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In data center networks, the solution for converting 400G SR8 to 4x100G SR2 is primarily implemented through the use of fiber breakout cables or specific combinations of optical modules. It is important to note that 400G SR8 utilizes eight lanes of 50G PAM4 signals, which differs in modulation scheme from traditional 100G interfaces (such as 4x25G NRZ, which is SR4).
Interface Types at Both Ends
This “1-to-4” breakout cable typically features the following physical interfaces:
- 400G End (Trunk Side): Employs a QSFP-DD interface. QSFP-DD is the most prevalent form factor in 400G networks and offers backward compatibility with the QSFP series like QSFP28 100G, QSFP+ 40G, etc.
- 100G End (Breakout Side): Employs four QSFP56 interfaces.
- Key Consideration: Since 400G SR8 utilizes eight lanes of 50G PAM4 signals, the resulting 100G connections must be via QSFP56 interfaces that support two lanes of 50G PAM4 (i.e., 100G SR2). Traditional QSFP28 interfaces (4x25G NRZ) are incompatible with SR8 at the physical layer modulation level, unless the networking equipment possesses a specialized automatic rate negotiation capability.
Based on data center cabling requirements, there are typically three implementation methods: AOC cables, DAC cables, and optical transceivers paired with MTP fanout cables. Users can select the cabling method best suited to their needs based on actual transmission distances and cost considerations.
Solution 1. Using Breakout Cables
The 400G end provides 8x50G PAM4 signals, while the breakout end splits into 4x100G QSFP56 ports, with each 100G port carrying 2x50G PAM4 signals.
- Interfaces: One end features an integrated QSFP-DD 400G SR8 transceiver, while the other end features four integrated 100G QSFP56 modules.
- Compatibility Note: The 100G modules at the breakout end typically utilize the QSFP56 form factor (supporting 50G PAM4), rather than the older QSFP28 form factor (which primarily supports 25G NRZ).
1.1 AOC Breakout Cable
AOC breakout cable represents the most direct and cost-effective solution for short-distance connections, and pre-terminated by the manufacturer and comes with integrated optical modules permanently attached at both ends. It is lighter than copper cabling, offers a typical transmission range of 1 to 100 meters, and is suitable for inter-cabinet or cross-cabinet connectivity.
1.2 Fiber DAC Cable
Fiber DAC Cable use Twinax copper wire, no need optical to Ethernet converter, so it is lowest cost effective solution for short distance connection within 3 meters—making them suitable only for short-distance connections within the same cabinet. The 400G DAC breakout cable is also pre-terminated by the manufacturer and comes with integrated optical modules permanently attached at both ends.
Solution 2: Using Optical Transceivers + MPO/MTP Fanout Cables
If you require greater cabling flexibility or longer transmission distances (up to 100 meters), you can utilize discrete optical transceivers paired with fiber optic patch cables.
400G Side: Use a 400GBASE-SR8 QSFP-DD optical transceiver; this QSFP DD-SR8-400G module typically features an MPO-16 interface.
100G Side: Use four 100GBASE-SR2 QSFP56 optical transceivers (Note: this utilizes the SR2 2-lane scheme).
Connecting Cable: A single MPO-16 to 4x MPO-12 breakout patch cable is required.
Key Point: Within those four MPO-12 connectors, only the four central (or specific) fiber ports are populated with fibers, while the remaining eight ports are empty (unpopulated). When a 100G SR2 optical module is connected, the laser inside the module aligns only with the 4 active fibers for data transmission.
The 12-fiber (MPO-12) configuration you see refers merely to the physical standard of the connector; it does not necessarily imply that there are 12 optical fibers contained within. This housing fewer fibers within a larger connector shell is adopted to ensure compatibility with the physical interfaces of existing 100G optical fiber modules.
Key Considerations and Technical Core
- Modulation Scheme Compatibility: The 400G SR8 utilizes an 8x50G PAM4 architecture. When implementing a 4x100G breakout configuration, the 100G receiver on the receiving end must be capable of processing 50G PAM4 signals (i.e., based on a 100G-SR2 or 100G-DR architecture); it cannot be directly connected to legacy 100G SR4 ports that support only 4x25G NRZ.
- Port Configuration: On the switch, the 400G port must be manually configured to “Breakout” mode (e.g., “split 4x100G” or “channelized”) so that the system recognizes it as four independent 100G logical interfaces.
- Interface Specifications: QSFP DD-SR8-400G modules typically utilize MPO 16 APC (Angled Physical Contact) or MPO 24 connectors. When selecting MPO/MTP fanout cables, it is critical to verify that the polarity and connector type (APC vs. UPC) are compatible.
If you require longer distances (>100m) or a single-mode solution, you would typically switch to a 400G DR4 module to break out into 4x100G DR1 module via an 12 MTP fiber breakout cable, Please refer to the article below for inspiration:
