Fan-out Technology Used in 40GbE Deployment

Obvious is the growing need for bandwidth required to accommodate various cloud services, smooth multimedia downloads or uploads, video streaming, as well as growing wireless usage. 10Gbps links are not sufficient enough to handle the increased bandwidth. As such, it’s imperative to deploy Higher Speed Ethernet (HSE) network, say 40GbE, by using devices that scale to match customer demands. Besides adding more devices and cables, higher port density is also the ideal solution to get greater bandwidth. To deliver the higher HSE port density for 40GbE interfaces, fan-out (breakout or harness) cabling technology is introduced here which also meets the need to keep costs at a manageable level.

Fan-out Technology Basics

Being one of the latest enabling technologies to help increase port densities and lower costs, fan-out technology is easy to be grasped. At first sight, a fan-out cable looks very simple. It takes one (large bandwidth) physical interface and breaks it out into several (smaller bandwidth) interfaces. A typical example of this is QSFP to SFP+ cable assembly which takes one 40G QSFP interface on one end and uses a fan-out cable to break it into four SFP+ interfaces one the other end. In this QSFP+ assembly, a fan-out cable enables splitting out the 4 lanes into Tx/Tx pairs usable for 10GE transmissions.

However, the truth is not definitely just as its face. This complex fan-out technology is critically important in the distribution layer of 40G data center. The following passages discuss two fan-out assemblies that are widely used in 40G data center.

40G MPO/MTP Fan-out Cables

Here goes to the MPO/MTP fan-out cables in 40GbE transmission firstly. A MPO/MTP fanout cable is a multi-fiber optical cable that contains several individual tight buffered optical fibers with one end terminated with a male/female MPO connector and the other end usually terminated with several LC connectors. Existing 40Gbps transmission over this multi-mode backbones is accomplished by using an LC to MPO fan-out assembly to connect the existing LC connectors in the installed cabling to the MPO connector in the 40Gbps.

40G connection with LC distribution module

There are many MPO fan-out cable types. For 40G cabling, the most common breakout cable type is the one which is able to fan out into 12 fibers. (a typical 12-fiber MPO breakout cable often with OM3 optical fiber as the transmission media is shown below). This 12-fiber MPO fan-out cable is terminated with 6 duplex LC connectors on one end and a male MPO connector on the other end, which can work from MPO trunk backbone assemblies to LC fiber rack system in high density backbone cabling from 40G device to 10G devices.

12-fiber MPO fan-out cable

40G Direct Attach Fan-out Cables

Direct attach cable (DAC), as a kind of optical transceiver assembly, is also available in fan-out design. It’s a form of high speed cable with “transceivers” on either end used to connect switches to routers or servers. Although the “transceivers” on both ends of DACs are not real optics and their components are without optical lasers, they perform the similar functions as the real transceivers. DAC fan-out cables can be divided into direct attach copper breakout cables and active optic fan-out cables.

40G DAC fanout cable: DAC copper vs. AOC

Converting one form factor to a different form factor is necessary in many cases. For example, a 40G device may be connected to one or several 10G devices for distribution or adapting. With fan-out DAC, this process would be much easier. This pre-terminated components can also increase the reliability of data center effectively.

Fan-out Technology Consideration

One important consideration for fan-out technology is that for each group of fan-out ports, there must also be a MAC (media access control) engine and the entire IEEE reconciliation layers behind it. For example, a 40GE QSFP port in fan-out to 4x10GE links must have four individual 10GE MACs running behind each group of electrical lanes. The port electronics must be able to switch from a single 40GbE MAC to 4 10GbE MACs, and back again to a single 40GE MAC. This process is integral in assuring that the lanes have a correct and trackable MAC assignment — it is crucial to make fan-out technology actually work.


40G fan-out technology separates a single physical interface into multiple physical interfaces that are 410GbE-capable, providing high-density interfaces over multiple speeds. This increases interface flexibility at lower costs. Fiberstore offers a wide selection of fan-out MPO and DAC cables (QSFP to SFP+ cable) for your 40G transmission. Besides these 40G cables, 40G modules which are fully compatible with major brands can also be found, such as Cisco QSFP-40G-SR-BD. This compatible module just costs you US$ 300.00 at Fiberstore, but when you search it on Amazon or somewhere else, it seems that you have to pay more than US$ 400.00 for a new one.

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