Why MPO Polarity is the #1 “Silent Killer” in Data Center Cabling
In high-density data centers, MPO/MTP connectors are the standard for 40G, 100G, and 400G links. But this solution introduces a critical, and often misunderstood, technical challenge: polarity.
Polarity is the simple concept of matching the “Transmit” (Tx) port on one end of a link to the “Receive” (Rx) port on the other. With a traditional duplex LC cable, if the link doesn’t come up, you just flip one of the connectors.
You cannot do this with an MPO cable.
A 12-fiber MPO connector is a single-body connector. If the polarity is wrong, the entire link is dead—and it can’t be fixed on-site. Getting it wrong during the design phase means costly downtime and re-orders. This guide will ensure you get it right, every time.
[Note: This guide assumes standard 12-fiber MPO connectors. The TIA-568 standard defines three methods to manage polarity: Type A, Type B, and Type C.]
Type A (Straight-Through): The “Trunk” Cable
A Type A MPO cable is a “straight-through” cable. The fiber at Pin 1 on one end arrives at Pin 1 on the other end. Pin 2 arrives at Pin 2, and so on.
- Diagram Description:
- End 1 (Key Up): [1] [2] [3] … [12]
- End 2 (Key Up): [1] [2] [3] … [12]
- Key Feature: This cable does not flip polarity. A Tx-Rx pair (e.g., Pins 1-2) remains a Tx-Rx pair.
- Use Case: A Type A cable is never used for a direct switch-to-switch connection. It is designed to be a “trunk” cable in a structured cabling system, where it connects two MPO cassettes. The polarity flip is then handled by the duplex patch cords at the front of the cassette.
Type B (Reversed/Key-to-Key): The “Direct Connect” Cable
A Type B MPO cable is a “reversed” cable. The fiber at Pin 1 on one end arrives at Pin 12 on the other end. Pin 2 arrives at Pin 11, and so on.
- Diagram Description:
- End 1 (Key Up): [1] [2] [3] … [10] [11] [12]
- End 2 (Key Up): [12] [11] [10] … [3] [2] [1]
- Key Feature: This cable performs a full, end-to-end flip. This is the mechanism that ensures Tx (e.g., Pin 1) connects to Rx (e.g., Pin 12).
- Use Case: This is the most important type. Type B cables are used for direct 40G/100G/400G connections between two transceivers (e.g., connecting two 100G-SR4 switches directly).
Type C (Pairs-Flipped): The “Specialist” Cable
A Type C MPO cable is a “pairs-flipped” cable. It looks like a Type A, but each adjacent pair of fibers is crossed. Pin 1 arrives at Pin 2, and Pin 2 arrives at Pin 1. Pin 3 arrives at Pin 4, and Pin 4 arrives at Pin 3, etc.
- Diagram Description:
- End 1 (Key Up): [1] [2] | [3] [4] … [11] [12]
- End 2 (Key Up): [2] [1] | [4] [3] … [12] [11]
- Key Feature: It flips polarity by crossing pairs.
- Use Case: This type is less common. It’s typically used in specific vendor-driven pre-terminated systems and is not the standard for direct transceiver connections.
Practical Application: Which MPO Cable Do I Buy?
Here are the two most common scenarios data center engineers face.
Scenario 1: Connecting Two 100G Switches Directly
- Your Link:
[100G-SR4 Transceiver] <--> [MPO Cable] <--> [100G-SR4 Transceiver] - The Answer: You need one (1) Type B MPO Trunk Cable.
- Why? The 100G-SR4 transceiver uses Pins 1-4 for Tx and Pins 9-12 for Rx. The Type B cable’s end-to-end reversal ensures that the Tx (1-4) on one side correctly maps to the Rx (12-9) on the other.
Scenario 2: Connecting via a Patch Panel (Structured Cabling)
- Your Link:
[Switch] <-> [MPO Jumper] <-> [MPO Cassette] <-> [MPO Trunk] <-> [MPO Cassette] <-> [MPO Jumper] <-> [Switch] - The Answer:
- MPO Trunk Cable (the long run): Type A
- MPO Cassettes & Patch Cords: You must use a “Method A” system, where the cassettes and duplex patch cords are designed to manage the flip.
- Why? In this setup, the Type A trunk acts as a simple extension. The polarity is managed at the endpoints by the cassettes and patch cords.
FAQ: MPO Polarity
Q: Is MTP® polarity different from MPO polarity?
A: No. MTP® is a high-performance, trademarked brand of MPO connector. It adheres to the exact same TIA standards for polarity: Type A, Type B, and Type C. The terms are often used interchangeably in the industry, and the polarity methods are identical.
Q: What happens if I use the wrong polarity cable, like a Type A, to connect two switches?
A: The link will not come up. The Tx (Transmit) fibers on one end will be connected to the Tx fibers on the other end, and the Rx (Receive) fibers will be connected to the Rx. Since no signal is being received, the link will remain down.
Q: Can I “fix” or change the polarity of an MPO cable in the field?
A: On most standard MPO trunk cables, no. This is why it’s a critical design choice. While some specialized (and more expensive) “polarity-changeable” patch cords exist, your main trunk cables are fixed. Ordering the correct polarity from the start is essential.
Q: This guide uses MPO-12. What about MPO-8 or MPO-24?
A: The principle is identical, but the application differs. MPO-8 is a base-8 system used for some 40G/100G applications. MPO-24 is used for very high-density trunks or 400G SR8. However, the most common direct 40G/100G link (SR4) uses an MPO-12 connector (with 4 fibers dark). In that standard application, Type B is still the correct choice.
Conclusion: How to Avoid a Polarity Disaster
For 90% of engineers looking for a direct switch-to-switch connection, the rule is simple:
When in doubt, you need a Type B MPO Cable.
- Rule 1: Never mix polarity types (e.g., connecting a Type A to a Type B).
- Rule 2: Always plan your links before ordering.
- Rule 3: When ordering, always specify the polarity you need.
A polarity mistake is a 100% avoidable problem.
PHILISUN provides TIA-standard MPO Trunk Cables in all polarity types. Every cable is 100% factory-tested for correct polarity and performance.
Don’t risk your network uptime. If you are unsure about your MPO polarity design, contact our fiber optic specialists today. We will validate your design for free.
