200G Data Centers: Choosing Between QSFP56 and QSFP-DD as the Dominant Standard

With the rapid advancement of optical communication and the Internet, there has been a corresponding increase in network demand, leading to a significant surge in telecom backbone network traffic at a rate of 50% to 80% annually. In order to cater to user needs, the transmission rate of optical communication has seen continuous development, progressing from 10G, 25G, and 40G to the current 100G, 200G, 400G, and beyond. While 100G optical transceivers have become the prevalent choice in the market, the requirements for bandwidth and port density continue to escalate, thereby propelling the advancement of technology towards 200G, 400G, and higher-speed systems.

What Are the Form Factor Types for 200G Optical Transceivers?

At present, the market features two primary forms of 200G optical transceivers: the 200G QSFP56 and the 200G QSFP-DD. The QSFP56, introduced in 2017, signifies a notable design progression from earlier QSFP transceivers. In contrast, the QSFP-DD was still undergoing development during that timeframe. These transceivers are specifically crafted for high-performance computing and data centers, and they offer backward compatibility with earlier QSFP versions, including the QSFP28.

The 200G QSFP56 transceivers feature four channels for both transmitting and receiving, with each channel capable of operating at 53.125 Gbps, culminating in a total data rate of 212.5 Gbps. This transceiver operates over wavelengths of 850nm, 1310nm, CWDM, or LWDM. For optical connectivity, the QSFP56 utilizes an MPO interface, while electrically it connects through a 38-pin connector. In contrast to its QSFP predecessors, the QSFP56 employs the PAM4 digital modulation technique to enhance data transmission capabilities.

The 200G QSFP-DD transceivers are compliant with IEEE802.3bs and QSFP-DD MSA standards. The dual density architecture essentially increases the number of channelsof electrical interfaces. 200G QSFP-DD has an eight-channel electrical interface totaling 212.5Gb/s of total bit rate. The optical interface is either MPO or duplex LC. QSFP-DD is backward compatible with most QSFP formspecifications, including QSFP56. QSFP-DD electrical interfaces feature eight lanes of up to 25 Gbps each, using the NRZ modulation scheme.

Comparison of 200G QSFP56 and QSFP-DD

Modulation Technology

200G QSFP56 uses PAM4 modulation, while 200G QSFP-DD typically adopts NRZ modulation. NRZ (Non-Return-to-Zero) uses two voltage levels to represent logic 0 and 1 (also known as PAM2), whereas PAM4 (Pulse Amplitude Modulation) uses four voltage levels to represent the four two-bit combinations: 11, 10, 01, and 00.

This enables PAM4 signals to transmit faster than traditional NRZ. Although PAM4 offers higher transmission speed, 200G NRZ stands out in reducing power consumption, lowering latency, and easy deployment, offering a cost-effective data center interconnect solution.

Transmission Performance

Compared with 200G QSFP56 transceivers, 200G QSFP-DD transceivers offer lower power consumption, latency, and bit error rate (BER), enhancing transceivers' overall performance.

Backward Compatibility

QSFP-DD is backward compatible with QSFP, QSFP+, QSFP28 and QSFP56 transceivers. QSFP56 offers backward compatibility with QSFP28 but not QSFP-DD.

Scalability

Not all switches and routers support QSFP-DD, making upgrading link rates quite costly. Considering the initial setup expenses, opting for QSFP56 might be more economical. However, if the budget is enough, QSFP-DD offers straightforward upgrades for future expansion.

Cost

Compared to the eight channels required for QSFP-DD, QSFP56 transceivers need to use only four channels for 200G transmission, enabling fiber cost savings and reduced link loss. QSFP56 modules' price is generally more affordable. However, QSFP-DD has lower operational and deployment costs than QSFP56 for higher port density and better compatibility.

Applications

QSFP56 is designed to support 200G applications but cannot accommodate network upgrades to 400G and beyond. In contrast, QSFP-DD supports 200G and 400G applications, allowing for incremental network upgrades. QSFP-DD can be split into lower-speed legacy form factors, offering network and system engineers greater flexibility and facilitating easier network upgrades.

FS 200G Optical Transceivers/DAC/AOC

FS offers a full range of 200G InfiniBand and 200G Ethernet Transceivers, which support various application scenarios.

Products	Form Factor	Max. Power Consumption	Modulation (Electrical)	Modulation (Optical)	Length	Application
200G QSFP56 SR4	QSFP56	5W	4x50G PAM4	4x50G PAM4	100m	InfiniBand Ethernet 200G-to-200G links 200G to 4x50G links
200G QSFP56 FR4	QSFP56	6.5W	4x50G PAM4	4x50G PAM4	2km	Ethernet 200G-to-200G links
200G QSFP56 LR4	QSFP56	1.5W	4x50G PAM4	4x50G PAM4	10km	Ethernet 200G-to-200G links
200G QSFP-DD SR4	QSFP-DD	4.5W	8x25G NRZ	8x25G NRZ	100m	Ethernet 200G-to-200G links 200G-to-two 100G links
200G DAC/AOC cables	QSFP56 to QSFP56 QSFP-DD to QSFP-DD	4.5W	PAM4, NRZ		1m~100m	InfiniBand Ethernet 200G-to-200G Links
	QSFP56 to 2x QSFP56 QSFP-DD to 2x QSFP28	4.5W	PAM4, NRZ		1m~70m	InfiniBand Ethernet 200G-to-Two 100G Links
	QSFP56 to 4x SFP56 QSFP-DD to 4x QSFP28	5.5W	PAM4, NRZ		1m~70m	InfiniBand Ethernet 200G-to-Four 50G Links
	QSFP-DD to 8x SFP28 QSFP28-DD to 8x Duplex-LC	4W	NRZ		1m~70m	InfiniBand Ethernet 200G-to-Eight 25G Links