Fiber Optic Components

40G QSFP+: Data Center Bandwidth Provider

With time passing by, the world’s data centers witness the rapid flowing of digital information whose volume grows at an ever increasing rate. The increase of server virtualization and cloud computing applications, coupled with the trend toward network convergence, all these are boosting today’s data center networks to become faster and more efficient than ever before. Date back to the past years, 100Mbps Ethernet is replaced by 1Gbps Ethernet which is then substituted by 10Gbps Ethernet. With 40GbE and 100GbE being available in the market, some companies or organizations can implement cut-through switching and flatter network architectures which can deliver more bandwidth, so as to reduce traffic latency and meet wireless computing needs.

To ensure the smooth 40GbE networking performance, one device is fundamental—40G QSFP+ (Quad Small Form-factor Pluggable Plus) transceiver. This QSFP+ transceiver interfaces a network device motherboard (for a switch, router, media converter or similar device) to a fiber optic cable, providing sufficient bandwidth to enable fully non-blocking switch fabrics.

40G QSFP+ Working Principle

40G QSFP+ transceiver is a hot-swappable module, integrating 4 independent 10Gbps data lanes in each direction for 40Gbps aggregated bandwidth. QSFP+ modules offer customers a wide variety of high-density 40Gbps connectivity options for data center, high-performance computing networks, enterprise core and distribution layers, and service provider transport applications. The release of the IEEE 802.3ba physical layer standard for 40G QSFP+ in 2010 specifies both fiber and copper cabling solutions, such as 40GBASE-SR4, 40GBASE-LR4 and 40GBASE-CR4.

40G QSFP+ Port types

Commonly used QSFP+ transceiver port types are 40GBASE-SR4 and 40GBASE-LR4.

  • 40GBASE-SR4

40GBASE-SR4 physical medium dependent (PMD) variant defines a 4 lane parallel optical interconnect for operation up to 100m link length over OM3 multi-mode fiber (MMF) and to 150m over OM4 MMF. It establishes high-bandwidth 40G optical links over 12-fiber parallel fiber terminated with MPO/MTP multi-fiber female connectors. Four of the twelve fiber for receive, other four for transmit, leaving the middle four fiber unused. Each of the four lanes operates at a data rate of 10.3125Gbps which is the same serial bit rate that was defined for 10Gb Ethernet links in the IEEE 802.3ae standard ratified in 2002.


40GBASE-SR4 PMD addresses the modern need for 40Gbps interconnects in the data center and takes advantage of lower cost 850nm Vertical Cavity Surface Emitting Laser (VCSEL) technology that is widely deployed throughout networking industries. As each of the 4 lanes in 40GBASE-SR4 operates at the same serial bit rate as a 10Gb Ethernet link, there is an opportunity for switching hardware vendors to utilize 40GBASE-SR4 as 4 separate 10Gb Ethernet interconnects. The problem is that 40GBASE-SR4 standard is not defined to be backward compatible with the existing 10GbE short reach interconnect standard.

To address this issue, QSFP BiDi transceivers have been released into the market. A QSFP BiDi transceiver (e.g. Cisco QSFP-40G-SR-BD) transmits full-duplex 40-Gbps traffic over one dual-fiber LC-connector OM3 or OM4 MMF cable. It provides the capability to reuse 10-Gbps fiber infrastructure. In other words, it enables data center operators to upgrade to 40-Gbps connectivity without making any changes to the previous 10-Gbps fiber cable plant.

QSFP BiDi technology

  • 40GBASE-LR4

40GBASE-LR4 has two link options: coarse wavelength division multiplexing (CWDM) and parallel single-mode fiber (PSM). This passage mainly talks about the first one option.

40GBASE-LR4 CWDM QSFP+ uses a duplex LC connector as the optical interface, able to support transmission distance up to 10km over single-mode fiber (SMF). It converts 4 inputs channels of 10G electrical data to 4 CWDM optical signals by a driven 4-wavelength distributed feedback (DFB) laser array, and then multiplexes them into a single channel for 40G optical transmission, propagating out of the transmitter module from the SMF. Reversely, the receiver module accepts the 40G CWDM optical signals input, and demultiplexes it into 4 individual 10G channels with different wavelengths. The central wavelengths of the 4 CWDM channels are 1271, 1291, 1311 and 1331 nm as members of the CWDM wavelength grid defined in ITU-T G694.2. Each wavelength channel is collected by a discrete photo diode and output as electric data after being amplified by a transimpedance amplifier (TIA).


Cable Types for 40G QSFP+

IEEE has specified standards for 40G transmission in both long distance and short distance, which are 40GBASE-SR4 and 40GBASE-LR4. As what has been mentioned above, the latter is suggested for 40G transmission over SMF in long distance up to 10km, while the former is for 40G transmission in short distance over MMF (often in with a 12-fiber MPO connector version)—OM3 (up to 100 meters) and OM4 (up to 150 meters). OM3 and OM4, which are usually aqua-colored, are accepted economic solutions for 40G in short distance with lower insertion loss and higher bandwidth. Besides, DACs are also chosen as the 40G cabling solutions sometimes, available in QSFP+ to QSFP+ DAC and QSFP to SFP+ cable versions.


40G QSFP+ transceivers support 40GBASE Ethernet, allowing flexibility of interface choice and great bandwidth for data centers. For your smooth 40GbE performance, Fiberstore supplies various cost-effective QSFP+ transceivers which are fully compatible with major brands, such as Cisco, Juniper, Brocade, and so on. Besides these 40G modules, SFP transceivers are also offered, like GLC-T, a Cisco compatible 1000BASE-T SFP. All these products are rigorously checked for quality and compatibility assurance. If you want such a module for your networking use, you can consider Fiberstore.

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