Copper DAC and LACC Cables Overview
Copper cabling is the lowest cost, lowest latency, and lowest power way to interconnect high-speed systems together. DAC cables “directly attach” the electrical subsystems together, hence the name Direct Attach Copper cables (DACs). DAC copper cables are mainly used for inside system racks linking compute servers to storage subsystems within the 3-meter maximum length.
Offered in twin-port OSFP, OSFP, and QSFP112 ends
DACs offer length of 0.5m to 2-meters for straight cables, and up to 3-meters for splitters
Power consumption is 0.1 Watts per end
Thin 30AWG wire is used for short lengths up to 1-meter for 800G-to-800G cables
Thicker 26AWG wire with more shielding is used for 2- to 3-meters and splitters
DACs 1:2 and 1:4 QSFP112 splitters have been discontinued – use transceivers.
DAC Twin-port OSFP Straight and Splitters using OSFP
LACC cables are used to extend the low-cost, low-power, and low latency of copper cables from 3- to 5-meters which can span several racks. LACCs are essentially DAC cables with an additional IC in each end to boost the signal power and noise reduction. This cable uses a pre-emphasis type of circuit, aka linear ACC, is a signal amplifier and boosts the signal well above the noise enabling a longer length cable.
Unlike HDR/200GbE ACCs that use DSPs, pre-emphasis LACCs have very low power and low latency.
LACC configuration shadows the DAC line at longer
Offered in twin-port OSFP for 800G-to-800G switch-to-switch links at 3, 4, 5-meter lengths.
LACCs 1:2 and 1:4 OSFP splitters have been discontinued – use transceivers.
Thin 30AWG wire is used for short lengths of 3- and 4-meters.
Thicker 26AWG wire with more shielding is used for 5-meter splitters.
AOC splitters are offered for backwards compatibility by downshifting the modulation rate of switches to 50G-PAM4 enabling links Quantum-2 and SN5600 OSFP Ethernet switches to link to 200G HDR Quantum QSFP56 switches and HDR ConnectX-6/ConnectX-7 and BlueField-3 DPU adapters.
1:2 AOC splitters using twin-port OSFP 2x200G-to-two 200G QSFP56s based on 50G-PAM4 is offered up to 30-meters for linking 400GbE/NDR switches to HDR/200GbE switches, and adapters. In the Quantum-2 and Spectrum SN5600 switches, the 8-channel cages are down shifted to 50G-PAM4 modulation and split into 1:2 200Gb/s using QSFP56. This AOC can also support 200G-to-two 100Gb/s QSFP56 links as 2x 50G-PAM4. AOCs are offered in IHS and RHS versions at the 2x200G end.
These 2 AOCs are the only optical links available to link between 100G-PAM4 OSFP and 50G-PAM4 QSFP56 systems as separate transceivers are not available.
AOCs for backwards compatibility to HDR/200GbE
No AOCs for 100G-PAM4 Series
AOCs consist of two or more transceivers with the fibers bonded inside and not detachable. This creates and entire, plug-and-play cable assembly like copper cables but at much longer lengths. While used extensively in EDR, HDR and 100GbE, 200GbE systems, AOCs are not offered for 100G-PAM4 Ethernet or InfiniBand systems. The twin-port OSFP and OSFP ends are heavy and very likely to break the fibers during installation. An 800G-to-4x 200G AOC would have five large OSFP connectors. Additionally, AOCs are typically used for <10m reaches and AI datacenters are much larger.
Additionally, complex fiber infrastructures are better addressed with connectorized fibers, and they can support future line rate upgrades with minimal changes.