Understanding SFP, SFP+, SFP28, QSFP+ and QSFP28 — DAC, AOC and Fibre Optics

This overview explains the key differences between common transceiver form factors (SFP to QSFP28) and cabling media (DAC, AOC, and standard fibre) used in modern data centres, helping you choose the right physical infrastructure based on bandwidth, distance, and latency requirements.

Choosing the correct physical medium is just as important as selecting the right form factor. The choice largely depends on required distance, environmental conditions, and latency requirements.

DAC (Direct Attach Copper): Factory-terminated cables using copper twinax wiring. Highly cost-effective and low power consumption, making them the standard choice for short connections within the same rack. Because DACs transmit electrical signals directly without signal conversion, they offer the lowest possible latency — highly desirable for HFT, HPC, and other ultra-low latency environments. Not suitable for environments with exceptionally high EMI.

AOC (Active Optical Cable): Permanently attached transceiver heads using multimode fibre. Completely immune to EMI, noticeably thinner and lighter than DACs. Typically used for medium-distance links up to 30–100 metres across adjacent or nearby racks.

Standard Fibre Optics (with discrete transceivers): The only viable solution for distances beyond DAC or AOC limits. Multi-mode fibre suits short-to-medium building links; single-mode fibre suits long-haul connections spanning kilometres. The modular approach provides the greatest reach and flexibility.

The SFP module (often referred to as mini-GBIC) is a compact, hot-pluggable transceiver used for both telecommunication and data communications.

• Supports data rates up to 1 Gbps
• Works with both copper and fibre optic cables
• Standardised by the Multi-Source Agreement (MSA) for cross-vendor compatibility

An enhanced version of SFP supporting significantly higher data rates in the same physical dimensions.

• Supports data rates up to 10 Gbps
• Widely deployed for 10 Gigabit Ethernet
• Backwards compatible — most SFP+ ports accept standard 1G SFP modules

Designed for 25G Ethernet performance using the same physical footprint as SFP and SFP+.

• Supports data rates up to 25 Gbps per single electrical lane
• Cost-effective upgrade path for next-generation networks
• Backwards compatible — SFP28 ports can often run at 10G with SFP+ optics

Integrates four transmit and four receive channels, multiplying network capacity in a compact but slightly larger footprint.

• Supports data rates up to 40 Gbps via four parallel 10 Gbps lanes
• Ideal for 40G core switch and distribution applications
• Can be split into four separate 10G connections via breakout cable

The modern standard for 100G networks, using the same quad-channel design at significantly higher lane speeds.

• Supports data rates up to 100 Gbps via four parallel 25 Gbps lanes
• Same physical dimensions as QSFP+
• Breakout options available — e.g. one 100G port split into four 25G SFP28 connections