SFP-DD (Small Form-Factor Pluggable Double Density) and SFP28 are two different form factors for optical transceivers used in high-speed data communication applications. While both are designed for similar purposes, there are differences between SFP-DD and SFP28: 

25G SFP28

1. Form Factor:

• SFP-DD:

• SFP-DD is an evolution of the traditional SFP form factor. It is designed to double the electrical interface by adding a second row of contacts, effectively allowing for two lanes (two channels) of electrical communication.

• SFP-DD can support various data rates, including 25 Gbps and 50 Gbps per channel, and can aggregate multiple lanes to achieve higher overall data rates.

• SFP28:

• SFP28 is a single-lane version of the SFP+ form factor. It is specifically designed to operate at data rates of 25 Gbps per channel.

• SFP28 is often used in applications like 25 Gigabit Ethernet (25GbE) and is part of the same family as SFP+ (10GbE) but operates at a higher data rate.

2. Electrical Interface:

• SFP-DD:

• SFP-DD features a dual-row electrical interface, providing connectivity for two independent channels.

• The additional row of contacts allows for increased data rates and higher overall bandwidth compared to traditional SFP.

• SFP28:

• SFP28 features a single-row electrical interface, supporting a single channel with a data rate of 25 Gbps.

3. Data Rates:

• SFP-DD:

• SFP-DD can support various data rates, including 25 Gbps and 50 Gbps per channel.

• The aggregate data rate can reach up to 200 Gbps or more, depending on the implementation.

• SFP28:

• SFP28 is specifically designed for a data rate of 25 Gbps per channel.

4. Use Cases:

• SFP-DD:

• SFP-DD is designed to provide a higher-density solution with increased overall bandwidth, making it suitable for applications requiring higher data rates and capacity, such as 100 Gigabit Ethernet (100GbE) and beyond.

• SFP28:

• SFP28 is commonly used for 25GbE connections, and its form factor is similar to SFP+, making it a straightforward upgrade path for 10GbE networks.

In summary, while both SFP-DD and SFP28 are designed to address the need for higher-speed optical transceivers, SFP-DD achieves this by doubling the electrical interface and supporting higher data rates per channel. SFP28, on the other hand, operates at a fixed data rate of 25 Gbps per channel. The choice between the two depends on the specific requirements of the network and the desired data rates.

How do they compare?

SFP and SFP+: SFP is for 100BASE or 1000BASE applications while SFP+ is used in 10GBASE applications. SFP+ ports can accept SFP optics but at a reduced speed of 1 Gb/s, but an SFP+ transceiver cannot be plugged into an SFP port.

SFP+ and SFP28: SFP28 is designed for use with 25GBASE connections. SFP+ and SFP28 have the same form factor, and compatible pinouts. SFP28 transceivers will work with SFP+ optics but at a reduced speed of 10 Gb/s.

QSFP and QSFP+: QSFP carries 4 x 1 Gb/s channels. QSFP+ supports 4 x 10 Gb/s channels and the channels can be combined into a single 409 Gb/s connection. A single QSFP+ can replace 4 SFP+ transceivers resulting in greater port density.

QSFP-DD, QSFP28, and QSFP56: QSFP-DD transceivers have the physical dimensions and same port densities as the QSFP, QSFP28, and QSFP56 but double the number of lanes to eight. QSFP-DD modules are available that support 400 Gb/s and 800 Gb/s. To accommodate the greater number of lanes, the mechanical interface of QSFP-DD on the host board is slightly deeper than that of the other QSFP transceivers to support an additional row of contacts.