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Can a QSFP56 optic work with QSFP28 cable

What is QSFP28

In 2014, QSFP28 (an abbreviation for quad small form-factor pluggable 28) was published to carry 100 Gigabit Ethernet, EDR InfiniBand, or 32G Fibre Channel. It offers 4 channels of high-speed differential signals with data rates ranging from 25 Gbps up to potentially 40 Gbps, and meets 100 Gbps Ethernet (4×25 Gbps) and 100 Gbps 4X InfiniBand Enhanced Data Rate (EDR) requirements.

QSFP28 is specified by a multi-source agreement (MSA) under the auspices of the SFF Committee. The ‘QSFP’ means the form factor; ‘28’ represents that each lane has a maximum transmission rate of 28Gbps.

There are many products with QSFP28 form factor like the SR4, LR4, PSM4, CWDM4 and AOCs. ‘SR’ means short reach (100m), ‘LR’ is long reach (10km) and ‘ER’ is extended reach (40km). ‘4’ represents that the module has 4 lanes. For example, the QSFP28 SR4 means that it is a long reach 100G module which can transmit 100G signals over four wavelengths with a transmission distance of 100m.

How does QSFP28 work

The most straightforward way to connect 100G traffic is by using standard transceivers and there is a wide range of cables and transceivers suitable for this. To offer cost-optimized solutions for connecting switches together in a rack or data center, Ethernet switch vendors want to maximize the density of optical transceiver ports on their equipment. This is so they can offer maximum channel count and lowest cost per bit traffic.

Most of these switch vendors choose the smallest form factor transceiver: the QSFP28 transceiver. It’s the smallest module available, and has the lowest power consumption among those that are capable of handling 100G traffic. It’s also notable that the QSFP28 has the same physical size as the QSFP+ commonly used for 40G traffic. This means that switch vendors can increase the traffic throughput by a factor 2.5 without the need to redesign the front panel of their switches.

Types of QSFP28 products

QSFP28 AOC & DAC cable assemblies

Active Optical Cables (AOCs) are a good fit for short reach data interconnects. Terminated with transceiver style ends, AOCs range from 3m to 100m lengths. It transmits error-free parallel 4×25 Gb/s data over multimode fiber (MMF) cables. Its’ small bend radius makes for easy installation and fiber management.

QSFP28 Transceivers

QSFP28	SR4	LR4	PSM4	CWDM4	ER4
Standards	IEEE 802.3	IEEE 802.3	MSA	MSA	IEEE 802.3
Connector	MTP/MPO-12	LC Duplex	MTP/MPO-12	MTP/MPO-12	MTP/MPO-12
Cable Type	MMF	SMF	SMF	SMF	SMF
Wavelength	850nm	1310nm	1310nm	1271nm, 1291nm, 1311nm and 1331nm	1310nm
Transmission Distance	70/100m	10km	500m	2km	40km
Transmitter Type	VCSEL	EML	DML/FP	DML	DML

QSFP28 Application Guide

DAC Passive	DAC Active	AOC MM	MMF Transceivers	SMF Tranceivers
Cable Assembly	Cable Assembly	Cable Assembly	Discrete transceivers & cables MPO structured cabling	Discrete transceivers & cables LC SMF structured cabling
Lowest Cost Lowest Distance	Low Cost Better performance (CDR)	Higher cost than DAC Longer distance than DAC	Low Cost	Most expensive transceivers Best performance
Inside Rack	Inside Rack and Rack-to-Rack	Rack-to-Rack	Rack-to-Rack	Data Center to Data Center
0-3m	0-15m	0-70m	0-100m	0-40km

FAQs about QSFP28

Is QSFP56 compatible with QSFP28

No, the QSFP56 adopts PAM4 modulation technology, which can achieve transmission rates of up to 50G per channel while QSFP28 adopts the previous NRZ modulation technology.