Data Center

Use of Multimode fibers in Data Center Cabling System

These days, the data traffic explosion is happening due to the growth of data centers. The Data centers being managed by hyperscale, Web 2.0 companies such as Google, Facebook, Amazon, Apple and Microsoft etc. are constantly demanding and moving toward faster speeds and higher densities. Demand for higher density had resulted in redesigning of the concepts in fiber optic cable designs. More and more fiber cable manufacturers are entering in to the high density high fiber count cables. Speeds as high as 25G and 100G Ethernet have already become mainstream in data centers, and the industry is working collaboratively on next-generation Ethernet development, such as 200G and 400G Ethernet. Experiments are going on to achieve 800G and above transmission rates. While optical fiber itself can carry such amounts of data, associated equipment capable to support such data rates shall be developed. For long-haul transmission, systems to support 40G and 100G can easily be deployed using single mode fiber, data centers look for cost effective alternatives using multimode fibers.

Multimode fiber (MMF) is a cost-effective solution in data centers due to their large core diameter and therefore high tolerance for fiber misalignment and relatively low connection loss at each connector interface. Multimode fibers commonly deployed for telecom/data center system usually have a core diameter of 50 micrometers or 62.5 micrometers. Multimode fiber cabling systems, combined with LEDs and VCSEL (vertical-cavity surface-emitting laser)-based optical transceivers are ideal for short-reach optical interconnects.

Multimode fiber has its own limitation due to the fact that it allows multiple modes to travel. The transmission distance is limited due to the modal dispersion. The channel capacity of multimode fiber has multiplied by using parallel transmission over several fiber strands (four or 16, with collections of 25 Gbps carried in each direction). But this method raises cabling system prices.

ITU-T G.651 fiber deployed at the initial stage of telecommunication using optical fiber was multimode. Initially developed to support Fast Ethernet (100BASE-FX and 1000BASE-SX), OM1 and OM2 multimode fiber cable can no longer support 10 Gbps and 25 Gbps data transmission speeds. In the ANSI/TIA-568.3-D Standard, OM1 and OM2 multimode fiber types have been “grandfathered” in, and are not recommended for new installations.

Until recently, OM3 and OM4 (laser-optimized multimode fiber [LOMMF]) were the mainstream multimode fiber cabling choices to support 10G, 40G and 100G Ethernet, InfiniBand and Fibre Channel protocols. As bandwidth requirements increase much faster than the VCSEL-based transceiver technology curve, however, it’s becoming more costly for optical fiber cabling systems to support next-generation Ethernet speed migration. For example, in the IEEE 802.3bs Standard draft, 400GBASE-SR16 has been specified to reuse 100GBASE-SR4 technology, but it calls for a new MPO-32 connector instead of an MPO-12 connector.

A Potential Alternative is Wideband Multimode Fiber. Wideband multimode fiber (WBMMF) is an ANSI/TIA development that can deal with escalating data rates and the infrastructure required to support higher bandwidth. It uses wavelengths to increase each fiber’s capacity by at least a factor of four, which allows at least a fourfold data-rate increase (or a fourfold reduction in the number of fibers required to achieve a given data rate. Instead of using four separate fibers to transmit four optical signals, the signals can be sent down one fiber over four separate operating windows.

ANSI/TIA-492AAAE, the new wideband multimode fiber standard, was approved for publication in June 2016 after a 20-month, industry-wide study carried out by a special TIA taskforce within TR-42.11 (Optical Systems Subcommittee) and TR-42.12 (Optical Fibers and Cables Subcommittee). The International Organization for Standardization/International Electrotechnical Commission (ISO/IEC) has recently decided on the nomenclature for wideband multimode fiber cable: OM5. This new fiber cable standard has already been referred to by the IEEE 802.3 working group for next-generation Ethernet standard development.

Please note that the TIA-492AAAE document specifies the raw glass fiber performance for the wideband operation, while ISO/IEC OM5 and TIA 568.3-D specify the cabled fiber performance containing the wideband multimode fiber.

Chief Editor of Fiber Optic Mania Magazine

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