What Ethernet standard to use with QSFP28?

The 100G Ethernet

When 10G Ethernet has appeared it was considered a great step forward from 1Gbps standard considering the small factor implementations with SFP.  After the  standardisation of 100G Ethernet, the first hardware implementations where no so optimal regarding the cost, consumption and footprint in the form of CFP’s. ‘C’ - was the Latin symbol for ‘Cento’ - meaning the 100. Although the next implementations versions of CFP,  the CFP2, CFP4 had and CPAK  were improved in terms of the above characteristic, their adoption was not massive due to SFP+ and 40G - QSFP -better design.

The 40G QSFP is a major intermediate step towards modern 100G adoption. ‘Q’ letter points out about the existence of four lanes that are multiplexed to obtain the final throughput. In this way the 4 lanes of 10Gbps leads to 40G QSFP. The 100G QSFP28 is implemented with four 25-Gbps lanes. In all QSFP versions, both the electrical lanes and the optical lanes operate at the same speed, eliminating the costly gearbox found in CFP, CFP2, and the CPAK. The QSFP28 module has an upgraded electrical interface to support signaling up to 28 Gbps signals, yet keeps all of the physical dimensions of its predecessor.

QSFP28 - Main drives

When compared to any of the other alternatives, 100G QSFP28 increases density and decreases power and price per bit. It is fast becoming the universal data center form factor.The form factor is the same and the maximum number of ports is the same, but the lane speeds are increased from 10 Gbps to 25 Gbps.

The QSFP28 standard is designed to carry 100 Gigabit Ethernet or EDR InfiniBand. This transceiver type is also used with direct-attach breakout cables to adapt a single 100GbE port to four independent 25 gigabit ethernet ports.The cables can be either direct-attach copper cables, commonly referred to as DACs, or active optical cables (AOCs). DACs offer the lowest cost but are limited in reach to perhaps 3 m. They are typically used within the racks of the data center, or as chassis-to-chassis interconnect in large switch and routers. AOCs are much lighter and offer longer reaches up to 100 m and more.

Connection to servers & switches

QSFP28 refers to the 100G DAC cable using the QSFP+ form factor, and SFP28 refers to the 25G DAC cable using the SFP+ form factor. While the form factor remains the same, the new standard will require improved cables and connectors capable of handling this additional bandwidth. Existing QSFP+ and SFP+ cable assemblies will not be compatible with QSFP28 and SFP28 ports.

Currently the cabling application is one QSFP28 to four SFP28 cables. The target application for the CFI is 100G switches connecting to four 25 Gigabit in cabinet or adjacent cabinet servers using these breakout cables. There is also expected to be SFP28 to SFP28 cabling, assuming equipment vendors develop a ToR 25G switch. A 25G fiber option is also being developed based on duplex LC that would mate to the SFP28 ports.

100G and 25G Cable Variants

100G Transceivers applications

25G Transceiver Selection: