Why Migrate to 40G
With the quick development in data center, cabling infrastructures should provide manageability, flexibility and reliability. Deployment of optical connectivity solutions enables for an infrastructure meeting these requirements for current applications and data rates. Scalability is another key factor that needs to consider when choosing the type of optical connectivity. It refers to not only the physical expansion of the data center with respect to additional servers, switches or storage devices, but to the scalability of the infrastructure to support a migration path for increasing data rates. As technology evolves and standards are completed to define data rates such as 40G/100G, Fibre Channel (32G and beyond) and InfiniBand (40G and beyond), the cabling infrastructures installed today need to provide scalability to accommodate the need for more bandwidth in support of future applications. Moreover, current data rates cannot meet the needs of the future with the rising demand to support high-bandwidth applications. 40G technologies and standards, however, can support future networking requirements. Thus, a migration to 40G is required.40 Gigabit Ethernet Standard
Ratified in June 2010, 802.3ba standard provides a guidance for 40G transmission with multimode and single-mode fibers. And this standard does not have guidance for Cat UTP/STP copper cable. OM3 and OM4 are the only multimode fibers included in the standard. Due to the 850nm VCSEL modulation limits, multimode fibers utilize parallel optics transmission instead of serial transmission. Single-mode fiber guidance utilizes duplex fiber WDM (wavelength-division multiplexing) serial transmission.
Compared to single-mode fiber, multimode fiber offers a significant value proposition for short length interconnects in the data center. Unlike traditional serial transmission, parallel optics transmission utilizes an optic module interface where data is simultaneously transmitted and received over multimode fibers. The 40GBASE-SR4 supports 4 x 10G on four fibers per direction.
Cabling Performance Requirements for OM3/OM4
When evaluating the performance needed for the OM3 and OM4 cabling infrastructure, the following criteria should be considered. Each of the criteria would have an impact on the cabling infrastructure’s ability to meet the standard’s transmission distance of 100 meters over OM3 fiber and 150 meters over OM4 fiber.
Bandwidth is the primary criteria. OM3 and OM4 fibers are optimized for 850nm transmission and have a minimum 2000 MHz∙km and 4700 MHz∙km effective modal bandwidth (EMB). Fiber EMB measurement techniques are utilized today. The minimum EMBc (Effective Modal Bandwidth calculate) method combines the properties of both the source and fiber. With a connectivity solution using OM3 and OM4 fibers that have been measured using the minEMBc technique, the optical infrastructure deployed in the data center will meet the performance criteria set forth by IEEE for bandwidth.
Insertion loss is a critical performance parameter in current data center cabling deployments. Total connector loss within a system channel impacts the ability to operate over the maximum supportable distance for a given data rate. The supportable distance at data rate decreases with total connector loss increasing. The 40G standard specifies the OM3 fiber to a 100m distance with a maximum channel loss of 1.9 dB, which includes a 1.5 dB total connector loss budget. OM4 fiber is specified to a 150m distance with a maximum channel loss of 1.5 dB, which includes a 1.0 dB total connector loss budget. The maximum cable fiber attenuation is 3.5 dB/km at 850 nm. So the insertion loss specifications of connectivity components should be evaluated when designing data center cabling infrastructures. With low-loss connectivity components, maximum flexibility can be achieved with the ability to introduce multiple connector matings into the connectivity link.
Summary
Cabling deployed in the data center today must be selected to provide support of data rate applications of the future. To achieve this purpose, OM3 or OM4 is a must. They provide the highest performance for today’s needs. With 850nm EMB of 2000 MHz∙km and 4700 MHz∙km, the fibers provide the extended reach required for structured cabling installations in the data center. Except the performance requirements, the choice in physical connectivity is also important. Utilizing MTP-based connectivity in today’s installations provides ways to migrate to multifiber parallel optic interface when needed. Therefore, MTP-based connectivity using OM3 and OM4 fiber is the ideal solution in the data center. It can be installed for use in today’s applications, while providing an easy migration path to future higher speed technologies.