The QSFP+ transceiver is the dominant transceiver form factor used for 40GbE (Gigabit Ethernet) applications. In 2010 the IEEE standard 802.3ba released several 40GbE standard and along with the general specification, defined a number of fiber optic interfaces. These standard interfaces including 40GBASE-SR4 (QSFP SR4) and 40GBASE-LR4 (QSFP LR4) attempted to satisfy a number of different objectives including support for MMF (multimode fiber) and SMF (single-mode fiber) compatibility.
The basic versions of QSFP are SR4 and LR4 that are respectively designed for MMF and SMF 40GbE applications. SR is short for short reach and LR represents Long Reach. 4 means 4 Tx and Rx. The interfaces designed by 40GBASE-SR4 and 40GBASE-LR4 are not the same. The most obvious difference is that 40GBASE-SR4 is with MPO/MTP port while the 40GBASE-LR4 is with the Duplex LC port. The following table summarizes the differences between QSFP+ transceiver optical interfaces and electrical interfaces.
The cost of 40GBASE-SR4 QSFP+ is more cheaper than the 40GBASE-LR4 QSFP+. Additionally, the fiber cost of the cabling with them also have a big difference. Because smf cabling is more expensive than the mmf cabling.
With the wide deployment of 40 GbE, the 40GBASE-SR4 and 40GBASE-LR4 transceivers are more and more widely used. 40GBASE-LR4 QSFP+ transceivers (such as QSFP-40G-LR4) are most commonly deployed between data-center or IXP (Internet exchange point) sites with single-mode fiber. 40G SR4 QSFP+ transceivers (e.g. Cisco QSFP-40G-SR4) are used in data centers to interconnect two Ethernet switches with 8 fiber parallel OM3/OM4 multimode fiber cabling.