The Evolution of QSFP Form Factors

QSFP (Quad Small Form Factor Pluggable)

The QSFP (Quad Small Form Factor Pluggable) has become the dominant form factor for high-speed networking transceivers.  By enabling 4 optical lanes in a single transceiver, QSFP created a solution to increasing bandwidth significantly from its SFP predecessors. The form factor continues to be used, finding mainstream success with QSFP+ (40Gps), now achieving 800Gps in the QSFP-DD (Quad Small Form Factor Pluggable Dual Density).  The QSFP form factor also kept the same standard of sizing, in which the newest QSFP-DD supports all previous models for housing and operation, providing an optimal grow to scale model for networking need.

Timeline

2012
2014
2015
2018
2020
2020

QSFP+

40GBASE

QSFP28

100GBASE

QSFP56

200GBASE

QSFP-DD

400GBASE

QSFP112

400GBASE

QSFP-DD

800GBASE

The Need for Speed

The QSFP transceiver has evolved significantly since its introduction, primarily to meet the increasing demands for higher data rates and improved network efficiency.  As network demands grew, the QSFP+ variant was developed offering 10 Gbps per channel and providing an overall capacity of 40 Gbps. Further advancements led to the QSFP28, which supports 25 Gbps per channel, achieving a total throughput of 100 Gbps. This progression continued with the QSFP56, doubling the channel rate to 50 Gbps for a total of 200 Gbps, and the QSFP112 doubling the channel rate again for a total 400Gbps. The latest iteration is QSFP-DD (Double Density), which incorporates eight channels to deliver an impressive 800 Gbps. These advancements reflect the ongoing need for faster data transmission and increased bandwidth in data centers and high-performance computing environments, enabling more efficient and scalable network architectures.

Form Factor Comparison

Similarities

  • Form Factor

    All QSFP share the same basic "Quad Small Form-factor Pluggable" (QSFP) form factor, ensuring physical compatibility with the same ports on switches and network adapters.

  • Hot-swappable

    All QSFP are hot-swappable, allowing for easy insertion and removal without powering down network equipment.

  • Lanes

    Each QSFP version supports 4 electrical interface lanes for transmit / receiving. QSFP-DD (dual density) now incorporates 8 electrical lanes producing more available bandwidth.

  • Backwards Compatibility

    All QSFP are designed to be fully backwards compatible. QSFP-DD being the latest generation supports all previous models.
    Newer versions will not work in older ports (Example: QSFP56 will not work in QSFP+ ports)

  • Breakout Capability

    All forms excluding QSFP112 support lower speed breakout methods.

Differences

With evolution in speeds, power consumption has increased for optical transceivers.

QSFP+

Up to 40Gbps
  • Power Consumption
  • 1W to 3.5W

QSFP28

Up to 100Gbps
  • Power Consumption
  • 2.5W to 4.5W

QSFP56

Up to 200Gbps
  • Power Consumption
  • 4.5W to 7.5W

QSFP112

Up to 400Gbps
  • Power Consumption
  • 7W to 10W

QSFP-DD400

Up to 400Gbps
  • Power Consumption
  • 7W to 12W

QSFP-DD800

Up to 800Gbps
  • Power Consumption
  • 12W to 18W

* Power consumption ranges rated at max data speeds

Dimensions

The QSFP112 and QSFP-DD cases both increase in width over their predecessors, however, the case with and height remain the same allowing for compatibility across QSFP ports. 

QSFP+ / QSFP28 / QSFP56

18.35mm x 72.3mm x 8.5mm

QSFP112

18.35mm x 83.85mm x 8.5mm

QSFP-DD

18.35mm x 89.4mm x 8.5mm

QSFP Form Factor Specifications

Form Factor QSFP+ QSFP28 QSFP56 QSFP112 QSFP-DD400 QSFP-DD800
Maximum Data Rate 40 Gbps 4x25 Gbps / 1x100 Gbps 200 Gbps 400 Gbps 400 Gbps 800 Gbps
Lanes 4 4 4 4 8 8
Maximum Bit Rate (per lane) 10 Gbps 25 Gbps 50 Gbps 100 Gbps 100 Gbps 100 Gbps
Modulation 10G NRZ 25G NRZ / 100G PAM4 50G PAM4 100G PAM4 25G NRZ / 50G PAM4 / 100G PAM4 50G PAM4 / 100G PAM4
Power Consumption <= 3.5W <= 4.5W <= 7.5W <= 10W <= 12W <= 18W
Backward Compatibility N/A QSFP+ QSFP28; QSFP+ QSFP56; QSFP28; QSFP+ QSFP112; QSFP56; QSFP28; QSFP+ QSFP112; QSFP56; QSFP28; QSFP+
Breakout Capability 4x SFP+ (10G per lane) 2x QSFP28 (50G per lane)
4x SFP28 (25G per lane) 		2x QSFP28/56 ( 100G per lane)
4x SFP56 (50G per lane) 		Target Application from
OSFP / QSFP-DD Form Factors Only 		2x QSFP28 (100G Per Lane)
2x QSFP56 (200G Per Lane)
4x QSFP28/56 (100G Per Lane)
8x SFP28 (25G Per Lane)
8x SFP56 (50G Per Lane) 		2x QSFP-DD400 (400G Per Lane)
8x QSFP28 (100G Per Lane)
Size (W x L x H) mm 18.35 x 72.3 x 8.5 18.35 x 72.3 x 8.5 18.35 x 72.3 x 8.5 18.35 x 83.85 x 8.5 18.35 x 89.4 x 8.5 18.35 x 89.4 x 8.5
Date Released 2012 2014 2015 2020 2018 2020

Did you know?

40G QSFP+
Provided an economical upgrade path from 10G Ethernet.
100G QSFP28
Became the backbone for many modern network infrastructures. Compatible with 25G SFP28 all the way to the latest 800G QSFP-DD form factor, allowing for flexible and future-proof network designs.
200G QSFP56
Intermediate step towards even higher bandwidth. QSFP56 offers a good balance between performance and cost, especially in scalable networks.
400G QSFP112
Investing in QSFP112 allows data centers to prepare for future bandwidth demands looking towards 800G and beyond
400G / 800G QSFP-DD
Widely used in hyper-scale data centers and core networks requiring ultra-high-speed connectivity. Fully backward compatible with previous generations all the way to 100G QSFP28.

Optical Channel Modulation

NRZ Modulation (QSFP+, QSFP28, & QSFP-DD)

NRZ (Non-Return to Zero) is a digital line coding scheme used in telecommunications.  In NRZ, binary data values are directly represented by distinct signal levels:

Binary 1: Represented by a specific signal level (often a positive voltage).

Binary 0: Represented by a different signal level (often a negative voltage).

The key point is that the signal level does not return to zero between transmitting consecutive bits of the same value.

Imagine a classic light switch, on or off. That’s essentially NRZ, using two voltage levels to represent data: high for 1 and low for 0. Simple, reliable, and widely used for lower data rates under 100G.

NRZ incorporated 1G, 10G, and up to 25G per lane speeds, but was not suited for higher levels of data due to the simplicity of design.

Eye Diagram for NRZ data

PAM-4 Modulation (QSFP56, QSFP112, & QSFP-DD)

PAM4 (Pulse Amplitude Modulation, 4 Levels) is a multi-level signaling technique used in high-speed data communication.

Instead of representing bits with just two distinct levels (high and low) like in NRZ, PAM4 uses four distinct voltage levels allowing it to transmit two bits of information per symbol.

00: Lowest voltage level

01: Second lowest voltage level

10: Second highest voltage level

11: Highest voltage level

Think of this as a dimmer switch with multiple levels vs NRZ method of “on & off”. This doubles the bandwidth compared to NRZ, perfect for transmitting more data at the same speed of modulation.

PAM4 in QSFP-DD utilizes up to 8 electrical lanes over the 4 optical channels.  Broken down in speeds of 50Gb/s, 100Gb/s, and soon 200Gb/s to achieve speeds of 400G, 800G, and soon to be 1.6TB collectively.

Eye Diagram for PAM4 data