Data transmission with higher density and bandwidth has become the trend under present networking environment. With 40 Gigabit Ethernet commonly deployed in most data centers, various network devices designed for 40 Gigabit Ethernet (GbE) link are available on the market. Among them, 40G QSFP+ transceivers play an important role in driving the bandwidth to a mounting point. There are mainly two interfaces adopted by 40G QSFP+ transceivers—MTP/MPO and LC. What is the difference between these two interface types? This article will have an analysis of the 40G QSFP+ transceivers with LC interface and 40G QSFP+ transceivers with MTP/MPO interface.
From the figure below, we can easily understand the working principle of 40G QSFP+ transceivers with LC interface. In the transmit side, 4 channels of 10G serial data streams at different wavelengths are passed to laser drivers. The laser drivers control directly modulated lasers (DML) with wavelengths. Then the output of the four DMLs are optically multiplexed to a SMF through an industry-standard LC connector, combining as 40G optical signal. In the receive side, the 40G optical signal is demultiplexed into four individual 10G optical data streams at different wavelength. And each wavelength light is collected by a discrete photo diode and amplified by a TIA, and then outputted as electric data. In this process, a 4-wavelength CWDM multiplexer and demultiplexer is used over a pair of single-mode fibers. For transmission distance of this type of 40G QSFP+ transceiver, take 40G LR4 QSFP+ transceiver as an example, it can support an optical link length up to 10 kilometers over the single mode fiber.
We can easily understand the working principle of 40G QSFP+ transceiver with MTP/MPO interface from the figure below. In 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 parallelly through the multimode fiber ribbon terminated with MPO/MTP fiber optic connector. In the receive side, the receiver converts parallel optical input signals via a photo detector array into parallel electrical output signals. Generally, 40G QSFP+ transceivers with MTP/MPO interface are utilized for short distance transmission over multimode fiber (MMF), like 40G SR4 QSFP+ transceiver, it can support a link length up to 100 meters on OM3 cable and 150 meters on OM4 cable.
Note: there are also some 40G QSFP+ transceivers with MTP/MPO interface supporting long distance transmission over SMF. For example, 40G LR4 PSM QSFP+ transceiver, a parallel single-mode optical transceiver with an MTP/MPO fiber ribbon connector, it offers four independent transmit and receive channels, each capable of 10G operation for an aggregate data rate of 40G over SMF. That is to say, eight single-mode fibers are used to achieve parallel transmission, with transmission distance up to 10 kilometers. From the perspective of cost, this kind of 40G QSFP+ transceivers cost more than 40G QSFP+ transceivers with LC interface. Besides, in the data center fiber infrastructure, MTP patch panel has to be used to accommodate MTP cables, which would cost more than LC connectors and regular SMF cables.
For the 40G QSFP+ transceivers with LC interface, they cannot be split into 4x10G as they use 4 wavelengths on a pair of single-mode fibers and do not lend themselves to “splitting” into 4 pairs without substantial complexity to split out the wavelengths. For the 40G QSFP+ transceivers with MTP/MPO interface, they can be used in 4x10G connectivity via an external 12-fiber parallel to 2-fiber duplex breakout cable, which connects the 40G module to four 10G optical interfaces.
Generally speaking, the 40G QSFP+ transceivers with LC interface are used for long distance transmission over single-mode fiber (SMF), and 40G QSFP+ transceivers with MTP/MPO interface are utilized for short distance transmission over multimode fiber (MMF). However, for some 40G QSFP+ transceivers with MTP/MPO interface, such as 40GBASE-LR4 PSM QSFP+ transceiver, it can support long distance transmission over SMF.