40G QSFP+ MTP vs 40G QSFP+ LC Transceivers

Since 40 Gigabit Ethernet has been deployed in most data centers, a variety of network devices designed for 40GbE links are available on the market. 40G transceivers is bound to be an indispensable component. Generally speaking, there are two interfaces adopted by 40G QSFP+ transceivers—MTP/MPO and LC. Then QSFP+ MTP vs QSFP+ LC transceivers: what are their differences?

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).