In network switching, various connectivity options are available, including optical modules paired with fiber, Active Optical Cables (AOC), and Direct Attach Cables (DAC). DACs can be further classified into Active Copper Cables (ACC), Active Electrical Cables (AEC), and passive DACs. But what exactly are these solutions, and how do they differ from each other? This article delves into their definitions, advantages, applications, and distinctions to help you make informed decisions about selecting the most suitable and cost-effective connectivity solution for your network architecture, particularly in AI clusters where AOC, DAC, ACC, and AEC are frequently used.
1. Overview of Active Optical Cable (AOC)
An Active Optical Cable (AOC) combines fiber-optic cables with optical-electrical converters at both ends, enabling long-distance, high-speed, and low-power data transmission. AOCs integrate optical transceivers and fiber optic cables into a single unit, enhancing signal quality and reliability while mitigating the distance and signal degradation issues common with traditional cables. Each AOC features two integrated optical modules connected by a fiber optic cable, with embedded laser components designed to minimize optical port contamination and boost overall reliability. AOCs reduce optical components and eliminate the need for Digital Diagnostic Monitoring (DDM), balancing cost and performance.
Advantages and Disadvantages of AOC
AOCs offer high transmission speeds, long-distance capabilities, low power consumption, and are lightweight and easy to install, making them ideal for data centers and HPC environments. Unlike bulkier copper-based technologies, AOCs provide high-density connectivity in space-constrained environments and are resistant to electromagnetic interference (EMI), reducing packet loss and enhancing data transmission. Consequently, AOCs are particularly suitable for connecting data centers and HPC setups.
A network setup using Cisco Nexus 3432D-S switches for both spine and leaf layers, with 400G AOCs and 400G-4x100G AOCs connecting the devices
NADDOD 400G-QDD-AOC Application in 400G Switch to 400G Switch Link
While AOCs have notable advantages, they also present challenges. The integrated optical ports eliminate cleaning needs, but if an AOC module fails, the entire cable must be replaced, reducing convenience compared to detachable optical modules. Additionally, AOCs require predetermined transmission distances before shipping, limiting flexibility after delivery. Their design complexity and higher costs result from optical signal transmission and conversion, leading to increased power consumption compared to DACs. Currently, 800G OSFP AOCs are not available due to the physical constraints of OSFP connectors, which are large and heavy, making them prone to breakage during installation and unsuitable for AOC formats. Consequently, OSFP to OSFP/2xOSFP AOC products are not currently offered by manufacturers or the market.
2. Overview of Direct Attach Cable (DAC)
Direct Attach Cables (DAC) are high-speed copper cables designed for short-range connectivity in data centers, connecting networking devices like switches, servers, and storage units. DACs feature fixed copper connectors and are ideal for low-latency, high-performance data transmission. They are commonly used for distances up to 7 meters due to their efficient cost and reliability. DACs come in passive and active variants, with Active Copper Cables (ACC) and Active Electrical Cables (AEC) further enhancing signal quality.
NADDOD's DAC Cables
DAC cables do not incorporate optical-electrical conversion modules, making them highly cost-effective. The simple copper connectors at each end keep costs down while still providing high-speed performance. In data centers, DACs are commonly used to connect servers to storage area networks (SANs) and between switches and routers. DAC's cost efficiency and reliability for short-distance connections have made it a favored solution for interconnectivity within data centers, especially for distances up to 7 meters. For instance, during the construction of 128 HGX H100 clusters, we discussed how using DACs and single-mode modules could reduce costs by approximately 35% compared to multimode modules configurations.
Advantages of DACs in Large AI Clusters
- High-Speed Performance:DACs support data rates up to tens of Gbps, providing high bandwidth and rapid data transfer over short distances, surpassing traditional copper and fiber cables.
- Cost-Effective:They are more affordable than fiber optic solutions, making them ideal for short-range, high-speed connections.
- Low Power Consumption: DACs operate at lower temperatures and consume less power than fiber optics, which is ideal for energy-efficient data centers. For instance, Quantum-2 InfiniBand switches use 747W with DACs versus 1,500W with multimode optics.
- Efficient Heat Dissipation and Stability: The copper construction allows excellent heat dissipation, reducing overheating risks and enhancing stability. DACs are also more robust and durable than optical modules, minimizing jitter, latency, and faults common with fiber optics, as they directly transmit electrical signals without conversion delays. This reliability makes DACs particularly valuable in large-scale AI clusters, where low latency and minimal fault risk are crucial.
- Installation and Maintenance: DACs eliminate the need for complex fiber infrastructure, simplifying deployment and reducing costs. Their durability and straightforward setup further reduce maintenance complexity and enhance overall network stability in high-density environments, making them increasingly popular in large AI clusters for enhancing performance while maintaining cost-effectiveness.
Disadvantages of DACs
- Distance Limitation: DAC cables are limited by the inherent properties of copper, restricting their effective transmission range to short distances (typically less than 7 meters). This makes them unsuitable for longer-distance connections.
- Limited Flexibility: DAC cables are bulkier and less flexible compared to fiber optics, making them harder to manage in dense data center environments where cable management and flexibility are critical.
- Susceptibility to EMI: Since DAC cables use copper conductors, they are more vulnerable to EMI, particularly in high-density environments where numerous electronic devices are in close proximity. This can negatively affect signal stability and data integrity.
The limitations of DACs have led to the development of ACC and AEC, which will be explained in the following sections.
Difference Between AOCs and DACs
AOCs and DACs use the same form factors and electrical interfaces at both ends, such as SFP, QSFP, and other standards, ensuring compatibility with system components like switches and NICs. AOCs contain electro-optical conversion chips within the module, including key components like CDR, Retimer/Gearbox, Laser, and Photodetector (PD), modulating electrical signals into optical signals for transmission. In contrast, DACs are passive copper media, consisting of high-speed differential coaxial cables (twinax cables) directly soldered within the module, with shielding and outer coverings, allowing direct end-to-end electrical signal transmission without conversion.
3. Understanding Active Copper Cables (ACC) and Active Electrical Cables (AEC)
Passive DACs have been vital in data centers for their low power use and cost-effectiveness, even at 800G. However, as data rates rise, DACs' effective range has shrunk, now limited to just 3 meters at 800G. The increase in channels from 4 to 8, and eventually to 16 also causes cables to thicken, complicating management and airflow within server racks. While AOCs are suggested for long distances, their high power consumption and cost make them less ideal for mid-range links, leading to the development of ACC and AEC, which offer a balanced solution for medium distances.
Difference Between AECs and ACCs
ACC (Active Copper Cable): ACCs use a redriver chip architecture, employing Continuous Time Linear Equalization (CTLE) to boost signals on the receiver (Rx) side, acting as analog signal amplifiers.
AEC (Active Electrical Cable): AECs are more advanced, using a retimer chip architecture to amplify and equalize signals at both transmitter (Tx) and receiver (Rx) ends, with added clock data recovery (CDR) to reduce jitter, offering higher signal integrity and clearer data transmission.
Both AEC and ACC are active cables. ACC amplifies electrical signals, while AEC not only amplifies signals but also includes clock data recovery (CDR) to reduce signal jitter. Unlike copper cables that suffer from RF losses due to the skin effect, AECs use high-frequency carriers to minimize loss. AECs offer longer reach (up to 5-7 meters) due to retimers and FEC capabilities, making them suitable for demanding applications. They provide higher signal integrity by resetting loss and timing planes. While AECs are more efficient and consume less power than optical solutions, they do require some power (6-12 watts). DACs are better for shorter connections (2-3 meters), are cheaper, and do not require power, making them ideal when power consumption is a concern.
AOC/DAC/ACC/AEC Comparison
DAC vs. AOC vs. ACC vs. AEC
4. NADDOD’s AOC&DAC Cables
NADDOD stands at the forefront of providing cutting-edge connectivity solutions tailored for the evolving demands of AI-driven data centers, edge computing, and high-performance computing (HPC) environments. Our range of Active Optical Cables (AOC) and Direct Attach Cables (DAC) is engineered to deliver unmatched performance, reliability, and scalability.
AOC Cables
400G QSFP-DD to 2x200G QSFP56 |
400G QSFP-DD to 4x100G QSFP56 |
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200G QSFP56 to 2x100G QSFP56 |
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100G QSFP28 to 4x25G QSFP28 |
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40G QSFP+ to 4x10G SFP+ |
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DAC Cables
800G OSFP DAC |
800G Breakout DAC |
400G QSFP-DD/QSFP112 DAC |
400G Breakout DAC |
200G QSFP56 DAC |
200G Breakout DAC |
100G SFP-DD DAC |
100G Breakout DAC |
100G/56G/40G QSFP+ DAC |
40G Breakout DAC |
25G/10G SFP28 DAC |
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At NADDOD, we are committed to advancing the future of network connectivity by providing products that meet the stringent requirements of next-generation data centers. Whether you’re scaling your AI infrastructure, enhancing your HPC environment, or optimizing your edge computing capabilities, NADDOD’s optical connectivity solutions offer the performance, reliability, and flexibility you need to succeed.
Our products are rigorously tested to ensure they deliver exceptional performance in the most demanding environments. With NADDOD, you’re not just investing in connectivity; you’re partnering with a leader in network technology innovation. Explore how our solutions can help you stay ahead in a rapidly evolving digital landscape.
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