LC Interface or MTP/MPO Interface for 40G QSFP+ Module

petak , 16.12.2016.

As we know, duplex LC interfaces are widely used in 10G SFP modules, for instance, SFP-10G-LR-S module and SFP-10G-SR module, which occupy the majority of 10G module market. However, for 40G QSFP+ modules, both LC interface and MTP/MPO interface play important roles in meeting the high speed transmission, which are available on 40G QSFP module market. What are the differences between these two interface types? Which one should be chosen when deploying 40G Ethernet network? Do they function similarly in the same application? Let’s talk about this topics and find one kind of 40G QSFP+ module with the most suitable interface type so as to make a smooth 40G connection for our network.

Differences Between LC Interface and MTP/MPO Interface
From the following figure, we can see the examples of QSFP+ modules with two different interfaces. The left one is the module with with LC interface, while the right one is with MTP/MPO interface. The differences between these two kinds of modules vary from the fiber types they work with to the working principles, which will be analyzed in the following text.

Different Fiber Types
As for the QSFP+ module designed with LC interface, it usually works with single mode fiber (SMF). Since SMF is able to support 40G network for a very long distance, the LC interface module is widely applied in long transmission. However, the QSFP+ module with MTP/MPO interface design is strongly recommended in the short distance application where the 40G signals are transmitted through multimode fiber (MMF). What should be paid attention to are some special QSFP+ modules, like QSFP-40G-PLRL4 and QSFP-40G-PLR4 module, which don’t fit the rule. Although the two kinds of modules mentioned above are designed with MTP/MPO interfaces, they are also capable of supporting 40G long distance transmission through SMF.

Different Working Principles
As shown in the following figure, QSFP+ module with LC interface has a very complicated working principle for transmitting 40G signals. At the beginning of the transmission, four 10G serial data signals via four channels are transmitted to laser drivers in the transmitting side. Then they are directly controlled by the laser drivers as four modulated lasers (DML) with different wavelengths. After that, the output of four DMLs are optically multiplexed as a total 40G optical signal and transmitted through the LC connector and the SMF. While in its receiving side, the 40G optical signal is demultiplexed into four individual 10G signals with different wavelengths, then collected by the discrete photo diode, and finally amplified by the TIA and outputted as electric data.



As for QSFP+ module with MTP/MPO interface, its working principle is much easier than the previous one as it doesn’t require CWDM technology. In its working process, the transmitter firstly converts four 10G parallel electrical input signals into parallel optical signals by using the laser array. Then the four 10G parallel optical signals are directly transmitted through the MMF ribbon in a parallel mode. Finally, these parallel optical signals are transmitted through the photo detector array and converted into parallel electrical output signals in the receiving side. To better understand the working principle, here offers the figure of its detailed working process.



Besides, the mentioned above QSFP+ module with MTP/MPO interface, like QSFP-40G-PLRL4 and QSFP-40G-PLR4, also works in a parallel mode that features four independent transmitting and receiving channels to finish 10G operations, achieving a whole 40G connection. The only difference of the working principle is that the signals are transmitted and received through eight SMF ribbons, hence this kind of module can also achieve a long distance transmission.

Conclusion
From this article, we can easily find that QSFP+ module with LC interface is more commonly used for long distance application, while QSFP+ module with MTP/MPO interface is a cost effective solution for short distance application. Except that, there is still a great difference for applications between the two kinds of modules due to their different working principle. As QSFP+ module with LC interface uses CWDM technology to transmit signals with different wavelengths through a pair of single mode fibers, the 40G signals can’t be separated into four 10G signals and transmitted to 10G devices. However, QSFP+ module with MTP/MPO interface can be used in the 40G to 10G application by using an external 12-fiber parallel to 2-fiber duplex breakout cable.