The QSFP-DD (Quad Small Form Factor Pluggable Double-density) optical transceiver supports double the number of high-speed electrical interfaces compared to the standard QSFP28 module. This article will introduce QSFP-DD optical transceiver in detail and understand its differences with QSFP+ / QSFP28 / QSFP56 and OSFP / CFP8 / COBO.
Contents
1. What is QSFP-DD optical transceiver?
QSFP-DD is forward and backward compatible with the QSFP port and is also compatible with the existing QSFP28 optical modules and AOC/DAC, etc. With NRZ modulation technology, the data rate can be achieved for 200G( 25Gx8 channels) network transmission. Meanwhile, with PAM4 modulation technology, the data rate can reach 400G( 50Gx4 channels) network transmission for high-performance computing data centers and cloud networks. As the best option for the 400GE optical transceiver form factor, this QSFP-DD connector enables data centers to effectively grow and expand cloud capacity as needed.
With fully considered the using flexibility for users, the MSA group adopted ASIC design, made the QSFP-DD supports multiple interface rates so as to reduce port cost and equipment deployment cost. In a nutshell, QSFP-DD module is a high-speed, small pluggable and low-power transceiver, which will become one of the mainstream form factors for 400G network. So what is the difference between QSFP-DD and previous QSFP+/QSFP28/QSFP56? And what is the difference with other 400G modules such as OSFP/CFP8/COBO?
QSFP-DD vs. QSFP+ vs. QSFP28 vs. QSFP56
Although QSFP-DD and QSFP+/QSFP28/QSFP56 belong to the same QSFP package and have the same size, there are still some differences between them.
- Structure
Main-board Mechanical Interface Depth: when designing QSFP-DD, MSA team members made it has the same port density as QSFP+/QSFP28/QSFP56 and can accommodate an additional row of conductive metal. The mechanical interface on the main board is a little deeper than that of QSFP+/QSFP28/QSFP56.
Number of electrical interface channels: QSFP-DD module is equipped with 8-channel electrical interface. Compared with QSFP+/QSFP28/QSFP56 which have only 4-channel electrical interface, the number of electrical interface channels of QSFP-DD is doubled, which is also due to a row of additional conductive metal. Such as 400G SR8 optical transceiver.
Number of integrated circuits: since QSFP-DD has 8-channel electrical interfaces, the number of integrated circuits (i.e. ASIC) and density are doubled.
- Bandwidth&Application
QSFP-DD is generally used as the form factor of 400G optical module and 400G high-speed cables (i.e. DAC and AOC), which is used for the interconnection of 400G data centers to solve the problem of mass data migration between data centers; while QSFP+/QSFP28 /QSFP56 are respectively used for the 40G/100G/200G optical modules and high-speed cables, and for 40G/100G/200G network interconnection.
- Compatibility
Because QSFP-DD is backward compatible, modules or connectors in QSFP+/QSFP28/QSFP56 form factors can be plugged into QSFP-DD ports. In contrast, modules or connectors in the QSFP-DD form factors cannot be plugged into QSFP+/QSFP28/QSFP56 ports.
In short, QSFP-DD optical transceiver has been upgraded on the basis of continuing the advantages of previous form factor of modules, which improves the bandwidth and can better meet the needs of large bandwidth network applications. At the same time, backward compatibility can effectively reduce the replacement of equipment and save the cost of network upgrading.
3. QSFP-DD vs. OSFP vs. CFP8 vs. COBO
- OSFP
The OSFP optical transceiver also adopts 8-channel electrical interface and the data rate of each channel is 50Gbps. It has its own radiator, which can greatly improve the heat dissipation performance. However, the new interface standard is incompatible with the existing photoelectric interface and is larger than the size of QSFP-DD, so it requires a larger area of Printed Circuit Board (PCB) and high power consumption.
- CFP8
The CFP8 optical transceiver is equivalent to a high-speed evolution version of the CFP4, its electrical interface can be 8 channels or 16 channels (16 channels mostly), and it suitable for telecom backhaul; but with higher cost, size and power consumption.
- COBO
Among all the 400G optical transceivers, the COBO form factor is kind of different for that it places the optical module in the PCB board, which is not limited by the interface density of the front panel, and can use the main-board for heat dissipation. However, it does not support hot-plug, which is difficult to maintain.
It can be seen from the table that when we score the four types of optical modules respectively, QSFP-DD has the highest score, and OSFP is the second better. For this reason, QSFP-DD and OSFP have become the preferred optical modules for most suppliers. However, QSFP-DD is more suitable for data center applications, while OSFP is more suitable for telecommunications applications.
4. Will QSFP-DD be the mainstream form factor of the 800G optical module?
With the large-scale commercialization of 400G will inevitably drive the single-wave 100G technology to maturity, which also lays a certain foundation for the arrival of 800G. By now, the QSFP-DD800 Multi-Source Agreement (MSA) organization has released the first version of QSFP-DD800 transceiver hardware specification, which is dedicated to the continuation of the current QSFP-DD form factor to support a single channel rate of 100Gbps for the new generation of QSFP-DD800, which also means that 800G optical transceiver will still use QSFP-DD form factor, bringing greater cost advantages and business value to network operators. Nevertheless, the large-scale production of 400G modules requires a breakthrough of corresponding chips, so does 800G.
