Understanding Transceiver Form Factors: SFP+, QSFP, and Micro QSFP

Communications technology is constantly evolving to accommodate the ever-growing need for higher data rates and increased bandwidth. A crucial element in this progress is the development of new pluggable transceiver form factors, such as the SFP+—an enhanced version of the widely-used SFP—and the rise of QSFP and QSFP+ interfaces. In this article, we'll explore the innovations behind these advancements and discuss the challenges of electromagnetic interference (EMI) and thermal management. We'll also take a glimpse into the future of transceivers with the exciting development of micro QSFP. Join us as we uncover the driving forces behind the communications industry's growth and the promising opportunities ahead.

SFP+ is now the most popular socket for 10GE systems

The communications industry's demand for increased data rates and bandwidth has driven the development of new pluggable transceiver form factors like SFP+, an enhanced version of the widely-used SFP.

While SFP has been used for a long time for 1.125 Gbit/s fibre channel and 2.5 Gbit/s Ethernet applications, SFP+ supports data rates up to 10-14 Gbit/s and components for SFP+ systems are now widely available for 8 Gbit/s fibre channel and 10 Gbit/s Gigabit Ethernet.

SFP+ shares a form factor with SFP (with a higher port density than other form factors, such as XFP), so it means that some legacy SFP designs can still be used with SFP+. However, SFP+ modules only convert optical signals to electrical signals. In other words, they don’t do clock and data recovery, and these tasks must be done by the host board.

What is the Difference Between SFP and QSFP? 

In the realm of transceiver form factors, SFP and QSFP serve distinct roles with unique characteristics. SFP, or Small Form-factor Pluggable, is a compact transceiver utilised in various telecommunications and data communication applications. It supports data rates up to 2.5 Gbit/s and is commonly employed in 1.125 Gbit/s fibre channel and 1 Gbit/s Ethernet connections.

QSFP, on the other hand, stands for Quad Small Form-factor Pluggable. As the name suggests, QSFP is essentially a four-channel SFP interface capable of transferring four times the data of a single SFP channel. This increased data throughput results in greater port density and overall system cost savings compared to traditional SFP products. QSFP is designed to support data rates up to 40 Gbit/s, making it suitable for high-speed applications like 40 Gbit/s Ethernet connections and InfiniBand.

In summary, the key differences between SFP and QSFP lie in their data transfer capabilities and port densities. While SFP is suitable for lower-speed applications, QSFP excels in high-speed, high-density environments, meeting the ever-growing demand for higher data rates and increased bandwidth in the communications industry.

The Emergence of QSFP and QSFP+ Form Factors

QSFP stands for Quad SFP, and it’s essentially a four-channel SFP interface which can transfer four times the data of a single channel. The result is greater port density and overall system cost savings over traditional SFP+ products².

The relationship between QSFP and QSFP+ is similar to SFP and SFP+; QSFP+ can support 10 Gbit/s data rates, but otherwise, the form factor is the same.

Addressing EMI Challenges in Transceiver Form Factors

EMI is the enemy of all signal transmission systems, particularly high data rate systems. Potential EMI sources include improperly shielded connectors, which is why stacked connectors like SFP+ connectors are usually made of sheet metal.

TE Connectivity has gone a step further and made some enhancements to its connectors to ensure good EMI performance for stacked connectors. The gasket retention plate, which holds the gasket in place, was redesigned with more attachment points to minimise any leakage between the cage body and the gasket retention plate.

The latch plate, which separates the upper ports from the lower ports and houses lightpipes from the PCB to the cage, was also identified as a source of EMI. Another component was added to the latch plate without changing the way the latch plate functions to prevent leakage. The result was a reduction in EMI in the 10-15GHz range.

Good design considers thermal management

System architects for networking and data centre applications need to be very familiar with the thermal requirements of their equipment. Since today’s SFP+ modules operate at higher wattages than SFP modules, this must also be considered.

Most still operate under 1W, but this can be up to 1.5W for extended reach and fixed DWDM (dense wavelength division multiplexing) transceivers. Cooling these transceivers to stay below their (typical) 70°C operating point is a challenge when they are stacked very close together. Different configurations will affect the thermal properties of the interface; the number and density of ports is a key variable, as is the spacing between them, and other properties of the system, such as ambient temperature and airflow.

Stacked connectors typically come with perforations in the sheet metal body to allow heat to escape, and there may also be holes in the internal structure to allow for additional airflow.

There’s a new version of QSFP under development

A brand new form factor called micro QSFP will improve the port count by a third, compared to QSFP – 48 micro QSFP ports will fit in the space previously occupied by 36 QSFP ports³. It’s aimed at switch platforms wanting to increase speed from QSFP’s 3.2 Tbit/s to 4.8 or 6.4 Tbit/s.

The main challenge for micro QSFP is how to deal with waste heat – it squeezes a 3.5W interface into the same width occupied by a 1.5W SFP interface. One idea is to add a heat sink to the modules with fins that help to distribute the heat.

Although heat sinks are used in QSFP, today they are part of the cage and inserting the module into the cage brings it into contact with the heat sink, making a less-than-perfect join between the module and the heat sink. Since this interface is so important, adding the heat sink to the module itself means it can be manufactured with a much better join so that heat can be radiated more efficiently.

Although micro QSFP is still under development, it looks promising.

In conclusion

The continuous evolution of transceiver form factors like SFP+, QSFP, and the promising micro QSFP showcases the communications industry's commitment to addressing the growing demand for higher data rates and bandwidth. By overcoming challenges such as EMI and thermal management, manufacturers are successfully pushing the boundaries of port density and system performance. 

As we look ahead, we can expect further innovations in transceiver technology, ensuring that our networking and data centre infrastructure remains capable of supporting the ever-increasing data transmission requirements of our interconnected world. With rapid advancements and creative solutions, the future of transceiver form factors is undoubtedly bright and promising.

References:

1. Cisco Systems, "SFP and SFP+ Transceiver Module Installation Notes," Cisco.com, https://www.cisco.com/c/en/us/td/docs/interfaces_modules/transceiver_modules/installation... ↩

2. Transition Networks offers 40-Gbps QSFP+ optical transceiver https://www.lightwaveonline.com/optical-tech/transmission/article/16654699/transition-networks... ↩

3. Micro QSFP subject of new optical transceiver MSA https://www.lightwaveonline.com/optical-tech/transmission/article/16651134/micro-qsfp... ↩