Array Polarity Systems
Addendum to ANSI-TIA-568-C.0 outlines three distinct ways they can be established using optical fiber.
May 1, 2013
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In August, 2012 TIA published Addendum 2 to the ANSI/TIA-568-C.0 “Generic Telecommunications Cabling for Customer Premises Standard” with a mysterious title called general updates. This Addendum contains a lot of valuable information including a comprehensive section on Array Polarity Systems.
Typically, 12-fiber, pre-terminated array cabling is used in a data centre for connections between racks of equipment. For this month’s column, I wanted have a look at of the different methods that exist to establish polarity when using optical fiber array systems. The standard provides three example methods to do this, Connectivity Method A, B and C without a preference for one or another of these methods. All these methods have the same goal, that is to establish an optical path from the transmit port of one device to the receive port of another device. Each of these methods requires a specific combination of components to establish polarity. The standard recommends that a method be selected in advance and maintained consistently throughout an installation, otherwise it won’t work.
Multiple duplex signals:
Figure 1 illustrates Connectivity Method A, B and C for maintaining polarity of multiple duplex signals, e.g. 10 Gigabit Ethernet or 8G, 10G & 16G Fibre Channel. The components of the system includes a breakout cassette at each end (MPO adapter to multiple duplex adapters), a duplex patch cord at each end that plugs into the equipment, and a pre-terminated 12-fiber trunk cable (or multiples thereof) between the two cassettes. The components are color coded in Figure 1 to distinguish between the different Type designations, as follows:
• A-to-B duplex patch cord (KeyUp-to-KeyUp)
• A-to-A duplex patch cord (KeyUp-to-KeyUp)
• Type A MPO adapter (KeyUp-to-KeyDown)
• Type B MPO adapter (KeyUp-to-KeyUp)
• Type-A:1-1 trunk cable (KeyUp-to-KeyDown)
• Type-B:1-1 trunk cable (KeyUp-to-KeyUp)
• Type-C:1-1 trunk cable (KeyUp-to-KeyDown)
A simple way to look at these polarity schemes for multiple duplex systems is that method A requires a cross-over patch cord at one end. Method B uses the same patch cord at both ends. Type B cassettes, as shown circled in red in Figure 1, need to be flipped over at one end to map position 12 to position 1, position 11 to position 2, etc. Method C is a variant of Method A except that the fiber pair cross-overs are implemented in the Type C trunk cables instead of in the patch cord. Both Method B and Method C have the advantage in that the same A-to-B patch cord is used at both ends. You will also notice that a suffix 1-1 is added to the Type designation for the array cables and cords. The 1-1 suffix refers to an interface that employs a single row of 12 fibers for the MPO connector.
Figure 2 illustrates the corresponding Connectivity Methods A, B and C to establish polarity for parallel signals using an MPO transceiver interface with one row of fibers. For example, 40 Gigabit Ethernet over multimode fiber uses 4 transmit and 4 receive fibers in a 12-fiber array, or 4 lanes at 10 Gbps. For these parallel array systems, the breakout cassette and the duplex fiber patch cords are replaced with 12-fiber array patch cords that plug directly into the MPO adapters at the patch panel and into the equipment interface. An important point to remember is that MPO plugs use alignment pins. For an MPO connection, one plug is pinned and the other plug is unpinned. As MPO transceivers typically have pins, this convention leads to the following implementation on initial build out 1) Patch cords from transceiver to patch panel are typically unpinned (Female) on both ends 2) Trunk cables are typically pinned (Male) on both ends. Maintaining polarity for Array systems is not as simple as it seems. For more information consult ANSI/TIA-568-C.0-2 Addendum. CNS
Paul Kish is Director, Systems and Standards at Belden. The information presented is the author’s view and is not official TIA correspondence.