With the tremendous requirement for high bandwidth in 5G, loT and cloud data center, the focus of 400G Ethernet has been lasting for a couple of years. Vendors such as Cisco, Arista, and juniper are developing and testing technologies for 400G Ethernet networks. As the key hardware devices for interconnecting optical networks, there is no dispute that 400G transceiver will become the mainstream of the industry. Still curious about 400G transceivers? This paper will give you a comprehensive introduction to the different 400G transceiver types of different characteristics including applications, interface standards, and form factors.
According to the transceiver application, optical modules can be classified into two categories: client-side transceivers and line-side transceivers.
Client-side transceivers are used to interconnect between the metro networks and the optical backbone. The term "client side" refers to relatively short distances compared with the line side, generally from 50m to 10km and with only one transceiver connected to fiber thus no coherent optics is needed. There are various transceiver interfaces that have been standardized by IEEE and MSA. Most importantly, it has an agreed and standardized interface that is used for the network connection. PAM4 has been chosen by IEEE 802.3bs for 400GE client side transmission.
Different from client side, line side reaches transmission distances of 80km or even longer using DWDM. Coherent technology is expected to implement 400G line side transmission. OIF has been working on standardizing the 400G coherent DWDM interface for DCI and other metro/access applications. The signal processing of coherent transport is much greater than that of short reach PAM4 data center transmission, which requires more DSPs and power than in client side transmission.
The transceiver interfaces are defined by the interface standards. The following chart lists the common 400G Ethernet standards and the corresponding interfaces.
Interface standard Interface Link Distance Media Type Optical Architecture IEEE 802.3bs 400GBASE-SR16 100m MMF 16× 25G NRZ 850nm 400GBASE-DR4 500m SMF 4× 100G PAM4 1300nm 400GBASE-FR8 2km SMF 8× 50G PAM4 WDM 400GBASE-LR8 10km SMF 8× 50G PAM4 WDM IEEE P802.3cm 400GBASE-SR8 100m MMF 8× 50G PAM4 850nm 400GBASE-SR4.2 100m MMF 8× 50G PAM4 BiDi 850/910nm IEEE P802.3cn 400GBASE-ER8 40km SMF 8× 50G PAM4 WDM IEEE P802.3ct 400GBASE-ZR 80km SMF Coherent DWDM 100G Lambda MSA 400GBASE-FR4 2km SMF 4× 100G PAM4 CWDM 400GBASE-LR4 10km SMF 4× 100G PAM4 CWDM CWDM8 MSA 400G-CWDM8-2 2m to 2km SMF 8× 50G CWDM 400G-CWDM8-10 2m to 10km SMF 8× 50G CWDMNote: 400GBASE-SR16 has not been released by any transceiver vendors. As 400GBASE-SR16 interface requires a high fiber count (32 fibers per duplex link), this standard is not expected to enter the 400G transceiver market.
There are several mainstream 400G form factors，400G QSFP-DD, OSFP, CFP8, COBO, etc., some of which have been put in the market and some are still as a design.
CFP8 is the first generation 400G transceiver, with a relatively large physical size, offering the lowest port density.
COBO is named for Consortium for On-Board Optics, installed internally to the line-card equipment in a controlled environment, thus lacking flexibility.
OSFP stands for Octal Small Form Factor Pluggable, which is a new kind of pluggable form factor. There are some companies that have already sold 400G OSFP transceivers on the website.
400G QSFP-DD transceivers are now one of the most popular optical modules in the market, which have been launched and manufactured by Finisar, Innolight, FS.COM, etc.
The table below includes detailed comparisons of size, compatibility, power, etc. for the three main form factors: OSFP, QSFP-DD and CFP8.
OSFP QSFP-DD CFP8 Application Scenario Data center & telecom Data center Telecom Size 22.58mm× 107.8mm× 13mm 18.35mm× 89.4mm× 8.5mm 40mm× 102mm× 9.5mm Max Power Consumption 15W 12W 24W Backward Compatibility with QSFP28 Through adapter Yes No Electrical signaling (Gbps) 8× 50G 8× 50G 8× 50G Switch Port Density (1RU) 36 36 16 Media Type MMF & SMF MMF & SMF MMF & SMF Hot Pluggable Yes Yes Yes Thermal Management Direct Indirect Indirect Support 800G Yes No NoAmong these three transceiver form factors, it is obvious CFP8 lacks density, unlike the other two 400G transceivers. OSFP modules have been designed with 800G in mind. The QSFP-DD form factor has the main advantages of its high density, small size, and back forward compatibility that it supports QSFP28 enabling easier migration to 400G Ethernet, which addresses the industry need for high speed and high-density networking. Therefore it is expected that QSFP-DD form factor will become the most appropriate form factor for the 400G Ethernet applications.
Apart from the above categories of 400G transceivers, fiber mode, wavelength, etc. are also the common characteristics that are used in optical transceiver classification, which are not further explained. The demand for high-speed data transmission is rocketing. As the transceiver market is pushed to shift, we can expect the 400G Ethernet deployment in the next-generation data centers and the popularity of 400G optical transceivers in the near future. Though both opportunities and challenges in the 400G transceiver test exist in the research stage, 400G Ethernet is still an inevitable trend.