Small Form-factor Pluggable - Wikipedia
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QSFP (non-plus) is 4gb/s, and it not directly compatible with SFP. It was designed for Fibre Channel, Infiniband, or to carry 4 GBE channels).
QSFP+ uses the same signalling system (CGMII) as SFP+. A QSFP+ module is technically 4 SFP+ modules in one box (4 “lanes” for QSFP vs 1 “lane” for SFP+). They are compatible with passive adapters.
Likewise, QSFP28 is 2 SFP28 modules in one box, using 2 (QSFP28, 50 gb/s) or 4 lanes (QSFP28, 100gb/s) instead of 1 lane (SFP28, 25gb/s).
QSFP28 is backward compatible with QSFP+.
QSFP56 follows along with SFP56, where it is 4 lanes (QSFP59 200gb/s) instead of 1 lane (SFP56 , 50gb/s).
The same should follow for QSFP112/SFP112 and QSFP-DD/SFP-DD112.
Note that the QSFP/QSFP+/QSFP28/QSFP56 are designed to be electrically backward compatible with SFP/SFP+/SFP28 or SFP56 respectively. Using a simple adapter or a special direct attached cable it is possible to connect those interfaces together using just one lane instead of four provided by the QSFP/QSFP+/QSFP28/QSFP56 form factor. The same applies to the QSFP-DD form factor with 8 lanes which can work downgraded to 4/2/1 lanes. — https://en.wikipedia.org/wiki/Small_Form-factor_Pluggable#SFP_types