Much like how SFP28 is an enhanced version of SFP+, the QSFP28 form factor is the next iteration of QSFP+. QSFP28 transceivers essentially have the same form factor as QSFP+ transceivers. Consequently, QSFP28 ports in switches and routers accept both 100G QSFP28 transceivers as well as 40G QSFP+ transceivers.
Similar to QSFP and QSFP+, QSFP28 supports 4 channels due to its quad form factor. However, unlike QSFP and QSFP+, QSFP28 is capable of transmitting 25G per channel to get a combined 100G of data throughput. In addition, the QSFP28 interface is very flexible in that it can achieve 100G in various ways.
By leveraging a breakout connection, a QSFP28 transceiver can use 2 channels at 50G. On the other hand, it is also capable of achieving a single channel of 100G. QSFP28 100G transceivers are available in various different standards including SR4, LR4, PSM4 and CWDM4. With these available standards, you can achieve 100G in many different applications from short reach to extended reach. You can also consider dual rate transceiver solutions such as 40G/100G QSFP28 SR4, which is compatible with legacy 40G SR4 optics.
QSFP28 is an efficient solution for network operators to seamlessly upgrade to 100G without ripping and replacing their existing infrastructures.