Data Center/SAN

Multiple Fiber Cabling Approaches for Data Centers & SANs

Laser optimized 50 µm multimode fiber (LOMF) is designed to support 10 Gbps data transmission at distances exceeding 300 meters, the typical distance encountered in enterprise LAN backbones. Link lengths in data centers and SANs are typically shorter, in the range of 100 to 150 meters. At these shorter distances, LOMF offers a compelling advantage…much improved link loss budgets compared to conventional multimode fiber (MMF). Furthermore, the industry-leading 4,900 MHz-km effective modal bandwidth of Berk-Tek's GIGAlite-10XB LOMF, guarantees 10 Gbps to 600 meters with 2 LC connections.[1] This additional headroom enables the use of an additional interconnect, as well as the rapidly deployable and popular Ortronics Momentum^® modular fiber optic cassette systems. These systems incorporating MTP^® 12-fiber connectors and ribbon backbone cables have proven very popular in DC/SAN applications. They greatly reduce installation time by allowing the installer to connect 12 fibers simultaneously, but they have slightly higher insertion loss than single fiber connectors. The NetClear MM10 solution with GIGAlite-10XB fiber supports the greatest number of connections in the Data Center and SAN environment.

Alternatively, NetClear Fiber Trunk Cable Assemblies, available in a wide range of configurations and connector types, may also be used in these applications where cable plant loss budget remains an issue due to greater length and/more connections. Trunk cable assemblies incorporate state-of-the-art low insertion loss connectors such as the LC. NetClear fiber trunk cable assemblies utilized the most advanced cable design with the best quality optical fibers and industry-leading factory termination processes. The resulting product has a simple design, is less expensive to install than a multi-component system, and provides unsurpassed optical performance.

Some data center/SAN designers prefer fully field-terminated fiber solutions. For these installations, NetClear offers a wide range of fiber optic connectors and installation techniques including both no-polish OptiMo^® LC Field-installable Connectors and Berk-Tek Field-terminable Adhesive/polish Connectors. No-polish connectors such as the LC offer the advantage of proprietary factory polishing techniques for ideal tip geometry verified with laser interferometry inspection and state-of-the-art insertion and return loss testing. Adhesive/polish connectors offer superior pull-strength with no fiber pistoning, ensuring excellent long-term reliability.

Maximizing Airflow and Minimizing Congestion

Heat dissipation and cable congestion in data center and SAN equipment racks, below data center floors, or in ceiling-mounted cable trays are increasingly of concern due to the increased deployment of blade servers and systems. Thermographs of hot spots in blade server racks confirm that smaller cable bundles result in less airflow restriction; hence reduced cooling loads. The smaller outside diameter of Berk-Tek's Adventum™ plenum-rated fiber optic cables and BT pre-terminated fiber assemblies can help reduce congestion by consuming less space in equipment racks and increasing fill capacity in cable trays. Despite smaller fiber optic cable diameters, fiber cable pull strengths are generally up to an order of magnitude greater than unshielded twisted pair (UTP) copper cable. Conduit and innerduct can be eliminated with Berk-Tek's Armor-Tek™ interlocking armored fiber optic cable, saving space and providing overall cost savings of 25-30% compared to standard distribution cables installed in conduit or plenum innerduct systems.

State-of-the-Art NetClear Fiber Technology the Key to Superb Data Center/SAN Performance

Many factors determine the performance of reliable, high speed fiber optic cabling systems in the data center and SAN. One key element of NetClear systems that provides "above the standards" performance is the best-in-class fiber used in these systems. In order to support 1 and 10 Gbps Fibre Channel and Ethernet systems used in data centers and SANs, a new type of multimode fiber was required as conventional fibers could not support the speeds and distances required for typical link lengths. This new fiber type is called 850 nm laser optimized 50 µm multimode fiber or "LOMF". During the development this new fiber, new link modeling techniques were developed by the TIA and IEEE (Institute of Electrical and Electronics Engineers) in order to accurately characterize and guarantee the performance of this new fiber type and the transceivers used in these systems. Development of this new link model resulted in identification of a variety of parameters affecting the performance of these systems. One of the limiting parameters determining the performance of 1 and 10 Gbps Fibre Channel and Ethernet systems is Inter-Symbol Interference or ISI.

Inter-Symbol Interference

What is ISI?

ISI is an impairment of data communications systems that limits the reach and bandwidth of the network. Ethernet and Fibre Channel systems transmit information using digital transmission (1s and 0s). When the transmission laser is on, a digital "1" is transmitted, and when it is off, a digital "0" is represented. For example, the pattern "1010100" represents the letter "T". If the transmitter transmits "1010100" and the receiver detects the same pattern, then "T" is successfully transmitted. A digital laser transmitter is like a flashlight that is turned on and off very quickly to send these bit patterns – up to 10 Billion times per second for 10 Gigabit Ethernet transmitters! But if the laser pulses spread as they travel down the fiber they may overflow into adjacent bit periods, confusing the detector at the receiving end of the fiber and resulting in bit errors and system failure.

Increasing system speeds from 1 to 10 Gbps results in reduction of the system bit period by a factor of 10, requiring that each bit in a 10 Gbps system remain completely within its assigned and much smaller bit period with no overlap into adjacent bit periods. In multimode fiber, single bits are represented by multiple "modes" or rays of light traversing the fiber from the transmitter to the receiver. Reliable 10 Gbps performance in the data center and SAN requires that all modes that represent a single pulse remain within their very small assigned bit period.

What Causes ISI?

Pulse spreading causes ISI, and the primary cause of pulse spreading in multimode based systems is differential model delay, or DMD. Multimode fiber is so named because it has hundreds if light pathways, or modes, in which light can travel through the fiber. If the speed of the light in each mode is equal, the fiber will have zero DMD. But imperfections in fiber manufacturing and design result in large differences in modal speed, causing some amount of DMD. If the laser transmits a "1" into a fiber with too high a DMD, the various modes of light representing this laser pulse will travel along the fiber at different speeds. As a result, some modes of light representing the binary "1" may spread into the adjacent bit periods, causing the system to fail, as shown in Figure 1.

Why is it Important to Reduce ISI?

ISI is the primary impairment limiting 1 and 10 Gigabit system performance. By minimizing ISI, the reach of such systems can be increased by up to 12 times. Or of particular importance in data centers where link lengths are shorter, minimizing ISI can enable an increase from 1 Gigabit to 10 Gigabit speeds at the same distance.

What is the Best Way to Reduce ISI and Increase Bandwidth?

Controlling and minimizing DMD maximizes the bandwidth and minimizes the ISI of a multimode fiber system. The laser pulses representing each binary digit or bit are contained within their respective time slots, and do not overlap into adjacent bit periods. Using a fiber with low DMD can dramatically improve system performance is shown in figure 2.

NetClear Fiber Solution for Data Centers & SANs Offers State-of-the-Art Performance

Data Center and SAN applications are growing rapidly and increasing in speed from 1 to 4, 8, and 10 Gbps. The NetClear Solution for Data Center and SANs features low-DMD GIGAlite fibers that will allow you to design data center and SAN fiber optic structured cabling systems with additional connections or greater reach, while preserving the low system cost benefits of multimode fiber based systems.

Industry Leading LOMF Fiber Technology Offers Clear Advantage in Data Centers & SANs

Understanding the IEEE 10 Gbps Multimode Fiber Link Model

Figure 3 shows the 10 Gbps IEEE multimode fiber link model for 850 nm serial transmission using 2,000 MHz-km 50 µm multimode fiber. The model shows maximum “reach” or supported distance. It plots power penalty, in dB as a function of link length. The various impairments are plotted individually but are added together to quantify the total power penalty for the optical link. Clearly the majority of loss associated with this model is attributable to ISI (Pisi) or intersymbol interference) and Ch IL (channel insertion loss) which together account for 75% of the total power penalty at 300 meters. Intersymbol Interference is the tendency for optical pulses to spread or “smear” across bit period boundaries. This pulse spreading can cause indeterminate results at the receiver, leading to bit errors. Channel insertion loss is the total loss due to the fiber cable plant including the fiber cable attenuation as well as the insertion loss of all connections and splices in the optical link. Other penalties including cross noise (Pcross), relative intensity noise (Prin), and mode partition noise (Pmpn) are also shown but each represents a relatively small portion of the total budget. The sum of all these penalties is shown by the teal line and must not exceed the 7.3 dBm budget represented by the blue line. The passive portion of the total link penalty has been defined by the IEEE to be 2.6 dB at a distance of 300 meters using 2000 MHz-km “OM-3” 50 µm laser optimized multimode fiber (LOMF).

While the IEEE model highlights a link length of 300 meters, practical lengths within data centers/SAN typically do not exceed 100 meters (328 feet). This important distinction has many hidden benefits to the data center designer when considering the advantage of low DMD optical fiber such as the GIGAlite family of fibers and their impact on the ISI portion of the IEEE model. NetClear offers several levels of advanced optical fiber (GIGAlite, GIGAlite-10, and GIGAlite-10XB) that effectively increase cable plant loss budget by reducing ISI. The net effect is added cable plant loss budget that can be utilized in your structured cabling design for additional connection points and greater flexibility in system design. Reduced ISI coupled with low attenuation cables and industry leading channel insertion loss provides system robustness required in mission critical data centers and SANs.

ISI is reduced by high fiber modal bandwidth and increased by chromatic dispersion from wide source spectral width. Channel insertion loss is a function of connection loss and cable attenuation. Fiber cable attenuation varies little between manufacturers, and spectral width typically meets the standard. The major factors affecting the fiber link design are fiber bandwidth and channel insertion (connection) loss. These can vary significantly between various fiber grades, connector designs, and termination methods. High bandwidth LOMF available in the NetClear Solution for Data Centers and SANs combined with low loss connections such as the LC prepared with state-of-the-art polishing processes maximize reach and performance.

Dispelling the 2.6 dB Cable Plant Loss Budget Myth for 10 Gbps Ethernet Using LOMF Fiber

The cable plant loss budget in a fiber optic link is the amount of attenuation or loss that may occur between the optical transmitter at the near end of the link and the receiver at the far end. The budget is consumed by fiber cable attenuation plus the insertion loss of all connections and splices in the link. As long as the cable plant loss budget is not completely exhausted by these elements and a small amount of “dB” remains as margin, the link has been designed properly and will operate from the standpoint of the passive cable plant. Applications vary significantly but the margin is 0.8 dB for 10 Gbps Ethernet using serial transmission, and may be as low as 0.1 to 0.2 dB for other fiber applications.

The IEEE 10GBASE-SR standard defines the requirements for 10 Gbps Ethernet operating at the 850 nm wavelength using serial transmission. Operating at the short wavelength with serial transmission has proven to be the most cost-effective way to support both 1 and 10 Gbps Ethernet using multimode fiber to industry leading distances. Accordingly, it is estimated that approximately 90% of 1 Gbps fiber Ethernet systems installed today use short wavelength (850 nm) serial transmission systems. For this reason, it is expected that the overwhelming majority of 10 Gbps fiber Ethernet systems will also adopt cost-effective serial transmission operating at 850 nm.

As discussed previously, the passive component of the total link penalty for short wavelength 10 Gbps fiber Ethernet systems using serial transmission has been defined by the IEEE as 2.6 dB at 300 meters. It is incorrectly assumed by many that these numbers represent the maximum cable plant loss budget and distance allowable respectively for these systems independent of system length. The 2.6 dB value, however, is for the maximum distance specified in the standard and this value can vary based on the actual distance and number of connections (i.e., number of mated pairs of connectors). In addition the use of higher performance laser optimized fiber than what is specified in the standards can result in significant increases to available cable plant loss budget.

The NetClear Solution for Data Centers and SANs offers the highest performing LOMF in the industry, supporting up to 600 meters with 2 LC connections.[1] In data centers and SANs the available cable plant loss budget is typically more important as typical links are considerably shorter (< 150 meters). For data center and SAN applications, the NetClear Solution offers increased performance by effectively increasing cable plant loss budget through a combination of industry leading bandwidth and lowest mated pair insertion loss. The net effect is more reliable 10 Gbps transmission and greater flexibility in designing the structured cabling system. Consider a NetClear MM10 system utilizing GIGAlite-10XB fiber having a rated distance of 600 meters with 2 LC connections.[1] In a data center/SAN application where shorter fiber link lengths are common, operating this system at 300 meters yields an additional 1.4 dB of cable plant loss budget. Consequently, when added to the IEEE-specified 2.6 dB, a total of 4.0 dB channel insertion loss is available in the cable plant loss budget. Similarly, for a NetClear MM10 system utilizing GIGAlite-10 fiber, an additional 1.4 dB of headroom (for a total of 4.0 dB) is available at a fiber link length of 150 meters, one-half this fiber’s maximum rated distance.

Conventional 50 µm multimode fiber supports 10 Gbps at only up to 82 meters, while conventional 62.5 µm multimode fiber supports 10 Gbps at a maximum distance of only 25 meters. However, NetClear solutions include three high performance fiber types: 50 µm GIGAlite, 50 µm GIGAlite-10 and 50 µm GIGAlite-10XB, with all three solutions providing increased cable plant loss budgets. NetClear 50 µm GIGAlite (“LB” grade) LOMF offers robust 10 Gbps operation for up to 150 meters and is offered in cabling solutions typically priced no higher than conventional 50 µm multimode fiber. GIGAlite-10 (“EB” grade) LOMF offers performance beyond that specified for industry-standard OM-3 fibers used to support 10 Gbps transmission up to 300 meters. And NetClear’s highest performing fiber, GIGAlite-10XB (“XB” grade) LOMF offers 10 Gbps performance at up to 600 meters with 2 LC connections.[1] For data center designers and operators the superior performance of solutions using GIGAlite-10XB fiber enables support for additional mated pairs or higher insertion loss connections than that available with standard LOMF fibers. This additional performance may provide the capability design a structured cabling system infrastructure not possible with other LOMF fibers.

Link Budget Reallocation Increases Structured Cabling System Design Flexibility for Data Center & SAN Applications

Multi-fiber connectors such as the MTP or its generic equivalent, the MPO, are often used in mission-critical data centers & SANs to facilitate more rapid deployment, achieve higher fiber packing densities, and provide maximum flexibility in terms of system design. Recognizing that insertion loss associated with MPO technology is generally higher than that offered by single fiber connectors such as the LC, NetClear compensates with systems design incorporating high bandwidth LOMF fibers to create a high performance cassette based structured cabling system that is simple to design and even easier to install.

As discussed earlier, the two major link loss components are channel insertion loss and ISI. The higher the bandwidth of the LOMF, the greater the amount of ISI penalty that can be reallocated or reused to increase insertion loss. NetClear fibers, especially GIGAlite-10XB support the greatest amount of ISI penalty reallocation in the industry. Therefore, in a NetClear Data Center and SAN solution, more optical link loss budget is available, freeing the system designer to use some of that additional insertion loss budget for time and labor saving cassette based solutions.

In Figure 5 you see a typical data center or SAN system with a Fibre Channel hub connected to a switch through a multi-fiber ribbon backbone cable using MTP/MPO connectors. By reallocating a portion of our ISI penalty for the shorter 10 Gbps Fibre Channel link shown here with LOMF and low loss connectors, the link distance may be extended or additional connections may be supported with remaining margin to support maintenance and future moves adds and changes, compared to fiber optic links with connectors that are simply standards compliant.

1. Support for 10 Gbps transmission at 600 meters assumes a maximum link connection loss of 0.65 dB using IEEE 802.3ae compliant 10GBASE-SR transceivers.