400G QSFP-DD optical transceiver: the history and future

400G QSFP-DD optical transceiver: the history and future

On March 21, 2016, QSFP-DD MSA team began to plan to develop QSFP-DD high-speed interface. On September 18, 2018, QSFP-DD MSA team released version 4.0 of QSFP-DD hardware specification and version 3.0 of 8/16 channel pluggable optical module general management interface specification. So far, QSFP-DD MSA is relatively perfect, and QSFP-DD optical transceivers of major leading optical module manufacturers are also listed in this period. QSFP-DD is the main packaging type of 400G optical transceiver. Its emergence and development is due to the increasing requirements of network bandwidth and data transmission rate in today’s society. In the future, with the development of more advanced technologies such as big data, artificial intelligence and 5G, 400G QSFP-DD optical transceiver will become more and more important as an important connector for data center communication.

From 100G to 200G to 400G: QSFP28 vs QSFP56 vs QSFP-DD

Before the emergence of 400G Ethernet, the optical module in the form of QSFP28 package applied to 100G Ethernet was the most on the market, and then the QSFP56 optical module as the transition from 100GE to 400GE appeared. We must understand the two forms of 400G QSFP-DD optical module before it is packaged.

D:\外链\配合发外链的文章\blog外链\400G transceiver today tomorrow.jpg400G transceiver today tomorrow


The standards beginning with 100G QSFP28 are proposed by IEEE802.3 in 2013. The QSFP28 (Quad Small Form Factor Pluggable) transceiver module integrates 4 transmit and 4 receive channels. “28” means each lane carries up to 28G data rate. QSFP28 can do 4x25G breakout connection, or 1x100G connection depending on the transceiver used. The QSFP28 interface is very flexible as it can achieve 100G in other ways. Using a breakout connection, QSFP28 transceivers can use just 2 channels at 50G each or they can even achieve a single channel of 100G depending on the transceiver type being used. There are many QSFP28 products on the market including SR4, LR4, PSM4 and CWDM4, etc.


The QSFP56 (Quad 50 Gigabits Small Form-factor Pluggable) optical module is an upgraded version of 40G QSFP+ and 100G QSFP28 optical module. It is an optical module specially designed for 200G Ethernet interconnection. The QSFP56 denotes 4×50 to 56Gb/s in a QSFP form factor. Sometimes it can also be referred to as 200G QSFP for sake of simplicity. Generally, two QSFP56 modules can be used with an SMF or MMF to realize a 200G link. According to different transmission distances, 200G QSFP56 optical modules are divided into QSFP56 CR, SR, DR, FR and LR, which can realize transmission at different distances on single-mode and multimode optical fibers. And the latest iteration of the optical module form factor is from QSFP56 to QSFP56-DD, which is also called 400G QSFP-DD.


In terms of packaging form, 400G optical transceiver can be divided into CDFP, CFP8, OSFP, QSFP-DD, etc. Compared with other 400G transceiver packages, QSFP-DD has great advantages in size, heat capacity, power consumption, backward compatibility and bandwidth, and has gradually become the mainstream solution. QSFP-DD (Quad Small Form Factor Pluggable Double Density) is a package of high-speed pluggable modules defined by QSFP-DD MSA team. QSFP-DD can be compatible with QSFP, QSFP28 and QSFP56. Moreover, compared with QSFP56, QSFP-DD provides two rate schemes of 200GB/s or 400GB/s. It adopts 8-channel electrical interface, and the rate of each channel is up to 25GB/s (NRZ modulation) or 50GB/s (PAM4 modulation).

NRZ PAM4In addition, although the QSFP-DD and QSFP+/QSFP28 systems have the same port density. However, since each QSFP-DD port can accommodate 8 channels instead of 4 channels, QSFP-DD doubles the number of ASIC ports of existing interfaces (such as CAUI-4) it supports. Compared with QSFP+/QSFP28, QSFP-DD has only a small increase in length, which increases the bandwidth to 10 times or 4 times of the latter without changing the port density, and is backward compatible, which means that customers skip QSFP and directly deploy QSFP-DD system, which reduces a lot of equipment costs.

The future of QSFP-DD: data-center’s main choice

The large-scale and flat data center puts forward more and higher speed requirements for optical modules. The large-scale data center increases the demand for long-distance transmission. The transmission distance of multimode optical fiber is limited by the increase of signal rate, and will be gradually replaced by single-mode optical fiber, mainly using PSM4 and CWDM4 schemes. The CWCM optical module is mainly used for longer distance transmission. Through the external wavelength division multiplexer, the optical signals of different wavelengths are combined together and transmitted through a wide signal to save optical fiber resources.


Fiber Mall 400GBASE DR4 optical transceiver

400G QSFP-DD package adopts 8-channel electrical interface. When NRZ modulation is adopted, the rate of each channel is up to 25Gb/s (200GE transmission can be realized), and when PAM4 modulation is adopted, the rate of each channel is up to 50Gb/s (400GE transmission can be realized). QSFP-DD can achieve an aggregate bandwidth of up to 14.4Tb/s in a single switch slot. The bandwidth of QSFP-DD can be up to ten times that of QSFP+ or four times that of QSFP28. In addition, QSFP-DD can not only realize the forward and backward compatibility of QSFP, but also be compatible with the existing QSFP28 optical module and AOC/DAC. The QSFP-DD and QSFP+/QSFP28 systems have the same port density. Therefore, it has incomparable advantages in the interconnection scenario of 400G data center. It is expected that the world’s leading super large-scale data center will soon deploy 400G, and QSFP-DD may become the mainstream package.

Moreover, the 5G era is coming, and we can expect this trend of network use to snowball more and more. All this means that we need to take 400GE seriously and seize the opportunities it brings. Optical transceiver is the most key factor for 400G deployment, and QSFP-DD is the best choice for data center to deploy 400GE.

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