Q1: What is SFP28?

A: SFP28 is the standard form factor of 25G optical modules. Its size is the same as 10G SFP+ (such as SFP-10G-SR) and 1G SFP, but the electrical interface supports a transmission rate of up to 28Gbps, which is suitable for 25Gbps signals. Most 25G optical modules have a nominal rate of 25.78125Gbps.

Q2: What 25G Products does 6COM Provide?

A: 6COM provides 25G optical modules and high-speed cable products that comply with IEEE standards, including standard 25G SFP28 (include SR, LR, LH, ER, etc.), 10/25G dual-rate SFP28, BiDi SFP28, DWDM SFP28, CWDM SFP28, LWDM SFP28 optical modules, DACs (direct-attached copper cables) and AOCs (active optical cables). 25G optics are used in transmission scenarios with different distances.

Q3: Why is the Use of 25G Increasing?

A: Network operators choose 25G instead of multiple 10G because 25G provides 2.5 times the bandwidth at a power consumption similar to 10G and uses the familiar SFP form factor. This enables switches and routers to provide more high-bandwidth connections on a single 1RU panel, with common port configurations of 12, 24, or 48.

Q4: What is the Cost per Bit of 25G?

A: Although the total cost of 25G is slightly higher than 10G, its bandwidth is 2.5 times that of 10G, so the cost per bit is reduced by about 50%, making it a more cost-effective choice.

Q5: Is 25G Standardized?

A: Yes, IEEE has set standards for 25G, and the main reference standards are IEEE 802.3by and IEEE 802.3cc.

Q6: What are the Types of Common 25G SFP28 Optical Modules?

A: There are many options for standard 25G SFP28 optical modules, including:

MMF (multi-mode fiber): SFP-25G-SR-S (up to 100 meters), SFP-10/25G-CSR-S (up to 400 meters)

SMF (single-mode fiber): SFP-25G-LR (up to 10 kilometers), SFP-10/25G-LR (up to 10 kilometers), SFP-25G-ER (up to 40 kilometers).

Q7: What are the Types of 25G Cables?

Standard 25G high-speed cables include SFP28-25G-AOCxM AOC cables and SFP-H25G-CUxM DAC cables. The former are optical fiber cables, and the latter are copper cables.

Q8: What do SR and LR Mean?

A: SR (short distance) usually refers to transmission through multimode fiber up to several hundred meters. LR (long distance): usually refers to transmission through single-mode fiber, up to 10 kilometers.

Q9: What is the Difference Between DAC and AOC?

A: DAC (direct attach copper cable) is a copper cable using copper as the medium to deliver electrical signals. AOC (active optical cable) is an optical fiber cable using multi-mode fiber as the medium to transmit data with optical signal. 

DAC is divided into two types: active and passive cables: active DAC requires external power to display its characteristics, and electronic components are set in the SFP connectors at both ends of it: passive DAC has no electronic components in the SFP connectors at both ends, does not require signal conditioning, and has the characteristics of low power consumption, so passive DAC is the most cost-effective high-speed cable. AOC requires external power, and the transceivers at both ends contain electronic components, and data is transmitted through internal electrical-optical-electrical conversion. The length of DAC is limited by the copper wire medium it uses, and the maximum length is generally only 15m; while AOC optical cable can reach 100m.

The connector of DAC does not have expensive optical lasers, which greatly saves costs and becomes a low-cost and high-efficiency high-speed data communication solution to replace optical modules. Active optical cables ensure the stability of transportation and the flexibility of application, and are more common in high-density applications.

Q10: Does the 25G Module Support Dual Rate?

A: Yes. The 25G dual rate modules (ie. 6C-SFP28-10/25G-SR, 6C-SFP28-10/25G-LR) are compatible with 10G and 25G rates and can be switched on the ports of the corresponding devices.

Q11: Under What Circumstances Does the 25G Optical Module Need to Turn on FEC?

A: FEC (forward error correction) can reduce the bit error rate (BER) to 10^-12. When the pre-FEC BER of the 25G module exceeds 5×10^-5, RS-FEC should be enabled; when the pre-FEC BER exceeds 10^-8, FC-FEC should be enabled.

Q12: What is the Maximum Transmission Distance of 25G DAC and AOC?

A: Generally speaking, the maximum transmission distance of 25G DAC is 15 meters, while AOC can reach up to 100 meters.

Q13: Is the 25G Port Compatible with 10G Modules?

A: Generally speaking, 25G ports are compatible with optical modules with 1G, 10G, and 25G rates and support multiple-rate switching.

Q14: How to Interconnect 25G SFP28 and 100G QSFP28 Ports?

A: Interconnection can be achieved by using modules + breakout cables. For example, a 100G port can be interconnected with 4 25G SFP28 ports through a 100G to 4x25G AOC or 100G to 4x25G DAC cable.

Q15: What are the Technical Features of 25G Transceivers?

A: 25G optical modules are usually equipped with CDR (clock data recovery) circuits, and in some cases, FEC (forward error correction) functions are also required to ensure stability and low bit error rate of data transmission.

Recommended Reading:

In-Depth Guide to SFP-10/25G-CSR-S Dual-Rate Transceiver