QSFP28 Breakout Cable Guide: Splitting 100G into 4x25G with Hirundo DAC & AOC Solutions
Data centers in 2026 are being built around two converging trends: the migration of server access ports from 10G to 25G, and the steady increase of 100G uplinks at the leaf and spine layers. The speed mismatch between these two layers is exactly where the QSFP28 breakout cable proves its value. By splitting one 100G QSFP28 port into four independent 25G SFP28 links, a single breakout assembly connects four servers, NIC ports, or storage nodes from one switch port — without adding extra line cards or wasting valuable port capacity.
Hirundo's AOC/DAC series covers the full 10~200 Gb/s range, and the QSFP28 to 4*SFP28 breakout assemblies sit at the center of that portfolio. Their low power consumption, small size, and high reliability help increase data center capacity, improve port density, and reduce overall power consumption — the three metrics that decide how far a modern facility can scale.
What Exactly Is a QSFP28 Breakout Cable?
QSFP28 is a four-lane module format. Each lane carries approximately 25 Gb/s, giving an aggregate 100 Gb/s per port. A breakout cable takes those four electrical lanes and fans them out to four separate SFP28 interfaces, each running at 25 Gb/s. The result is a transparent 4-in-1 fan-out: the switch simply sees four independent 25G links, and the far-end devices require nothing more than a standard 25G port.
Hirundo builds these breakout assemblies in both direct attach copper (DAC) and active optical cable (AOC) variants. The same fan-out concept is also available in the QSFP+ to 4*SFP+ format for legacy 40G-to-10G deployments. Standard QSFP28 to 4*SFP28 assemblies are offered alongside QSFP28 to QSFP28, QSFP56 to QSFP56, QSFP+ to QSFP+, SFP28 to SFP28, and SFP+ to SFP+ cables, giving network planners one consistent cabling family across every tier of the network.
DAC vs AOC: Choosing the Right Breakout Media
Direct Attach Copper (DAC)
Copper breakout cables are the default choice for in-rack and adjacent-rack connections. Most are passive by design, which means no additional power draw and effectively negligible latency. Hirundo supplies DAC breakout assemblies in 24AWG, 26AWG, 28AWG, and 30AWG conductor gauges. Thinner gauges are easier to route through dense cable trays, while heavier gauges support longer runs. Both passive and active copper options are available, with passive variants handling short reaches and active variants extending the distance copper can cover reliably.
Active Optical Cable (AOC)
When the distance grows beyond what copper can handle, AOC breakout cables take over. These assemblies use OM2, OM3, OM4, or OM5 multimode fiber with 850 nm VCSEL sources, while presenting the same four-lane electrical interface as their copper counterparts. Because the signal travels as light rather than as an electrical pulse, AOC cables are immune to EMI, are considerably lighter and thinner than copper at equivalent reach, and support much longer spans — making them a natural fit for inter-rack and cross-row links. Cable lengths across the Hirundo breakout family range from 1 m and 3 m for in-rack patching up to 15 m, 30 m, and 100 m for longer optical runs, with custom lengths available on request.
Key Specifications Behind a Reliable 100G Breakout Link
A breakout cable is only as good as the transceiver endpoints and the optical budget behind them. Hirundo's 100G QSFP28 modules, which pair with these breakout cables, operate at 25.78 Gb/s per lane with a center wavelength of 840–860 nm, use LC or MPO optical ports, and conform to the QSFP+ MSA and IEEE 802.3. The table below summarizes the essential optical parameters.
| Parameter | Typical Value | Unit |
|---|---|---|
| Signaling rate per lane | 25.78 | Gbps |
| Center wavelength | 840–860 | nm |
| Average launch power per lane | -6 to 2.4 | dBm |
| Transmit OMA per lane | -4 to 3 | dBm |
| Optical extinction ratio | ≥2 | dB |
| Relative intensity noise | ≤-128 | dB/Hz |
| Average receive power per lane | -10.3 to 2.4 | dBm |
| Receiver sensitivity per lane | ≤-10.3 | dBm |
| Receiver reflectance | ≤-12 | dB |
| LOS assert / de-assert | -30 / -13 | dBm |
On the 25G side, Hirundo's SFP28 optical module family covers SR, LR, ER, CWDM, DWDM, LWDM, and MWDM variants with LC optical ports. These modules consume less than 1 W, operate from a 3.14–3.46 V supply, deliver an optical output power between -6.4 dBm and 2.4 dBm with an extinction ratio of at least 2 dB, and provide receiver sensitivity of at least -10.3 dBm with a receiver overload point of 3 dBm. Their wide operating temperature range makes them suitable for both data center server access and 5G fronthaul, and they are a natural partner for any QSFP28-to-4xSFP28 breakout deployment.
Brand Compatibility: One Cable, Many Platforms
Breakout cables have to interoperate with whatever switch or server platform is already installed. Hirundo codes its QSFP28 and SFP28 breakout assemblies for Generic, HUAWEI, H3C, CISCO, RUIJIE, ZTE, NOKIA, HP, MELLANOX, Arista, and DELL equipment, and also accepts custom coding requests. That breadth of compatibility matters during mixed-vendor refreshes, where a single rack may contain hardware from three or four different suppliers and the cabling layer must not become the bottleneck.
Deployment Use Cases for 2026 Networks
- Leaf-spine with 100G uplinks and 25G access: one QSFP28 port on the leaf switch feeds four 25G server links, matching the port economics of a 25G access layer without extra optics.
- Server NIC fan-out: a single 100G switch port serves four 25G NICs, which is often the most cost-effective way to light up quad-port servers.
- Storage and SAN connectivity: four 25G storage targets share one 100G port, simplifying rack-level cabling and reducing cable congestion.
- AI and HPC cluster interconnect: short DAC breakouts handle in-rack GPU-to-switch links, while AOC breakouts cover cross-row traffic where copper reach runs out.
- Legacy 40G migration: QSFP+ to 4*SFP+ breakout cables let existing 10G infrastructure continue to serve while the fabric is upgraded.
Conclusion
As 25G becomes the standard access speed and 100G remains the standard uplink, QSFP28 breakout cables are no longer a niche accessory — they are fundamental building blocks of the data center cabling plan. Whether the requirement is a passive copper DAC for a short in-rack hop or a multimode AOC for a cross-row run, Hirundo's breakout assemblies deliver low power consumption, high port density, and broad multi-vendor compatibility, backed by a product family that scales from 10G all the way to 200G.
