Deep Dive: 400GBASE-LR4 QSFP-DD Optical Transceiver


The 400GBASE-LR4 is a specific optical transceiver interface type, available in the QSFP-DD (Quad Small Form Factor Pluggable – Double Density), supporting an aggregate transmission rate of 400G. With a reach of up to 2km over single-mode (SM) fiber, its primary application is high-speed intra-data center connectivity. This article will provide a comprehensive description of the important features and attributes of this important new technology.

LR4 Versus Other 400G Interface Types

Before we dive into the deep end on the LR4, let’s review some other popular 400G interface types. In the interest of brevity, this list is limited to 400G interfaces with 100G optical channels (we will take on 400G interfaces with 50G optical channels in a separate post). The following table lists the key attributes that define and differentiate these interfaces:

The 400G-LR4 (highlighted in yellow) is differentiated from the DR4 in three critical respects: nominal optical transmission wavelength, reach, and optical connector. The reach of the LR4 is 2km, four times the reach of the DR4. The LR4 uses four different CWDM wavelengths in the O-band (Original band, 1260nm-1360nm) for transmission of their 4 full-duplex 100G lanes (versus the DR4 using only 1310nm). 400GBASE-LR4 devices include an integrated CWDM Mux to combine (Tx direction) these 4 wavelengths onto a single fiber and a CWDM de-mux to separate them on the Rx side. This allows the use of the much more convenient Duplex LC optical connector versus the MPO-12 connector of the DR4. The primary disadvantage of this arrangement is, it is difficult to “break-out” the component 100G channels, a simple matter of using an MPO-to-4XDuplex-LC cable with the 400GBASE-DR4. The only significant difference between the 400GBASE-LR4 and 400GBASE-FR4 is maximum transmission distance (aka “reach”), at 10km and 2km, respectively.

400GBASE-LR4 – What’s Inside?

Now let’s take a look inside to see the main components of the transmission path of a 400GBASE-LR4.

The left side of this block diagram shows the 50G electrical lanes connecting to the host system into which the 400G DR4 module is installed. The blue block is circuitry (ASIC), referred to as a ‘gearbox’ the purpose of which is to combine 2 x 50G lanes into one 100G lane in the Tx direction and separate each 100G lane into 2 x 50G lanes in the Rx direction. The LR4 supports 4 x CWDM full-duplex 100G optical transmission lanes. The four CWDM wavelengths (1270nm, 1290nm, 1310nm, 1330nm) are multiplexed onto a single fiber in the transmit direction and de-multiplexed in the receive direction. This integrated CWDM Mux/DeMux allow the 400GBASE-LR4 to utilize a Duplex LC connector with which to connect to a duplex Single-Mode Fiber (SMF).

Standards Defining the 400GBASE-LR4

Several standards apply to define the attributes necessary to create multi-vendor plug-and-play 400GBASE-LR4 optical transceivers, as follows:

  • QSFP-DD MSA – This Multi-Source Agreement (MSA) defines the electrical and optical connections, electrical signals, power supplies, mechanical and thermal requirements of the pluggable QSFP Double Density (QSFP-DD/QSFP-DD800). It builds on the prior QSFP form factor, connector and cage system.
  • IEEE 802.3bs – This standard defines the 400GAUI-8 (GAUI – Gigabit Attachment Unit Interface) defining electrical clock and data interface between the 400G LR4 and the host system
  • OIF CEI-56G-VSR-PAM4 – The Optical Internetworking Forum CEI-56G-VSR-PAM4 (Common Electrical I/O, 56Gbps, Very Short Reach, Chip-to-Module, PAM4 Modulation) standard, or Implementation Agreement in OIF parlance, defines the SERDES (Serializer/De-serializer) function, including PAM4 modulation, for compatibility across the QSFP-DD to Host interface.

Optical Connector

As stated in the table above, the duplex LC is the standard optical connector defined for the QSFP-DD 400GBASE-LR4. One of the advantages of this arrangement is it greatly simplifies data center upgrades as existing duplex SMF cabling/connectors may be use for new 400GBASE-LR4 interconnections. As mentioned earlier, the biggest downside, versus the 400GBASE-DR4, is that it does not support “break-out” connections, where each 100G lane may be connected to a separate 100G QSFP endpoint. There is an alternative LR4-type interface, the QSFP-DD 4x100GBASE-LR1 which does provide this convenience (the subject of a future article).


The QSFP-DD 400GBASE-LR4 optical transceiver is an excellent choice for high-speed interconnection within and between commercial and enterprise data centers. This 400G interface type is excellent for links of up to 10km for which it is desirable to use standard duplex SMF cabling terminated with duplex LC connections.

Feel free to browse Fluxlight’s wide array of 400G Optical Transceivers and start building a bigger, faster optical networks sooner than you may have imagined.

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