QSFP+ stands for Quad Small Form-Factor Pluggable Plus. A parallel and hot-swappable fiber optical module that provides high-density 40G connectivity options for high-performance computing networks and modern data centers. It has four independent receive and transmit optical channels that can terminate to another 40G QSFP+ transceiver, or can be distributed to four separate 10 Gigabit SFP+ transceivers. The QSFP+ modules , in a nutshell, connect the entire electrical circuitry of your system to an optical external network.

QSFP+ Interface: LC & MPOThere are a variety of 40G transceivers currently available on the market. However, 40G QSFP+ optics follow the QSFP MSA standard which dictates that 40G QSFP+ connectors can be either 12-fiber MPO or duplex LC. Which are the MPO and the LC.

What are the differences between these two interfaces?

To put it simply, 40G QSFP+ transceivers that have an LC interface are used in transmitting data over long distances via single mode fiber (SMF), while transceivers with an MPO interface (except for 40GBase-PLR4 and 40GBase-PLRL4) are deployed for data transmission over short distances via multimode fiber (MMF). In addition to that, MPO interfaced QSFP+ transceivers can be used in 4x10G connectivity using a breakout cable, compared to LC interfaced ones that can’t be divided into 4x10G since they use 4 wavelengths on one pair of SMF.

Let’s delve deeper and understand how 40G QSFP+ transceivers with LC interface work. From the transmitting side, 4 channels of 10G serial data streams that have different wavelengths are sent over to laser drivers. These same laser drivers control the DML or directly modulated lasers using wavelengths. Then the DML outputs are multiplexed optically to a singlemode fiber via a standard LC connector, combining as 40G. On the receiving side, the 40G signal is demultiplexed into four separate 10G streams at different wavelengths. Each single wavelength light is acquired by a diode and amplified by a TIA and then comes out as electric data.

The working principle of a 40G QSFP+ transceiver with an MPO interface can be explained by starting from the transmitting side, when it transmutes parallel electrical input signals into parallel optical signals via a laser array. The optical signals are then sent through the MPO-terminated MMF ribbon. In the other side, the receiver transforms parallel optical input signals into parallel electrical output signals using a photo detector array.

What Are 40G QSFP+ MTP and 40G QSFP+ LC Transceivers?

The connector type that attached to both ends of the patch cords is decided by the interface of the 40G transceivers. Normally speaking, 40G QSFP+ transceivers for short-transmission distance are equipped with MTP interfaces, and LC interfaces are employed for long-transmission distance. The specific modules and some exceptions will be meticulously introduced in the second part.

40G QSFP+ MTP modules are QSFP+ transceivers with MTP interfaces, which are collocated with MTP/MPO cables. MTP is an enhanced version of generic MPO (multi-fiber push-on), and they are called interchangeably. In most cases, we'll just use "MTP" for convenience. 40GbE QSFP+ MTP modules typically connect with 8 or 12 fibers MTP cables that are terminated with 12-fiber parallel (female) MTP connectors.

40G QSFP+ LC modules are QSFP+ transceivers with LC interfaces, which are armed with LC cables. 40GbE QSFP+ LC modules typically connect with 2-fiber duplex LC cables that are terminated with 2-fiber duplex LC connectors.

QSFP+ MTP vs QSFP+ LC Transceivers: What Are Their Differences?

Working Principle

40G QSFP+ MTP transceiver—On the transmit side, 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 simultaneously through the MTP parallel cable. On the receive side, the receiver converts parallel optical input signals via a photodetector array into parallel electrical output signals. It's worthwhile to mention that MTP interfaced 40G QSFP+ transceivers can be used in 4x10G connectivity via an external 12-fiber parallel to 2-fiber duplex breakout cable.

40G QSFP+ LC transceiver—On the transmit side, 4-channel 10G serial data streams are passed to laser drivers. The laser drivers control directly modulated lasers (DML) with wavelengths. Then the output of the four DMLs is optically multiplexed to a fiber cable through an LC connector, combining as a 40G optical signal. On the receive side, the 40G optical signal is demultiplexed into 4 individual 10G channels with different wavelengths. Each wavelength light is collected by a PIN and then outputted as electric data after amplified by a TIA (transimpedance amplifier). Unlike 40G QSFP+ transceivers with MTP interfaces, 40G QSFP+ transceivers with LC interfaces cannot be split into 4x10G as they use 4 wavelengths on one pair of SMFs.

Single Mode vs Multimode QSFP+

Single mode fiber (SMF) has much tighter tolerances for optics used. The core is smaller and the laser wavelength is narrower, which means that SMF has the capability for higher bandwidth and much longer distances in transmission. 40G QSFP+ transceivers that have LC interfaces are typically used in data transmission over long distances via SMFs, except that 40GBASE-BiDi transmits data with an LC interface over short distances (100 m@OM3; 150 m@OM4) via multimode fiber (MMF).

MMF uses a much bigger core and usually uses a longer wavelength of light. Because of this, the optics used in MMF have a higher capability to gather light from the laser. 40G QSFP+ transceivers with MTP interfaces are generally deployed for short transmission via MMFs, except for 40GBASE-PLR4 and 40GBASE-PLRL4, they support long-distance transmission over SMFs.

The table below summarizes the cable type, the connector type, and the max. transmission distances of nine common QSFP+ types (Note: 40GBASE-UNIV can operate with both MMF and SMF).

QSFP+ MTP vs QSFP+ LC transceivers: What Are Their Matched Patch Cables?

Advanced switches, matched patch cords and transceiver modules are all indispensable in the 40G transmission network. Learn to select proper patch cords for 40G transceivers is still of great significance. Generally speaking, the selection of patch cords is influenced by two main factors: transmission distance and connector type.

The aforementioned nine transceivers are available in short or long 40G transmission, such as 40GBASE-SR4 (for short reach) and 40GBASE-LR4 (for long reach). MMF is suggested for 40G QSFP+ short transmission distance, while SMF is advisable for 40G QSFP.

For 40G QSFP+ MTP transceivers, the MTP cables can be either MTP trunk cables or MTP harness cables. The MTP trunk cable is a cable with two MTP connectors at both ends serving as a permanent link connecting the MTP interfaced modules to each other. An MTP trunk cable together with the 40G QSFP+ transceiver modules is used for 40G-40G connection. The MTP harness cable, also known as fanout cable or breakout cable, has a single MTP connector on one end and 4 LC duplex connectors on the other end. An 8-fiber MTP to LC duplex breakout cable with one 40G QSFP+ transceiver and four 10G SFP+ transceivers can achieve 40G-4x10G connection.

Unlike the previous 40G QSFP+ MTP module, 40G QSFP+ LC transceivers are applicable to 40G-40G connection only. The matching between transceivers and patch cords are listed below: 40GBASE-BiDi transceiver can transmit full-duplex 40Gbps traffic with OM3 or OM4 multimode LC duplex patch cords, 40GBASE-LR4/LR4L/ER4 are often accompanied with single mode LC duplex patch cords, and LC-interfaced 40GBASE-UNIV supports both SMF and MMF LC patch cords.

See the table below for detailed information of nine common QSFP+ modules' patch cords selection:

Note: The polarity must be considered when selecting MTP cables for 40G QSFP+ MTP modules. There are three different MTP array patch cords defined in the TIA standard—Type A, Type B, and Type C used for the three different connectivity Methods A, B, and C respectively. For your reference, here offers another article that is informative and exhaustive about MPO polarity: Polarity and MTP Technology in 40/100G Transmission.

Conclusion

There is a wide variety range of network solutions on the market. QSFP+ 40G optical modules, QSFP+ 40G DAC, and 40G AOC are popularized with their advantages and provide a high-performance and cost-effective interconnection between servers, switches in data centers. 40G BASE-LR4 QSFP+ optical transceivers differentiate from 40GBASE-SR4 QSFP+ mainly in their interfaces. 40G QSFP+ DAC cable is well known for its strengths such as lower cost and power consumption and good heat dissipation. While 40G AOC cables enable longer transmission distance and adopt highly integrated array optical engine core device technology. May the analysis on these 40G QSFP+ network solutions is meaningful for your selection.