Why Polarity Matters (Part 2)

If you’ve read my previous posts on the subject, you know that polarity can be a tricky matter and it’s even more complicated when you try to choose it for your data center cabling. You really have to choose based on several factors:

1. Patch cords – method A has two different patch cords that you have to stock, but the upside is that it’s pretty simple to follow where the signal is going and if you happen to be out of one type of patch cord, you can really take the one you have and just flip the fibers as a temporary fix until you can get the other patch cords. Of course this isn’t recommended, but if you’re in a bind and need to get up-and-running right away, it will work. With methods B and C you have the same patch cord on each end so no need to worry about this, but if you happen to have the wrong cassettes or backbone, nothing will work and you'll have to wait to get the correct ones.
2. Cassettes and backbone cables – you need to make sure you buy all of one method of polarity or your system won’t work. If you’re concerned about supply, all three polarity methods are available from multiple vendors, but Method A is “preferred” by most.
3. Upgradability – this is where it can get dicey. Typically your pre-terminated assemblies are running Gigabit applications today and a few may be running 10G. Any of the polarities will work at these data rates. But when you move to 40/100G, methods A and B have straight forward paths, while C does not. Also, you’ll want to make sure you use the highest grade of LOMF available, which is OM4 – this will give you the best chance of being able to reuse your backbones up to 125m. If you need something longer, you’ll need to go to SMF.

If you are thinking about installing pre-terminated cassette-based assemblies now for 10G with an upgrade path to 40 and 100G, you need to consider the polarity method you use. Unlike today's 2-fiber configurations, with one send and one receive, the standards for 40G and 100G Ethernet implementations use multiple parallel 10G connections that are multiplexed. While 40/100G equipment vendors will tell you that polarity is not an issue, care must be taken if you want to reuse this installed base.

40G will use four 10G fibers to send and four 10G fibers to receive, while 100G uses either four 25G fibers or ten 10G fibers in each direction. Because 40 and 100G will be using the MPO connector, if the polarity method is carefully chosen, you will be able to reuse your backbone cables. This is enabled by the fact that the IEEE took much care in specifying the system so that you can connect any transmit within a connection on one end of the channel to any receive on the other end.

Those selecting fiber to support 10G now and 40G in the near future need to understand what will be involved in transitioning and repurposing their cable plant. In order to upgrade using method A, you can replace the cassettes with MPO-to-MPO patch panels and MPO-to-MPO patch cords and it will enable flexibility to address moves, adds and changes as well as promoting proper installation best practices. The polarity flip will need to be accomplished in either an A-to-A patch cord or possibly with a key up/key down patch panel.

Method B multimode backbone cables can also readily support 40G applications. For a structured cabling approach, method B will still use a patch panel and patch cords, though as with current method B, both patch cords could be A-to-B configuration. While Method C backbones could be used, they are not recommended for 40G as completing the channel involves complex patch cord configurations.

It appears that 100G will use either the 12-fiber (4x25G) or the 24-fiber (10x10G) MPO connector. With transmits in the top row and receives in the bottom row, the connection will still be best made using a standardized structured cabling approach as described above.

There are many suppliers of pre-terminated optical assemblies including Belden, Berk-Tek, a Nexans Company, CommScope, Corning, Panduit, Siemon, Tyco Electronics NetConnect as well as many smaller shops that give quick-turn assemblies like Cxtec CablExpress and Compulink.