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The rapid growth in data generation and consumption has led to an ever-increasing demand for higher data rates and bandwidth in data centers. Historically, copper interconnects have been the backbone of high-speed data transmission within data centers, providing reliable and cost-effective solutions for short-reach connections. However, as data rates continue to climb, the physical limitations of copper are becoming increasingly apparent, prompting a shift towards optical interconnects.
This technical article provides an overview of the transition from copper to optical interconnects, focusing on key performance metrics for SerDes IP, latency considerations, power consumption, and the emergence of linear optical interfaces.
Critical Performance Metrics Influencing the Transition from Copper to Optics
The transition from copper to optics is influenced by several critical performance metrics. Historically, Ethernet connections on the front end have predominantly used optical solutions, while scaling up within data centers has relied on copper. However, the reach of copper interconnects is diminishing rapidly. At 112 Gbps, the length for copper backplane cables is approximately 2.5 meters. As data rates increase to 224 Gbps, this length shortens to about one meter.
To address these limitations, three primary options are available:
- Direct Attached Copper cables (DAC)
- Active Electrical Cables (AEC)
- Active Optical Cables (AOC)
While copper solutions like DAC and AEC are still prevalent for short-reach, high-speed connections, the future of scaling out is undeniably optical, driven by the need for longer reach and higher bandwidth.
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Latency Considerations
Latency is a crucial factor in the deployment of optical interconnects. While optics offer significant advantages in reach and bandwidth, they introduce higher latency compared to copper solutions. This increased latency is primarily due to the Reed-Solomon (RS) effect and Hamming error correction used in optical transmissions.
In high-speed data transfer scenarios, such as load and store operations that are highly latency-sensitive, copper remains the preferred choice. The lower latency of copper interconnects makes them suitable for these critical applications. On the other hand, scale-out operations, which involve less latency-sensitive tasks like data partitioning and distribution, are increasingly adopting optical solutions.
Power Consumption
Power consumption is another vital consideration when comparing copper and optical interconnects. Active optical modules typically consume 2.5 to 4 times more power than their active copper counterparts. This discrepancy is due to the additional components required for optical transmission, such as laser drivers and transimpedance amplifiers (TIA).
Despite the higher power consumption, the demand for optical interconnects is growing due to their ability to support higher data rates and longer distances. The development of more power-efficient optical components is an ongoing area of research, with the goal of narrowing the power gap between copper and optical solutions.
Figure 1: Comparison of power savings with linear drive optics. From OFC 2023: Revolutionizing Networking for AI Workloads - Ethernet Alliance
Emergence of Linear Optical Interfaces
Linear optical interfaces have been a topic of discussion since the early 2010s, particularly at 10 Gbps speeds. However, the technology did not see widespread adoption until recently. The increasing data rates and the limitations of traditional re-timed interfaces have rekindled interest in linear direct drive optics.
Figure 2: Utilizing Host (Switch) Electrical SerDes to Compensate for Optical Impairments
In conventional re-timed interfaces, a DSP chip is used to drive the optics, introducing additional complexity and latency. Linear drive optics eliminate the need for intermediate DSP components, which are traditionally used to re-time and equalize signals before they are transmitted over optical fibers. By integrating these functions directly into the host chip's SerDes, linear drive optics simplify the transmission path, reducing latency and power consumption.
The primary advantage of linear drive optics is their ability to handle high data rates with reduced complexity. At 112 Gbps, linear drive optics can directly interface with the optical components, providing a clean and efficient signal path. This direct interface is achieved by incorporating advanced analog and mixed-signal design techniques within the SerDes, enabling it to drive the optical modulators and detect signals with high fidelity.
As data rates increase to 224 Gbps, the industry is exploring a hybrid approach. The higher Nyquist frequencies at these speeds introduce nonlinearity in the optical domain, necessitating additional components for signal integrity. A hybrid model, incorporating Automatic Gain Control (AGC) and Clock Data Recovery (CDR) chips, is emerging as a viable solution to handle these challenges.
Co-Packaged Optics (CPO) and UCIe
The discussion on Co-Packaged Optics (CPO) has gained momentum in recent years. CPO involves integrating optical components directly with the host chip, reducing the distance between electrical and optical interfaces and thereby improving performance.
The Universal Chiplet Interconnect Express (UCIe) standard is playing a pivotal role in the development of CPO systems. UCIe facilitates die-to-die communication within multi-die systems, enabling high-density, high-speed connections. Two primary approaches are being explored for CPO:
- Serial PHY Direct/Linear Optical drive: In this approach, UCIe is used for die-to-die communication within the host chip, which then interfaces with an I/O chip containing the SerDes and optical components. This method offers longer reach but introduces higher latency and power consumption due to the additional SerDes.
Figure 3: CPO use case: Longer reach but higher latency and pJ/b and lower Gb/s/mm
- Parallel PHY Direct/Linear Optical drive: This approach eliminates the SerDes, using UCIE for direct communication between the host chip and the I/O chip with integrated silicon photonic components. This method offers shorter reach, lower latency, and higher power efficiency but requires advanced optical multiplexing techniques like CWDM (Coarse Wavelength Division Multiplexing) for effective fiber management.
Figure 4: CPO use case Shorter reach but lower latency and pJ/b and higher Gb/s/mm
Challenges and Solutions at Higher Data Rates
The transition from copper to optics is not just a matter of replacing one medium with another; it requires a comprehensive approach to ensure that the new optical systems can deliver the required performance while addressing the inherent challenges. Linear drive optics represent a critical innovation in this transition, offering a streamlined and efficient means of optical data transmission.
As data rates increase to 224 Gbps and beyond, the challenges associated with linear drive optics become more pronounced. The higher Nyquist frequencies at these speeds exacerbate signal integrity issues, such as nonlinearity and loss, which can degrade the performance of the optical link.
To address these challenges, a hybrid approach is being developed, which is half-retimed. This approach combines the benefits of linear drive optics with additional signal conditioning components, such as AGC and CDR chips. These components help to mitigate the effects of nonlinearity and loss, ensuring that the optical signals maintain their integrity over longer distances.
Figure 5: Half retimed linear optical interface variant
Linear Drive Optics: A Reality
Synopsys’ recent demonstrations of PCIe 7.0 and 224 Gbps linear optical drive solutions have showcased the practical implementation of these technologies. These demos highlight the integration of passive components, linear amplifiers, and modulated lasers, emphasizing the feasibility of linear optical interfaces for next-generation data centers.
The future of high-speed interconnects lies in the continued evolution of optical technologies. As the industry progresses towards higher data rates and more complex system architectures, the adoption of optical solutions will become increasingly prevalent. The development of power-efficient, low-latency optical components will be crucial in addressing the challenges of scaling out modern data centers.
Summary
The transition from copper to optics is driven by the need for higher bandwidth, longer reach, and improved performance. While copper remains relevant for latency-sensitive applications, optical interconnects are poised to dominate scale-out scenarios. The emergence of linear optical interfaces and the adoption of standards like Ultra Ethernet and UCIe are paving the way for the next generation of high-speed data transmission solutions. The ongoing advancements in optical technology will play a pivotal role in shaping the future of data center connectivity, enabling the seamless and efficient transport of data at unprecedented speeds.
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