AOC Cable Interface Classification: Differences Between SFP+, QSFP28, and QSFP56 Versions and Selection Recommendations

 In data centers, high-performance computing clusters, and high-speed network cabling, AOC cables (Active Optical Cables) have become the mainstream solution for short-distance interconnections due to their low power consumption, high bandwidth, and resistance to electromagnetic interference. However, faced with multiple interface types such as SFP+, QSFP28, and QSFP56, many engineers and procurement professionals are often confused: What exactly are the differences between these versions? And how should one make an accurate selection based on actual requirements? 

 

Overview of AOC Interface Categories: Understanding SFP+, QSFP28, and QSFP56

 

Before diving into a detailed comparison, let’s briefly review the basic roles of these three interfaces:

· SFP+ AOC cables: Support a single-channel data rate of 10 Gbps and are commonly used for interconnecting 10-gigabit network devices.

· QSFP28 AOC Cables: Support 100 Gbps (4×25 Gbps) and are the mainstream choice for current 100G data centers.

· QSFP56 AOC Cables: Support 200 Gbps (4×50 Gbps), are designed for 200G/400G evolution architectures, and offer higher energy efficiency.

These three interfaces look similar but have completely different pin configurations, data rates, and application scenarios. 

 

Detailed Explanation of SFP+/QSFP28/QSFP56 Version Differences

 

Differences in Data Rates and Channel Architecture

Key Differences: Both QSFP28 and QSFP56 feature a 4-channel design, but the data rate has doubled, placing higher demands on PCB layout, optical component bandwidth, and DSP (Digital Signal Processor) performance.

 

Comparison of Power Consumption and Transmission Distance

 

SFP+ AOC: Typical power consumption <0.5W, with a maximum transmission distance of 300 meters (OM3 multimode fiber).

QSFP28 AOC: Typical power consumption 2.5W–3.5W, supporting transmission distances from 100 meters (OM3) to 300 meters (OM4).

QSFP56 AOC: Typical power consumption 4W–5W. Due to the use of PAM4 modulation (50G PAM4), it requires higher fiber quality for the same transmission distance; OM4/OM5 fiber is recommended.

QSFP56 is typically backward compatible with QSFP28 ports (if supported by the switch firmware), but the reverse is not true—lower-speed modules cannot automatically up-rate on higher-speed ports.

 

Evolution of Protocols and Management Interfaces

 

· SFP+: Based on the SFF-8431 specification, it uses simple I²C management.

· QSFP28: Introduces the SFF-8636 specification with Digital Diagnostic Monitoring (DDM), supporting more robust real-time parameter reading.

· QSFP56: Upgraded to the CMIS 4.0+ protocol (Common Management Interface Specification), coordinating complex functions such as retiming and PAM4 link training, resulting in more comprehensive management capabilities.

 

AOC Cable Selection Recommendations: Matching Scenarios and Equipment

 

Based on different network scales and budgets, the following AOC cable selection recommendations are provided:

 

Small and Medium-sized Enterprise Data Centers or Storage Networks (10G Level)

 

Recommended Selection: SFP+ AOC Cables

Reasons:

· Lowest cost, compatible with the vast majority of existing 10G switches and server network cards.

· Low power consumption, requiring no additional power supply or cooling design.

· Suitable for 10 Gigabit interconnections within a rack or between adjacent racks (≤100 meters).

 

Mainstream Data Center Aggregation Layer or TOR (Top of Rack) Uplink (100G Class)

 

Recommended Solution: QSFP28 AOC Cables

Reasons:

· Currently the most cost-effective 100G interconnect solution, saving approximately 30% in costs compared to optical modules plus fiber.

· Supports 4×25G breakout mode, allowing connection to four 25G SFP28 ports for flexible networking.

· Mature and reliable, with widespread certification from mainstream brands (such as Cisco, Huawei, and Arista).

 

Hyperscale clusters or 200G/400G evolution networks (200G class)

 

Recommended Solution: QSFP56 AOC cables

Reasons:

· 200G bandwidth per port, doubling port density and saving rack space.

· Seamless future upgrade to 400G (QSFP112 or QSFP-DD) architectures.

· Better signal integrity for switches already deployed with PAM4 DSP chips (e.g., Nvidia Spectrum-3, Broadcom Tomahawk 4).

If existing equipment only supports QSFP28 and lacks PAM4 capability, QSFP56 AOCs cannot be used directly; verify the compatibility list.

 

Mixed-Generation Device Environments

 

Selection Tips: Use adapter modules or branching cables. For example:

· QSFP28 AOC on the switch side, SFP+ on the server side? → Direct connection is not possible; a QSFP28-to-4×SFP+ branching cable is required.

· Connecting a QSFP56 port to a QSFP28 device → It is recommended to purchase AOC cables that support dual-mode operation (some vendors offer 56/28 auto-negotiation products).

 

How Can the Three Key Interfaces Make Quick Decisions?

Before purchasing, be sure to verify the supported interface types, FEC (Forward Error Correction) modes, and maximum transmission distances of your switches and network interface cards. In multi-vendor environments, we recommend requesting compatibility test reports from your AOC cable supplier or borrowing samples for hands-on testing.

By understanding the differences between SFP+, QSFP28, and QSFP56 versions and selecting AOC cables based on your network speed, power budget, and future upgrade path, you can not only save 30% to 50% on cabling costs but also significantly improve data center operational efficiency. We hope this article helps you cut through the confusion surrounding these interfaces and make an informed decision.