Two Short-reach Solutions for 40GbE Transmission


Because of the large amount of storage that is needed for high-performance networking services, such as smooth video streaming and rapid file upload or download, the demands for even higher data rates are increasing rapidly in telecommunication and datacoms networks. To meet such fast-growing bandwidth demands, the IEEE ratified standards to support 40GbE (Gigabit Ethernet) in 2010, known as 802.3ba.

The IEEE standard 802.3ba released several 40-Gbps based solutions, including 40GBASE-LR4 optic transceivers over single-mode fiber (SMF), 40GBASE-SR4 optics solution for multi-mode fiber (MMF). The former is suitable for significantly longer distances, while the latter is appropriate for short distances. Actually, there is another component available to replace 40GBASE-SR4 optics when the required distances are short in network interconnections, that is 40GBASE QSFP+ AOC (active optic cable). This text mainly differs from these two short-reach solutions for 40GbE transmission: 40GBASE-SR4 optics and 40GBASE QSFP+ AOC.

40GBASE-SR4 Optics

40GBASE-SR4 optics uses a 12-fiber MPO connector interface where all the 12 fibers are aligned in a single row. Four fibers on one side are used to transmit, while another four on the other side are utilized to receive, leaving the middle four fibers unused. In total, eight of the twelve fibers are used.

40GBASE-SR4 diagram


As a kind of DAC (direct attach cable), AOC can be applied in 10G, 40G, and even 120G. When used in 40G transmission, two versions of AOC cabling assembly are available. The first one is QSFP to QSFP, with a QSFP+ connector on one end and another QSFP+ connector on the other end. The second is QSFP to four SFP+, with one end connected with a QSFP+ connector and the other end with several SFP+ connectors. Like QSFP-4X10G-AOC10M, this 40G cabling product is the second cabling assembly. Although the “transceivers” on both ends of AOCs are not real optics and their components are without optical lasers, they have a similar function to the real optic transceivers, and also can transmit signals through fiber optic cables.

40G Attached optical fiber cable

40GBASE-SR4 Optics vs. 40GBASE QSFP+ AOC

Both 40GBASE-SR4 optics and 40GBASE QSFP+ AOC are good solutions for interconnection. But everything has two sides just like a coin. Each solution has its pros and cons. Figure out the differences and choose the better component for your applications.

  • Transmission Distance

The first point is the transmission distance. As has been mentioned above, 40GBASE-SR4 optics and 40GBASE QSFP+ AOC are designed to support short-distance transmission. When the required distance is less than 100m, these two solutions have similar performance. But when link length is longer than 100m, then 40GBASE-SR4 optics is the better choice. 40GBASE-SR4 transceiver (eg. MC2210411-SR4) is able to realize 150m reach over OM4 MMF. Currently, most 40G AOC provided by the telecommunication equipment manufacturers are less than 100m.

  • Reliability

Reliability comes as the second factor to be considered. For interconnection use, both of the two components should be inserted into a switch or a server. And the repeating plug of them is necessary for daily use and maintenance, which might affect the performance of the component. As such, reliability is of great importance during these daily actions. 40G AOC connectors are factory pre-terminated, while QSFP+ SR4 transceivers are connected by additional MPO connectors and fiber optic cables. In contrast, AOC is less affected by the repeating plug during daily use. AOC has better reliability than that of transceivers.

  • Installation and Maintenance

It’s clear that 40G AOC is easier for installation since 40G AOC connectors are factory pre-terminated. For working use, customers just need to plug the two connectors in the switches. In comparison, as for QSFP+ SR4 transceivers, additional patch cords with MPO connectors are needed to establish the link. When talking about maintenance, in case there was a fault in the interconnection, for AOC, you can just replace it with another AOC. However, for interconnection using QSFP+ SR4 transceivers, you have to locate the fault firstly by testing the patch cords and optics, and then find the fault, which means a time-consuming task.

  • Cost

In most cases, cost means a lot for users. Here cost includes two aspects: material cost and the possible maintenance cost in daily use. As for the material cost, the price for 40G AOC is generally cheaper than 40GBASE-SR4 optics, as the “transceivers” on both ends of AOCs are not real optics. In a word, 40G AOC has advantages over 40GBASE-SR4 optics in both material cost and maintenance cost.


According to the transmission distance, 40GBASE-SR4 optics delivers better performance than 40G AOC can. But when talking about reliability, installation, maintenance, and cost, 40G AOC is a better choice, since it’s cheaper and reliable. For your 40GbE applications, Fiberstore supplies an ocean of 40GBASE-SR4 transceivers (eg. MC2210411-SR4) and 40G QSFP+ AOC (eg. QSFP-4X10G-AOC10M). You can buy them at Fiberstore at lower prices and enjoy high performance. Please visit Fiberstore or connect me directly at Linkedin @Fern Xu (Fiberstore) if you want such a product.



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