Ready for 40G but Not New Jumper Cables?

Networks and data centers have relied on 10Gbps connectivity speeds for the past 5+ years. With ever increasing network access rates (e.g., GoogleFiber and 5G Mobile) and the increasing trend to remote “Cloud” storage and computing, fiber connectivity rates are on the rise. While speculation abounds whether 40Gbps or 100Gbps (or something higher) will be the next-big-thing, 40Gbps transceivers in the QSFP (Quad Small Form-Factor Pluggable) package are a mature technology with thousands QSFP slots being shipped each month.

One thing slowing down adoption of QSFP-based 40G solutions for Short-Reach (SR) applications is that customers have had to both acquire new switching gear with the QSFP ports and switch out their interconnect cabling. Existing 10GBASE-SR SFP+ transceivers use OM3 multimode fiber terminated with LC connectors for interconnection. 40GBASE-SR4 QSFP modules are defined with an MPO-12 connector. Not only are MPO-12 jumper cables bulkier and stiffer, they also eliminate the possibility of a drop-in 40G replacement of a previous 10G installation. The lowest cost and most time-efficient upgrade for network installers/operators is to simply disconnect fiber jumpers from previous generation 10G SFP+ transceivers, slot in the 40G equipment, and plug those same jumpers right back in to 40G QSFPs.

Finally, a new 40Gbps bidirectional optical (BiDi) module provides a QSFP-based package compatible with previous 10Gbps fiber infrastructure. This 40-Gbps BiDi QSFP supports an aggregate of 40Gbps capacity over a single pair of OM3 MMF with LC connectors. Now network and datacenter operators have a migration path to 40G with no additional fiber cost. In fact, since each pair upgraded to 40G carries 4X the data, this technology achieves a 4:1 reduction in fiber cost!

Previous 40-Gbps Short Reach Transceivers
As defined in the QSFP MSA, previous short-reach (SR) transceivers for 40-Gbps connectivity in a QSFP form factor, such as QSFP SR4, use 4 parallel fiber strands in each direction, for a total of 8 active fibers per MPO-12 link. One result is 4 fiber strands in each cable are wasted. Figure 1 shows the cabling for existing short-reach 40-Gbps QSFP cabling.

Figure 1. Earlier 40-Gbps Transceivers (MMF, 40GBASE-SR4)

Beyond the four wasted fibers, the 12-fiber ribbon in larger and substantially more ridged than the typical 2mm duplex fiber jumpers used with previous generation 10Gbps SFP+ transceivers. This compounds an already often tightly congested cabling environment.

The 40-Gbps QSFP BiDi Solution
The QSFP BiDi Transceiver, Fluxlight PN: QSFP-40G-SR-BD (for Cisco compatible version) is a short-reach optical transceiver that delivers 40 Gbps over either a duplex OM3 or OM4 MMF connection. Connections of up to 100 meters on OM3 MMF or up to 150 meters on OM4 MMF are supported. The QSFP BiDi Transceiver is identified by a gray bail latch or pull tab. Figure 2 shows a Fluxlight QSFP BiDi Transceiver.

Figure 2. Fluxlight QSFP BiDi Transceiver (QSFP-40G-SR-BD)


The QSFP BiDi Transceiver combines the four 10Gbps data lanes from the host device into two 20-Gbps channels. Each of these 20Gbps channels are transmitted and received simultaneously on two wavelengths over a single MMF strand. The result is a full duplex link totaling 40-Gbps link over a single pair of MMF strands. Figure 3 shows is a depiction of these two bi-direction 20Gbps links forming a total 40Gbps on two fibers.

Figure 3. Bidirectional Transmit and Receive Concept of the QSFP BiDi Transceiver


For More Information
For more information about Fluxlight’s Cisco-compatible 40-Gbps BiDi Transceiver, please visit
For information about QSFP 40Gbps BiDi solutions compatible with other vendors equipment, or about transceiver in general, please call 888-875-7574 or email: or

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