DACs, ACCs, AOCs, and Transceiver Interconnects

There are two main ways to link switches and adapters by using either copper wires or optics. Copper has a length or reach limitation of less than 5 meters and two different optical technologies enable using different technologies for the least cost to fit the application. Multimode short reach optics typically have 50m-100m maximum reaches. Single-mode, mid to long reach optics are typically 100m, 500m 2km, 10km, and 40km maximum reaches.

There are many different technology combinations of optical connector, plugs, optical connectors, electronics, and optics. This document concentrates on high-volume products offered by specifically NVIDIA for accelerated AI data center-oriented cables and transceivers.

Direct Attach Copper cables (DACs) consist of a connector plug (QSFP or OSFP) and basically copper wires and shielding. DAC wires tend to radiate high-speed electrical signals, like radio antennas, hence are limited to 2, 3, 5-meter lengths depending on the speed. DAC cables are very popular due to their low cost, almost no power consumption and latency delay. DAC cables are complete assemblies and cannot separate into plugs and wires.

Active Copper Cables (ACCs) are DAC copper cables but include a signal booster IC in the end to extend the length to 3, 4, and 5-meters depending on the speed. ACC are very popular due to their lower cost than optics, very low power consumption, and low latency delay. ACC cables are complete assemblies and cannot separate into plugs and wires. NVIDIA’s 100G-PAM4 ACCs, aka linear ACCs, use a pre-emphasis IC that offers very low latency, only 1.5-Watts, and can reach lengths of 5-meters.

Multimode Transceivers use a large, 50-um light carrying core in the optical fiber. A digital pulse consists of many individual photons that travel down the fiber in different paths or “modes” bouncing off the fiber walls. As the fiber length increases, the different photon paths arrive at the receiver at different time and distort the signal pulse at the receiver receiver limiting the maximum reach.

• Multimode maximum length is 100-meters for 50G-PAM4 and 25G-NRZ but reduces to 50-meters for 100G-PAM4. Multimode transceivers and AOCs use 850nm vertical cavity surface emitting lasers (VCSELs) and due to the ease of aligning fibers with lasers and detectors are significantly less expensive than single-mode transceivers.

• Multimode transceivers are described as Short Reach or SR, SR4, SR8, etc.

• Parallel optical connectors use 8 or 16-fiber Multiple-Push-On (MPO) connector or 2-fiber Lucent Connector (LC). UPC is an Ultra flat polish and APC is an Angled polish.

Active Optical Cables (AOCs) consist of two multimode optical transceivers with the optical fibers bonded inside and not removable. AOCs offer lower costs than two transceivers and separate fibers as only electrical testing is required in manufacturing. AOCs are offered up to 100 meters and are typically used in configurations with easy cabling access. AOC cables are complete assemblies and cannot separate into plugs and fibers. AOCs are very popular at 100G, 200G, and 400GbE speeds.

• AOCs based on 100G-PAM4 are not offered with 800G twin-port OSFPs, due to the large OSFP connector size and possibility of breaking the fiber during installation. A 4x 200G AOC splitter would have five, large OSFP connectors and ne very difficult to install in crowded infrastructures.

• OSFP AOCs are offered for backwards compatibly links of 2x200G-to-2x200GbE and 2xHDR and 2xHDR100.

Single-mode Transceivers use a tiny 9-um light carrying core in the optical fiber, which is difficult to align lasers and detectors in manufacturing, hence are more expensive than multimode transceivers. An individual 1310nm light pulse travels down the fiber in a single path or single mode and can travel great distances without distorting the pulse. Single-mode optics used in data centers is offered up to 40km.

For 100G using 4x25G-NRZ single mode transceivers, the optics was defined by several individual industry groups and the IEEE.

For 50G-PAM4 and 100G-PAM4, the IEEE group standardized on different naming terminologies that everyone agreed to:

• PSM4 became Datacenter Reach 4-channel (DR4) for 500m

• CWDM4 became Far Reach 4-channel (FR4) for 2km

As a result, the naming of an optical transceiver consists of many descriptors such as:
“400G DR8 single-mode 500m, 8-channel electrical, 4-channel multiplexed optical, MPO-12/APC optical connector”.