With the rapid development of optical communication and Internet technology, the demand for network data traffic is growing exponentially, and the average annual growth rate of telecom backbone network traffic is as high as 50% to 80%. In response to the increasing demand for data transmission, optical communication rates have achieved continuous evolution from 10G, 25G, 40G to the current mainstream specifications of 100G, 200G, 400G, and even higher.

In this context, when building a 200G data center, the selection of QSFP56 and QSFP-DD as key interface standards for achieving 200G speed is particularly important. The QSFP56 module is an improvement on the original QSFP+design, aimed at improving data transmission speed through optimization; QSFP-DD introduces a dual density design, effectively improving port density and maintaining downward compatibility with the QSFP series products. Both have their own advantages and characteristics in performance, cost control, energy efficiency, and heat dissipation management. In practical applications, a comprehensive evaluation and reasonable selection should be made based on the specific business needs, expansion plans, and future development trends of the data center.

Packaging types of 200G data center optical modules

The mainstream 200G optical modules in the current market mainly adopt two packaging forms, namely 200G QSFP56 and 200G QSFP-DD. Among them, QSFP56 was officially released in 2017, which is a major technological upgrade based on the early QSFP series optical module design; At the same time, QSFP-DD is in the research and development stage and gradually emerged. Both of these optical modules are designed to meet the demanding requirements of high-performance computing and data center scenarios, and both have good backward compatibility, seamlessly integrating with early QSFP versions including QSFP28.

The QSFP56 optical module optimized for 200G Ethernet applications is equipped with four independent transceiver channels, each supporting a maximum data rate of 53.125 Gbps, achieving a total transmission capacity of 212.5 Gbps. This module is suitable for the wavelength range of 850nm, 1310nm, CWDM or LWDM, and uses an MPO interface for optical signal transmission. It is connected to the electrical interface through a 38 pin electrical connector. Compared to the previous generation QSFP products, QSFP56 adopts advanced PAM4 digital modulation technology, significantly improving data transmission efficiency.

On the other hand, the QSFP-DD (Quad Small Form Factor Pluggable Double Density) optical module follows the IEEE802.3bs standard and QSFP-DD MSA specification. Its core innovation lies in the dual density structure design, which increases the number of electrical interface channels. Specifically, the 200G QSFP-DD has eight electrical interface channels and a total bit rate of up to 212.5Gb/s. In terms of optical interfaces, MPO or multimode duplex LC interface forms can be selected. It is worth mentioning that QSFP-DD is not only compatible with most QSFP specification versions, such as QSFP56, but its electrical interface includes up to eight channels with a maximum speed of 25 Gbps, using NRZ modulation to ensure efficient and stable signal transmission.

Comparison between 200G QSFP56 and 200G QSFP-DD

In the field of 200G data centers, the optical modules packaged in QSFP56 and QSFP-DD exhibit significant differences in using different digital modulation technologies. NRZ (Non Return to Zero) is a fundamental and widely used modulation method that achieves data transmission through two voltage levels corresponding to logic 0 and 1 (i.e. PAM2). However, with the increasing demand for bandwidth, PAM4 (Four Level Pulse Amplitude Modulation) has emerged, which can encode all four states of two bits using four different voltage levels at the same time unit: 11, 10, 01, and 00, allowing PAM4 signals to transmit data at twice the rate compared to traditional NRZ signals.

The main advantage of PAM4 compared to NRZ is that it can achieve higher data transfer rates. However, for the 200G NRZ, although it may be slightly inferior to PAM4 in absolute speed, it also has some undeniable advantages, such as lower power consumption, smaller signal delay, and relatively simple deployment process. Especially in the scenario of internal interconnection in data centers, the solution using 200G NRZ modulation, due to its optimized energy efficiency and cost-effectiveness, can provide an economically efficient interconnection solution for data centers, especially suitable for application environments with high requirements for energy consumption control, real-time performance, and limited budget.

Advantages and limitations of 200G QSFP56 and QSFP-DD

QSFP56 is designed specifically to meet the needs of 200G applications, but its technical architecture does not support direct upgrades to network environments with speeds of 400G and above.

In contrast, QSFP-DD is compatible with both 200G and 400G speed versions, and allows users to gradually upgrade according to their needs, with stronger scalability and flexibility.

In terms of modulation, QSFP56 adopts PAM4 technology, while QSFP-DD typically uses NRZ modulation to achieve a 200G rate.

In terms of channel configuration, QSFP56 only requires 4 channels to complete 200G data transmission, which is more advantageous in terms of fiber cost and link loss compared to QSFP-DD that requires 8 channels.

However, QSFP-DD has a series of advantages: lower maintenance costs, high efficiency (pre FEC bit error rate as low as E-8, post FEC bit error rate as low as E-12), low power consumption, low latency, and easy deployment and management. In addition, it can flexibly adapt to traditional specification interfaces of different speeds through splitting, enhancing the network's scalability and compatibility. It can be backward compatible with early QSFP series optical modules, including QSFP56, but not QSFP-DD.

On the price side, the price of QSFP-DD is about 15% to 30% higher than that of QSFP56. Although the initial investment is relatively high, considering long-term operation and maintenance costs, QSFP-DD can offset this investment to some extent with lower energy consumption and delay performance. It is also worth noting that if QSFP-DD is not widely supported by current network devices, choosing QSFP56 may be more cost-effective, as increasing connection speed may face high cost pressures. However, if the budget is sufficient and the focus is on future network expansion and performance optimization, QSFP-DD is an ideal choice that is more convenient for subsequent upgrades.