First realized 100GBASE-SR10

"100GBASE-SR10" (*1) is a standard called "100GBASE-SR10" that transmits 100m using 24-core ribbon fiber, and the form factor is CFP or CFP2.

(* 1) The standard standardized by IEEE802.3 or the standard under discussion for standardization is called "100GBASE".

The S in SR10 means short range. The 10 in SR10 indicates that the optical signal has 10 lanes. In other words, the optical signal consists of 10 GbE x 10 lanes.

100GBASE-SR4 for short distances with 4 lanes

Technology has advanced since then, and "100GBASE-SR4", which consists of 25GbE x 4 lanes, has been standardized. The 4 in SR4 indicates that the optical signal has 4 lanes. In other words, the optical signal consists of 25GbE x 4 lanes.

The number of light-emitting laser elements and receivers has been reduced from 10 (SR10) to 4 (SR4), resulting in significant miniaturization. The form factor is CFP4 or QSFP28, and like the SR10, it transmits 100m. Currently, this QSFP28 100GBASE-SR4 (Figure 1) is often used for short-distance connections within 100m.

Figure 1. 100G SR4 block diagram

100G LR4 for long range and 100G ER4 for ultra long range

Subsequently, ``100GBASE-LR4'', which transmits 10km with four optical signal lanes, and ``100GBASE-ER4'', which transmits 40km, were standardized for long-distance connections. L in R4 stands for long distance (Long), E in E R4 stands for extended long distance (​Extended), and transmission is performed using two-core optical fiber.

I mentioned that ribbon fiber is used for SR4, which transmits 100m, but for LR4 and ER4, which transmit long distances such as 10km or 40km, using ribbon fiber is not practical due to operational and cost considerations.

Therefore, we use a technology called WDM (Wavelength Division Multiplexing) to multiplex four lanes of optical signals and send them over a single optical fiber (Figure 2).

This technology uses an optical multiplexer (MUX) to multiplex four different signals onto a single optical fiber by slightly shifting the wavelengths of the four lanes of light. On the receiving side, on the other hand, an optical demultiplexer (DeMUX) is used to separate the signals into different wavelengths and separate them into signals for each lane. Click here to learn more about WDM.

Figure 2. 100G LR4 / ER4 block diagram

To reduce the cost of optical transceiver modules, many optical transmission standards use FEC (Forward Error Correction) to perform error correction. With FEC, redundant bits are added on the transmitting side to detect and correct errors, and errors that occur during transmission are corrected on the receiving side. Using FEC has the advantage of reducing the cost of optical components.

However, LR4 does not specify the use of FEC. We use a high-performance laser and receiver to extend the transmission distance to 10km without using FEC.

1. 100G CWDM4

Many users, including Hyper Scale Data Center, have told us that SR4's 100m transmission distance is not enough, but LR4's 10km is not necessary. There was also a request that it would be nice if it was cheaper. CWDM4, which has a transmission distance of 2km, was proposed as a 100G optical transceiver module that satisfies these demands.

In LR4, the laser wavelength spacing was 800 GHz (approximately 4 nm), whereas in CWDM4, the wavelength spacing was widened to 20 nm, eliminating the need for a complex wavelength control mechanism. (Figure 3)

Additionally, by limiting the transmission distance from 10km to 2km, we are now able to use inexpensive lasers instead of expensive ones.

Figure 3. Wavelength arrangement of LR4 and CWDM4​

2. 100G ER4f

In the explanation of 100GBASE-LR4, which is for long distance use, I mentioned that LR4 does not specify the use of FEC. The extended long range ER4 also does not specify the use of FEC. In order to extend the transmission distance to 40 km without using FEC, we use a more powerful laser and receiver.

However, the MSA standard called ER4f (or 4WDM-40) is an application of FEC to ER4 that has made it more compact and lower in cost. You may see optical transceiver modules sold as "QSFP28 100G ER4" on the web, but most of them cannot transmit 40km without FEC, and the limit is 25km to 30km. In other words, even though it is called ER4, it is actually "ER4f" so be careful.

Figure 4.​ ​CWDM4 and ER4f, which are lower cost versions of LR4 and ER4.

3. 100G SWDM4

SR4 uses a 12-core ribbon fiber to transmit 100m. SWDM4 was proposed in response to the desire to transmit using 2-core multimode fiber instead of ribbon fiber.

100G SWDM4 is a technology that applies WDM, which was previously a technology for long wavelength bands, to short wavelength bands for transmission using two-core fibers.

The biggest advantage of 100G SWDM4 is that you can upgrade from 10G to 100G without changing the fiber from 10G SR, which uses 2-core multimode fiber.

Both transmission methods use 2-core multimode fiber, so the fiber can be used as is.

Figure 5.​ ​SWDM4 applying WDM technology to SR4