It is well known that 40GBASE-SR4 QSFP+ transceiver uses a parallel multimode fiber (MMF) link to achieve 40G. It offers four independent transmit and receive channels and each channel is capable of 10G operation for an aggregate data rate of 40G with distances up to 100 meters on OM3 MMF or 150 meters on OM4 MMF. However, for 40GBASE-LR4 QSFP+ transceivers, two kinds of links are available. One is parallel single-mode fiber (PSM), and the other is coarse wavelength division multiplexing (CWDM). What is the difference between the two? Keep reading this article and you will find the answer.
PSM QSFP+ transceiver is a parallel single-mode optical transceiver with an MTP/MPO fiber ribbon connector. Moreover, it offers four independent transmit and receive channels and each channel is capable of 10G operation for an aggregate data rate of 40G on 10 km of single-mode fiber. The guide pins inside the receptacle could ensure proper alignment. Usually, the cable cannot be twisted for proper channel to channel alignment. For a PSM QSFP+ transceiver, the transmitter module accepts electrical input signals compatible with common mode logic (CML) levels. All input data signals are differential and internally terminated. The receiver module converts parallel optical input signals via a photo detector array into parallel electrical output signals. The receiver module outputs electrical signals are also voltage compatible with CML levels. All data signals are differential and support a data rates up to 10.3G per channel.
The 40GBASE-LR4 CWDM QSFP+ transceiver like QSFP-40GE-LR4 is compliant to 40GBASE-LR4 of the IEEE P802.3ba standard. It has a duplex LC connector for the optical interface and its maximum transmission distance is 10 kilometers. Single-mode fiber (SMF) has to be used to minimize the optical dispersion in the long-haul system. This transceiver converts 4 inputs channels of 10G electrical data to 4 CWDM optical signals by a driven 4-wavelength distributed feedback (DFB) laser array, and then multiplexes them into a single channel for 40G optical transmission, propagating out of the transmitter module from the SMF. Reversely, the receiver module accepts the 40G CWDM optical signals input, and demultiplexes it into four individual 10G channels with different wavelengths. The central wavelengths of the four CWDM channels are 1271, 1291, 1311 and 1331 nm as members of the CWDM wavelength grid defined in ITU-T G694.2. Moreover, each wavelength channel is collected by a discrete photo diode and output as electric data after being amplified by a transimpedance amplifier (TIA).
From the perspective of an optical transceiver module structure, PSM seems more cost effective as it uses a single uncooled CW laser splitting its output power into four integrated silicon modulators. Moreover, its array-fiber coupling to an MTP connector is relatively simple. However, from the perspective of an infrastructure, PSM would be more expensive when the link distance is long, because it uses 8 optical single-mode fibers while CWDM only uses 2 optical single-mode fibers. The following table illustrates the main differences between CWDM and PSM.
In addition, the caveat is that the entire optical fiber infrastructure within a data center, including patch panels, has to be changed to accommodate MTP connectors and ribbon cables, which are more expensive than conventional LC connectors and regular SMF cables. Moreover, it is not a straightforward tack to clean MTP connectors. So CWDM is a more profitable and popular 40G QSFP link.
For 40GBASE-LR4 QSFP+ transceivers, either CWDM link or PSM link, the maximum transmission distance is both 10 km. 40GBASE-LR4 PSM QSFP+ transceiver uses an MTP/MPO fiber ribbon connector via 8 optical single-mode fibers to reach 40G, while 40GBASE-LR4 CWDM QSFP+ transceiver uses a duplex LC connector via 2 optical single-mode fibers to achieve 40G. Thus, CWDM QSFP+ enables data center operators to upgrade to 40G connectivity without making any changes to the previous 10G fiber cable plant, which is more cost-effective and widely used by people.