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Fiber Optic Cassette Polarity
Last Updated: 10/04/2016

Fiber Optic Cassettes and MPO Polarity

Cassette MPO Breakout Method C
Panduit Panduit Cassette Panduit MPO Breakout Panduit MPO to MPO
Ortronics Ortronics Cassette Ortronics MPO Breakout Ortronics MPO to MPO


Fiber Optic Cassette – fiber optic connectivity housing consisting of 1 or more MPO connectors that fan out to an array of standard fiber optic connectors (LC, SC)

MPO – multi-position optical (a.k.a. multi-fiber push-on) fiber optic connector used for high density applications. The MPO connectors may terminate 12, 24, 36, 48, 60 or 72 fiber strands in a single housing. There are male and female versions of the MPO more commonly referred to as “pin” and “no pin” variations. These pins and holes serve to align the fiber strands when mated. The MPO will also be keyed for either up or down installation as related to the numbering sequence of the fibers to be aligned (polarity).

Panduit MPO Connector Panduit MPO Front View

(Drawings courtesy of Panduit)

Fiber cassettes provide plug and play connectivity in data centers and closets. However, there are different “methods” of polarity design to be considered. The cassettes and trunks need to be laid out properly in order to make the system work.


In high-density data centers, bulk fiber is key to handling 10/40/100G transmissions. Cassettes and pre-terminated trunks eliminate the need to terminate large quantities of fiber in these data centers. The trunk cables have MPO connectors which contain 12-72 fibers.

Each fiber requires a transmitter at one end and a receiver on the other, and that connectivity transition is handled through one of three standardized methods. TIA 568 standards provide these three methods of configuring the trunks and cassettes for polarity: Methods A, B and C.

See charts below for comparisons.

Installation Considerations:

Method B (Core) and Method B (Edge) cassettes offered by Leviton - because the polarity flip for Method B occurs in the cassette Leviton distinguishes the two configurations as Core and Edge for ease of installation and maintenance. Core cassettes are typically placed at the main connection point at or close to the main switching equipment, while Edge cassettes are placed at the end connection point near access layer equipment.

Fiber optic cable incorporates strengthening components within its construction (typically Kevlar aramid yarn) for pulling. The cable should be pulled by these components unless the cable is specifically designed for pulling by the jacket.

Do not twist the cable, which can stress the fibers. Pulling eyes should be attached to prevent the cable from twisting while pulling.

If maximum bend radius or pulling tension is exceeded and results in damage to the cable, it is assumed to be irreversible and the cabling is typically replaced. Repositioning or easing the tension does not necessarily undo the damage.

Cable manufacturers will have a specific maximum bend radius that must not be exceeded. In the absence of a specified max. the general rule is that the bend radius should be at least 20 times the cable diameter.

Hook and loop fasteners are preferred for bundling and securing fiber optic cables. Cable ties may be used but should be hand-tightened only and should remain loose enough to move along the cable.

Common Options:

  • Multi-Mode OM1

  • Multi-Mode OM2

  • Multi-Mode OM3

  • Multi-Mode OM4

  • Multi-Mode OM4+

  • Single-Mode OS1/2

(See What are the differences between Fiber Optic Cables for comparisons of these cable types.)

Popular Manufacturers:

Related Industry Terms/Acronyms:

  • MTP – trade mark protected MPO connector manufactured by US Conec.

  • Trunk - pre-terminated cable with multiple strands (12-144 fibers).

  • Cassette - housing with MPO female connectors on one side and LC or SC female connectors on the other for patching.

  • Fiber Enclosure - rack or wall mount metal or plastic box (housing) that houses the cassettes and allows a place for cables to land securely.

  • Jumper - short fiber patch cable, usually duplex, that allows patching from network gear to the cassette.

  • Meter - fiber optic distances are typically expressed in meters. 1 meter is equal to 3.28 feet (39.37 inches).

  • Wavelength - the distance from one energy wave to another. Fiber optic cable uses longer wavelengths in the infrared to minimize attenuation.

  • Bandwidth - refers to the range of signal frequencies within which a fiber optic channel will operate.

  • Nanometer - equal to 1 billionth of 1 meter. It is used in fiber optics to express the wavelength of transmitted light.

  • Attenuation - loss of signal over the length of a network link.

  • Insertion Loss - signal loss due to the insertion of an additional component or the creation of a splice within a continuous path.

  • Bend Radius - the minimum radius a cable can be bent without kinking it, damaging it or shortening its life. When cabling is bent beyond the specified minimum it can cause transmission failures.

Comparison Charts:


Method A Method B Method C
Polarity Flip: In patch cord In cassette In fiber pairs of trunk cable
Trunk Cable: Straight through Flipped Adjacent pairs flipped
Trunk Key Positions: Key up / key down Key up / key up Key up / key down

Mating Adapters (usually in the form of cassettes)

Type A Type B
Mates Connectors 1 & 2: Key up to key down Key up to key up
Connection: Straight through Flipped
Position 1 of Connector 1: Aligns to Pos. 1 of Conn. 2 Aligns to last Pos. of Conn. 2

Path (alignment) of Fibers in 12 Strand Method A Trunk Cable:

Position Connector 1: 1 2 3 4 5 6 7 8 9 10 11 12
Position Connector 2: 1 2 3 4 5 6 7 8 9 10 11 12

Path (alignment) of Fibers in 12 Strand Method B Trunk Cable:

Position Connector 1: 1 2 3 4 5 6 7 8 9 10 11 12
Position Connector 2: 12 11 10 9 8 7 6 5 4 3 2 1

Path (alignment) of Fibers in 12 Strand Method C Trunk Cable:

Position Connector 1: 1 2 3 4 5 6 7 8 9 10 11 12
Position Connector 2: 2 1 4 3 6 5 8 7 10 9 12 11

See Also:

Further References:

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