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Quick Answer
A Direct Attach Cable (DAC) is a factory-assembled high-speed copper cable with fixed connector “module-style” ends. It’s widely used for short-reach links in data centers because it delivers low latency, simple deployment, and cost-efficient interconnects-especially for rack-level connectivity.
To understand DAC quickly, classify it by
- (1) connector family (SFP/QSFP/OSFP)
- (2) connection shape (1-to-1 vs breakout)
- (3) speed generation (10G→800G).
What is a DAC (Direct Attach Cable)?
A DAC is a copper interconnect assembly where the connectors are permanently attached to the cable. In practice, it behaves like a “plug-and-connect” link: you insert the ends directly into switch/NIC ports and get a short-reach connection without separately sourcing transceivers and fiber patch cords.
Why it matters: in many rack environments, the “fastest” link isn’t the one with the most parts-it’s the one with fewer moving pieces.
Is DAC copper or fiber?
In day-to-day data center language, “DAC” almost always means copper.
A Direct Attach Cable (DAC) is typically a twinax copper cable assembly with fixed connector ends (module-style heads) that plug directly into switchfiber.
Because naming in the industry can be confusing, you’ll often see related terms that look similar but refer to different technologies:
- AOC (Active Optical Cable): a fiber-based fixed cable assembly with integrated optical components at the ends.
- ACC (Active Copper Cable): a copper cable assembly that includes active electronics for signal conditioning (often used when copper needs more help to meet a link budget).
- AEC (Active Electrical Cable): also electrical (not optical), but typically refers to a newer class of cables used at higher speeds (e.g., 400G/800G ecosystems) where active retimers/equalization may be used to improve signal integrity over practical short-reach electrical links. In practice, AEC is still an electrical/copper-oriented solution category, distinct from fiber-based AOC.
Simple takeaway:
DAC / ACC / AEC = electrical (copper/twinax) family
AOC = optical (fiber) family
What is twinax used for in data centers?
Twinax is used for high-speed, short-distance connections where operators want:
- predictable performance within a controlled physical path (rack / near-rack)
- low latency characteristics typical of short copper links
- operational simplicity (fewer components, faster turn-ups)
Twinax-based DAC often shows up in:
- ToR / adjacent-rack switch-to-server or switch-to-switch cabling
- dense compute/storage racks where changes happen frequently
- high-port-count environments where operational clarity matters
Passive vs Active DAC: what’s the actual difference?
The cleanest way to think about it:
- Passive DAC: no active electronics for signal conditioning inside the cable assembly.
- Active DAC (ACC): includes electronics to condition the signal (and can help extend practical short-reach boundaries in certain designs).
You don’t need a long debate to understand the impact: passive is “simpler in-cable,” active is “more engineered in-cable.” Which is “better” depends on the physical environment and the platform’s signal expectations-so treat it as a design boundary, not a marketing label.
How to read DAC “types” without getting lost
Most DAC confusion goes away if you sort by three axes:
1) Connector family (what plugs into your ports)
- SFP+ / SFP28 (common in 10G / 25G access)
- QSFP+ / QSFP28 / QSFP56 / QSFP-DD (common in 40G / 100G / 200G )
- OSFP / QSFP112 (common in newer high-speed generations depending on platform)
2) Connection shape (how endpoints are wired)
- 1-to-1 (straight-through): one port to one port (same speed class).
- Breakout (fanout): one high-speed port splits into multiple lower-speed ports (e.g., 100G → 4×25G).
3) Speed generation (what “era” the link belongs to)
10G/25G/40G/100G are the “classic” data center generations; 200G/400G/800G appear more as you move into modern leaf-spine expansions, AI/HPC fabrics, and high-density cluster designs.
Common DAC cable types
Instead of a long “one-by-one list,” here’s the set engineers actually recognize-grouped by how they’re used.
A) Straight-through DAC (1-to-1): the workhorse short link
These connect two devices with matching port families:
- 10G SFP+ ↔ SFP+ DAC: Often used for legacy/edge connectivity and short switch-to-server links in older designs.
- 25G SFP28 ↔ SFP28 DAC: Common for 25G server access where the rack stays within short-reach copper paths.
- 40G QSFP+ ↔ QSFP+ DAC: Shows up in established 40G environments and short inter-switch links.
- 100G QSFP28 ↔ QSFP28 DAC: Used for short uplinks or switch-to-switch interconnects where 100G is the baseline.
- 200G QSFP56 ↔ QSFP56 DAC: Appears in newer fabrics where 200G becomes a standard building block.
- 400G QSFP-DD ↔ QSFP-DD DAC: Used in high-throughput racks where 400G ports connect within short reach.
(You’ll see additional variants in modern platforms-OSFP, QSFP112-depending on the ecosystem.)
B) Breakout (fanout) DAC: turning one fast port into multiple endpoints
Breakout is a topology tool. It’s not “just a cable”-it expresses a port-mode intent:
- 40G QSFP+ → 4×10G SFP+ breakout DAC: A common bridge between 40G uplinks and 10G endpoints.
- 100G QSFP28 → 4×25G SFP28 breakout DAC: One of the most common modern patterns for turning a single 100G port into multiple 25G access links.
- 200G → 2×100G breakout DAC: Used where the fabric runs higher speeds but the next layer still consumes 100G.
- 400G → multiple 100G (breakout families): Widely used as 400G becomes a spine/uplink baseline while parts of the network remain at 100G.
Breakout exists because data centers rarely upgrade all layers at once. It lets a higher-speed layer “feed” the next layer efficiently.
C) Transitional / special cases
Some environments include interface transitions driven by platform history (older optics families, unusual port mixes, or niche fabric requirements). These exist to solve a specific interoperability or lifecycle problem rather than being the default data center pattern.
Why many teams choose DAC over “transceivers + fiber” for short reach?
For short, controlled physical paths, DAC often wins on engineering practicality:
- Fewer components: fewer parts to source, track, and replace
- Simpler operations: straightforward patching and labeling
- Low-latency feel: short copper links behave predictably inside a rack plan
- Cost efficiency: you avoid building an optical short-reach chain where it’s not needed
That said, DAC is not a universal answer. Once distance, routing complexity, or site rules push beyond short-reach practicality, optical solutions become more appropriate.
If your design intent is specifically 100G to four 25G endpoints, this is a practical product entry point:
100G to 4 x 25G Direct Attach Coppers
(That page serves as a procurement entry where customers can match the configuration to their platform environment.)
FAQs
Q1: Is a DAC basically a transceiver?
A: No-DAC is a cable assembly with fixed “module-style” ends, not a removable transceiver + separate cable system.
Q2: What does “breakout” mean in physical terms?
A: It maps a high-speed port’s signaling into multiple independent lower-speed interfaces (e.g., one 100G port into four 25G links).
Q3: Does every QSFP28 port support 4×25G breakout?
A: No-breakout support is platform- and mode-dependent; treat it as a capability to verify.
Q4: After breakout, are the 25G legs “real ports”?
A: Yes-each leg behaves like a normal interface from the switch’s perspective once the port mode is applied.
Q5: Passive vs active: what’s the one-sentence difference?
A: Passive has no in-cable signal conditioning; active adds electronics for conditioning.
Q6: Why do higher generations (200G/400G/800G) have more variants?
A: Because fabrics upgrade in layers, and newer port families are designed to support multiple link patterns (straight-through and breakout) for staged migration.
Q7: When should I stop thinking about DAC and move to optics?
A: When physical distance, routing unpredictability, or facility standards exceed what’s practical for short-reach copper operations.
Q8: What’s the fastest way to confirm a breakout plan is feasible?
A: Validate platform port-mode capability and confirm the design’s intended endpoint behavior (speed/mode expectations) before deploying at scale.
Summary
DAC remains a core data center interconnect option because it’s a short-reach, operationally simple way to connect high-speed ports-especially when you classify it correctly by connector family, connection shape, and speed generation. Once you understand the “type map,” the popular patterns (like 100G QSFP28 → 4×25G SFP28 breakout) become easy to place in your architecture-and easy to standardize across racks.
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