MTP (MPO) Polarity Management

For 10G and 25G duplex networking

WHAT IS POLARITY?

Maintaining transmit to receive channels (Tx to Rx)

Polarity in its simplest form

To connect two SFP 1Gbps or SFP+ 10Gbps transceivers together a duplex (2 fibre cores) cable is typically used. For a short distance of say 5m or 10m the ideal cable would be a duplex patch cord. In this example the patch cord would have a basic polarity of A-B.

When the patch cord is plugged into optical transceivers at each end of the path cable, the LC key of the patch cable would be KEY UP at both ends. Both transceivers would transmit into connector B and the signal would be sent out of connector A. Patch cables are sometimes described as either crossed or straight through. This terminology can often cause confusion. An A-B patch cord does not have the fibres physically crossed but they run parallel to each other. But the fibres do connect connector A to connector B.

LC duplex connector
LC Patch Cord Key
A to B Polarity Drawing LC Connector
Duplex patch cord with A-B polarity

Transmit (Tx) and receive (Rx) working in pairs

Maintaining the correct polarity across a fibre network ensures that a transmit signal (Tx) from any type of active equipment will be directed to the receive port (Rx) of a second piece of active equipment and vice versa. Typically, for lower data rates the Tx and Rx signals transmitted in pairs and are therefore networked in pairs. The fibre infrastructure is designed to maintains these as pairs.

For most situations, the connection between two transceivers will require a longer link. It is not considered best practice to directly connect two transceivers together but to use an “isolated” trunk  or backbone fibre cable. This ensures the trunk cable is unlikely to get damage if further changes occur and also provides greater flexibility in the use of the trunk. A typical set up could look like the image below.

Transmit Receive Polarity Diagram
Fibre network standard Polarity

The diagram above shows that 3 cables used in this way will maintain the Tx/Rx polarity across the network. In fact, any number of cables networked in this way will maintain the polarity between two optical transceivers with a duplex LC interface.

POLARITY IN MTP (MPO) NETWORKS

& across different data rates

For fibre networks based on MTP (MPO) multi-fibre connectors maintaining polarity becomes a little more complicated.

Firstly, MTP (MPO) connectors are used in both duplex transmission and in multi-fibre parallel transmission and so there are more than 2 polarity types.

Secondly, both networks and individual components within a network can have polarity types and just to make it confusing both are called A, B or C. For example a single MTP (MPO) cable can have polarity C but also a network containing 2 cassettes, one trunk and 2 patch cords could be polarity C.

Thirdly, MTP (MPO) connectors are used across many different data rates and the same data rate can have a different polarity. For example, 100G could use an MTP (MPO) 24 fibre connector or a 12 fibre connector.

There are however, two main types of application:

1. 10G and 25G Duplex networks
2. 40G or 100G SR4 and PSM4 networks

These are defined within international cabling standards. For example TIA/EIA 568-b-1-7 defines 3 standard methods for maintaining polarity within multi-fibre connector fibre networks, (A, B, C).

The majority of brands of MTP (MPO) based systems use these methods for standard ranges. This means that it is possible to mix and match between different cabling brands and still ensure the correct network polarity is maintained.

10G AND 25G DUPLEX NETWORKS USING MTP (MPO) FIBRE

Polarity Method C

Duplex patch cord

Polarity A-B

Cassette 1

Polarity A/C

Front: 12 x LC ports
Rear: 2 x MTP (MPO) ports

24 fibre MTP (MPO) trunk cable

Polarity C

Cassette 2

Polarity A/C

Front: 12 x LC ports
Rear: 2 x MTP (MPO) ports

Duplex patch cord

Polarity A-B

Polarity Method C

Polarity Method C Network: the most common MTP (MPO) choice for
duplex networks

In the simplest form, an MTP (MPO) network could be used to connect network equipment (server/ switch / storage) at two locations. Two MTP (MPO) to LC 24 fibre cassette modules, one MTP (MPO) 24 fibre trunk and LC patch cords would give a 12 channels. It would connect together 24 SFP or SFP+ transceivers.

Each transceiver would require 2 fibres: one for transmit (Tx) and one for receive (Rx). The cassette and trunk have 24 fibre capacity and are set up so that these 24 fibres work in pairs.

Port 1 (top left) on Cassette 1 connects to Port 1 on Cassette 2. Similarly port 2 to port 2 and so on up to port 12 to port 12.

The key on the MTP (MPO) connectors (shown below) means that the MTP (MPO) trunk cables can only be connected to the MTP (MPO) cassettes in one orientation. This means that the polarity is fixed when the cables and cassettes are manufactured.

All components within the network have a component polarity; patch cords (A-B), MTP (MPO) cassettes (A/C) and MTP (MPO) trunks (C). For the link to work correctly all these component polarities must be correct.

MTP 12 Fibre Female Connector
Complete Connect MTP (MPO) Technical Support

Polarity Method C Network in detail

The diagram below shows the internal wiring of the cassettes and trunk cable. In order to follow the signal path, an understanding of MTP (MPO) adapters is required.

Polarity C Duplex Signals Trunk and Patch Cables

MTP (MPO) ADAPTERS

There are two types of MTP (MPO) adapters:

1. Key-Up to Key-Up (aligned)
2. Key-Up to Key-Down (opposed)

The “key” refers to the MTP (MPO) connector which is asymmetrical. MTP (MPO) adapters are used in the rear of most MTP (MPO) cassette modules and in MTP (MPO) fibre panels for MTP (MPO) cable extensions and to connect MTP (MPO) patch cords to MTP (MPO) trunks.

A Key-Up to Key-Up adapter will flip all the fibres when two 12 fibre cables are connected through the adaptor, i.e. fibre no. 1 (of the 12 fibres) in the first cable will connect to fibre no. 12 in the second cable. At every point in a fibre link where a key-up to key-up adaptor is used the fibres 1 to 12 will be flipped and reverse the polarity of the link.

A Key-Up to Key-Down adapter maintains the fibres so that fibre 1 in cable 1 is joined to fibre 1 in cable 2.

Key-Up to Key-Down adapters are far easier to use when trying to maintain a consistent polarity across an installation both at the time of installation and for subsequent moves, adds and changes.

Technical Support

Polarity Method C Network with a Cross Connect

Normally, equipment racks would be connected via a distribution rack which would hold a patching area (cross connects) as shown below:

MX Series MTP MPO Distribution Rack

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