40GBASE-LR4 QSFP+ Transceiver Links: CWDM vs. PSM
It’s known that the demand for higher data rates in telecommunications is skyrocketing, as the massive amount of storage is needed for high-performance consumption, such as video streaming. In response, the IEEE in 2010 ratified standards supporting 40 and 100 Gigabit Ethernet (GbE), known as IEEE 802.3ba to support the fast-changing and growing bandwidth demands in data centers.
In this standard, 40GBASE-SR4 port is specified to use a parallel multi-mode fiber (MMF) link to achieve 40G. It offers 4 independent transmit and receive channels, each capable of 10G operation for an aggregated data rate of 40G over 100 meters of OM3 MMF or 150 meters of OM4 MMF, like E40GQSFPSR. This 40GBASE-SR4 QSFP+ transceiver is used for 40G applications with links up to 150 meters over OM4. And for 40GBASE-LR4 port type, there are two kinds of links. One is coarse wavelength division multiplexing (CWDM) and the other is parallel single-mode fiber (PSM). How much do you know about them? Do you understand the differences between them? Follow this article and take a chance to know their working principles respectively.
40GBASE-LR4 CWDM QSFP+ Transceiver
Compliant to 40GBASE-LR4 of the IEEE P802.3ba standard, this 40GBASE-LR4 CWDM QSFP+ transceiver takes a duplex LC connector as its optical interface with the maximum transmission distance up to 10km. To minimize the optical dispersion in the long-haul system, single-mode fiber (SMF) has to be used.
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 4 individual 10G channels with different wavelengths. The central wavelengths of the 4 CWDM channels are 1271, 1291, 1311 and 1331 nm as members of the CWDM wavelength grid defined in ITU-T G694.2. Each wavelength channel is collected by a discrete photo diode and output as electric data after being amplified by a transimpedance amplifier (TIA).
40GBASE-LR4 PSM QSFP+ Transceiver
Different from CWDM QSFP+ transceiver which uses a LC connector, PSM QSFP+ is a parallel single-mode optical transceiver with an MTP/MPO fiber ribbon connector. Similarly, PSM QSFP+ also offers 4 independent transmit and receive channels, each capable of 10G operation for an aggregate data rate of 40G on 10km of single-mode fiber.
In terms of a PSM QSFP+, the transmitter module accepts electrical input signals, and the receiver module converts parallel optical input signals via a photo detector array into parallel electrical output signals. Both the input signals and output signals are compatible with common mode logic (CML) levels.
CWDM QSFP+ vs. PSM QSFP+
As far as the transceiver module structure is concerned, PSM seems more cost-effective, since it uses a single uncooled CW laser which splits its output power into four integrated silicon modulators. Besides, its array-fiber coupling to an MTP connector is relatively simple. But when allowing for the infrastructure viewpoint, PSM would be more expensive when the link distance is long, mainly because PSM uses 8 optical single-mode fibers while CWDM uses only 2 optical single-mode fibers.
In addition, the key point 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. What’s more, cleaning MTP connectors is not a straightforward task. Therefore, CWDM is a more profitable and popular 40G QSFP link.
Conclusion
For 40GBASE-LR4 QSFP+ transceivers, both CWDM QSFP+ and PSM QSFP+ support the maximum transmission distance of 10km. The former establishes 40G links over 2 optical SMFs with a duplex LC connector, and the latter achieves 40G links via 8 optical SMFs with a MTP/MPO fiber ribbon connector. Therefore, 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. Fiberstore provides 40GBASE-LR4 CWDM QSFP+ transceivers, which are fully compatible with major brands, such as Juniper JNP-QSFP-40G-LR4. Additionally, brands-compatible 40GBASE-SR4 QSFP+ transceivers are also offered, like Mellanox MC2210411-SR4. For more information or quotation, please visit Fiberstore.
