What MPO cabling means for fiber testing in the data center

MPO cabling can create a testing, certification, and migration nightmare. The standard testing process – and you are testing your pre-terminated cables, right? – can be time consuming, error prone and, once you throw polarity of all 12 fiber connections into the mix, almost a hit-and-miss manual affair. And if you migrate from 10 Gbps to 40/100 Gbps on the same cable? You need to test and validate performance all over again.

Lwfluke071912

Enterprise data centers have an insatiable appetite for bandwidth. Virtualization, cloud computing, storage area networks, and a host of other factors conspire to drive that appetite, and meeting the need means a steady proliferation of fiber connections and ever-faster links.

Up until recently, 10 Gbps was considered a top-end connection; now 40 Gbps is gaining traction, and some are even starting to look at 100 Gbps. In fact, according to a recent Cisco report (Cisco: "Market Need for 40 Gigabit Ethernet," 2012), shipments of servers with 40-Gbps Ethernet interfaces are expected to grow by more than 40% in the near term.

Multifiber Push-On (“MPO”) fiber trunks have become the default cabling solution to these ever-increasing data center bandwidth requirements. Because they are a natural fit for parallel optics, these fiber links are compact, pre-terminated, able to handle bandwidth all the way up to 100 Gbps, and even plug and play by design. What could go wrong?

Well, it turns out that this type of MPO cabling can create a testing, certification, and migration nightmare. The standard testing process – and you are testing your pre-terminated cables, right? – can be time consuming, error prone and, once you throw polarity of all 12 fiber connections into the mix, almost a hit-and-miss manual affair. And if you migrate from 10 Gbps to 40/100 Gbps on the same cable? You need to test and validate performance all over again.

The rise of MPO cabling

Lwfluke071912
Figure 1. MPO connectors

To understand the challenges of MPO cable validation, it’s necessary to understand MPO cables and how they’re tested in the field. An MPO connection is about the size of a fingernail and contains 12 optical fibers, each less than the diameter of a human hair – and each one needs to be tested separately. That traditionally means the use of a fan-out cord to isolate each fiber, followed by tedious manual testing, tracing, and error-prone calculations.

The actual fiber test is quick enough: typically under 10 seconds per fiber once you’re in process. But you better be cruising: While one of our enterprise customers has data centers with as little as 48 MPO fiber trunks (x12 fibers each), that same customer also has a 30,000-MPO data center installation. That’s 30,000 connections with 12 fibers each, or roughly 3,120 hours in labor (and $343,200 in cost) if you had to test them all individually.

And at some point, you better have tested them. There were two primary drivers behind development of MPO fiber trunks. The first was the ever-increasing need for cabling density in the data center. Cabling blocks airflow, so the denser the cable, the better the thermal management. And, as data center bandwidth steadily climbs to 10, 40, and 100Gbps, a dense multi-fiber cable becomes the only option.

But the second, perhaps more important factor, is the difficult and highly technical nature of field termination for fiber. We’re talking curing ovens, adhesives, microscopic fibers, etc. Given that expensive and time-consuming “craft” process, modular factory-terminated MPO cables promise simplicity, lower cost, and true plug-and-play fiber connectivity.

The challenge is that pre-terminated fiber is only guaranteed “good” as it exists in the manufacturer’s factory. It must then be transported, stored, and later bent and pulled during installation in the data center. All kinds of performance uncertainties are introduced before fiber cables are deployed. Proper testing of pre-terminated cables after installation is the only way to guarantee performance in a live application. In short, investing in factory-terminated fiber trunks to save time and decrease labor costs doesn’t really offer an advantage if the testing becomes an expensive bottleneck.

Testing and determining fiber polarity is another challenge. The simple purpose of any polarity scheme is to provide a continuous connection from the link’s transmitter to the link’s receiver. For array connectors, TIA-568-C.0 defines three methods to accomplish this: Methods A, B, and C. Deployment mistakes are common because these methods require a combination of patch cords with different polarity types.

More bandwidth means more testing
The use of MPO cables for trunking 10-Gbps connections in the data center has steadily risen over the past 10 years. That trunking requires use of a cassette at the end of the MPO cable designed to accommodate legacy equipment connections. Now that 40-Gbps and 100-Gbps connections are coming on the market, a migration path has emerged: Remove the 10-Gbps cassette from the MPO cable and replace it with a bulkhead accommodating a 40-Gbps connection. Then it might be possible to remove that bulkhead and do a direct MPO connection for 100 Gbps at a later date.

The problem is that while this migration strategy is an efficient way to leverage the existing cabling, in comparison to 10-Gbps connections, the 40-Gbps and 100-Gbps standards call for different optical technology (parallel optics) and tighter loss parameters.

In short, each time you migrate you need to verify the links to ensure the performance delivery the organization requires.

Proper MPO testing
So what would a proper MPO test look like? The answer is simple: Test all 12 fibers – the whole cable – simultaneously and comprehensively (including loss, polarity, etc.). That sort of test capability changes the fiber landscape, enabling installers and technicians to efficiently validate and troubleshoot fiber – flying through the process by tackling an entire 12-fiber cable trunk with the push of a button.

The tools to perform this type of test are just emerging on the market, and promise to reduce the time and labor costs up to 95% over individual fiber tests (according to internal research based on the average list of standard competitive products). Characteristics to look for in such a tool include:

  • An onboard MPO connector to eliminate the complexity and manual calculations associated with a fan-out cord.
  • A single “Scan All” test function that delivers visual verification via an intuitive user interface for all 12 MPO fibers in a connector.
  • Built-in polarity verification for end-to-end connectivity of MPO trunk cables.
  • “Select Individual Fiber” function that enables the user to troubleshoot a single fiber with more precision.

Demand for fast and reliable delivery of critical applications is driving data center technology to evolve at an ever-increasing pace. And that insatiable need for bandwidth ensures that the integrity of the data center has become inextricably linked to the strength of the fiber cabling infrastructure. The growing use of MPO fiber trunks – and the migration from 10-Gbps to 40/100-Gbps connections – means that it’s time to stop the cumbersome verification of individual fibers. After all, it’s a single MPO connection. You should be able to test it as one.

David Luk is fiber product manager for the DCI business unit at Fluke Networks.

More in High-Speed Networks