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The 200G QSFP56 port is the standard interface for 200G network interconnection in current data centers and AI clusters, widely used in server network cards, switches, and various high-speed interconnection device.
What is QSFP56?
QSFP56 is a member of the QSFP (Quad Small Form factor Pluggable) packaging family, with the same physical size as QSFP28 and QSFP+, but with a single channel rate increased to 50Gbps (PAM4 modulation) and 4-channel aggregation achieving a transmission rate of 200Gbps.
| Pack type | Year of release | Single channel rate | Modulation technology | Total speed |
| QSFP+ | 2013 | 10Gbps | NRZ | 40G |
| QSFP28 | 2016 | 28Gbps | NRZ | 100G |
| QSFP56 | 2017 | 56Gbps | PAM4 | 200G |
Core difference: QSFP56 adopts PAM4 (4-stage pulse amplitude modulation) technology instead of traditional NRZ, which can transmit 2 bits of information per symbol period, thus increasing the single channel rate from 28G (NRZ) to 56G (PAM4) and doubling the bandwidth.
Core technology: PAM4 modulation
PAM4 (Pulse Amplitude Modulation 4-level) is a key technology for QSFP56 to achieve 200G:
- Traditional NRZ: using 2 signal levels (0/1), transmitting 1 bit per symbol period
- PAM4: Use 4 signal levels (00/01/10/11) to transmit 2 bits per symbol period
This means that when the same transmission rate is achieved, the symbol rate of PAM4 is only half of that of NRZ, and the transmission loss is greatly reduced.
Typical signal path of 200G QSFP56:
- Electrical interface side: 4 × 53.125Gbps PAM4 (200GAUI-4)
- Optical interface side: Depending on the module type, it can be 4 × 50G or 2 × 100G PAM4
Connection types supported by QSFP56 port
The 200G QSFP56 port supports multiple interconnection schemes and can be flexibly selected according to the transmission distance:
| Connection type | Interface | Media | Transmission distance | Typical power consumption | Applicable scenarios |
| Breakout DAC cable | QSFP56（integrated） | Twinax copper cable | 1-5 m | <0.1-1.5W | Interconnection between servers in the cabinet and TOR switches |
| AOC fiber optic cable | QSFP56（integrated） | Multimode fiber | ≤ 100 m | ~4.5-5W | Plug and play across cabinets, no need for separate optical modules |
| QSFP56 200G SR4 transceiver | MPO-12 | Multimode optical fiber (OM3/OM4) | 70-100 m | <5W | Standard interconnection within 100 meters of the data center |
| Single mode SFP transceiver FR4 | Duplex LC | Single mode fiber | 2 km | <7W | Park interconnection, local data center |
| Single mode SFP transceiver LR4 | Duplex LC | Single mode fiber | 10 km | <7W | Metropolitan area interconnection, long-distance cross park |
In addition to the native support of 200Gbps, the QSFP56 port typically has good backward compatibility due to its physical form factor being identical to the previous QSFP series, provided that it is supported by switch firmware. This backward compatibility feature mainly stems from two aspects:
- The QSFP56 package is physically compatible with QSFP28 and QSFP+;
- Its electrical channel supports both PAM4 and NRZ modulation methods, enabling seamless integration with existing low-speed network devices.
Supported scenarios for each speed
| Transmission rate | Support modes and scenarios | Key Instructions |
| 100G | Standard single port mode Can insert 100 GBPS optical transceiver or use DAC AOC cable. Breakout mode Split one 200G port into two 100G ports through MTP breakout cables. | This is the most mainstream compatibility method, which can directly replace 100G modules for use. |
| 50G | Breakout mode Split into 4 x 50G SFP56 ports through MPO to 4LC breakout cables. Standard single port mode In rare cases, specific 50G modules can be inserted. | Mainly used to connect 50G server network cards. |
| 40G | Standard single port mode Can be plugged into 40G QSFP optical module or QSFP+ DAC cable. | Commonly used to connect old 40G switches or devices. |
| 25G | Breakout mode Split into 4 x 25G SFP28 ports through breakout cable. | This is the standard practice for data centers to connect 25G servers. |
| 10G | Breakout mode Split into 4 x 10G SFP+ports through breakout cables. | Mainly used for reusing old 10G network equipment. |
Two key ways of “downward compatibility”
- Automatic speed negotiation: The port can automatically reduce the speed to 100G based on the inserted module (such as 100 GBPS optical transceiver), achieving plug and play functionality.
- Breakout mode: Split the four high-speed channels of QSFP56 port into independent low-speed interfaces. For example, by connecting four 25G servers through a branch cable, one 200G port can serve four 25G devices, greatly improving port density and flexibility.
Practical application cases
- LR-LINK LRES4160PF-2QSFP56 network card: The specifications show that its QSFP56 port supports a link speed of 200/100/50/40/25/10/1Gbps.
- Edgecore DCS511 switch: Its QSFP56-DD port supports multiple splitting modes such as 4x 10G, 4x 25G, and 2x 50G through branch cables.
- Cisco 200G QSFP56 module: Official data shows that its ports are compatible with running modules such as 100G and 50G.
Compatibility Comparison Table
| Form facgtor | Transmission rate | Modulation | Channel qty | QSFP56 port support or not |
| QSFP+ | 40G | NRZ | 4x10G | Yes (depending on the device) |
| QSFP28 | 100G | NRZ | 4x25G | is |
| QSFP56 | 200G | PAM4 | 4x50G | native support |
| QSFP112 | 400G | PAM4 | 4x100G | No (different electrical characteristics) |
Note: Although it can be physically plugged in, whether the port can function properly depends on the logical configuration of the switch chip (ASIC) and the software locking strategy of the manufacturer.