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Backward Compatibility: QSFP-DD/QSFP28/QSFP+/SFP+

AuthorDerek

Date07/21/2022

Backward compatibility is vital for optical modules. It is closely bound up with the network system upgrading and operation. Transceivers with backward compatibility will provide great flexibility for future data center deployment. This article will give a comprehensive backward compatibility guide to the most common transceiver types, including QSFP-DD backward compatibility, QSFP28 backward compatibility, QSFP backward compatibility, and SFP+ backward compatibility.

SFP+ Backward Compatibility

SFP+ Wiki

SFP+ (enhanced small form-factor pluggable) is designed for 10G networking applications. Before 10G SFP+, 10G optical transceivers have experienced the version of 300Pin, XENPAK, X2, and XFP. The most significant enhancement of SFP+ is realizing the 10 Gbit/s data rate in a very small form factor, which is the same as SFP. The SFP+ specification was first introduced in 2006. With the advantages of high density and low cost, SFP+ optical modules are always popular throughout the market for more than a decade. Today, 10G SFP+ can be seen everywhere in data centers. SFP+ transceivers are used in various applications, including Ethernet, Fibre Channel, and InfiniBand. The common SFP+ types include 10GBASE-SR, 10GBASE-LR, 10GBASE-T, 10G BiDi SFP+, and 10G SFP+ CWDM/DWDM.

Is SFP+ backward compatible with SFP? 

Since SFP+ is exactly the same dimensions as SFP, it is backward compatible with SFP, but provides much higher data rates. SFP+ ports can work with 1G SFP modules, but it will work at 1Gbps instead of 10Gbps. The data rate configuration is not automatic generally, and you need to configure the port to 1G speed manually.

Is SFP+ compatible with XENPAK, X2?

How about the previous 10G module version of XENPAK, X2? Is SFP+ compatible with them? The answer is definitely no. If your SFP+ modules need to connect to the older equipment with XENPAK ports or X2 ports, a XENPAK to SFP+ converter adapter or X2 to SFP+ converter adapter will be needed.

QSFP+ Backward Compatibility

QSFP+ Wiki

QSFP+ (Quad Small Form-factor Pluggable Plus) supports data rates up to 40Gbps by four 10 Gbit/s channels. IEEE 802.3ba defines QSFP+. It is a litter bigger than SFP+, but it is still the highest density optical transceiver for 40 Gigabit Ethernet. The common QSFP+ optical transceiver types include 40G QSFP+ SR4, LR4, and ER4.

Is QSFP+ compatible with SFP+? How to convert a QSFP+ port to an SFP+ port?

Since the different physical dimensions, QSFP+ is not compatible with SFP+. Does this mean that the QSFP+ port can’t connect with the SFP+ port? No, there are many solutions for converting a QSFP+ port to SFP+.

 

Solution 1: Using QSFP to SFP+ breakout cable 

QSFP to 4 SFP+ copper direct attach breakout cable (DAC) and QSFP to four SFP+ active optical breakout cables can be used to connect to a 40G QSFP switch port on one end and to four 10G SFP+ ports on the other end. These two types of DAC and AOC breakout cables are both pre-terminated with a QSFP+ transceiver on one end and four SFP+ transceivers on the other end. Their main difference is that DAC uses copper cables while the AOC uses fiber cables. These breakout cables are perfect for those who need to connect short distances and they offer an extremely cost-effective and convenient way of doing so.

 

Solution 2: Using optical transceivers + MTP to LC breakout cable

This solution needs one QSFP optical transceiver and four SFP+ transceivers, then uses an MTP to LC 8 fiber breakout cable to connect them. The fiber breakout cables include MMF and SMF types, which to choose is determined by the two ends of optical transceivers. This solution can provide a much longer distance than the prior, but also costs more.

 

Solution 3: Using QSFP to SFP+ adapter

QSFP to SFP+ adapter converter is also a cost-effective way to convert QSFP+ to SFP+ port. It allows 10 Gigabit Ethernet connectivity on QSFP-only platforms. Plug and play, no configuration required. The QSA adapter interoperates with all major SFP+ optical transceiver modules, SFP+ direct attached copper cable (DAC), and active optical cables (AOC). With this clever adapter, you can flexibly use any SFP+ transceivers or cables to connect with a lower-speed port on the other end of your network.

QSFP28 Backward Compatibility

QSFP28 Wiki

QSFP28 is designed for 100G Ethernet and can support data rates up to 100 Gbps over four channels, each channel support 25Gbit/s. 100G QSFP28 transceivers are gradually replacing SFP and SFP+ as the top popular optical module type. The common 100G QSFP28 transceivers include 100G SR4, LR4, CWDM4, and PSM4. QSFP28 to QSFP28 cable is also popular in short-range 100G applications, including QSFP28 DAC and AOC. The typical 400G QSFP-DD transceivers include 400G QSFP-DD SR8, DR4, FR4, LR4, and LR8. And 200G QSFP-DD transceivers include 200G QSFP-DD SR8, PSM8, and LR8.

Is QSFP28 backward compatible with QSFP?

The answer is yes. QSFP28 has the same factor and size as QSFP. Both QSFP28 and QSFP have four lanes. QSFP28 supports 25Gbit/s per lane, while QSFP supports 10Gbit/s per lane. Similar physical form factor and working principle make QSFP28 backward compatible with QSFP.

QSFP-DD Backward Compatibility

QSFP-DD Wiki

QSFP-DD (Quad Small Form-factor Pluggable Double Density) is a new form factor, designed for the next-generation and high-density 200G and 400G Ethernet. It is compliant with the IEEE802.3bs and QSFP-DD MSA. As its name implies, compared to its predecessors of 4-lane QSFP28 and QSFP, QSFP-DD doubles the number of high-speed electrical interfaces, providing an eight-lane electrical interface. 

 

Based on different lane data rates and modulation technology, QSFP-DD supports data rates up to 200Gbps or 400Gbps:

 

200Gbps: 8 lanes, supporting 25Gbps per lane, using NRZ modulation

400Gbps: 8 lanes, supporting 50Gbps per lane, using PAM4 modulation

Does QSFP-DD support backward compatibility? Which form factors does it support?

QSFP-DD is designed based on QSFP pluggable form factor. It has the same pinout as the QSFP connector. Therefore, QSFP-DD quadruples aggregate switch bandwidth while maintaining the port density. Also, QSFP-DD is backward compatible with the current QSFP form factor types, including 40G QSFP+, 100G QSFP28, and 200G QSFP56. This means that equipment designed with QSFP-DD connectors will be backward compatible, allowing them to support existing QSFP and QSFP28 modules and provide great flexibility for end users.

 

Conclusion

 

Form Factors

Backward Compatibility

SFP+

SFP

QSFP+

-

QSFP28

QSFP+

QSFP56

QSFP+, QSFP28

QSFP-DD

QSFP+, QSFP28, QSFP56

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