What Is SWDM4?

Actually, SWDM, whose whole full name is short wavelength division multiplex, is a new multi-vendor technology that promises to provide the lowest total cost solution for enterprise data centers upgrading to 40G and 100G Ethernet with the existing 10G duplex OM3/OM4 MMF infrastructure. What’s more, it can cost-effectively increase bandwidth density for new data center builds and extend the reach when used with OM5 wideband multimode fiber (WBMMF) as well. By the way, OM5 fiber also future-proofs the infrastructure for possible future 200G, 400G and 800G interfaces.

To upgrade data centers to 40G/100G Ethernet without changing the existing duplex MMF infrastructure being used for 10G Ethernet, pluggable optical transceivers with SWDM technology matters a lot. This approach consists of multiple vertical-cavity surface-emitting lasers (VCSELs) operating at different wavelengths in the 850nm window (where MMF is optimized). The four-wavelength implementation of SWDM is called SWDM4, and these four wavelengths (850, 880, 910 and 940 nm) are multiplexed/demultiplexed inside a transceiver module into a pair of MMFs (one fiber in each direction, i.e., a standard duplex interface). Each of the four wavelengths operates at either 10G or 25G, enabling the transmission of 40G (4 x 10G) or 100G (4 x 25G) Ethernet over existing duplex MMF, using standard LC connectors.

SWDM4 transceivers are a smart choice for data center and enterprise networks faced with potential cable upgrades.

In the data center application environment, more than 90% of fiber links in small and medium-sized data centers are less than 100 meters in length, more than 70% of fiber links in large data centers are less than 100 meters in length, and more than 80% of them are less than 125 meters in length, so multimode fiber can meet the needs of most links. And that’s why in 100G modules, the largest demand is for 100G multimode modules. The QSFP28 100G multimode module has three mainstream solutions: 100GBASE-SRBD, 100GBASE-SR4, and 100GBASE-SWDM4.

100GBASE-SR4 is the most widely used among the three 100G multimode modules. 100G QSFP28 SR4 is a hot-pluggable, full-duplex optical module with a center wavelength of 850nm, compliant with IEEE 802.3ba standard, and specifically designed for short-range transmission in 100G Ethernet. The 100G QSFP28 SR4 optical module can provide four independent transmit and receive channels, each operating at up to 25Gbps. The 100G QSFP28 SR4 optical module adopts MTP interface (8 cores) and has a transmission distance of 70m when used with OM3 multimode fiber and 100m when used with OM4 multimode fiber. its operation principle is shown as in the following picture.

Unlike 100G SR4 which requires eight fibers for transmission, the 100G BiDi (bidirectional) module uses WDM technology to enable each LC port to simultaneously transmit and receive optical signals of different wavelengths from a single multimode fiber, and the two LC interfaces together can transmit and receive 100G signals; the maximum transmission distance is 70 meters when used with OM3 fiber patch cable. When used with OM4 fiber patch cable, the transmission distance can reach up to 100 meters, and with OM5 fiber patch cable, the transmission distance can reach up to 150 meters. Its working principle is shown as in the following picture.

As can be seen, although all three modules need to be used with multimode fiber, the technical principles of the modules are different.

All three modules are compatible with mainstream 100G QSFP28 ports, and while there are major differences in technical specifications, there are minimal differences in applications: the 100GBASE-SR4 needs to be used with MPO 8/12 core multimode patch cables, while the 100GBASE-SRBD and 100GBASE-SWDM4 both need to be used with duplex LC multimode fiber. From the perspective of fiber resources and cabling cost, 100GBASE-SRBD and 100GBASE-SWDM4 cabling requires only 1/4 of the fiber resources of 100GBASE-SR4, which can significantly save the capital expenditure of fiber infrastructure. However, the cost of 100GBASE-SRBD and 100GBASE-SWDM4 modules is much larger than 100GBASE-SR4, so from the overall cost of optical module + fiber, using 100GBASE-SRBD or 100GBASE-SWDM4, the cost is not necessarily lower than using 100GBASE-SR4.