Active Optical Cables Create Efficiencies

Active optical cable(AOC) or optical active cable (OAC) are synonymous terms used to refer to a new class of cables. These electrical interface cables differ from passive copper wiring. They are ‘active’ cables. These cables use electrical power along with the signals at the copper interface, but convert the signal to the optical domain with a transceiver within the cable.

Manufacturers of these products include Avago, Emcore, Finisar, Hitachi, Luxtera, Merge Optics, Molex, Reflex Photonics, Tyco Electronics, Zarlink Semiconductor and others. Although some AOCs may be as simple as a multi-source agreement (MSA) standard form factor, shrink-wrapped to a pair of strands with LC (Lucent connector, IEC 61754-20), most AOC products offer material and time-saving benefits that reduce total cost.

AOCs offer the same density as existing MSA ports, with full compatibility for XFP (10 Gigabit small form-factor pluggable) and SFP+ (small form-factor pluggable, plus) MSAs. AOC is also promoted as an alternative to all copper solutions because of the higher port density, better bend radius, and lower electrical power consumption of the AOC solution compared to an all copper 10GBASE-T CX4 standard.

Risk mitigation

AOCs are driven in part by some of the same trends as MSAs.

What has made the selection of fixed optics a risky choice for vendors is the large and growing variety of optics required for different reach (or length) applications; different types of optics, including single mode (SM) or multimode (MM) and even different types of connectors. Those would include not only LC, but also Fibre Channel (FC), square or standard connector (SC, IEC 61754-4) and sub-miniature A (SMA). To manage the types of connectors, optics and applications, vendors can reduce their risk by standardizing on an MSA interface, such as SFP, SFP+ or XFP.

AOC takes the MSA a step further by replacing the technical decisions of transceiver, connector, and fiber with a single integrated product selection. The transceiver is matched to the appropriate type of fiber and the connectors are no longer relevant. The resulting ‘system’ is an optical transport that acts like an MSA-to-MSA wire with an electrical interface.

For applications in data centers, optical wire will simplify the installation and operation of networks. Of course, applications that require patch panels and risers cannot be solved with optical wires. In most networks, however, risers are a significant minority of the total connections.

The first market for AOC is storage area networks (SANs).

Including FC and now InfiniBand, SANs have unique and complex cabling configurations not unlike those in some carrier networks. However the market for SANs is much larger, encompassing customers that sometimes do not have the expertise to order custom cables or whose staff time could better be used on other tasks.

Many of the connectors, such as multi-fiber termination push-on (MTP) for MMF ribbon cable, do not have test sets designed to inspect fiber tips. Although the SAN network interfaces are standard, they are not widely understood. AOC provides an integrated solution that removes fiber selection, connector selection, optics selection and maintenance issues.

The result is a product with nearly all of the benefits of the modular approach and only two limitations.

Two disadvantages

AOCs are integrated products and so cannot be used in structured wiring (distribution) because they cannot be used with path panels. This limits their use largely to in-room applications for data centers, wiring closets, or carrier facilities.

The second disadvantage is that MSA purchasing is mixed with optical fiber purchasing. Comparing prices of the separate and integrated solution can be tricky. The AOC total cost benefits from eliminating the need for connectors, tipping and cleaning. But the fixed application could also limit the life to a specific application. These are factors that operators should consider before purchasing AOCs.

The majority of network connections, including optical, are in LAN applications from switches to host (computers) and between network elements, such as switches and routers. For these, the optical wire provides a valuable solution. The value is primarily in the reduced time for configuration, ordering, installation and maintenance of fiber optics and MSA modules.

Innovation, less loss

With planar optical advancements and now silicon-photonic integration, we can expect to see continued innovation in AOC.

The most significant trends will be in lower-power devices and the possible integration of system-on-a-chip (SoC) solutions to provide additional functions. How long will it be before we find an Ethernet switch embedded in the middle of an eight-legged cable?

Anyone who has worked in operations with Gigabit interface converters (GBICs) and later SFPs may appreciate what may be perhaps the best benefit of AOC. In my case, whether by accident or on purpose, I’ve seen a combination of hording and stealing of optical packages.

They are all-too-conveniently small. Recall the "S" in SFP. One easily can leave a few in a pocket by accident and possibly even forget about where they are. By contrast, AOC have those pesky fiber cables attached and do not fit neatly into pockets. Most operators have lost optics in their inventory. Individual optical packages costs in the hundreds to thousands of dollars. A pocketful could easily be in the tens of thousands.

Per portable ounce, MSA optics are the single most costly tangible asset on the balance sheet. AOC solves this problem by putting a long lease — fiber — between every pair of MSAs.

-Victor Blake is an independent consultant.