As cloud services expand at an unprecedented pace, the need for high-performance, cost-effective data center interconnects has intensified. This has led to the increased adoption of Direct Attach Copper (DAC) cables. Unlike optical interconnects, which rely on optical modules for signal conversion, DAC cables provide a direct physical layer connection between two ports using twin-axial cables, ensuring signal integrity over specified lengths without the need for active optical components.
1. Advantages of DAC Cables in Modern Data Centers
Data centers face significant demand for both intra-rack and inter-rack connections. DAC cables are well-suited for these environments, offering high-speed connectivity with minimal power consumption and heat generation—less than 0.1W. This efficiency reduces the cooling load on data center air conditioning systems. Additionally, the durability of DAC copper cables is a critical advantage, as they are less prone to damage from bending or other physical stresses, thereby minimizing the risk of failures in high-density settings. Compared to optical modules or Active Optical Cables (AOC), DAC cables are more cost-effective, reliable, and stable, making them the preferred interconnect solution.
2. Overcoming DAC Distance Limitations with ACCs
One challenge with DAC cables has been their theoretical distance limitation of 5 meters, which has restricted their application. In response, Active Copper Cables (ACC) were developed to extend these distances, particularly in server-to-switch connections that often need to reach or exceed 7 meters.
2.1 Enhancing 25g NRZ Connections
Extending the distance of copper cable connections while maintaining low costs can be achieved through two methods: signal retiming (Retimer) and signal amplification (Redriver).
Retimer Technology
Redriver Technology
The Repeater solution, balancing cost, power consumption, and functionality, was selected due to its clear advantages across these parameters.
Cost |
Power Consumption |
Transmission Distance |
System Adaptation |
25G DAC |
★★★★★ |
★★★★★ |
<5m |
25G ACC-Repeater |
★★★★ |
★★★★ |
<12m |
25G ACC-Retimer |
★★ |
★ |
<10m |
By adjusting output parameters at various frequency points and ranges, we successfully extended the DAC cable's effective distance to 10 meters under ideal conditions. However, the theoretical design distance does not always translate to practical application, as production consistency and system redundancy also need to be considered.
After multiple rounds of testing and validation, along with factoring in real-world fault tolerance, we determined that the 25G Linear-ACC is effectively applicable for distances up to 8 meters, covering 80% of 25G server applications.
Ultimately, products related to 25G achieved the expected benefits. The DAC+ACC cabling solution reduced costs by 40% compared to traditional AOC solutions and lowered the network failure rate by an order of magnitude from the previous 0.3%.
2.2 Upgrading to 50G PAM4
Building on the 25G Linear-ACC success, transitioning to 50G PAM4 necessitates more refined ACC designs due to higher costs and stricter signal quality requirements. Key advancements include:
2.2.1 Cost Control
In 50G PAM4 applications, using DAC and ACC interchangeably depending on the length offers a cost advantage. In the telecommunications industry, material costs are significant: InP is more expensive than Si, and Cu is more expensive than Si. As with 25G, there is a crossover point where ACC becomes cheaper than DAC for the same length. For 50G PAM4, this cost crossover point is at 2.5 meters. Beyond this length, using ACC not only provides greater performance margin but also lowers overall costs.
2.2.2 System Parameter Standardization
Ensuring cables meet IEEE 802.3cd standards for lengths over 2.8 meters requires higher redundancy. The IEEE 802.3cd standard mandates that 50G PAM4 cables have an SDD21 of less than 17dB at 13.28GHz. However, mass-produced cables longer than 2.8 meters often exceed this standard. Although the baseline frequency for 50G PAM4 modulation is similar to 25G NRZ, 50G PAM4 is more sensitive to signal output strength, and mid-to-high frequency attenuation results in higher bit error rates (BER). Consequently, greater redundancy is necessary for 50G PAM4 applications. The new generation of ACCs allows for more flexible modulation of cable parameters, ensuring better system compatibility.
When the system identifies copper cables, it needs to calibrate the Signal Integrity (SI) of the cables—a process known as Training. If the SI parameters of the cables are known and standardized, a matching parameter can be quickly input on the system side, reducing identification time and error rates, and preventing potential link downs. The main feature of the new generation of ACCs is the ability to standardize the SI of cables of different lengths into a very narrow range, making cables of varying lengths appear as if they are of the same specification. This is the essence of the “T” concept.
2.2.3 Noise Reduction
50G PAM4 signals demand lower noise and higher bandwidth. Introducing Noise Cancelling Technology (NCT) in new Redriver chips reduces high-frequency noise by 30%. Compared to 25G NRZ signals, 50G PAM4 signals are more sensitive to noise and require higher signal output strength, leading to greater high-frequency attenuation and bit error costs. New ACC designs offer more flexibility in modulating cable parameters for system compatibility. Specifically, PAM4 signals require better noise performance and linearity from Redriver chips to maintain signal integrity across the three eyes of PAM4 modulation. The higher rise and fall time requirements of PAM4 signals necessitate more high-frequency compensation.
Redriver chips for 50G PAM4 ACCs need to achieve lower noise, higher bandwidth, and better linearity. However, noise and bandwidth, along with the high-frequency gain provided by the Redriver, are inherently contradictory. To address this, the new generation of Redriver chips incorporates noise cancellation technology. The working principle involves feedback and feedforward networks to cancel out noise while enhancing the signal. Ideally, the noise from transistors in the equalizer is entirely canceled, reducing equivalent high-frequency noise by 30%. This allows the linear Redriver to improve bandwidth and high-frequency gain without degrading the signal-to-noise ratio (SNR). Additionally, a novel push-pull transconductance structure is used in the new linear equalizers, significantly enhancing large signal linearity and maintaining the consistency of PAM4 signal eyes.
2.2.4 Practical Performance Testing
To validate the performance of the new 50G PAM4 linear Redriver chip, a 7-meter 28AWG active cable was tested in a laboratory setting for S-parameters and BER. The test results are as follows:
All performance indicators exceeded expectations. The return loss and insertion loss of the 7m 28AWG ACC met and surpassed the requirements of the 802.3bj standard, achieving a COM value of 6dB. Without FEC, the BER was in the 10^-9 range, well below standard requirements, and with FEC, there were no errors. The overall power consumption was less than 0.4W, approximately one-tenth that of 56G PAM4 AOC.
2.3 Transition to 100G PAM4
As data rates continue to climb with the advent of 400G, 800G, and beyond, 50G PAM4 has become insufficient, necessitating the development of 100G PAM4. This transition involves multiple technological advancements:
Increased Signal Rate
The single-channel signal rate for 50G PAM4 is 25 GBaud, whereas 100G PAM4 increases this to 50 GBaud. Doubling the signal rate is crucial for achieving double the bandwidth. Specifically:
50G PAM4: 25 GBaud × 2 bits/symbol = 50 Gbps
100G PAM4: 50 GBaud × 2 bits/symbol = 100 Gbps
Modulation Optimization
Although both use PAM4 modulation, 100G PAM4 optimizes the modulation process with smaller signal level spacing, requiring more precise signal processing. Gray coding is employed to reduce bit errors between adjacent levels, and pre-equalization and adaptive equalization techniques are introduced to address signal distortion at higher speeds.
Technological Upgrades
To support higher signal rates, 100G PAM4 ACC requires advanced technologies, including more robust forward error correction (FEC) capabilities and improved clock recovery circuit precision to meet stricter jitter requirements.
Material and Manufacturing Improvements
100G PAM4 necessitates higher quality materials and more precise manufacturing processes. This includes using low-loss, low-dispersion copper conductor materials, improved shielding designs to reduce crosstalk and electromagnetic interference, and enhanced mechanical precision and electrical performance of connectors.
These phased technological upgrades have successfully doubled the single-channel rate from 50 Gbps to 100 Gbps, providing higher bandwidth and lower latency for data centers and high-performance computing applications. This also lays the groundwork for future 400G and 800G data transmission.
3. Copper and Optical Solutions Maximizing Cost-Effectiveness in Data Centers
While the trend in the telecommunications industry favors the transition from copper to optical solutions, this does not mean the complete disappearance of copper. In 800G networks with 128 nodes, most customers may opt for multimode solutions, but solutions that combine single-mode with DAC can save over 30% in costs, making it a highly advantageous solution.
As data centers rapidly standardize and hardware designs become more refined, both copper and optical technologies need to leverage their respective strengths to best serve data center networks. NADDOD provides a comprehensive range of optical connectivity products, covering all speeds with multimode transceivers, single-mode transceivers, and DAC, AOC, ACC, and AEC solutions. We are committed to offering the most suitable solutions based on your specific needs, achieving higher speeds, lower power consumption, and greater reliability to meet the demands of enterprise networks, data centers, 5G, and high-performance computing applications.
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