MTP® (MPO) Polarity for Duplex Networks
Network polarity options: A, B, C, Universal
There are three methods for creating duplex networks that are within the international standards (TIA, IEC and EN). There is also a commonly used 4th method (Universal) which is outside the standards and has been created to accommodate the increasing demand for upgrading duplex networks to Base-8 network.
Below we run through each of the four network polarity types as laid out in the standards where applicable.
A note on polarity terminology
Polarity can refer to both a network type and an individual network component (for example an MPO cable). Unfortunately, when speaking about either a network polarity type or a component the naming is uses A, B and C. So you can have a network polarity Type A and also an individual MPO cable with polarity A.
Topics covered
- Polarity options summary
- Method Type C
- Method Type A
- Method Type B
- Method Type Universal
For clarity, when referring to individual components we use the term Polarity A (B or C) and when referring to a network we use the term Method Type A (B, C or Universal).
General principals of the method types
Maintain consistency within each component type
Generally, MPO networks consist of five different types of component: MPO-LC cassette, MPO trunk, MPO-LC breakout, MPO adapter module, duplex patch cord. The standards are designed to keep these component’s polarity consistent across the network irrelevant of which network method type is used. Although outside the standards, the Universal network type also follows this principle.
The standards have been written based around using MPO-12 fibre connectors. They can also be applied to using MPO-16 or MPO-24 fibre connectors. To make explanation easier we have focused on the MPO-12 fibre connector.
Where does the “FLIP” occur
For a duplex network to work and ensure every Tx needs to connect to an Rx and vice versa. In order to achieve this there needs to be the needs to be a “flip” in each fibre pair within the network. The different network types introduce this “flip” at different points and this is the main difference between each network type.
Male and female connectors
As well as keeping the polarity of components consistent within a network type, the standards also keep the gender of the MPO connectors within the components consistent. For example, all MPO-LC cassettes have rear MPO ports with one gender or all MPO trunks have one gender.
Mapping fibre connections
Connecting key down to key up
MPO port presentation, MPO patching and trunk extensions
The information below covers the basic network across two cassettes and a trunk cable. From a networking perspective this would be considered to be the main building block of a modular MPO network. These blocks can be daisy-chained together to create more complex channels such as introducing an End-of-row (EOR) distribution area between edge equipment.
The information below doesn’t include introducing front of rack MPO ports and MPO cross connects which are commonly used in distribution areas. Also excluded are MPO trunk extensions and direct connections with MPO-LC breakouts which are common in SANs.
For more information on how to create more complete networks chat to us via our online chat function (bottom right). You will connect to a person, there are no AI or annoying chatbots on our site.
Network Method Type C
The “Flip” is in the trunk
When MPO connectors were first introduced and until the appearance 40G SR4 transmission in the early 2010s, network type C was the preferred choice for many installations.
The basic network is shown below. Two MTP® (MPO) to LC 24 fibre cassette modules, one MTP® (MPO) 24 fibre trunk and LC patch cords would give 12 duplex channels. The network shown here uses only MPO-12 fibre connectors at all points and so the cassette has two MPO-12 rear ports and the trunk has two MPO-12 connectors at each end.
The cassettes have male connectors and key up / key down (opposed) adapters. The trunk cables have female connectors. The polarity C trunk cable creates the “flip” as the fibres are connected in pairs. End A fibre 1 is connected to end B fibre 2 and vice versa. End A fibre 3 is connected to end B fibre 4 and so one to fibres 11 and 12.
Components Spec
- one type of cassette – polarity A
- one type of trunk – polarity C
- one type of duplex patch cord – A to B
- Cassettes MPO male, trunk MPO female
- Key up / key down (opposed) adapters
Duplex A-B patch cord
MPO – LC cassette Pol. A
MPO-MPO trunk Pol. C
Fibre “flip” within the trunk
MPO – LC cassette Pol. A
Duplex A-B patch cord
From left to right (green Tx):
Tx connects to fibre no.2 in port 1 on the cassette which is position 2 on the MPO connector in the cassette. This connects into fibre 2 on the MPO in the trunk which connects to fibre 1 (the flip) on the MPO at the other end. Fibre 1 connects to fibre 1 on the MPO in the 2nd cassette and this goes to port 1 fibre 1 and the Rx. The same happens in the other direction.
Network Method Type A
The “Flip” is in the patching
This network type maps ports at each end directly to each other. The flip occurs within the patching.
The cassettes have male connectors and key up / key down (opposed) adapters. The trunk cables have female connectors.
The polarity A trunk cable maintains the fibre positions; End A fibre 1 is connected to end B fibre 1, 2 to 2….12 to 12. The End A cassette port 1 fibre 1 is connected to End B port 1 fibre 1. Fibre 2 to fibre 2.
End B patch cord is A-A and this is where the flip occurs to enbable Tx to Rx and Rx to Tx.
Components Spec
- one type of cassette – polarity A
- one type of trunk – polarity A
- End A patch cord – A to B
- End B patch cord – A to A
- Cassettes MPO male, trunk MPO female
- Key up / key down (opposed) adapters
Duplex A-B patch cord
MPO – LC cassette Pol. A
MPO-MPO trunk Pol. C
MPO – LC cassette Pol. A
Duplex A-A patch cord
Fibre flip in the patch cord
From left to right (green Tx):
Tx connects to fibre no.2 in port 1 on the cassette which is position 2 on the MPO connector in the cassette. This connects into fibre 2 on the MPO in the trunk which connects to fibre 2 on the MPO at the other end. Fibre 2 connects to fibre 2 on the MPO in the 2nd cassette and this goes to port 1 fibre 2. The flip then occurs in the patch cord to ensure the signal goes to the Rx.
Network Method Type Universal
The “Flip” is in the trunk
This method type is outside the standards but has been created to enable an easy upgrade to Base-8. The flip occurs within the trunk via a polarity B cable.
As there is no official standard to work to, the choice of where to use male connectors and where to use female is often application dependent.
As this method type is often chosen to enable upgrade to Base-8, the trunk cables often are specified with male connectors because the standard for Base-8 has trunks with male connectors and MPO patch cords or MPO breakouts with female connectors.
Components Spec
- one type of cassette – polarity Universal
- one type of trunk – polarity B
- one type of duplex patch cord – A to B
- Key up / key down (aligned) adapters
Duplex A-B patch cord
MPO – LC cassette Pol. Universal
MPO-MPO trunk Pol. B
Fibre “flip” within the trunk
MPO – LC cassette Pol. Universal
Duplex A-B patch cord
Network Method Type B
The “Flip” is in the trunk
This network type is not commonly used as it requires two types of cassette. The flip occurs within the trunk via a polarity B cable.
There are several reasons why this method type is unpopular. The two main reasons are
- Use of key up – key up adapters means this is not suitable for singlemode networks which have MPO with APC ferrules
- Using two types of cassette creates an inconsistent network when the channel has more connections, such as via distribution areas
Components Spec
- two types of cassette – polarity B1, B2
- one type of trunk – polarity B
- one type of duplex patch cord – A to B
- Cassettes MPO male, trunk MPO female
- Key up / key down (aligned) adapters
Duplex A-B patch cord
MPO – LC cassette Pol. B1
MPO-MPO trunk Pol. B
Fibre “flip” within the trunk
MPO – LC cassette Pol. B2
Duplex A-B patch cord
Summary / Further information
The above information is intended as an introduction to polarity management of MPO networks to give an overview of the difference in the options according to the standards.
As mentioned above, just looking at a cassette-to-cassette link isn’t comprehensive and doesn’t cover the requirements of most data center networks. For a more complete view of building an MPO network for creating duplex channels view our MX Series Fibre System via the links opposite.
Complete Connect’s offering
Although we offer all of the above methods, method C and the Universal method dominate across our customer base for duplex networking.
With the introduction of 400G and 800G data rates, the need for even higher ports density is driving the adoption of MPO-16 and MPO-24 connectors and this is leading to the use of products that can breakout 16f and 24f to multiple 8f and then down to 2f ports. Managing the polarity consistently across an entire network is becoming more complex.