Industry exclusive! H3C releases CXL-O optical interconnect solution

Recently, New H3C Group, a subsidiary of Tsinghua Unigroup, released the industry’s first CXL-O (Compute Express Link Over Optical) optical interconnect solution. This innovative achievement breaks through the distance limitation of CXL copper cable physical connection, greatly improving the efficiency and flexibility of data centers. By integrating CXL and optical interconnect technology, efficient, long-distance, and low latency CXL connections across servers and cabinets in data centers have been achieved, significantly improving overall computing performance and efficiency, solving the problem of memory expansion and transmission in high-performance computing, and setting a new technological benchmark for the data center field.

CXL technology evolution: advantages and challenges coexist

As an open standards based cache consistent memory protocol, CXL (Compute Express Link) aims to achieve high-speed and efficient interconnection between computing units such as CPU, GPU, FPGA, and ensure memory consistency. The emergence of CXL has solved bottleneck problems in high-performance computing, including memory capacity, bandwidth, and I/O latency, providing powerful support for data centers.

CXL technology has gone through multiple stages of development since its inception. The CXL 1. x era has achieved initial changes in computing architecture, but expanding memory is limited to the internal of a single server and cannot achieve larger scale expansion. CXL 2.0 introduced CXL switches, breaking this limitation and achieving cross server memory expansion and pooling. Nowadays, CXL technology has entered the 3. x stage, further enhancing the cascading function of switches and effectively promoting the transformation of internal interaction in data centers from CPU centric to memory centric architecture, where all calculations are tightly executed around data and can ensure the consistency of cache across all devices.    

CXL specification release schedule

In the era of CXL 2.0 and CXL 3. x, facing the demand for high-performance computing, traditional copper cable connection technology has limited the deployment range and scale of CXL due to its severe signal attenuation. In addition, with the increasing complexity of data center architecture, ensuring high-speed and effective connections between various components has become a major challenge.

New H3C CXL Over Optical Interconnection Solution: Breaking the Connection Distance Barrier

The current CXL solution development in the industry mainly faces the following technical challenges:

• Receiver Detect: RC needs to detect a 50 ohm load to determine if there is a terminal. In the optical link environment, the 50 ohm termination characteristic of its EP cannot be transmitted to the receiving end.
• The transmission of sideband signals: For low-speed sideband control signals such as PERST (reset), PRST (bit), clock, I2C, etc., high accuracy and stability are usually required, while the transmission of optical signals may introduce additional noise or delay, affecting the accurate execution of control commands.
• Link training: Traditional high-speed optical link training strategies are difficult to adapt to the characteristics of CXL optics, and the receiver’s adjustment of the equalization parameters at the transmitter cannot adapt to specific optical transmitters, resulting in a decrease in signal quality. In addition, different types of optical components, such as VCSEL (vertical cavity surface emitting laser), DML (direct modulation laser), MZM (Mach Zehnder modulator), and EA (electroabsorption modulator), each have unique on/off characteristics and nonlinear responses, which increases the complexity of link training design.

Based on its profound technological accumulation and perfect ecological cooperation system in the ICT field, New H3C Group has joined hands with Shanghai Xizhi Technology Co., Ltd. to conduct in-depth research on the combination of CXL and optical interconnect technology. It has successfully verified the CXL 2.0 optical interconnect solution, which uses servers as hosts to read and write CXL switch memory bandwidth, delay, pressure testing, etc. At the same time, it has verified the transmission of low-speed signals with control signal outputs such as in place, clock, reset, I2C, etc. It has taken the lead in the industry to launch the CXL Over Optical (CXL-O) optical interconnect solution.

Schematic diagram of optical interconnection within and across cabinets

Optical interconnect technology has the advantages of unlimited bandwidth potential, ultra-low latency, high efficiency and energy saving, and long-distance transmission, which can effectively solve the limitations of traditional copper cable connections. The close cooperation between the optical transmission module and the CXL PHY (physical layer) interface enables the conversion from electrical signals to optical signals, and efficient transmission through optical carriers in the link. This process involves the controller interface of the optical transmission module, which is responsible for encoding protocol signals, bus information, and user data into optical signals. It also has the ability to decode control information received from the optical link, convert it back into electrical signals, and supply them to the PHY interface.

Principle of CXL optical transmission module

• Breaking through the limitation of connection distance: By utilizing optical interconnect technology, the long-distance and efficient transmission of CXL connections has been achieved, greatly expanding the layout flexibility of data centers.
• Improved computing performance: By optimizing memory expansion and transmission mechanisms, the computing performance and efficiency of data centers have been significantly improved.
• Reduce overall cost of ownership: Memory pooling and efficient utilization reduce redundant configurations and energy consumption, lowering the operating costs of data centers.
• Enhanced security: The CXL protocol itself supports integrity and data encryption, combined with the physical isolation characteristics of optical interconnects, further enhancing the security of data transmission.