There are lots of QSFP-DD variants in the market, they are able to be classified by the suffixes, the following text briefly introduces those suffixes.
- SR8: SR is an abbreviation of Short Range, it refers to transmitting over multimode fiber up to 100m, while 8 means 8x optical channel pairs (8x Tx and 8x Rx), as for 400G optical module, each optical channel transmits over 50Gb/s and with MPO-16/MPO-24 receptacle. The SR8 module can be configured for 400G-SR8, 2x 200G-SR4, and (operating at 1/2 rate) 2x 100G-SR4.
- DR4/XDR4 (FR)/XXDR4 (LR): They respectively refer to transmitting distances 500m/2km/10km via single mode fiber. 4x means 8x optical channel pairs (4x Tx and 4x Rx), 400G QSFP-DD transceivers transmitting 400G over 4 optical lanes and each lane for 100G, and normally using MPO-12 single mode receptacle. Besides the MPO-12 receptacle, some vendors are developing new models by using VSFF connectors, thus it can be realized by using 4x SN connectors, etc.
- FR4/LR4/ER4: They respectively refer to transmitting distance of 2km/10km/40km via single mode fiber. 4x means 8x optical channel pairs (4x Tx and 4x Rx), 400G QSFP-DD FR4/LR4/ER4 transceivers transmitting 400G over 4 optical lanes and each lane for 100G, however, the optical channels are multiplexed onto a single fiber, thus 400G QDD FR4/LR4/ER4 transceivers using duplex LC optical receptacle.