Comparing Three 100G Multimode Transceivers for Data Centers: 100GBASE-SRBD, 100GBASE-SR4, 100GBASE-SWDM4

With the rise of cloud computing and 5G networks, data centers are moving toward higher data rate, and the demand for 100G optical transceivers is increasing. Today, 100G optical transceivers account for a large proportion of network construction costs. In application environments in data centers, more than 90% of fiber optical links in small and medium-sized data centers are less than 100 meters in length, and more than 70% of fiber optical links in large data centers are less than 100 meters in length, more than 80% of the links are less than 125 meters, multimode fibers can meet the needs of most links, so multimode transceivers gain the most needs in 100G transceivers. The QSFP28 100G multimode module has three main solutions: 100GBASE-SRBD, 100GBASE-SR4,100GBASE-SWDM4.

01 Product Specifications

Among the three 100G multimode transceivers, 100GBASE-SR4 is the most widely used. 100G QSFP28 SR4 is a hot-swappable full-duplex optical module with a central wavelength of 850nm. It conforms to the IEEE 802.3ba standard and is designed for short-range transmission over 100G Ethernet. The 100G QSFP28 SR4 optical transceiver can provide four independent transmit and receive channels, each with a speed of 25Gbps. The 100G QSFP28 SR4 optical transceiver with MPO/MTP interface (8 cores) has a transmission distance of 70m when used with OM3 multimode fiber and 100m when used with OM4 multimode fiber. The working principle is as follows:

100G QSFP28 SR4 optical transceiver working principle schematic

Unlike 100G SR4, which requires eight optical fibers to transmit, the 100G BiDi (bidirectional) module uses wavelength division multiplexing technology to allow each LC port to simultaneously transmit and receive optical signals of different wavelengths from a single multimode fiber, and the two LC interfaces combined to complete 100G signal transmit and receive. When used with OM3 fiber patch cable it can transmit up to 70 meters,and with OM4 up to 100 meters, with OM5 up to 150 meters. Here's how it works:

100G QSFP28 SRBD optical transceiver working principle schematic

100GBASE-SWDM4 is similar to 100G BiDi, they both have duplex LC interfaces. SWDM (Short Wavelength Division Multiplexing) refers to short wavelength division multiplexing technology, similar to the CWDM4 and LWDM4 single-mode solutions, which transmits 4 wavelength bands of optical signals on a single multimode fiber by combining and dividing the waves through MUX/DEMUX. The 4 optical windows are 850nm, 880nm, 910nm, 940nm; the farthest transmission distance can be up to 70 meters when used with OM3 fiber patch cable, and up to 100 meters when used with OM4 fiber patch cable. Its working principle is as follows:

100G QSFP28 SWDM4 optical transceiver working principle schematic

On the face of it, although all three modules need to be used with multimode fiber, the technical principles of the modules are different. 100GBASE-SR4 uses four channels of parallel transmission, so at least eight fibers are needed to complete the transmission (four transmit and four receive). 100GBASE-SRBD uses a single fiber to transmit and receive at a rate of 50G, and the two interfaces transmit at a rate of 100G. 100GBASE-SWDM4 uses wavelength division multiplexing technology to multiplex 4 bands of signals into one fiber for transmission, requiring two fibers to complete receiving and transmitting.

02 Product Application Selection

All three transceivers are compatible with the mainstream 100G QSFP28 ports, and while the technical specifications differ greatly, the application differences are minimal: the 100GBASE-SR4 needs to be used with MPO/MTP 8 or 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 transceivers is much larger than 100GBASE-SR4, so from the overall cost of optical module and fiber, the cost of using 100GBASE-SRBD or 100GBASE-SWDM4 is not necessarily lower than using 100GBASE-SR4.

LC Duplex Multimode Cabling for Data Centers

Why would a customer choose 100GBASE-SRBD/100GBASE-SWDM4 when the overall application cost is higher than 100GBASE-SR4? After communicating with customers, NADDOD technicians found that the choice of 100GBASE-SRBD or 100GBASE-SWDM4 was due to network requirements of upgrading from 10G or 25G to 100G, while 10G or 25G network cabling was based on LC duplex multimode fiber, if using QSFP28 100G SR4 transceiver, it needs to make a big adjustment in the original wiring basis, the cost is very high. So they will choose duplex LC-interfaced 100GBASE-SRBD or 100GBASE-SWDM4.

Both 100GBASE-SRBD and 100GBASE-SWDM4 are duplex LC interfaces, but they are very different technically. Till the publication of this post, it is understood that only one manufacturer in the world produces 100GBASE-SWDM4, and the product ecology and supply are very unstable. 100GBASE-SRBD technology threshold is high, but there are many large factories investing in its production. 100GBASE-SRBD has a greater product ecological development. NADDOD recommends using the 100GBASE-SRBD module for data center 100G networks with LC duplex multimode cabling.