40G QSFP+: Data Center Bandwidth Provider

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40G Parallel & Bidirectional Optical Transceiver Introduction

Speeds in data centers have maintained their growth in the past years, and will continue to do so in the predictable future. High-data-rate systems have become increasingly popular among some enterprises for high-performance computing networks, such as 40 Gigabit Ethernet (GbE) infrastructure, in which 40G fiber optic transceivers and cables are needed to ensure the high-performance and great-bandwidth of the 40GbE system. This article mainly introduces 40G fiber optic transceivers: the pluggable optical Enhanced Quad Small Form-Factor Pluggable (QSFP+), with focus on the bidirectional optical transceivers and parallel optical transceivers.

40G Optical Transceiver Types

The transceiver is an electronic device that receives an electrical signal, converts it into a light signal, and launches the signal into a fiber. It also receives the light signal, from another transceiver, and converts it into an electrical signal. With the 40G QSFP being the dominant transceiver form factor used for 40GbE applications, the IEEE standard 802.3ba released several 40-Gbps based solutions in 2010, including a 40GBASE-SR4 parallel optics solution for multi-mode fiber (MMF). Another solution is a bidirectional 40-Gbps transceiver that uses a two-fiber LC optical interface.

  • 40G Parallel Optical Transceiver

40G parallel optical transceiver enables high-bandwidth 40G optical links over 12-fiber parallel fiber terminated with MPO/MTP connectors. 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 fiber are used. That is to say, when used for 40GBASE-SR4 and 40GBASE-CSR4, parallel optical transceiver has 10-Gbps electrical lanes that are mirrored in the optical outputs, causing the requirement of eight fibers with a MTP connector interface. Each fiber either transmits (Tx) or receives (Rx) 10-Gbps traffic at a single wavelength.

40G parallel optical transceiver, 40GBASE-SR4

Just as mentioned above, 40GBASE-SR4 QSFP+ transceiver belongs to 40G parallel optical transceivers, which uses multi-mode MPO trunks to establish 40G links. This port type 40G QSFP+ module can support link lengths of 100 meters and 150 meters over laser-optimized OM3 and OM4 MMFs respectively. It can also be used to connect with four 10GBASE-SR optical interfaces using an 8-fiber MTP to 4 duplex LC cable. Fiberstore listed 40GBASE-SR4 optical transceivers are fully compatible with such famous brands, as Cisco, Intel, Juniper (QFX-QSFP-40G-SR4), and so on. All are quality-and compatiblity-assured, offering the high performance to customers.

  • 40G Bidirectional Optical Transceiver

By contrast, 40G bidirectional optical transceiver consists of two 20-Gbps transmit and receive channels, enabling an aggregated 40-Gbps link over a two-strand MMF connection. That is, the bidirectional optical transceiver used for 40GBASE-SR-BD uses the same 10-Gbps electrical lanes, which are then combined in the optical outputs, thus requiring two fibers with an LC connector interface. Each fiber simultaneously transmits and receives 20-Gbps traffic at two different wavelengths.

40G bidirectional optical transceiver

Cabling Options for 40G Parallel & Bidirectional Optical Transceiver
  • Cabling Options for 40G Parallel Optical Transceiver

As previously mentioned, in 2010 IEEE 802.3ba approved the 40GBASE-SR4 physical-medium-dependent (PMD) multi-mode parallel optic solution, which uses eight fibers to transmit four duplex channels each at 10-Gbps. This is an economical path to 40GbE data rates, while using many of components of 10GbE solutions. The main advantage of the parallel optical transceiver over the bidirectional transceiver at 40GbE is the reach. For example, if you cable your data center with OM3 at 10GbE, you can support distances up to 300m. Then if you move to 40GbE, you can support the same 300m distance with the same OM3 fiber and a 40GBASE-CSR4 transceiver. However, if your cabling distances do not justify the extra distance capability, then the bidirectional solution would be used.

There exists a problem in this parallel optical cabling solution—MTP cable assemblies which built on 12-fiber position connectors, leaving four unused fibers in each link. There are several basic cabling options for parallel optics connectivity. One approach is to ignore the unused fibers and continue to deploy 12 fibers. Another approach is to use a conversion device to convert two 12-fiber links into three 8-fiber links.

  • Cabling Options for 40G Bidirectional Optical Transceiver

This two-fiber 40G bidirectional multi-mode solution tackles the challenge—polarity correction that occurs in a 12-fiber MTP connector., using two different transmission windows (850 and 900nm) that are transmitted bidirectionally over the same fiber. This approach allows the use of the same cabling infrastructure for 40GbE as was used for 1 and 10 Gigabit Ethernet. The pluggable bidirectional transceiver has the same QSFP+ format as the existing 40GBASE-SR4 transceivers. Therefore, the same switch line card with QSFP+ ports can support either parallel optics 40GBASE-SR4 or bidirectional optics 40GBASE-SR-BD solutions.

As such, while connecting a 40GbE bidirectional transceiver to another bidirectional transceiver, a Type A-to-B standard LC duplex patch cord can be considered, with one fiber in connector position A on one end and in connector position B on the other end. Such reverse fiber positioning allows a signal to be directed from the transmit position on one end of the network to the receive position on the other end of the network. However, this direct connectivity is recommended only within a given row of cabinets.


40G QSFP+ transceivers can meet the growing data center applications, such as big data and high-frequency trading (HFT) applications, and virtualized and clustered environments. With 40G bidirectional optical transceivers, no big changes to the existing cabling infrastructure are required, a cost-effective way for migration from 10-Gbps to 40-Gbps connectivity in data center networks. As a professional fiber optical product manufacturer and supplier, Fiberstore offers various 40G QSFP+ transceivers for your choice. Besides, many 40G cables (ie. QSFP-4SFP10G-CU5M) are also available for the smooth migration. For more information about 40GbE solutions (40G QSFP+ and 40G cables), you can visit Fiberstore.

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SMF&MMF 40G QSFP+ Transceiver Overview

The demand for better network throughput and performance has never ceased. Instead, it has become more and more vigorous. The server consolidation, virtualization, as well as networking-service performance improvements, all these have pushed the necessity for dense 40GbE switch connections in data centers.

But when migrating to 40GbE from10GbE, some companies or organizations are challenged by two main factors in re-configuring the physical layer of the network: firstly, the possible reduced reach of the OM3/OM4 multi-mode optics from 10GBASE-SR (300/400 m) to 40GBASE-SR4 (100/150m), and secondly, the need to upgrade the existing fiber optic cabling plant so as to support the IEEE-defined 40GBASE-SR4 parallel optics. In order to avoid these questions, SMF&MMF 40G QSFP+ transceiver is brought to the market.

SMF&MMF 40G QSFP+ Transceiver Definition

It’s know that a fiber optic transceiver may either operate on single-mode fiber (SMF) or multi-mode fiber (MMF). However, this SMF&MMF 40G QSFP+ transceiver is able to communicate with both SMF and MMF, without the need for any software/hardware changes to the transceiver module or any additional hardware in the network. It has 4 channels (1270, 1290,1310, and 1330nm) of 10G multiplexed inside the module to transmit and receive an aggregate 40G signal over 2 strands of fiber with a duplex LC connector.

SMF&MMF 40G QSFP+ transceiver, 4 channels

Based on IEEE defined 40GBASE-LR4 specifications, this supports distances up to 150m over OM3 or OM4 MMF and up to 500m over SMF. Certainly, different fiber optic equipment vendors may have different specifications.

SMF&MMF 40G QSFP+ Transceiver Advantages

SMF&MMF 40G QSFP+ transceiver is designed for seamless migrations from existing 10GbE to 40GbE networking without modification or expansion of the fiber network. It addresses several challenges faced by today’s data centers and the passages highlight the advantages of this transceiver.

  • No Redesign or Expansion of Fiber Network

Other short-reach 40G QSFP+ transceiver types, such as MMF 40GBASE-SR4 transceivers (100m over OM3 MMF), utilize four independent 10G transmitters and receivers for an aggregate 40G link. These 40GBASE-SR4 transceivers (eg. JG325B) use a MPO-12 connector and require 8-fiber parallel OM3 or OM4. As a result, customers installing MTP/MPO fiber systems may need to deploy new fiber while upgrading from 10G to 40G. However, SMF&MMF 40G QSFP+ transceiver uses duplex LC connector, which is consistent with the existing 10G connections. It works on existing OM3 and OM4 MMF infrastructure which is widely installed and used for 10GbE networks, thus free from redesign or expansion of the fiber network.

No Redesign or Expansion, SMF&MMF QSFP+

  • Increase in the Number of 40G Links

The existing MMF 40GbE solutions use of 8 fibers for a 40G link, and customers have to add additional fiber if they want to increase the number of 40G links. But if you deploy SMF&MMF 40G QSFP+ transceiver, the number of 40G links is 4our times of that existing MMF 40GbE solutions without any changes to their fiber infrastructure. During this link increase, the network scale and performance are also expanded.

  • A Cost-effective Solution for SMF Infrastructure

Limited in the distance reach that multi-mode transceivers can support, the migration from 10G to 40G, to 100G, or even 400G would become simpler with SMF. But single-mode transceivers typically cost up to 4 times more compared to multi-mode transceivers. Since SMF&MMF QSFP+ transceiver interoperates with QSFP-LR4 and QSFP-LR4L optics, it’s a cost effective solution for SM fiber infrastructure for distances up to 500m. And customers can deploy mixed connections without fiber concerns.

  • Simplification in Infrastructure Deployment

SMF&MMF QSFP+ transceiver boasts of the unique characteristic of working through both SMF and MMF without any requirement for additional fiber. Customers can consolidate their optics and use SMF&MMF QSFP+ transceiver in their network without concern about the fiber type, which makes the full use of existing cabling infrastructure, leading to the reduced equipment cost and simplification of deployment.


SMF&MMF QSFP+ transceiver allows data centers to migrate from 10GbE to 40GbE without redesigning or modifying the cabling infrastructure, providing companies or organizations a cost-effective solution to expand their fiber network. With SMF&MMF QSFP+ transceiver in hand, a smooth 40GbE migration at low cost is around the corner. Fiberstore SMF&MMF 40G QSFP+ transceivers are supplied to help you achieve smooth 40G migration. Besides, their interoperate QSFP-LR4 and QSFP-LR4L transceivers are also available, such as Cisco QSFP-40G-LR4 and WSP-Q40GLR4L. For more information about SMF&MMF QSFP+ transceivers, you can visit Fiberstore.

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For 40GBASE-LR4 QSFP+ Transceiver Link: CWDM or PSM?

Nowadays, the 40 Gigabit Ethernet (GbE) system comes as the popular deployment among some enterprises for their high-performance fiber optic networks. And for 40GbE system, fiber optic transceivers are the indispensable high-capacity modules for multi-lane communications, like 40GBASE-LR4 QSFP+ transceiver. It’s known that 40GBASE-LR4 QSFP+ transceiver has two link options: coarse wavelength division multiplexing (CWDM) and parallel single-mode fiber (PSM). How much do you know about them? Can you figure out the differences between them? Following this article and you will get something.

40GBASE-LR4 CWDM QSFP+ Transceiver Brief

Compliant to 40GBASE-LR4 (eg. QSFP-40G-LR4) of the IEEE P802.3ba standard, this 40GBASE-LR4 CWDM QSFP+ transceiver uses a duplex LC connector as the the optical interface, able to support transmission distance up to 10km over single-mode fiber (SMF) used to minimize the optical dispersion in the long-haul system.

This kind of 40GBASE-LR4 QSFP+ transceiver 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).

CWDM QSFP+, 2 optical SMFs with a duplex LC connector

40GBASE-LR4 PSM QSFP+ Transceiver Brief

Differently, PSM QSFP+ is a parallel single-mode optical transceiver and uses a MTP/MPO fiber ribbon connector instead of LC. Similarly, PSM QSFP+ also offers 4 independent transmit and receive channels, each capable of 10G operation for an aggregate data rate of 40G with 10km reach over SMF.

In a PSM QSFP+, the transmitter module accepts electrical input signals, while he receiver module converts parallel optical input signals via a photo detector array into parallel electrical output signals. Both the input signals and output signals are compatible with common mode logic (CML) levels.

PSM QSFP+, 8 optical SMFs with a MTP/MPO fiber ribbon connector


Allowing for the transceiver module structure, PSM seems more cost effective, since it uses a single uncooled CW laser which splits its output power into four integrated silicon modulators. Additionally, its array-fiber coupling to a MTP connector is relatively simple.

However, when taking the infrastructure into consideration, PSM would be more expensive when the link distance is long, because it uses 8 optical single-mode fibers while CWDM only uses 2 optical single-mode fibers. Besides, in the data center fiber infrastructure, the patch panel has to be changed to accommodate MTP cables, which would cost more than LC connectors and regular SMF cables. Besides, it’s a little difficult to clean MTP connectors. So CWDM is more ideal for 40GBASE-LR4 QSFP+ link.


For 40GBASE-LR4 QSFP+ transceiver link options, both CWDM QSFP+ and PSM QSFP+ support the maximum transmission distance of 10km. The former establishes 40G links over 2 optical SMFs with a duplex LC connector, and the latter achieves 40G links via 8 optical SMFs with a MTP/MPO fiber ribbon connector. Thus no change is required for migration from 10G infrastructure to 40G infrastructure, saving cost when CWDM QSFP+ is chosen. Fiberstore supplies a broad selection of 40GBASE-LR4 QSFP+ transceivers which are fully compatible with major brands, such as Finisar (FTL4C1QE1C). For more information about 40GBASE-LR4 QSFP+ transceivers, please visit Fiberstore.

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