---
title: "Why Your 400G DAC Cables Work at 3m But Not at 5m"
type: tutorial
target_audience: technical
score: 9/10
---

Direct Attach Copper cables at 400G have a physical cutoff point that is not a soft degradation but a hard failure boundary — a DAC cable that works perfectly at 3 meters will produce complete link failure at 5 meters under the same conditions, and the failure is not a flaky link or a marginal BER condition. It is a link that will not come up, or a link that comes up and immediately drops. Understanding why this happens requires understanding how PAM4 signaling interacts with the frequency-dependent attenuation characteristics of the coaxial or twinax copper medium, and why the number of signal levels in PAM4 makes this interaction far more consequential at 400G than it was at 100G with NRZ signaling.

Copper signal attenuation in the twinax medium used in SFP28, QSFP28, QSFP-DD, and OSFP DAC cables increases with frequency following a skin-effect model where attenuation in dB/meter scales approximately with the square root of frequency. At 26.5625 Gbaud — the symbol rate for 100G NRZ on a 4-lane QSFP28 DAC — the copper attenuation over a 5-meter 26 AWG twinax is approximately 18 to 22 dB depending on the specific cable construction. At the same cable length, a QSFP-DD 400G DAC operating at 53.125 Gbaud PAM4 per lane sees approximately 26 to 32 dB of insertion loss per lane, because the higher baud rate components of the PAM4 signal experience greater skin-effect attenuation. The SerDes and cable driver in the QSFP-DD module must overcome this with transmit equalization (pre-emphasis) and receiver equalization (CTLE and DFE) to reconstruct the original PAM4 signal at the receiver.

The equalization budget is finite and technology-specific. The receiver equalization in a QSFP-DD direct attach cable is implemented in the cable assembly's end connectors, not in the switch ASIC, because the host electrical interface for DAC cables is specified as a passive electrical specification — the cable assembly is responsible for meeting the signal integrity requirements at the host connector. The maximum equalization capability designed into typical QSFP-DD passive DAC cable assemblies supports insertion loss up to approximately 22 to 24 dB at Nyquist frequency (26.5625 GHz for 53.125 Gbaud PAM4). Below this loss limit, the cable assembly delivers a compliant signal to the host. Above it, the equalized eye remains open but with insufficient eye height and eye width to reliably decode PAM4 symbols with four distinct amplitude levels.

The reason the failure is sharp rather than gradual is the PAM4 amplitude level spacing. In a PAM4 signal with four amplitude levels — labeled 0, 1, 2, 3 — the spacing between adjacent levels is one-third of the total signal swing. After equalization that compensates for the bulk frequency roll-off but adds noise through the DFE tap adaptation, the effective noise floor relative to the inter-symbol spacing determines the symbol error probability. When the insertion loss is 22 dB (within equalization range), the equalized eye height at the decision threshold is above the noise floor with margin. When the insertion loss reaches 28 dB (beyond equalization range), the equalized eye height collapses to a small fraction of the noise floor and symbol error rate increases exponentially rather than gradually. This exponential behavior is why a 3-meter cable works and a 5-meter cable fails without a transitional zone of marginal performance.

The insertion loss versus length relationship for common AWG gauge twinax used in QSFP-DD DAC cables places the 22 to 24 dB Nyquist frequency insertion loss limit at approximately 3.0 to 3.5 meters for 26 AWG and approximately 4.0 to 4.5 meters for 24 AWG. This is why QSFP-DD passive DAC cables are typically available in 0.5, 1, 1.5, 2, and 3 meter lengths, but rarely in 4 or 5 meter lengths — the 4 to 5 meter range is where 26 AWG passive DAC cables fail and where 24 AWG passive DAC cables are at their limit. Manufacturers who sell 5-meter "passive" QSFP-DD DAC cables are either using 22 AWG cable (heavier, stiffer, harder to route in dense racks) or are actually selling active cables with integrated signal conditioning that they are labeling as passive for procurement simplicity.

Active Electrical Cables, also called active DAC or AEC, address the distance limitation by integrating a retimer or re-driver IC in the cable assembly connectors. The retimer fully reshapes the PAM4 signal, effectively resetting the signal integrity budget at each end rather than relying on passive equalization across the full cable length. AEC cables at QSFP-DD 400G support lengths of 5, 7, and in some implementations 10 meters, at the cost of power consumption — typically 1.5 to 2.0 watts per end connector, adding 3 to 4 watts total to the link power budget. AEC cables also require the host SerDes to operate correctly with the retimer's electrical interface characteristics, which is generally the case for production platforms but should be validated against the specific platform datasheet or QSFP-DD vendor qualification list. The latency of AEC cables is approximately 50 to 100 nanoseconds higher than passive DAC cables due to the retimer pipeline, which is irrelevant for most applications but matters for precision-timing applications and some high-frequency trading infrastructure.

Active Optical Cables at 400G QSFP-DD use the same form factor with optical fiber replacing the copper twinax core. AOC cables support distances of 10 to 100 meters and beyond, are immune to electromagnetic interference, and have consistent insertion loss across length that is not subject to the skin-effect copper attenuation penalty. The per-port cost premium over passive DAC is typically $80 to $150 for a 10-meter QSFP-DD 400G AOC versus $40 to $70 for a 3-meter passive DAC. For spine-leaf rack architectures where port-to-port distances are under 3 meters, passive DAC is the correct choice. For architectures where port-to-port distances range from 3 to 7 meters — as in some oversubscription-optimized pod designs where spine switches are mounted above the leaf switches with cable runs through overhead management — AEC fills the gap between passive DAC reach and AOC cost. For distances above 7 meters, AOC or structured optical cabling is the correct solution.

Specifying DAC cable lengths for spine-leaf port distances requires measuring actual port-to-port paths in the physical rack layout, not assuming a nominal rack-unit distance. A 3-meter cable specified for a port that is 14U above its peer in the same rack will need to route through a cable manager, potentially adding 0.5 to 0.8 meters of additional path length. A passive DAC specified at exactly 3 meters for a 2.6-meter measured port-to-port distance with cable management overhead becomes a cable that routes with cable ties creating 5 cm radius bends at every direction change — which does not cause electrical loss in passive copper DAC the way it would on optical fiber, but does cause mechanical stress at the connector boot over time. Specifying cables 0.5 meters longer than the measured path length gives routing latitude without pushing into the attenuation-limited length range.
