Nearly a third of the exam and the domain that decides real deployments. Master MPO/MTP fiber, when to use transceivers vs copper, polarity, insertion loss, and the connector cleanliness that keeps a link alive.
An AI fabric can involve thousands of fiber connections, and every one is a potential point of failure. A single mis-seated, mis-polarized, or contaminated connector can silently degrade a link and drag down an entire distributed training job. This domain tests whether you can install that cabling correctly, at scale, the first time.
We'll cover the four things this domain lives on: the connectors and fiber types, the choice between optical and copper media, polarity, and signal integrity (loss + cleanliness) — then how to install it cleanly.
High-density AI cabling runs on multi-fiber connectors, not single strands. The workhorse is the MPO/MTP connector.
MPO = Multi-fiber Push-On, a connector carrying many fibers in one ferrule (commonly 8, 12, 16, or 24). MTP is a branded, higher-performance MPO. One connector = many links, which is what makes high density possible.
Small core, laser source, long distances. Yellow jacket by convention. Used where runs are longer or future-proofing bandwidth.
Larger core, shorter reach, lower-cost optics. Aqua (OM3/OM4) or lime (OM5) jackets. Common for shorter in-row links.
The familiar two-fiber connector for lower-density links. You'll still meet it, but MPO/MTP dominates the high-density fabric.
MPO = Many Push On. When you see MPO/MTP, think "one connector, many fibers" — the whole point of high-density cabling.
Not every link needs optics. Matching the medium to the run length is a core competency — over-using optical transceivers wastes power and money; under-using them fails the link.
| Option | What it is | Typical reach | Best for |
|---|---|---|---|
| DAC (Direct Attach Copper) | Fixed copper cable with connectors built on; no optics | Very short (about ≤3 m) | In-rack / adjacent-rack links; lowest cost & power |
| AOC (Active Optical Cable) | Fixed cable with optics permanently attached at both ends | Medium (tens of metres) | Row-level runs too long for copper, fixed endpoints |
| Transceiver + fiber | Pluggable optic (e.g. OSFP / QSFP-DD) with separate fiber | Long / flexible | Inter-rack, spine links; swappable, longest reach |
Remember from Domain 1: the NVLink spine inside the rack is copper (short, low power), while scale-out links between racks use fiber with transceivers. DAC and AOC sit between those extremes.
"Always use fiber" is wrong. For a 2-metre link a DAC is cheaper, lower-power, and perfectly reliable; forcing an optical transceiver there wastes both. Choose by distance and endpoint flexibility, not by habit.
Every fiber link must connect a transmitter (TX) at one end to a receiver (RX) at the other. With multi-fiber MPO systems, keeping that mapping correct across patch panels and trunks is polarity management. TIA-568 defines three methods:
| Method | How it maintains polarity | Note |
|---|---|---|
| Method A (straight) | Straight-through trunk; one end uses a crossed patch cord to flip TX/RX | Requires mixed patch-cord types |
| Method B (reversed) | Trunk is reversed (key-up to key-down); same patch cords both ends | Common; watch keying |
| Method C (pair-flipped) | Fiber pairs are flipped within the trunk | Uses straight patch cords |
You don't have to memorize every wiring detail, but you must know that mixing polarity methods or the wrong patch-cord type breaks the TX→RX mapping and the link won't come up. Pick one method and apply it consistently across the whole channel.
Two things quietly kill fiber links: too much loss, and dirt.
Every connector and splice adds insertion loss (measured in dB). A channel has a loss budget — the total the link can tolerate and still work. More connectors, longer runs, and dirty or poorly mated connections all eat into it. Installations are tested against this budget (Domain 6).
Contamination is the single most common cause of fiber link problems. A speck of dust on a ferrule end-face — invisible to the eye — can scatter light, add loss, and even damage the mating connector under pressure. The rule the whole industry lives by:
"Inspect before you connect." Inspect every end-face with a scope, clean if needed, re-inspect, then mate. Never connect a dirty or unknown connector, and always cap unused connectors.
At 30%, cabling questions are where the exam is won. Practice MPO/MTP, media choice, polarity, and loss with explained answers.
Practice High-Density Cabling →NCP-ARI-style questions on Domain 4. Choose an answer to see the explanation.
Pick the cabling area you want to shore up.
Tap a card to flip it.
For educational use only · Not affiliated with or endorsed by NVIDIA Corporation. Connector types, polarity methods (TIA-568), and media reach figures follow industry standards and can vary by product; always follow the manufacturer's cabling documentation and the NVIDIA DGX SuperPOD cabling design guide for specifics.
