{"id":13132,"date":"2026-08-24T15:58:27","date_gmt":"2026-08-24T07:58:27","guid":{"rendered":"https:\/\/ascentoptics.com\/blog\/?p=13132"},"modified":"2026-08-24T15:58:27","modified_gmt":"2026-08-24T07:58:27","slug":"infiniband-cable-types","status":"publish","type":"post","link":"https:\/\/ascentoptics.com\/blog\/infiniband-cable-types\/","title":{"rendered":"InfiniBand Cable Types: DAC, AOC, and Optical Cable Selection Guide"},"content":{"rendered":"

InfiniBand<\/u><\/a>\u00a0cables primarily fall into four\u00a0categories: DAC (Direct Attach Copper), active copper cables (ACC and AEC), AOC (Active Optical Cable), and optical modules with fiber optic cables. DACs utilize copper cores, offering low cost and low power consumption, making them suitable for short-range interconnects within racks. AOCs integrate optical transceivers with fiber optics, combining flexibility with low power consumption and covering distances ranging from a few meters to several tens of meters. Optical modules paired with fiber optics support long-distance transmission; while they offer the highest performance, they also come at a higher cost.<\/p>\n

 <\/p>\n

 <\/p>\n

What Are InfiniBand Cable Types?<\/strong><\/h2>\n

InfiniBand cable types are the physical interconnect options that carry InfiniBand<\/u><\/a>\u00a0traffic between switches, network adapters, and accelerators. They fall into four categories: passive direct attach copper (DAC), active copper cables (ACC and AEC), active optical cables (AOC), and pluggable optical transceivers paired with fiber.<\/p>\n

The right choice depends almost entirely on distance and power. A cable that works perfectly inside a rack can fail outright one row over, and the power penalty for overreaching with optics is severe at 400G and 800G. The table below is the map you will come back to.<\/p>\n

 <\/p>\n\n\n\n\n\n\n\n
Cable type<\/b><\/strong><\/td>\nMaximum reach<\/b><\/strong><\/td>\nPower per end<\/b><\/strong><\/td>\nRelative cost<\/b><\/strong><\/td>\nBest use<\/b><\/strong><\/td>\n<\/tr>\n
Passive DAC<\/td>\n2-3 m<\/td>\nunder 0.5 W<\/td>\nLowest<\/td>\nIn-rack GPU to switch<\/td>\n<\/tr>\n
ACC \/ AEC (active copper)<\/td>\n3-7 m<\/td>\n1-4 W<\/td>\nLow<\/td>\nAdjacent racks<\/td>\n<\/tr>\n
AOC (active optical)<\/td>\n3-100 m (150 m for HDR)<\/td>\n12-17 W<\/td>\nMedium<\/td>\nMid-range, light cabling<\/td>\n<\/tr>\n
Optical transceiver + fiber<\/td>\n50 m-2 km+<\/td>\n8-30 W<\/td>\nHighest<\/td>\nLong reach, structured cabling<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

 <\/p>\n

 <\/p>\n

 <\/p>\n

The Four InfiniBand Cable Types, Compared<\/strong><\/h2>\n

Passive DAC: The In-Rack Workhorse<\/strong><\/h3>\n

A passive direct attach copper cable is a length of copper twinax with connectors on both ends and no electronics in between. It draws nearly zero power, adds almost no latency, and costs less than any other option.<\/p>\n

The tradeoff is reach. At higher speeds, the signal degrades quickly over copper, so passive DACs stay short. HDR passive DACs top out around 2 meters, and NDR DAC reach is shorter still. That is why passive DACs live almost exclusively inside a single rack, connecting GPUs to top-of-rack switches.<\/p>\n

 <\/p>\n

ACC and AEC: The Middle Ground Most Guides Skip<\/strong><\/h3>\n

When a link is too long for passive copper but too short to justify optics, active copper cables fill the gap. ACC (active copper cable) uses linear equalization to clean up the signal. AEC (active electrical cable) goes further with a digital signal processor that retimes the signal entirely.\u00a0ACC and AEC implementations may use different equalization, signal-conditioning, or retiming techniques depending on the vendor and cable design.<\/p>\n

This band matters most at NDR. Because NDR passive DAC reach collapses below HDR’s, the 3-to-7 meter span between adjacent racks needs an active copper solution. An ACC draws roughly 1.5 W at 800G, and an AEC draws under 1.5 W on the OSFP end. That is far cheaper than running optics for a 5-meter hop.<\/p>\n

 <\/p>\n

AOC: Active Optical Cable for Mid-Range<\/strong><\/h3>\n

An active optical cable looks like a DAC but replaces the copper with fiber and bonds an optical transceiver into each connector. It carries signal over 100 meters, or up to 150 meters for HDR, while staying lighter and more flexible than copper.<\/p>\n

AOCs earn their place in dense rows where heavy copper bundles would block airflow. The cost is power: an AOC draws 12 to 17 W per end at 400G and 800G, and its fixed length means you cannot patch it through a structured panel. AOCs generally consume more power than passive copper cables because each end contains active optical components.<\/p>\n

One important caveat applies at NDR. For NDR deployments, both AOCs and pluggable optical transceivers may be used, depending on the required distance, cabling architecture, power budget, and platform qualification. Pluggable optics are generally preferred when structured cabling and flexible link lengths are required.<\/p>\n

 <\/p>\n

Pluggable Optical Transceivers: Long Reach and Flexibility<\/strong><\/h3>\n

For anything beyond roughly 100 meters, the cable is no longer a single unit. You pair a pluggable optical transceiver with separate fiber, giving you reach from 50 meters to 2 kilometers or more, depending on the module and fiber type.<\/p>\n

This is the most flexible option and the only one that fits a structured cabling plant. It is also the most power-hungry, drawing 8 to 30 W per end at 400G and 800G. When a cluster spans rows, halls, or buildings, pluggable optics are the default.<\/p>\n

 <\/p>\n

\"The<\/p>\n

 <\/p>\n

 <\/p>\n

InfiniBand Cable Form Factors and Connectors by Generation<\/b><\/strong><\/h2>\n

InfiniBand cable types also differ by connector, and the connector is tied to the speed generation. Buying the right cable type with the wrong form factor is the most common procurement error.<\/p>\n

 <\/p>\n\n\n\n\n\n\n\n
Generation<\/b><\/strong><\/td>\n\n

Speed<\/b><\/strong><\/p>\n<\/td>\n

Form factor<\/b><\/strong><\/td>\n\n

Signal<\/b><\/strong><\/p>\n<\/td>\n<\/tr>\n

EDR<\/td>\n\n

100 Gb\/s<\/p>\n<\/td>\n

QSFP28<\/td>\n\n

4 x 25G NRZ<\/p>\n<\/td>\n<\/tr>\n

HDR<\/td>\n\n

200 Gb\/s<\/p>\n<\/td>\n

QSFP56<\/td>\n\n

4 x 50G PAM4<\/p>\n<\/td>\n<\/tr>\n

NDR<\/td>\n\n

400 Gb\/s<\/p>\n<\/td>\n

OSFP \/ QSFP112<\/td>\n\n

4 x 100G PAM4<\/p>\n<\/td>\n<\/tr>\n

XDR<\/td>\n\n

800 Gb\/s<\/p>\n<\/td>\n

OSFP<\/td>\n8 \u00d7 100G-class electrical lanes<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

 <\/p>\n

NVIDIA’s LinkX part numbers make the mapping concrete. The MCP1600-E is an EDR QSFP28 DAC<\/a>, available from 0.5 to 5 meters. The MCP1650-H is an HDR QSFP56 DAC<\/a>, capped at 2 meters. The MCP4Y10-N001 is a 400G NDR OSFP passive DAC<\/a>. Knowing these families helps you verify that a quoted cable matches the generation you are actually deploying.<\/p>\n

Two NDR-era details trip up buyers. First, OSFP and QSFP112 are not mechanically interchangeable, so an OSFP switch port and a QSFP112 adapter port need a breakout cable or a matched connector on both ends. NDR platforms may use OSFP or QSFP112 interfaces depending on the specific switch, adapter, and vendor implementation.\u00a0Second, NDR’s MPO\/APC fiber is not backward compatible with HDR or EDR transceivers. A mixed-generation deployment cannot reuse older patch cords.<\/p>\n

There is one more rule worth memorizing. For platforms using dual-port or twin-port configurations, follow the switch and adapter manufacturer’s port-mapping and cable qualification requirements. Avoid mixing media types or unsupported cable configurations within a validated port group.<\/p>\n

 <\/p>\n

 <\/p>\n

How to Choose an InfiniBand Cable<\/strong><\/h2>\n

Selection is a sequence, and following it in order eliminates most mistakes.<\/p>\n

    \n
  1. 1. Start with distance. <\/strong>Under 3 meters points to passive DAC. Three to 7 meters points to ACC or AEC. Up to 100 meters points to AOC. Beyond that, use pluggable optics and fiber.<\/li>\n
  2. 2. Match the form factor on both ends. <\/strong>Confirm whether each end is QSFP28, QSFP56, QSFP112, or OSFP, and use a breakout only when the two ends differ.<\/li>\n
  3. 3. Check the power budget. <\/strong>Count watts per end against your rack and switch power plan before ordering.<\/li>\n
  4. 4. Confirm the generation. <\/strong>HDR, NDR, and XDR each impose different reach limits on the same cable family.<\/li>\n
  5. 5. Decide on breakout needs. <\/strong>A fat-tree leaf-to-spine design may need to split one high-speed port into lower-speed links.<\/li>\n<\/ol>\n

     <\/p>\n

    Here is the cheat sheet in one sentence: in-rack is passive DAC, adjacent racks are ACC or AEC, mid-range is AOC, and long reach is pluggable optical transceivers with fiber.<\/p>\n

     <\/p>\n

     <\/p>\n

    \"How<\/p>\n

     <\/p>\n

     <\/p>\n

    InfiniBand Breakout and Splitter Cables<\/strong><\/h2>\n

    A breakout cable splits a single high-speed port into multiple lower-speed links. The most common patterns are 800G to 2 x 400G, 400G to 2 x 200G, and 800G to 4 x 200G.<\/p>\n

    Breakouts solve a structural mismatch that appears in almost every modern NVIDIA fabric. The switch side often uses OSFP, while the adapter side uses QSFP112. A single OSFP-to-QSFP112 breakout cable connects an NDR switch port to two ConnectX adapter ports, which is why the MCP7H50-H HDR splitter and similar NDR breakouts are staples of GPU cluster bills of material.<\/p>\n

    Use a breakout when the switch and the adapter run at different speeds or use different form factors. Skip it when both ends share the same form factor and speed, where a straight point-to-point cable is simpler and cheaper.<\/p>\n

     <\/p>\n

     <\/p>\n

    Deployment Caveats: Power, Firmware, and Qualification<\/strong><\/h2>\n

    The physical cable is only half the story. Three operational details separate a clean rollout from a late one.<\/p>\n

    First, budget power at the switch level, not the cable level. A fully populated 64-port XDR switch can add over 1,000 W from optics and active cables alone. A passive DAC adds almost nothing. An AOC adds 12 to 17 W per end. Those numbers compound across hundreds of ports.<\/p>\n

    Second, firmware has to match. Cable and transceiver firmware must be qualified against the exact switch and adapter firmware versions in your fabric. A cable that works in one firmware release can fail after an upgrade if the vendor did not validate the combination. This is the step that turns a two-hour install into a two-day troubleshooting exercise when skipped.<\/p>\n

    Third, test before you scale. Qualify one cable of each type and length on the actual hardware, then order in volume. The cost of a wrong cable type is not the cable itself. It is the power overrun, the airflow blockage, or the re-cabling of an entire row that a single failed qualification catches early.<\/p>\n

     <\/p>\n

    \"InfiniBand<\/p>\n

     <\/p>\n

     <\/p>\n

    Conclusion<\/strong><\/h2>\n

    InfiniBand cable types come down to four options: passive DAC for in-rack links, ACC and AEC for the 3-to-7 meter middle ground, AOC for mid-range, and pluggable optical transceivers for anything longer. Each choice trades reach, power, and cost, and the correct answer changes with every generation from EDR to XDR.<\/p>\n

    Keep three takeaways in mind. Match the connector to the generation, because QSFP28, QSFP56, QSFP112, and OSFP do not mix freely. Respect the NDR caveats, including MPO\/APC incompatibility and the fact that AOCs fade out at NDR. And always qualify firmware before you commit to a large order.<\/p>\n

    Choosing the right cable is not about buying the most expensive option. It is about buying the option that fits the distance, the form factor, and the power budget. That discipline is what keeps an AI cluster running on time and on wattage.<\/p>\n

     <\/p>\n

     <\/p>\n

    Frequently Asked Questions<\/strong><\/h2>\n

    1. What are the main types of InfiniBand cables?<\/b><\/strong><\/h3>\n

    The main InfiniBand cable options include passive DAC, active copper cables such as ACC and AEC, active optical cables (AOC), and pluggable optical transceivers used with fiber. The best option depends on link distance, data rate, power consumption, cabling architecture, and platform compatibility.<\/p>\n

    2. What is the difference between InfiniBand DAC and AOC?<\/b><\/strong><\/h3>\n

    DAC uses copper conductors and is generally preferred for short, in-rack connections because of its low cost and low power consumption. AOC integrates optical components and fiber into a fixed-length cable assembly, making it lighter and more flexible for short-to-medium optical links. However, AOC typically consumes more power than passive DAC.<\/p>\n

    3. When should I use an InfiniBand ACC or AEC cable?<\/b><\/strong><\/h3>\n

    ACC and AEC can be considered when a passive DAC cannot provide the required reach but an optical solution is not necessary. Active copper cables use electronics for signal conditioning or other signal-integrity functions to extend the practical reach of copper connections. The supported distance and power consumption vary by vendor and product.<\/p>\n

    4. Are QSFP112 and OSFP InfiniBand cables interchangeable?<\/b><\/strong><\/h3>\n

    No. QSFP112 and OSFP are different mechanical form factors and should not be treated as directly interchangeable. The cable or optical assembly must match the interfaces on both ends and be qualified for the specific InfiniBand platform.<\/p>\n

    5. What is an InfiniBand breakout cable?<\/b><\/strong><\/h3>\n

    An InfiniBand breakout cable connects one higher-speed port to multiple lower-speed ports. For example, a supported 400G port may be divided into two 200G links, while an 800G port may be divided into two 400G or four 200G links, depending on the platform and configuration. Breakout support should always be verified with the switch and adapter specifications.<\/p>\n

    6. How do I choose the right InfiniBand cable length?<\/b><\/strong><\/h3>\n

    Start with the physical distance between the devices and then check the qualified reach of the selected cable. Passive DAC is generally suited to short in-rack links, while ACC\/AEC can extend copper connectivity. AOC and pluggable optical transceivers provide optical alternatives when longer distances or more flexible cabling are required.<\/p>\n

    7. Are InfiniBand cables compatible across different generations?<\/b><\/strong><\/h3>\n

    Compatibility should not be assumed between InfiniBand generations. EDR, HDR, NDR, and XDR use different data rates, signaling technologies, form factors, and cable requirements. Always verify the cable’s data rate, connector, lane configuration, reach, and platform qualification before deployment.<\/p>\n

    8. What should I check before buying InfiniBand cables?<\/b><\/strong><\/h3>\n

    Before purchasing, check the InfiniBand generation, data rate, connector and form factor, required distance, cable type, power consumption, fiber and connector specifications for optical links, breakout requirements, and vendor qualification. For large AI or HPC deployments, it is also recommended to test representative cable configurations on the actual switches and adapters before placing a volume order.<\/p>\n

     <\/p>\n