As the demand for faster data transmission speed ever-increases, 40G network has gradually become ubiquitous in telecommunication field. 40G optical transceiver, especially, 40G QSFP+ transceiver, as an essential component to achieve 40G data rate, has been widely adopted in data communications applications. 40G QSFP+ transceiver uses four independent 10 gigabit per second data lane in each direction to provide an integrate 40Gbps bandwidth. There are various types of 40G QSFP+ transceivers available on the market, such as 40GBase-SR4, 40GBase-CSR4, 40GBase-LR4, etc, and each of them is designed to support long or short distance transmission. In this post, I am going to introduce some effective 40G short-reach transmission solutions, including 40GBase-SR4, QSFP-40G-CSR4, QSFP-40G-CR4.
QSFP-40G-SR4 is used by many vendors to identify 40G QSFP+ modules that support by 40GBase-SR4 standards. 40GBase-SR4 QSFP+ is the most popular 40G fiber optic transceiver for transmission in short distances. QSFP-40G-SR4 enables high-bandwidth 40G optical links over 12-fiber parallel fiber terminated with MPO/MTP multi-fiber female connectors, It can support link lengths of 100 meters and 150 meters over OM3 and OM4 multimode fibers respectively. Besides, 40GBase-SR4 QSFP+ transceiver can also be used to connect with four 10GBase-SR optical interfaces using an 8-fiber MTP to 4 duplex LC cable. Here is a Cisco compatible QSFP-40G-SR4 from Fiberstore.
Generally, QSFP-40G-CSR4 (shown in the following image) can be regarded as the upgraded version of QSFP-40G-SR4. Similar to the working principle of 40GBase-SR4 QSFP+ transceiver, 40GBase-CSR4 QSFP+ is used for native 40G optical link over 12-fiber parallel cables with MPO/MTP connectors or in a 4x10G mode with parallel to duplex fiber breakout cables for connectivity to four 10GBase-SR interfaces. But different from QSFP-40G-SR4 transceiver, QSFP-40G-CSR4 extends the 40G transmission distance to 300 meters over OM3 and 400 meters over OM4.
QSFP-40G-CR4 or 40GBase-CR4 QSFP+ is a port type for twinax copper cable, which uses four lanes of twinaxial cable delivering serialized data at a rate of 10.3125 Gbit/s per lane. This kind of QSFP+ module is permanently terminated at both ends of a copper cable, known as 40G QSFP+ cable or 40G QSFP+ direct attach cable (DAC). Much different from the above mentioned two 40G modules, it is not a real optics, since it is without expensive lasers and electronic components. Accordingly, the price of this module is much lower than 40GBase-SR4 QSFP+ and 40GBase-CSR4 QSFP+. But limited by the property of copper cable, QSFP-40G-CR4 can only reach a transmission distance of 7 meters. Here is a 1m 40G QSFP+ direct attach copper cable.
Generally, there are two types of transmissions in 40G network: 40G to 40G transmission and 40G to 10G transmission. All of these three modules can be used in the two 40G transmission types.
QSFP-40G-SR4 and QSFP-40G-CSR4 use the same connectivity methods for 40G to 40G and 40G to 10G transmissions. For 40G to 40G transmission, a length of MPO/MTP trunk cable will be required to connect 40GBase-SR4 QSFP+ and 40GBase-CSR4 QSFP+ to form the whole link. For 40G to 10G transmission, a length of 8-fiber MTP-4 duplex LC harness cable will be used. The following two images show the these two cables used to connect QSFP-40G-SR4 and QSFP-40G-CSR4 to achieve 40G to 40G and 40G to 10G transmissions.
As we have mentioned above, 40GBase-SR4 QSFP+ is permanently terminated at a length of a copper cable, so no additional fiber patch cables will be needed in 40G transmission. For 40G-40G transmission, we can use QSFP+ to QSFP+ direct attach cable to achieve 40G data rate between devices, and for 40G-10G transmission, we can use QSFP+ to 4xSFP+ direct attach cable to achieve connection between an available QSFP+ port on their 40Gbps rated switch and feed up to four upstream 10G SFP+ enabled switches.
40G Ethernet will definitely dominate the market sooner or later, since it can meet the increasing need for high bandwidth network. We have mentioned some 40G QSFP+ solutions and their applications in specific cases for 40G short distance transmission.