How improper fiber crossing degrades network performance

Network technicians often commit major errors crossing fiber cables during installation. If they don’t understand polarity or rush to get their network equipment powered up, they run the risk of using the wrong patch cord. That can be bad news.

 

This is the 15th in a Telect blog series, entitled The ABCs of Cable Management. Product Specialist Hugo Garcia explains the different types of polarity and how it can impact your fiber optic network.

 

Your network performance is at risk if fiber cable polarity isn’t a priority during installation. The wrong connection can result in signal degradation.

 

Or worse: damaged critical active equipment, which can lead to network downtime.

 

The challenges can arise if polarity is not properly maintained. This can be as simple as connecting an Rx transmitter to another Rx transmitter. Your signal won’t transmit.

 

WHAT IS POLARITY?

Polarity is often used to define a direction of flow. For example, a battery’s positive and negative polarity terminals determine the direction of its electrical current.

 

In fiber optics, polarity defines the direction the light signal travels through optical fiber. Unfortunately, this can cause confusion among technicians. Let’s break down the different types of polarity.

 

POLARITY IN DUPLEX FIBER OPTICS

Understanding polarity in duplex fiber applications, such as 10 GbE, is straightforward. Data transmission is bidirectional over two fiber cables, meaning each fiber connects the transmitter (Tx) on one end and to the receiver (Rx) on the other end.

 

Shown in the example below, the transmitter should always connect to the receiver, regardless of the number of patch panel adapters or cable segments in the channel.

 

Duplex Polarity Application

PLANNING FOR THE FUTURE

As networks strive to achieve 40 Gbps, or even speeds of 100 Gbps, network technicians must remove transition cassettes and harnesses from the link and replace them with the proper MPO adapters and patch cords.

 

Polarity becomes much more complex when you’re working with MPO cables. Some important characteristics for MPO connectors are:

 

Pins determine gender (male or female) and are necessary for achieving the correct fiber alignment

Polarization dot, oriented to Fiber 1

MPO Characteristics

 

THE THREE TYPES OF POLARITY

There are three types of MPO trunk cables and connectors to obtain proper MPO polarity:

 

1. Type A (Straight)

 

When components are Type A, the fiber identified as 1 (blue, according to the TIA color code) connects to Fiber 1. In other words, 1 goes to 1, also known as Key-up/Key-down. This type applies for adapters, cassettes and cables.

 

2. Type B (Crossed)

Fiber 1 goes to Fiber 12 or commonly called Key-up/key-up. This type also applies for adapters, cassettes and cables.

 

3. Type C (Cross pairs)

Type C refers to cross pairs, just like with ethernet connections. With Type C, Fiber 1 matches to Fiber 2, 2 to 1, 3 to 4 and so on. It only applies for trunk MPO cables.

 

POLARITY IN MPO

TIA 568 standard specifies three different methods for managing MPO polarity: A, B and C, each requiring different types of MPO adapters and cables.

 

Method A

 

Method A polarity uses straight-through MPO trunks and interconnect cables to map the fibers on both ends of the link. To flip the polarity, an A to B patch cord (LC to LC) at one end must be connected to an A to A cord at the other end. In this method, Fiber 1 arrives at Fiber 2 at the other end of the connection.

 

Since the fibers at each end have the same position, Method A offers the simplest deployment for multimode channels, easily supporting network scalability for the hyperscale future.

 

Method B

 

Method B uses three Type B MPO components, or three crosses for the transceiver-transceiver connection. Thus, two A to B patch cords are required on each side of the link. In Method B, the fiber located in Position 1 (Tx) arrives at Position 12 (Rx) at the other end of the link.

 

Method C

 

In Method C, two Type A MPO cassettes and one Type C trunk cable are necessary for the system. The polarity flip occurs within the array cable itself. Type C cords use reverse-pair positioning, through crossovers in the array cord, to swap the polarity of pairs of fibers. Thus, each pair of fibers is flipped so the fiber in Position 1 (Tx) arrives at Position 2 (Rx) at the opposite end.

 

While this method works well for 10 Gbps applications, it does not support parallel eight-fiber 40 and 100 Gbps applications where Positions 1, 2, 3 and 4 of the MPO connection are transmitting and Positions 9, 10, 11 and 12 are receiving.

 

As a result, Method C is not ideal for migrating your network for the hyperscale.

 

UNDERSTAND POLARITY OR RISK NETWORK DOWNTIME

Nearly half a billion (429 million) mobile devices and connections were added in 2016. That equals eight billion devices and connections globally.

 

Your network needs to make the switch to MPO to satisfy all that data consumption.

 

Deployment mistakes can happen, however. The simple way to avoid crossing fiber cables is to ensure you’re using the same type of patch cord throughout your facility. 

 

Or you could choose preterminated cabling assemblies and MPO cassettes, an option that quickens and simplifies installation.

 

Ultimately, it’s vital for techs to take the time and ensure they’re maintaining proper polarity to keep your network up and running.