40G QSFP+ – A Cost-effective Transceiver Solution

Data transmission with higher density and bandwidth has become the trend under today’s networking environment. And for better network performance, the existing bandwidth has been generated to 40Gbps. Among various network devices designed for 40 Gigabit Ethernet (GbE) links, 40G QSFP+ transceivers play an important role in driving the bandwidth to a mounting point. This passage is going to focus on this cost-effective transceiver. First, let’s move on to the overview of 40G QSFP+.

Overview of 40G QSFP+ Transceiver

The 40G QSFP+ (Quad Small Form-Factor Pluggable Plus) transceiver is a compact, hot-pluggable, parallel fiber optical transceiver with four independent optical transmit and four receive channels. Each channel is able to transfer data at 10Gbps. Thus, a QSFP+ transceiver with Four high-speed channels can support data rates up to 40Gbps and it supports Ethernet, Fibre Channel, InfiniBand and SONET/SDH standards. In addition, 40G QSFP+ is primarily used in switches, routers, and data center equipment where it provides higher density than SFP+ transceiver.

Three types of 40G QSFP+ Transceiver

With the development of the SFF-8436 Multi Source Agreement, many vendors are now offering a variety of IEEE- and MSA-compliant QSFP+ devices for fiber networks. And there are three basic 40G QSFP+ transceivers for this standard: 40G LR4 QSFP+ transceiver, 40G SR4 QSFP+ transceiver and 40G LR4 parallel single mode (PSM) transceiver.

40G LR4 QSFP+ Transceiver

The 40G LR4 QSFP+ Transceiver converts 4 inputs channels of 10Gbps electrical data to 4 CWDM optical signals, and multiplexes them into a single channel for 40Gbps optical transmission. Together with duplex LC connectors, 40G LR4 QSFP+ transceiver can support an optical link length up to 10 kilometers over the single mode fiber.

We can easily understand the working principle of 40G LR4 QSFP+ transceiver from the figure below. In the transmit side, four 10Gbps serial data streams at different wavelengths are passed to laser drivers. Then four data streams are optically multiplexed to a single mode fiber through LC connector. In the receive side, four 10Gbps optical data streams are de-multiplexed into four individual data streams by the optical de-multiplexer. And each data stream is collected by a PIN photodiode /TIA array and passed to an output driver.

Working Principle of 40G LR4 QSFP+ Transceiver

40G SR4 QSFP+ Transceiver

The 40G SR4 QSFP+ Transceiver provides a high-bandwidth 40G optical connection over fiber ribbon terminated with MPO/MTP connectors. Unlike the 40G LR4 QSFP+ transceiver, this kind of transceiveris used together with multi-mode fiber, supporting with a link length up to 100 meters on OM3 cable and 150 meters on OM4 cable.

We can easily understand the working principle of 40G SR4 QSFP+ transceiver from the figure below. The transmitter converts parallel electrical input signals into parallel optical signals through the use of a laser array. Then the parallel optical signals are transmitted parallelly through the multi-mode fiber ribbon. Reversely, the receiver converts parallel optical input signals via a photo detector array into parallel electrical output signals.

Working Principle of 40G SR4 QSFP+ Transceiver

40G LR4 Parallel Single Mode (PSM) Transceiver

The 40G LR4 PSM transceiver is designed with QSFP+ form factor, optical/electrical connection and digital diagnostic interface according to the QSFP+ MSA. As a highly integrated 4-channel optical module, this kind of transceiver can provide increased port density and total system cost savings. 40G LR4 PSM transceiver supports up to 10 kilometers over single mode fiber through MPO/MTP fiber ribbon connectors.

From the figure below, we can easily understand the working principle of 40G LR4 PSM transceiver which is nearly the same as that of 40G SR4 QSFP+ transceiver. The transmitter converts parallel electrical input signals into parallel optical signals and the receiver converts parallel optical input signals via a photo detector array into parallel electrical output signals. The difference is that the cable used in this link is single mode ribbon fiber cable. That’s to say, the parallel optical signals are transmitted parallelly through 8 single mode fibers.

When reading this, you may have found that both 40G LR4 QSFP+ transceiver and 40G QSFP+ PSM transceiver can support the maximum transmission distance of 10km. The obvious difference between these two transceivers is that the former establishes 40G links over 2 optical SMFs with a duplex LC connector, while the latter achieves 40G links via 8 optical SMFs with a MTP/MPO fiber ribbon connector. And we can easily find that 40G LR4 PSM transceiver costs more than 40G LR4 transceiver which uses only 2 single mode fibers to support an optical link. 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.

Working Principle of 40G LR4 PSM Transceiver


From the introduction above, 40G SR4 QSFP+ transceiver is suitable for short-distance transmissions. So it is often used in data centers to interconnect two Ethernet switches with 12 lane ribbon OM3/OM4 cables. While 40G LR4 QSFP+ transceiver and 40G LR4 PSM transceiver are often used in long-distance transmission applications. I hope this passage can help you know more about 40G QSFP+ and choose a suitable optical transceiver module according to your need.

Introduction to Fiber Optic Transceivers

Fiber optic transceiver includes both a transmitter and a receiver in a single module. The transmitter and receiver are arranged in parallel and they have their own circuitry so that they can handle transmissions in both directions independently. Fiber optical transceiver plays the role of photoelectric conversion, that is, the transmitter converts electrical signals into optical signals, and optical signals are converted into electric signals by the receiver. Fiber optic transceiver has different types which can be classified by the following aspects.

Optical Fiber Type

The single-mode fiber transceiver is used with single-mode fiber cable and adopts LD or LED as light source. It can transmit data signals with high-bandwidth at long transmission distance. The multimode fiber transceiver is used with multimode fiber cable and adopts LED or laser diode as light source. Though the transmission distance is shorter than that of single-mode fiber transceiver, the multimode fiber transceiver is cheaper and its power consumption is lower.

Optical Fiber Count

Simplex fiber transceiver is used with simplex fiber optic patch cable and receives the data sent in a single fiber transmission. Duplex fiber transceiver is used with duplex fiber optic patch cable and receives data transmitted on a dual fiber transmission. Single-fiber optic equipment can save half of that achieved to receive and transmit data over a single optical fiber, optical fiber strain on the resources in place very applicable. These products use the wavelength division multiplexing techniques, mostly using the wavelength 1310nm and 1550nm.

Transmission Rate

Transmission rate refers to the number of gigabits transmitted per second, per unit of Mbps or Gbps. Optical modules cover the following main rate: low rates, Fast, Gigabit, 1.25G, 2.5G, 4.25G, 4.9G, 6G, 8G, 10G and 40G.

GBIC(Giga Bit Interface Converter)

GBIC is an input/output transceiver. Its one end is to plug into a Gigabit Ethernet port such as on the switches, and the other end is to connect the fiber optic patch cords and link the fiber optic networks. Therefore, GBIC modules function is to transform the signals between the Ethernet network and fiber optic network.



SFP is a kind of compact, hot-pluggable transceiver used for both telecommunication and data communications applications and it can be simply interpreted as an upgraded version of GBIC. Compared with GBIC, the size of SFP is reduced by half and SFP can deploy more than double the number of ports on the same panel.



XENPAK is the standard specification for optical transceivers in 10 Gigabit Ethernet. It is a highly integrated, serial optical transponder module for high-speed, 10Gbit/s data transmission applications. And it is designed to transmit and receive optical data of long transmission distance.



X2 is mainly used for switch or router with Ethernet X2 port and network connection port. It is evolved from XENPAK standard and its internal function modules are basically same with XENPAK’s. However, the improved X2 that is only half the size of XENPAK can be placed directly on the circuit board, so it is suitable for high-density rack system and PCI network card application.



XFP are standardized 10GBASE hot-swappable and protocol-independent modules. It fully complies with the following standards: 10G Fiber Channel, 10G Ethernet, SONET/OC-192 and SDH/STM-64. XFP is mainly used for data communication and fiber optic transmission of telecommunication transmission network.



SFP+ is an enhanced version of the SFP that supports data rates up to 10 Gbit/s. SFP+ supports 8 Gbit/s Fiber Channel, 10 Gigabit Ethernet and Optical Transport Network standard OTU2. And SFP+ leaves part of circuit on the motherboard instead of inside the module.



QSFP interfaces networking hardware to a fiber optic cable and supports data rates from 4×10 Gbit/s. And QSFP+ is an evolution of QSFP to support four 10 Gbit/sec channels carrying 10 Gigabit Ethernet, 10GFC Fiber Channel, or QDR InfiniBand. The 4 channels can also be combined into a single 40 Gigabit Ethernet link.


Fiber optic transceiver, essentially just completed the converted of data between different media, can realize the connection between two switches or computers in the 0-120km distance. It is mainly applied in data communication, computer video, wireless voice communication and so on. And different types of fiber optic transceivers can be chosen according to different occasions.