FAQs on 400G Transceivers and Cables You Need to Know, Pt. 2


This is the second part of our series on 400G transceivers and cables. In this article, we’ll answer some more frequently asked questions about these technologies.

Q4: What are QSFP-DD cables?

QSFP-DD cables are assemblies including a cable with permanently attached QSFP-DD modules at each end. These cables may be twinax ‘copper’ cables or may be fiber optic. Cables are available in a variety of lengths. The copper versions typically support up to 3 meter lengths while optical versions support links up to 30 meters.

Q5: What are the Primary 400G Transceivers Packaging Forms?


The QSFP-DD is a next-generation, double-density version of the QSFP transceiver form factor. While using the same physical form-factor as the QSFP, an extra, staggered row of pins, allows for a doubling of the number of data lanes from 4 to 8. QSFP28 100G transceivers use 25G lanes while QSFP-DDs use 50G lanes. One of the great advantages the QSFP-DD versus the other 400G form-factors is its backward compatibility with the QSFP28. This means newer equipment, with QSFP-DD slots, to be populated with either QSFP28 or QSFP-DD modules on a per slot basis. The feature greater enhances deployment flexibility and migration of existing 100G networks to 400G.


OSFP, or Octal Small Form-Factor Pluggable, is a type of 400G transceiver used for data center applications. It is a pluggable module that can be used with both Ethernet and Fiber Channel networks. The OSFP supports 8 electrical data lanes either 50G or 100G, allowing it to support up to 800G if the inputs are 100G. The OSFP is almost exactly twice the size (in total volume) versus the QSFP-DD. It is a new form-factor meaning with no backward compatibility with any previous generation devices. On the positive side, its larger size allows it to include integrated thermal management which should allow the OSFP to support higher data rate and long transmission distances before those applications can be supported in the QSFP-DD form-factor.

3. CFP8

The CFP8 form factor is a multi-purpose form factor that was designed to support a variety of applications including 400G Ethernet and Optical Transport. The CFP8 form factor is based on the tried and trusted ‘C’ form factor, originally designed to support 40G and 100G. The CFP8 doubles the number of lanes from the CFP’s 4 lanes to 8 lanes and the electrical interface lane rates are 50G versus the 10G lanes of the original CFP. The CFP form factor also backward compatible with the C form factor, allowing for increased flexibility and scalability in the deployment of 400G Ethernet and Optical Transport networks. The biggest drawback of the CFP8 is its ‘bigness’. It is over 30% larger than the OSFP and about 65% larger than the QSFP-DD! This package is typically most successful among ‘early adopters’ that require high-power coherent optical interfaces such as long-haul telecommunication operators.

Q6: What Do the Suffixes “SR8, DR4, XDR4, FR4, LR4 and 2FR4” Mean in 400G Transceivers?

The suffixes SR8, DR4 / XDR4, FR4 / LR4, and 2FR4 are different optical interface types of 400G QSFP-DD transceivers.

SR8 – Short Reach Multimode 8x50G PAM4 16-fiber

SR8 transceivers are used for short-reach applications with data rates of up to 8x50Gbps, or 400g total. Both the electrical and optical interface of the SR8 consist of 8 Transmit (Tx) and 8 Receive (Rx) 50G optical lanes. On the optical side, each of which occupies a separate optical fiber. The SR8 includes an MPO-24 optical interface, of which 16 fibers are utilized, 8 x Tx and 8 x Rx. This interface type supports links of up to 100 meters on OM3 fiber.

DR4 / XDR4 – Short Reach Single-mode 4x100G PAM4 8-fiber

DR4 / XDR4 transceivers are used for short-haul links over single-mode fiber. The DR4 supports link lengths up to 500 meters while the XDR4 will push up to 2km. The DR4 and XDR4 both include and ‘gearbox’ that, for the Tx direction, combine the 8 x 50G PAM4 electrical inputs into 4 x 100G PAM4 optical outputs (opposite function in the Rx direction). Each 100G PAM4 optical lane is transmitted on a separate SM fiber for a total of 8 fibers (4 x Tx, 4 x Rx). These modules include and MPO-12 connector, of which 8 fibers are utilized for transmission.

FR4 – Single-mode 2km 4x100G PAM4 CWDM 2-fiber

The FR4, like the DR4 and XDR4 types, uses a gearbox to convert between 8 x 50G electrical lanes and 4 x 100G optical lanes (all PAM4). However, the FR4 converts each of the 100G lanes to/from different CWDM wavelengths (1271/1291/1311/1331nm). These four wavelengths are muxed/demuxed onto two single-mode fibers with link distances up to 2km. The optical connector on FR4 module is a duplex-LC type.

2FR4: Fiber Optic 2x 2x 100G PAM4

2FR4 transceivers are a variant of the FR4, the primary difference being, the 2FR4 transceives two of the 100G PAM CWDM wavelengths on one fiber pair and the other two wavelengths on another fiber pair. Since there is not sufficient space for 2 x duplex-LC optical interfaces on the front of the QSFP-DD package, a new connector type, called CS, is used. The CS connector is half the height and width of the duplex LC, allowing two of duplex CS connectors (total of 4 fibers) to fit in the space of one duplex LC.

LR4 – Single-mode 10km 4x100G PAM4 CWDM 2-fiber

The LR4 is almost identical to the FR4 but utilizes better optical transmitters and receivers and FEC (Forward Error Correction) to achieve link distances up to 10km on single-mode fiber.


400G transceivers and cables are becoming increasingly popular as the need for faster data speeds continues to grow. There are a few different types of 400G transceivers and cables, and each has its own benefits and drawbacks. Be sure to do your research before investing in any 400G equipment to ensure that it will meet your needs.

To ensure your network is functioning optimally, use products and accessories only from a reputable supplier like FluxLight. We carry a wide variety of 400G fiber optic transceivers, cables, testing and cleaning tools, and other connectivity solutions at reasonable prices. Browse our products today to order!

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