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From the article”All You Should Know About MPO” we know that there are two types of endface of the MT ferrule, which is the MPO connector polishing types:
- APC (Angled Physical Contact): 8-degree bevel, reduces reflection, green or yellow housing (low-loss, Elite), commonly used for single mode laser transceiver applications.
- UPC (Ultra Physical Contact): Flat, with black/beige/aquamarine/purple/rosy red, etc., commonly used for multimode laser transceivers.
40G MPO SFP Module Interface
In 40G Ethernet, the MPO/MTP patch cords used with qsfp+ modules primarily adopt either APC or UPC polish types, which are determined by the fiber types which will be used. Typically, single mode MPO modules use APC interfaces, while multi-mode MPO SFP modules employ UPC interfaces.
- Single mode 40G SFP+ (e.g., 40G-PSM4/PLR4): MPO APC connectors must be used. Since single-mode signals have extremely strict requirements regarding reflection, APC connectors effectively minimize reflection, typically achieving a return loss greater than 60 dB.
- Multi-mode 40G SFP+ (e.g., 40G-SR4/eSR4): MPO UPC/PC connectors (flat-polished; black or beige housings) are typically used. For short-distance multi-mode transmission, PC or UPC connectors are generally employed; these typically provide a return loss of 20 dB to 30 dB, which is sufficient to meet the requirements of standard applications.
However, in practical applications, MPO connectors are not limited to 40G networks; they are also utilized in higher-speed networks such as 100G, 200G, 400G, and 800G. Does this mean they follow the same conventions established for 40G—specifically, UPC for multimode and APC for single mode?
The answer: Not entirely.
As data rates increase, while single-mode fiber continues to utilize APC connectors, the connectivity standards for multimode fiber underwent a fundamental shift during the 200G and 400G eras. In high-speed networks, 100G QSFP28 SR4 modules still employ MPO UPC connectors; however, with the advent of 200G SR4 and 400G SR4, there has been a complete transition to MPO APC connectors. The specific evolution and selection guidelines for each speed tier are detailed below:
100G SR4: Still adopts MPO UPC
Endface type: MPO UPC (Flat/Ultra Physical Contact, typically with a black/beige/water blue head).
Reason: The 100G SR4 employs the traditional NRZ (Non-Return-to-Zero) modulation signal. NRZ signals have relatively high tolerance for optical reflection (return loss), and the reflection suppression provided by UPC is sufficient for short-distance transmission in multimode fiber.
200G SR4/400G SR4: Mandatory switch to MPO APC
End face type: MPO APC (8-degree slope/angle physical contact, usually green housing, although the SR4 modules use multi-mode fibere, but it is also green APC ferrule).
Reason: Starting from 200G and 400G, the MTP SFP transceivers has fully introduced PAM4 (four level pulse amplitude modulation) technology. Its main features are as following:
- Extremely sensitive to noise: PAM4 signal divides the level signal into four stages, and the vertical eye opening between signals decreases, resulting in extreme sensitivity to multipath interference (MPI) and optical echo reflection noise.
- Eliminating reflections: If the UPC planar connector is continued to be used, the resulting light reflections will directly hit the laser or be reflected multiple times in the link, disrupting the integrity of the PAM4 signal. Therefore, IEEE industry standards mandate the use of APC (inclined surface) in 200G/400G multi-mode parallel MPO MTP patch cords to allow reflected light to enter the cladding at an oblique angle and dissipate.
Does the 400G SR8 and 800G SR8 also use MPO APC?
Yes, both 400G SR8 and 800G SR8 still use MPO APC (slope, usually green housing).
Not only that, as the number of channels increases, their MPO MTP patch cables interfaces also introduce a new connector specification (MPO 16) and dual MPO 12 interface form. The following are detailed reasons and new changes that must be noted when wiring, direct connection, and branching connection:
Why do we have to use MPO APC?
Whether it is 400G SR8 or 800G SR8, the underlying signal modulation is all PAM4 (four level pulse amplitude modulation) technology.
- 400G SR8: using 8-channel x 50G PAM4.
- 800G SR8: using 8-channel x 100G PAM4.
Due to the extremely high single channel rate, the destructive power of multipath interference (MPI) and back reflection on signals increases exponentially. In order to introduce the angle deviation of reflected light into the cladding consumption, industry standards mandate that 400G/800G multi-mode short wave transceiver modules must be equipped with MPO APC.
The interface has undergone significant physical changes (key to avoiding pitfalls)
When conducting direct or branch cabling for 400G/800G SR8 transceiver modules, you cannot blindly purchase the commonly used MPO 12 cable, as their physical interfaces and channel arrangements have changed:
| Core change 1: Single MPO-16 interface | Core change 2: Dual MPO-12 Interface |
| Application scenarios: Most QSFP-DD packaged 400G SR8 modules and 800G SR8 modules.MPO patch cord requirements: Must be compatible with MPO 16 APC connector. The positioning key of this connector is eccentric in design, which is physically incompatible with traditional 12 core/24 core MPO and completely eliminates the possibility of wrong insertion.Channel occupancy: All 16 cores are fully utilized (8-core transmit TX, 8-core receive RX). | Application scenario: Especially common in NVIDIA Mellanox’s OSFP packaged 800G SR8 (2xSR4) module (such as in the H100/H200 computing cluster).MTP patch cord requirements: There are two parallel MPO/MTP ports on the module panel, so dual MPO/MTP 12 APC patch cords are required for direct connection.Channel occupancy: Each MPO MTP 12 port occupies 8 cores on the outer side (4 for sending and 4 for receiving), with 4 cores in the middle vacant. |
On-Site Cabling Pitfall Avoidance Guide
Due to the traditional convention that “single-mode uses green APC connectors, while multi-mode uses black or aqua UPC connectors,” errors are highly prone to occur when upgrading to a 400G backbone network:
1. 400G Multi-mode Also Uses Green Connectors: When procuring multi-mode patch cords for 400G SR4 or SR8 transceiver modules, you must explicitly specify “Multi-mode MPO APC.”
2. Strict Prohibition Against Cross-Mating: Under no circumstances should you attempt to connect legacy 100G MPO UPC patch cords to 400G SR4 MPO SFP modules. Doing so will not only prevent light transmission but will also directly damage the precision fiber end-faces located inside the SFP 400G module! Since 400G SR4 MPO modules feature MPO APC end-faces, APC and UPC connectors must never be intermixed; otherwise, the connector ferrule will be damaged, and signal transmission will be impossible.
3. MPO Male vs. MPO Female Connectors: MPO connectors are classified into male (featuring guide pins) and female (pin-less) types. Typically, when connecting a patch cord to a transceiver module, the module interface acts as the female end while the patch cord acts as the male end; however, this configuration should always be verified against the specific requirements of the actual equipment being used.
4. Polarity: 40G and 100G parallel optical networks typically utilize Type B (Cross-over) polarity.
Conclusion
Whenever a transceiver module name includes terms such as PAM4 modulation, 400G, 800G, SR4, or SR8, you can confidently select MPO APC connectors (featuring green housings) when purchasing multimode MPO MTP patch cords and Optical Distribution Frames (ODFs). If you are upgrading a 12-fiber system, be sure to verify whether the equipment modules require a switch to the MPO 16 specification.
| Transceiver model | Signal Modulation Method | Module Interface Types | MPO/MTP Patch Cord Endface | Common appearance recognition |
| 40G SR4 | NRZ | Single MPO-12 | MPO UPC | Light blue patch cable + black/beige connector |
| 100G SR4 | NRZ | Single MPO-12 | MPO UPC | Light blue/magenta patch cable + black connector |
| 200G SR4 | PAM4 | Single MPO-12 | MPO APC | Dedicated multi-mode patch cable + green connector (with PIN matching) |
| 400G SR4 | PAM4 | Single MPO-12 | MPO APC | Dedicated multi-mode patch cable + green connector |
| 400G SR8 (QSFP-DD) | PAM4 | Single MPO-16 | MPO APC | Dedicated multi-mode patch cable + green connector |
| 800G SR8(QSFP-DD) | PAM4 | Single MPO-16 | MPO APC | Dedicated multi-mode patch cable + green connector |
| 800G SR8 (OSFP 2xSR4) | PAM4 | Dual MPO-12 | MPO APC | Dedicated multi-mode patch cable + green connector |
