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Optical Distribution System Architecture: Cross-Connect

Fiber Optic
Posted by James Donovan on 9 November 2016 Connect with James on LinkedIn Estimated reading time: 5 minutes

When configuring an optical distribution frame (ODF), one of the first considerations is the decision between interconnect and cross-connect systems. As with the location of optical components, this decision has large implications for the network’s future growth, reconfigurability, cost, and reliability.

Cross-Connect Systems Architecture

A cross-connect ODF architecture provides a dedicated termination point for both the OSP fibers and the equipment fibers. The OSP and equipment fibers are connected via a cross-connect patch cord routed between the two ports on the front of the ODF.

This makes accessing the network elements much easier and more cost-efficient and improves the long-term reliability of the installed fiber network (see Figure 1).  

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(Figure 1) Optical Distribution Frame

Cross-Connect Configuration

A cross-connect configuration provides the greatest flexibility when it comes to future network reconfigurations. If reconfiguration is required, all the work is done from the front of the frame with a patch cord that is generally less than ten meters in length.

If by chance this cross-connect patch cord is damaged during handling, another patch cord can be easily used to replace it. This is not the case within an interconnect network, where the patch cord being rerouted is connected to the equipment that may be on the other side of the central office.

Additionally, having proper slack storage for the cross-connect patch cord will ensure that the network can be quickly reconfigured without inducing attenuation on adjacent fibers.

An ODF system with a strong, flexible slack storage system will require only a few standard-length patch cords for use in cross-connect routings. Having fewer standard lengths of short patch cords required means that keeping such an emergency supply of cross-connect patch cords on hand is much easier and cheaper than keeping many different lengths in store.

Cross-connect and multi-fiber cables

Using a cross-connect architecture also allows multi-fiber cables to be routed between the equipment and ODF. Using multi-fiber cable assemblies can reduce the total amount of time required to install the fiber network. They also provide additional protection to the fibers being routed.

At the same time, there are operational and economic disadvantages to using multi-fiber cables, in both interconnect and cross-connect applications.

For example, a rack of FO equipment may handle equipment using a certain number of fibers in a multi-fiber bundle. If in four years, that equipment is obsolete and replaced with equipment that has fewer terminations in the same frame, the excess fibers will be very difficult to redeploy.

The key factor when considering cross-connect and interconnect architectures is the future reconfiguration capability of the method chosen.

Network Growth and Evolution

As the network grows and evolves, new and different technologies will be incorporated into the FO equipment frames, and the existing equipment will become obsolete or will be redeployed one or more times until the oldest equipment is discarded or all fibers are used.

This network reconfiguration could involve moving large amounts of electronics and many long patch cords, or reconfiguring short patch cords on the front of the frame. The ease with which equipment is integrated into the network, and its potential effects on the installed network, will depend on the fiber cable management system.

Cross-Connect System Requirements

A cross-connect system with proper cable management features will allow the equipment within the fiber network to be redistributed simply by rerouting patch cords on the front of the ODF.

Additionally, with cross-connect, both the OSP and equipment terminations have dedicated permanent locations on the ODF. This means that even if the record keeping for a cross-connect patch cord reconfiguration is not properly done, the technicians will always know where the equipment terminations and the OSP terminations are.

This greatly reduces the time required to turn-up or restore services. It is true that in initial installation a cross-connect system is about 40-percent more costly than a comparable interconnect system because more equipment is needed.

A cross-connect system will also require more floor space, from 30- to 100-percent more, depending on the configuration, since there are more terminations required in the ODF network (see Figure 2).

In most OSP fiber networks, 50-percent of the fibers are spare or backup fibers (2:1 OSP:FOT ratio). These fibers are routed in the same sheath as the active fiber but are used if the connector or the fiber at the far end is damaged. Reconfiguring the network to use the spare fibers is done at the ODF termination panel.

Using cross-connect in this type of configuration will result in roughly a 35-percent increase in equipment costs, but will greatly improve network flexibility and the ability to reconfigure the network, while increasing network reliability.

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(Figure 2) Cross Connect System

Flexibility of the ODF system

The ODF system should be able to accept either interconnect or cross-connect, and allow both architectures within the same system.

This flexibility allows a network that starts out using interconnect to migrate to cross-connect when and if it is needed, without having to replace existing equipment. The ease with which the equipment can be redeployed and installed into the network depends largely on the ODF.

In a full cross-connect ODF, in which the equipment has a dedicated location in a termination panel, the existing equipment can be easily redeployed to a different OSP fiber via the cross-connect patch cord.

The accessibility of this patch cord directly affects the cost of this network reconfiguration. The ODF should allow the entire cross-connect patch cord, including excess stored slack, to be easily removed for rerouting. Accessing this fiber should be done without causing additional attenuation on any installed active fiber.

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