter the control of process parameters, the better the quality and the more consistent the optical fiber.

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Figure 1. The IEEE is expected to ratify approaches based on parallel ptics and multiple fibers for both 40- and 100-Gigabit Ethernet over multimode plant.

To ensure an OM3 (or OM4) cable will support parallel optics for 100GbE and above, each parameter discussed below must be well-controlled:

Inter-modal dispersion: Different modes traversing different optical paths in the MMF spread in time, causing pulse broadening. The parameter used to express pulse broadening due to inter-modal dispersion is differential mode delay (DMD).

For a fiber to be classified as OM3, the DMD measurement must comply with one of six DMD mask templates specified in TIA-455-220-A and IEC 60793-1-49. DMD is measured by launching a test pulse into a MMF at highly controlled radial positions across the fiber core, from the core center to the cladding region. From DMD measurements, one can determine the calculated EMB (EMBc) of the fiber, which is expressed in units of MHz·km. (The EMB of a fiber is specified as 1.13 times EMBc.)

Chromatic dispersion: When light of different wavelengths propagates in a material, it does so with different velocities. Vertical-cavity surface-emitting lasers (VCSELs) used for multimode fiber have a finite spectral width and as a result, a pulse of light containing spectral content will be dispersed. "Chromatic dispersion" describes this broadening of the pulse width, a factor that reduces signal quality and thereby degrades link performance.

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Figure 2. Inter-modal and chromatic dispersion measurement curves for two representative fibers. Such measurements are essential in determining whether fiber plant will accommodate high-speed requirements.

Pulse delay: The velocity of light is determined by the refractive index of the medium. Stress introduced during the manufacturing process, cabling process, or installation can result in variations in refractive index from fiber to fiber, which causes bits to arrive at the receivers at different times. In addition, any variation in individual fiber lengths will result in large differences in bit arrival times. The relative difference between pulse arrival times at the receiver is called pulse delay.

MMF performance capabilities

Using a high-resolution DMD measurement system, Panduit analyzed inter-modal dispersion, chromatic dispersion, and pulse delay in a 12-fiber OM3 ribbon cable typical of the type that might be found in a data center. To measure chromatic dispersion, Panduit adjusted the wavelength of its DMD measurement system and analyzed the wavelength dependence for a given propagating mode. The data was then used to calculate intermodal dispersion and chromatic dispersion and, when compared against other fibers, total skew. Initial bit-error rate (BER) system test measurements and DMD analysis of each of the individual fibers found that two of the 12 fibers were not, in fact, OM3 quality. Further investigation determined that the DMD measurement system used by the manufacturer to characterize these fibers made inaccurate measurements.

Measurements were then taken for time of flight and inter-modal and chromatic dispersion, and each of these measured contributions to skew was totaled. This resulted in two conclusions: First, the largest contribution to skew is the time of flight, which is independent of fiber bandwidth. Second, although not all fibers in the cable met OM3 bandwidth requirements, the maximum time-of-flight difference occurred between two OM3 fibers. These results indicate that use of high EMBc fiber cables (i.e., OM3 and above) will not automatically guarantee low skew or high performance. Certification testing of the ribbon cable is necessary to guarantee 40/100GbE performance for the cables.

Fiber certification testing cannot be done with commercially available equipment, and it cannot be assumed that high-bandwidth fiber will mask inefficiencies due to dispersion and skew. It is the vendor's responsibility to test the fiber with an eye toward control of uniform pulse delay and skew optimization, so contractors and designers can specify fiber that enables signal integrity across the channel.

The key to ensuring multi-lane 40/100GbE parallel optic signal integrity is strong control over the fiber manufacturing processes: The better the fiber manufacturing process and the tighter the control of process parameters, the better the quality and the more consistent the optical fiber.

Through its testing processes Panduit has identified that control over skew, an imortant parameter, is not sufficient to ensure good high-bandwidth performance, as there is not a close correlation between high EMB values and skew. Although the maximum allowable skew for 100GBase-SR10 remains to be specified, transceivers can correct for skew and that skew ultimately may not be identified as a critical parameter.

As customers develop future-ready cabling plants, they need to pay close attention to the quality of the fiber being installed. Best-in-class cabling vendors will be able to certify MMF for reliable multi-lane 40/100GbE performance through accurate characterization of fiber bandwidth, certification of BER system performance of raw fiber and cabled media, and establishing specifications for fiber media partners toward retaining lane-to-lane skew.

Rick Pimpinella, Ph.D., is the fiber research manager, and Gaston Tudury, Ph.D., is a fiber research engineer at Panduit Corp. (www.panduit.com).

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