QSFP-DD vs QSFP28/QSFP56 – What Are The Differences?

Intro

QSFP-DD vs QSFP28 / QSFP56 How we got here with so many different QSFPs? The rapid growth of cloud data center networking and interconnections is pushing the technology development to meet the ever higher bandwidth connectivity demands. From the rise of 40G-QSFP transceivers and ever successful advancement to the 100G-QSFP28 form-factor, the next major step is the prevalence of 200G and 400G Ethernet technology with QSFP-DD form-factor optical transceivers.

Current widespread use of 100G QSFP28 technology transceiver form-factor, followed by 200G QSFP56 transceiver form-factor, has led to a completely new optical transceiver form-factor, QSFP-DD.

Confused? Yep, it is a bit. This article will help to quickly understand the main differences between QSFP-DD, QSFP56, and QSFP28.

1. QSFP-DD vs QSFP28 / QSFP56 Quick Overview

2. QSFP-DD vs QSFP28 / QSFP56 Difference 1 – Data Rate

The first basic parameter that describes optical transceivers is data rate. Data rate is defined as the amount of data transmitted during a specified time period over a network. Different transceivers and respective networking technologies have different data rate values.

QSFP-DD has the maximum data rate of 200Gbps or 400Gbps, depending on the signal modulation technology used. Its main difference when compared to QSFP56 or QSFP28 is that the QSFP-DD name states – double density, which means that its electrical lines are doubled from 4 to 8.

• 8 channels modulated with NRZ results of 25Gbps per channel, which totals to 200Gbps.
• But 8 channels modulated with PAM4 result in 50Gbps per channel, which totals to 400Gbps.

(Respectively, QSFP56 maximum data rate is 200Gbps, but QSFP28 is 100Gbps).

Comparison of data rates based on networking technologies:

• Ethernet technology standardizes the following data rates for QSFP-DD – 212.5 Gbps, 425 Gbps.
• InfiniBand is an industry-standard, channel-based, switched fabric interconnect architecture for server and storage connectivity. InfiniBand standardizes the following data rates for QSFP28: 100G EDR, for QSFP-DD: 200G HDR, 400G NDR.
• The optical transport network (OTN) makes up a generic Layer-1 transport network, enabling different service types to coexist and share the same infrastructure transparently, without affecting each other’s performance.OTN Optical Transport Network specifies OTU4 for QSFP28 100G, OTUc2 for 200G, OTUc4 for 400G applications, QSFP-DD, respectively.

Fiber Channel: 128GFC (24850 56.1 PAM-4) released in 2021, with higher data rate generations to be released in the future – 256GFC in 2024.

Summary:

3. QSFP-DD vs QSFP28 / QSFP56 Difference 2 – Modulation Type

The next notable difference factor is transceiver modulation. An optical transceiver consists of a laser, which is able to emit light, and a modulator. An optical modulator is a device that can be used for manipulating a property of light. It determines the quality of transmissions between transmission equipment.

Regarding pluggable optical transceivers, it is the intensity of the light that is modulated. Engineers have long utilized non-return to zero (NRZ) modulation for 1G, 10G, and 25G, using host-side forward error correction (FEC) to enable longer distance transmissions. For higher data rates, 40G and 100G industry introduced signal parallelization of 10G and 25G NRZ modulated signals. In short, the lower speed optics 1G, 10G, 25G, and most 100G optics modulate the intensity of the light at NRZ two levels, and are therefore binary.

Downside for NRZ modulation technology for 200G, 400G, and other faster data rates is not as efficient as compared to smaller data rates. As a result, network scientists have developed PAM4 modulation for these high bandwidth data rates.

Different modulation technologies:

NRZ – Non-Return-to-Zero, is a 2-level binary code using low and high signal levels to represent the 1/0 information of a digital logic signal. NRZ can only transmit 1 bit, i.e., a 0 or 1, of information per signal symbol period.

PAM4 – Pulse Amplitude Modulation 4 is a 4-level signal modulation format used to transmit signals. Each signal level can represent 2 bits of logic information.

This means that with PAM4 modulation, information can be transmitted in half times shorter time. Or two times more information at the same time.

Practically, we can see this with a 400G transceiver. This transceiver has a built-in DSP gearbox that takes an 8x 25GBaud PAM4 signal and converts it to a 4x 50GBaud PAM4 signal. And so it is with PAM4 modulation 4x 50GBaud scales to 4x 100Gbps lines, which then are multiplexed and fully give 400Gbps data rate.

Picture – PAM4 has four distinct levels to encode 2 bits of data:

Summary:

4. QSFP-DD vs QSFP28 / QSFP56 Difference 3 – Power Consumption

Optical transceiver power consumption is closely related to the host device's total power budget. The bigger the transceivers' power consumption, the smaller the number of transceivers usable in host equipment. So the smaller power consumption of the transceiver and host equipment altogether, the better performance and smaller environmental impact.

QSFP-DD transceiver specification allows a maximum power consumption of 12 Watts. 

QSFP56 transceiver specification allows a maximum power consumption of <5 Watts. 

QSFP28 transceiver specification allows a maximum power consumption of <3.5 Watts. 

The general rule of thumb, the smaller the power consumption, the smaller the module temperature, the better the transceiver reliability, and the better overall performance of the transceiver.

Summary:

5. QSFP-DD vs QSFP28 / QSFP56 Difference 4 – Connector Types

Optical transceivers have two ends. One end is the electrical connector, which connects to the host equipment (switch, router, etc) other end is the laser output, which connects to the optical fiber. QSFP28, QSFP56 transceivers have a 38-pin electrical interface that connects to the host device. But QSFP-DD, as it is double density, has a double count of electrical pins – 76. This basically means that systems (host devices) that are designed with QSFP-DD ports are backward compatible with QSFP56 and QSFP28 form factor transceivers. Practically, this means that if you have a router with QSFP-DD ports, you can easily plug QSFP56 or QSFP28 in those same ports (before connecting, check the product data sheet if the port logically supports QSFP56 or QSFP28 transceiver).

The following picture shows us QSFP28 pads and additional pads, which are used for QSFP-DD, to accommodate the double line interface.

Optical transceivers on their respective ends have an optical connector compatible plug mechanism of female type. This connector can have the following types: LC, MPO, or CS. Regarding the MPO cables, we at EDGE Optics have a separate article about MPO and MPT cables.

CS connector is the newest generation technology for connecting 200G and 400G optical interfaces. The CS connector consists of a Double LC-type push-pull coupling mechanism, which is 40% smaller than our well-known Duplex LC connector. The smaller footprint doubles the connector density on the switch front panel if compared to the LC connector.

One QSFP-DD transceiver takes two CS connectors, one pair for transmission and one pair for receiving signals.

Dual CS connector profile:

CS connector:

6. Difference 5 – Dimensions

QSFP-DD is the latest developed and with the highest data rate transceiver; it has some dimension differences if compared with QSFP56 and QSFP28 form factor transceivers. QSFP-DD is a slightly longer transceiver because of four extra electrical lines, which makes it a double-density transceiver (the electrical interface contains a larger count of electrical pads – 76).

QSFP-DD 18.35 / 89.4 / 8.5mm

QSFP56 18.35 / 72.3 / 8.5mm

QSFP28 18.35 / 72.3 / 8.5mm

7. More on QSFP-DD Form Factor and Module Types

8. More on QSFP56 Form Factor and Module Types

9. More on QSFP28 Form Factor and Module Types

10. Conclusion

QSFP-DD, QSFP56, and QSFP28 transceivers differ in many ways. The most significant difference for the newest QSFP-DD transceivers is that this double-density form factor packs a double 4 lane electrical interface, which totals to 8 electrical lanes. This allows it to support data rates up to 400Gbps. The 8-lane technology packed in QSFP-DD increases its dimensions in length up to 89.4mm (if compared to QSFP56 and QSFP28, 72.3 mm). Additionally, QSFP-DD has a double count of electrical pad connector lines (76) to accommodate the 8 lane interface.

QSFP-DD unlocks its abilities to achieve 400Gbps data rate with the great help of PAM4 modulation. This modulation technique is new for optical transceivers, because previously the 2-level NRZ modulation technique was used in the last generation 100G-QSFP28 transceivers. Instead, PAM4 modulation is 4-level amplitude modulation, where each level can represent 2 bits of information. As a result, with PAM4 modulation,n information can be transmitted in half times shorter time, or respectively, two times more information at the same time. With this principle, QSFP-DD can achieve higher-than-ever data rates.

As the QSFP-DD packs in new technologies, its power consumption has risen to unseen levels. QSFP-DD peak power consumption can be up to 12 Watts. By comparison to QSFP28, 3.5 Watts is a notable increase. The increased power consumption allows it to achieve the promised 400Gbps data rate.

Both QSFP-DD and QSFP56 in the near future will be equipped with the latest technology CS connector. The CS connector consists of a Double LC-type push-pull coupling mechanism, which is by far smaller than the classical Duplex LC connector. The smaller footprint doubles the connector density on the switch front panel. As well, it enables two 400Gbps QSFP-DD transceivers connectivity with each other or signal break-out options to 2x 200Gbps QSFP56 transceivers.

QSFP-DD form factor is used for 400Gbps and a few 200Gbps transceivers. The 400G QSFP-DD transceiver types, most importantly, differ from each other with working distance and wavelengths used. For example, 400GBASE-SR8 is the only version of QSFP-DD transceiver that uses Multi-Mode fiber, and can achieve a 100-meter distance. Up until the top 400GBASE-ER8 version, which is a Single-Mode fiber 40km reach version. And many more versions in between.