by Donald K. Hall
As high-bandwidth applications-such as 10-Gigabit Ethernet; 2-, 4-, and 10-Gbit/sec Fibre Channel; and InfiniBand 4X-SDR and 4X-DDR-have emerged, link-loss budgets have been reduced. At the same time, there has been an increase in the use of factory-terminated cable assemblies. Early drivers of this latter trend included a move to structured cabling and the modularity benefits factory-terminated assemblies offer: rapid installation, scalable growth, and ease of fiber plant maintenance. However, as system designers increasingly find value in the flexibility of network topologies with a high degree of connectivity, a secondary driver has emerged: the ability to minimize connector-related insertion loss through superior factory polishing and assembly processes, which helps network designers meet the new smaller link-loss budgets.
The principles of passive plant design and installation using factory assemblies are essentially the same as when field termination is performed. However, there are some practical considerations that system designers and installers should be aware of, especially as low-loss cable assemblies are specified. These considerations are directly related to the fact that the assembly manufacturer is delivering finished components with guaranteed insertion loss and effective modal bandwidth performance when mated to other components. It is therefore important to understand the relationship between cable assembly specifications and expected system performance when multiple assemblies are linked together to form the passive plant.
Factory cable assemblies have been supplied for many years using simplex and duplex connectors. Increasingly popular are modular plug-and-play assemblies, which usually contain ribbons in multiples of 12 fibers to take advantage of the high density of MPO-style connectors. The four basic types of plug-and-play assemblies include the following:
• Trunks. These are cable assemblies of customer-specified length terminated on each end with 12-fiber MPO connectors. Fiber counts are typically up to 144 fibers.
• Harnesses. These are short, 12-fiber cable assemblies made from interconnect cables that are terminated on one end with a 12-fiber MPO connector and at the other end with simplex or, more commonly, duplex connectors. Harnesses mate to trunks via their MPO connectors. Although the simplex/duplex connectors can be mated into a patch panel, they are usually directly mated to an equipment port.
• Modules. Sometimes called breakout modules or transition modules, these assemblies offer a space-efficient means of transitioning from MPO connectors to simplex/duplex connectors. These cable assemblies are terminated in the same manner as harnesses, but they are usually made from bare optical-fiber ribbons rather than interconnect cables. The fiber portion is protected within a plastic or metal casing that mounts in a connector housing like a patch panel. The simplex/duplex connectors are accessed at the front of the module for mating with jumpers or patch cords. The MPO connector is accessed at the rear of the module, where it is mated to a trunk.
• Integrated trunk modules (ITMs). These cable assemblies combine the functional attributes of a module and a trunk. They are essentially modules with long interconnect cable tails terminated with an MPO connector. The protective casing mounts in a connector housing like a standard module but is deep enough to provide a means of storing trunk slack, thus allowing the installer to deploy only enough of the trunk to reach the intended MPO connection point. ITMs are especially useful in small data centers and enterprises between the main distribution area and the end equipment or in large data centers between a consolidation point and the end equipment.
These components are usually installed as part of a structured cabling network comprising multiple links per channel. For simplicity, Figures 1 and 2 illustrate the use of each of these components in a point-to-point system.
Those familiar with traditional field-terminated assemblies will quickly recognize that the static functionality of each of these systems can be achieved by terminating bulk cable with simplex/duplex connectors at patch panels. In such builds, calculation of an expected link budget is straightforward and is merely a summation of the fiber loss and connector losses. The fiber loss is calculated by multiplying the fiber length by the fiber attenuation coefficient, expressed in decibels per unit length. The fiber attenuation coefficient is wavelength dependent. The connector loss is simply a maximum loss specification per mated pair, frequently taken as 0.75 dB per mated pair in accordance with ANSI/TIA/EIA-568-B.1. This calculation does not include the connectors mated to the transceiver ports, because the loss of these connectors is accounted for in the transceiver specifications.
The static equivalent of both figures would be a cable terminated with simplex/duplex connectors and joined to the equipment at each end with jumpers mated through a patch panel. The link budget would be 1.5 dB plus calculated fiber loss. However, as noted, the traditional field-terminated assemblies, while achieving the desired static functionality, do not offer the flexible reconfigurability and scalability of a modular plug-and-play system.
It can be seen that the plug-and-play systems have more connector pairs per link compared to the field-terminated approach. Each breakout module contains two connectors. Each harness also contains two connectors, although if mated directly to equipment as in Figure 1, only the MPO connector pair would count in the link budget. Therefore, the link of Figure 1 contains three mated pairs. Although the link of Figure 2 is not directly terminated to equipment on either end, this link also contains three mated pairs, because the ITM is functionally equivalent to a trunk and a module, thus keeping the connector count for the link at three instead of four. If one allows 0.75 dB per connector pair, it can be seen that the link budget for these systems would be 2.25 dB plus fiber loss. In this case, there would effectively be a budget penalty associated with the increased modularity of the plug-and-play system.
For high-bandwidth systems, especially where multiple links form a single channel, the total channel loss penalty may be unacceptable, leading a system designer to specify a field-terminated system when the flexibility of a modular plug-and-play system is actually desired. For this reason, manufacturers of plug-and-play components may offer components with insertion loss specifications requiring connector losses well below 0.75 dB per connector.
To illustrate, consider the example of Figure 1. If the module has a specified insertion loss of 0.5 dB and the MPO pair shared by the trunk and harness has a specified loss of 0.35 dB, one would then calculate a link budget of 0.85 dB plus length-dependent fiber loss. Note again that the duplex connector pair shared by the harness and the equipment port doesn’t contribute to the link-loss budget because the harness is directly terminated into the equipment ports. This link budget is well below the 1.5-dB budget calculated for the field-terminated approach. Of course, it would be the prerogative of the system designer to specify a field-terminated system with maximum connector insertion loss of some value less than 0.75 dB per mated pair. However, the likelihood of achieving these lower insertion losses is very field craft dependent. Because of the superior control of insertion loss in the factory environment, there is a clear advantage to the specification of factory-built assemblies.
Product certification or QC measurements, when provided to customers, are usually the only customer-visible aspect of a manufacturer’s quality assurance program. Other aspects may include such controls as dimensional verification of fiber and connector geometry or other process monitoring means. The ability of individual cable assemblies to meet performance expectations when concatenated in the field must be proven as part of the product qualification during development. This is usually done by performing loss measurements before and after environmental exposure on randomly mated cable assemblies manufactured under standard process conditions.
ANSI/TIA/EIA-568-B.3, Annex A describes a broadly accepted, standardized procedure for qualification of cable assemblies under defined test conditions. These loss measurements can then be analyzed to ensure concatenated link performance. Once the cable assemblies have been qualified, QC pass/fail criteria can be set to ensure that they are manufactured to the same quality level as those that were evaluated during product qualification. Because individual component quality is assured by the manufacturer, it is not necessary to field-test individual components.
A final consideration is field link testing. Concatenated field links, which usually have simplex or duplex connectors at the ends, can be tested end-to-end using standard power-through test sets and well-known, industry-accepted measurement methods. One adjustment that should be made to standardized test practices is that higher-quality test jumpers should be used to test low-loss links. For example, if the supplier indicates that link performance guarantees are contingent upon the use of low-loss patch cords or equipment jumpers specified with 0.3-dB loss per connector pair, then the quality of the test jumpers should be verified by measuring the loss across the two mated test jumpers to confirm that the test jumpers meet this same specification.
In instances where link testing suggests an underperforming or damaged component, the component can most easily be identified through process of elimination by substituting modules from links confirmed to be good into the problem link and retesting. If the high attenuation disappears, this procedure quickly points to a problem module or harness. If the high attenuation persists, it points to a problem with the trunk.
Factory-terminated plug-and-play cable assemblies offer numerous benefits related to system scalability and maintenance. High-bandwidth multimode applications are driving demand for components and links with progressively lower losses. A clear understanding of suppliers’ product specifications will help system designers identify suitable components for their applications and will help installers avoid problems during link testing.
Donald K. Hall is a senior applications engineer at Corning Cable Systems (www.corningcablesystems.com).