400G DAC Cables Archives

1. Introduction

DAC (Direct Attach Copper) and AOC (Active Optical Cable) are two cost-effective, plug-and-play interconnect solutions widely adopted in data centers and enterprise networks. They play a critical role in 400G high-speed interconnects. Although emerging technologies like AI and HPC drive the evolution toward 800G and 1.6T optical communication, 400GbE remains dominant in most traditional and transitional enterprise data centers.

Due to their deployment convenience, 400G DAC and 400G AOC have become mainstream choices for short-reach interconnects. A deep understanding of their differences in physical structure, transmission performance, reach, power consumption, cost structure, and application scenarios is essential for network architects, procurement decision-makers, and technology selection teams.

2. Overview of 400G DAC and 400G AOC

Both DAC and AOC are essentially integrated cable assemblies with connectors on both ends. The mainstream 400G connector types include QSFP-DD, QSFP112, OSFP. The connectors at both ends can be homogeneous (same form factor) or heterogeneous (different form factors), depending on the port types of connected devices.

Based on application, these cables fall into two categories:

Direct-attach cables: Used for point-to-point connections between same-spec devices.
Breakout cables: Enable high-bandwidth ports to split into multiple lower-speed connections.

Direct cable with one connector at each end and breakout cable splitting from one connector into two cable branches.

2.1 What is DAC?

DAC cables are categorized into Passive DAC and Active DAC. Both use copper as the transmission medium, carrying electrical signals directly.

The key difference lies in: active DAC integrates signal amplifiers or retiming chips to compensate for high-frequency signal attenuation in copper, extending the effective transmission distance.

2.2 What is AOC?

AOC cables use fiber optics for transmission, converting electrical signals to optical signals via electro-optical components. They require host power to drive built-in lasers/receivers for O-E-O (optical-to-electrical-to-optical) conversion.

Currently, the vast majority of 400G AOC employs multimode fiber (MMF) transmission solutions, matching the standard MMF optical modules with a maximum reach of 100m.

Passive DAC, active DAC, and active optical cable shown as separate cable assemblies with different cable structures.

3. 400G DAC & AOC Types

3.1 DAC (Direct Attach Copper)

Model	Type	Rate	Form Factor	Transmission Distance
QDD-QDD-C	Direct	400G-to-400G	QSFP-DD to QSFP-DD	Passive: 0.5–3m Active: 1–5m
O400-QDD-C	Direct	400G-to-400G	OSFP to QSFP-DD	Passive: 0.5–2.5m Active: 1–5m
O400-2Q200-C	Breakout	400G-to-2x200G	OSFP to 2x QSFP56	Passive: 1–2m Active: 1–5m
Q400-2Q200-C	Breakout	400G-to-2x200G	QSFP-DD to 2x QSFP56	Passive: 1–3m Active: 1–5m

3.2. AOC (Active Optical Cable)

Model	Type	Rate	Form Factor	Transmission Distance
QDD-QDD-A	Direct	400G-to-400G	QSFP-DD to QSFP-DD	1–100m (MMF)
O400-QDD-A	Direct	400G-to-400G	OSFP to QSFP-DD	1–50m (MMF)
O400-2Q200-A	Breakout	400G-to-2x200G	OSFP to 2x QSFP56	1–50m (MMF)
Q400-2Q200-A	Breakout	400G-to-2x200G	QSFP-DD to 2x QSFP56	1–50m (MMF)

4. 400G DAC vs. 400G AOC

4.1 Power Consumption

DAC: No electro-optical conversion; transmits electrical signals directly. Ultra-low power: 0.1W (typical).
AOC: Requires power for lasers, drivers, and receivers. Average power: 8W (significantly higher than DAC).

4.2 Transmission Distance

DAC: Limited by copper signal attenuation. Passive DAC: ≤3m. Active DAC: ≤5m (suitable for intra-rack/adjacent rack connections).
AOC: Fiber-based, low-loss, EMI-resistant. Supports up to 100m, ideal for cross-rack/zone or small data center interconnects.

4.3 Weight & Cable Management

DAC: Heavy (hundreds of grams per cable) and stiff due to multi-core copper, posing challenges in dense cabling and maintenance.
AOC: Lightweight (1/3–1/2 of DAC’s weight). Easier to organize, bundle, and route, improving airflow and cooling efficiency.

4.4 Diameter & Bend Radius

DAC: Thick shielded twisted-pair structures (4.6mm diameter) to carry high-frequency differential signals. Rigid; minimum bend radius: 500mm (prone to signal degradation if bent).
AOC: Slim fiber optic design (~2.0mm diameter). Flexible; minimum bend radius: 30mm (space-efficient).

4.5 Signal Quality & EMI Resistance

DAC: Susceptible to electromagnetic interference (EMI) and signal attenuation at high frequencies.
AOC: Immune to EMI; lower bit error rate (BER) and higher signal integrity.

4.6 Cost Efficiency

DAC: Simple structure, requiring no expensive optical components. 30% cheaper than AOC (ideal for budget-sensitive short-reach links).
AOC: Higher cost due to fiber and optoelectronic components.

AOC and DAC compared by visible weight values, bend radius, cable diameter, and signal waveform examples.

5. Selecting DAC or AOC for 400G Data Centers

5.1 Selection Based on Distance Between Devices

The physical distance between devices is the primary factor in choosing DAC or AOC:

≤5m (same/adjacent racks): DAC is recommended for low cost and power efficiency.
Cross-rack or cross-room connections: AOC is mandatory to ensure signal integrity and reliable transmission.

5.2 Selection Based on Cabling Space

DAC: Suitable for deployments with ample cable trays, open pathways, and robust management systems.
AOC: Preferred in space-constrained, high-density environments due to its slim diameter, lightweight, and flexibility, which improve tidiness and airflow.

5.3 Selection Based on Budget

Cost-sensitive projects: DAC is the clear winner for its significant price advantage.

6. Conclusion

In the rapid evolution of modern data centers, DAC and AOC are not mutually exclusive competitors but complementary strategic partners. The optimal choice depends on application requirements (distance, bandwidth), physical constraints (space, bend radius) and budget priorities, both can serve as ideal cabling solutions for high-speed interconnects.

1. Necessity of Upgrading

The deep integration of cloud computing, AI, and 5G is driving exponential growth in global data traffic. Traditional 100G networks increasingly reveal bandwidth bottlenecks, struggling to meet demands for high throughput and low latency. Therefore, upgrading data center networks to 400G has thus become a strategic imperative to to accommodate future business expansion and ensure system performance.

2. Advantages of Upgrading to 400G Networks

2.1 Fourfold Increase in Throughput

400G optical modules doubles single-port bandwidth, delivering a four-fold increase over 100G. This transformation not only significantly enhances network transmission capacity but also enables data centers to more readily handle high-bandwidth scenarios, reserving ample space for future business evolution.

2.2 Doubled Port Density

Traditional 100G switches typically utilize a 1U form factor with 32 QSFP28 ports. In contrast, next-generation 400G switches employ a 2U size and can provide up to 64 QSFP-DD ports. This means a doubling of port count, with each port operating at four times the original speed.

2.3 Enhanced Performance

400G modules enable higher signal rates and lower processing latency, optimizing performance for massive parallel communication.

2.4 Optimized Resource Utilization

By consolidating multiple 100G links into fewer 400G connections, data centers can reduce physical interfaces/cables, simplifying topology and decreases cabling complexity. Such architectural streamlining not only minimizes potential failure points but also effectively lowers power consumption and thermal management demands.

3. 100G vs 400G Optics

3.1 Single-Channel Rate Enhancement

100G Modules typically use 4×25G NRZ modulation (4 channels at 25Gbps each), achieving a total throughput of 100Gbps.

400G Modules adopt 8×50G PAM4 or 4×100G PAM4, delivering breakthroughs in both channel count and per-channel data rate for higher throughput.

3.2 Cost-per-Bit Advantage

While 400G modules have a higher unit price than 100G, they offer lower cost per bit transmitted. As production scales and technology matures, per-bandwidth costs continue to decline, making 400G more economical for large-scale deployments (e.g., major cloud service providers, supercomputing centers).

3.3 Modulation Technology Evolution

100G primarily relies on NRZ (Non-Return-to-Zero) while 400G widely adopts PAM4 (Pulse Amplitude Modulation with four levels). PAM4 transmits twice the information of NRZ within the same bandwidth. Despite higher SNR (signal-to-noise ratios) requirements, advanced DSP algorithms ensure stable performance in high-speed links, making PAM4 the foundation for 400G+ rates.

3.4 Backward Compatibility

100G QSFP28 modules typically support downward compatibility with 40G speeds.

400G QSFP-DD modules offer greater flexibility, supporting 200G, 100G, and even 40G links.

4. 400G Ethernet Optics

4.1 400G QSFP-DD Optical Module

Type	Distance	Fiber Type	Channels	Interface	Use Case
SDR	≤50m	Multimode	4x	MPO-12/APC	Intra-rack server/switch links
SR8	≤100m	Multimode	8x	MPO-16/APC	High-density rack interconnects
DR4	500m	Single-mode	4x	MPO-12/APC	Intra-DC spine-leaf backbone
FR4	2km	Single-mode	4x/8x	LC Duplex	Campus/cross-building DCI
LR4/LR8	10km	Single-mode	4x/8x	LC Duplex	Metro DCI
ER4/ER8	40km	Single-mode	4x/8x	LC Duplex	Long-haul inter-city DCI
ZR	80km+	Single-mode	Coherent	LC Duplex	Ultra-long-haul backbone

4.2 400G QSFP-DD DAC (Direct Attach Cable)

Direct-Connect: 400G QSFP-DD to QSFP-DD DAC for short-distance interconnects between same-rate devices.
Breakout: Supports 400G-to-2x200G and 400G-to-4x100G, flexibly adapting to aggregation connections in leaf-spine architectures.
Key Features: Max length: 3m (copper-based, no optical conversion). Zero power consumption, minimal heat generation. Cost-effective alternative to optical modules, ideal for intra-rack or adjacent-rack connections.

4.3 400G QSFP-DD AOC (Active Optical Cable)

400G QSFP-DD to QSFP-DD AOC
400G QSFP-DD to OSFP AOC
Breakout AOC: 400G-to-4x100G
Key Advantages: Range: 1–100m (overcoming copper DAC limitations). Low latency and superior EMI resistance. Ideal for cross-rack/zone high-speed interconnects, bridging the gap between the distance shortcomings of DACs, serving as an ideal choice for medium-distance connections.

5. Migration Solutions: 100G → 400G

5.1 400G QSFP-DD SR8 + MPO-16 APC Fiber Patch Cords

Applicable Scenarios: Short-distance, high-density multimode environments (e.g., intra-data center interconnects).
Key Components: 400G QSFP-DD SR8 optical modules; MPO-16 APC trunk fiber cables.
Advantages:
Cost-effective, ideal for budget-sensitive projects.
Future-proof: Existing MPO-16 cabling can be reused for 800G upgrades, avoiding reinvestment.
Easy maintenance and standardized deployment.

5.2 400G DR4 + 100G DR + MPO-LC Breakout Cabling

Applicable Scenarios: Phased upgrades, heterogeneous network coexistence environments.

Design: Upgrade core/spine switches to 400G (using 400G QSFP-DD DR4 modules) , while leaf layer switches retain 100G (using 100G QSFP28 DR modules). Use MPO-LC breakout cables for rate adaptation.
Advantages:
Gradual migration, reducing upfront costs and risks.
Boosts backbone bandwidth without replacing access-layer devices.

6. Conclusion

In summary, the transition from 100G to 400G is no longer a question of “whether” but “how to execute efficiently”. This leap is not just about speed—it’s a holistic evolution in architecture, resource utilization, and sustainable development capabilities.