QSFP-DD Overview

QSFP-DD, or Quad Small Form-factor Pluggable Double Density, is a next-generation transceiver form factor that increases bandwidth capacity and meets the needs of data centers and high-performance computing environments. The device operates at 400 Gbps standard, which is achieved by doubling the electrical and optical interfaces of its predecessor – QSFP28, while maintaining compatibility with existing QSFP infrastructure. With a larger footprint than other form factors, this type can support more lanes and thus higher data rates, making it ideal for cloud computing applications, among others, such as enterprise networking and telecommunications, where long-range or short-range data transmission capabilities are required for modern networking solutions ensure continuous connectivity together with scalability.

Comparison of QSFP28 vs QSFP-DD vs QSFP56

Differences Quick View

The following table concludes the main differences of QSFP28, QSFP-DD and QSFP56.

The following will give a detail illustration to their differences.

Differences 1: Data rate and channel count

Data rate is the most fundamental difference among them. QSFP-DD is the max data rate one, as its name implies, double density, that is its channel count has doubled from 4-channel of QSFP28 and QSFP56 to 8 channels. As a result, QSFP-DD supports a max data rate of 400G (8x 50Gb/s) based on PAM4 modulation, and also supports 200G (8x 25Gb/s) based on NRZ modulation. And with 4 channels modulated with NRZ results of 50Gbps/25Gbps per channel, QSFP56 supports 200Gbps whereas QSFP28 supports 100Gbps.

Differences 2: NRZ vs PAM4 Modulation

As mentioned above, QSFP28 and 200G QSFP-DD use NRZ modulation technology while QSFP56 and 400G QSFP-DD use PAM4 technology. Then what are the differences between NRZ and PAM4?

Why we need PAM4

NRZ (Non-Return-to-Zero) is the main modulation technology in non-coherent optical communications. However, with the increasing data rate, such as 200G/400G, NRZ faced difficulties in dispersion, bandwidth, and costs. When the baud rate is above 25G, the influence of dispersion begins to be significant in medium and long-distance transmission. When the electrical-optical conversion bandwidth is more than 60 Gbit/s, the technical bottleneck comes. In terms of cost, under the same bit rate, the baud rate of PAM4 is only half of that of NRZ, which greatly reduces the bandwidth requirements of optical devices, thus reducing the cost of optical devices. As a result, PAM4 modulation technology has become an inevitable development direction.

Challenges of PAM4

PAM4 also has disadvantages, including,

• The SNR of PAM4 is worse than NRZ at -9.54dB.

• Reflections of PAM4 are 3x worse than NRZ.

• Higher power consumption than NRZ.

• Higher cooling conditions are required.

• More expensive equipment is required.

Since both QSFP56 and QSFP-DD support 200G data rates, factoring in SNR, power consumption, easy deployment, and cost, the 200G QSFP-DD type might be the better choice for the 200G short-range interconnection option.

Frequently Asked Questions (FAQs)

Q: What distinguishes QSFP28 from QSFP-DD the most?

A: The difference between QSFP28 and QSFP-DD is their data rates as well as lane configurations. Four lanes at 25 Gbps each give support to data rates up to 100 Gbps by QSFP28, while on the other hand, eight lanes at 50 Gbps each are used for a maximum of 400 Gbps in the case of QSFP-DD which makes it more suitable for higher bandwidth applications.

Q: Does OSFP have the same form factor as QSFP-DD?

A: No, OSFP is different from QSFP-DD in terms of form factors although both were designed with a target to support 400 Gbps data rates. While being backward compatible with other QSFP form factors, a bigger module size does not allow OSFP backward compatibility with QSFP systems.

Q: What are some key differences between QSFP56 and QFSP-DD?

A: Eight lanes supporting up to 400Gbps are used by Quad Small Form Factor Pluggable Double Density (QSFP-DD) transceiver modules, each operating at 50 Gbp whereas Quad small form-factor pluggable (QSFP56) can only run four lanes at a rate of 50 Gbp over distances of about two meters before needing repeaters or amplifiers. The lane count and maximum supported data rate represent major differences between them.

Q: Is there any backward compatibility between these two products –QSFP28 and QFSPDD?

A: Yes, this was one of its design goals, so that users would be able to replace old generation transceivers without having to change anything else in their infrastructure thus saving money on new installations where possible which also gives them more options when planning upgrades etcetera because they can mix different types together if need be knowing all work fine together easily but yes indeed! So even though QFSP28 is not as high performing, it still can be used with the QFSPDD.

Q: Which form factor would you say is more future-proof –OSFP or QSFP-DD?

A: Both OSFP and QSFP-DD are good options for future 400G networks, but the backward compatibility of QSFP-DD with previous QSFP form factors might make it more attractive for seamless upgrades. However, larger size allows for better thermal management which means that there will never be any overheating problems even when one needs higher performance in (future) applications where this may become necessary thus making them both equally viable choices depending on specific environments etcetera…

Q: What standards do QSFP56 transceivers comply with?

A: The IEEE802.3bs standard has been met by these devices as well as the Multi-Source Agreement (MSA) for Quad Small Form Factor Pluggable Double Density (QSFP-DD), thus providing bandwidths and power efficiencies that are not possible when only following one of those two alone also its good because they follow qsfpdd multi-company to ensure interoperability within different networking equipment types while adhering to industry performance & reliability guidelines etcetera!

Q: How does the electrical interface of QSFP-DD differ from that of QSFP28?

A: An advanced electrical interface is included in the QSFPDD module which supports eight simultaneous lanes unlike four in qsfp28 hence essentially doubling data throughput capability thereby enabling applications with higher bandwidth requirements so yes indeed!!

Q: Can applications requiring high data rates be supported by QSFP28?

A: Yes, it can support up to 100 Gbps which makes suitable for 25G, 50G and 100G Ethernet among others but for instance 200G or 400G Ethernet will need either OSFP or QFSPDD however i think osfpdd might do better due its bigger size allowing better thermal management and possible higher performance in future scenarios.

Q: What mechanical interface considerations are there for QSFP-DD?

A: The mechanical interface of QSFPDD includes enhancements such as an additional row of contacts and increased module depth designed to support the higher power and thermal constraints associated with 400G transmission while still being compatible with current QSFP ports.

Q: Why should someone choose QSFP-DD for data center applications?

A: Data centers need high-density, high-performance solutions that can be upgraded with time. This is where QSFP-DD comes in because it supports speeds of up to 400 Gbps, which are compliant with IEEE802.3bs and qsfpdd msa standards while also allowing users to use their existing systems without necessarily having any compatibility issues whatsoever, so its one size fits all kind of thing if you ask me!