Centralized vs. distributed electronics for fiber-to-the-desk

We are evaluating different designs for a fiber-to-the-desk project for a 24-story building with approximately 4,000 optical-fiber points using MT-RJ connectors. The main crossconnect (MC) is located on the eighth floor and the outlets from all floors will terminate in this MC.

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Q: We are evaluating different designs for a fiber-to-the-desk project for a 24-story building with approximately 4,000 optical-fiber points using MT-RJ connectors. The main crossconnect (MC) is located on the eighth floor and the outlets from all floors will terminate in this MC. A telecommunications room (TR) will be installed on each floor to house the interconnection equipment.

We have two proposals. The first is to use mechanical splices installed in splice trays to interconnect the 2 to 4 horizontal core cables to the 24-core vertical cables. The second is to use a patch panel, with the horizontal fiber connected to the front side, and the vertical fiber connected to the backside.

We understand that the standards allow both designs, but we have concerns about the patch panel solution.
Mohammed A. Azeem
Gulf Systems
Safat, Kuwait

A: In 1995, the fiber-cabling manufacturers introduced the concept of centralized electronics into the TIA standards as an attempt to more closely level the installed-channel cost of fiber with that of a copper-based solution. Seeing this as a threat to "structured cabling," the copper-cabling manufacturers made certain that the document emphatically stated that no horizontal cable could be longer than 90 meters. With this compromise reached, TIA/EIA TSB-72, Centralized Optical Fiber Cabling Guidelines, was published.

Suppose for a moment that the maximum distance from any work area in your building to the MC is less than 90 meters. TIA/EIA-568-B.1 allows you to place the cable from the work area, through the TR to the MC without having to splice, interconnect, or crossconnect between a backbone cable and a horizontal cable in the TR. But the moment your cable length exceeds 90 meters, TIA/EIA-568-B.1 requires you to cut a perfectly good fiber link into segments and install connecting hardware that exponentially increases the cost of your cabling system while degrading the channel performance.

Is this logical to you? Never has been for me, either. It seems that the end-user was the one compromised, by having to pay for all the additional connecting hardware and suffering the consequences of the additional 0.75-dB insertion loss per mated connector pair.

TIA/EIA TSB-72 has recently been rescinded, and centralized optical-fiber cabling is now addressed in Annex A of TIA/EIA-568-B.1, Commercial Building Telecommunications Cabling Standard General Requirementsellipse more or less the same text, but now it is part of the standard.

The TIA/EIA-568-B.1 offers three choices for connecting the backbone to horizontal cabling in the TR: splice, interconnection, and crossconnection, in addition to allowing installation of a cable in a continuous sheath from the work area through the TR to the centralized crossconnect. This cable is called a pull-thru.

Personally, I think that interconnection is the worst choice. You will want to terminate the four-strand horizontal cables on the back of the patch panels so that you would have them ready for patching from the horizontal cables to the network equipment, should you install it in the telecommunications room in the future. And no, simply plugging the horizontal cable termination into the network equipment is not allowed. This leaves the larger-strand-count backbone cables to be either buffered then connectorized, or spliced to pigtails and plugged into the front of the horizontal cables terminated on the patch panels. Cable management and labeling will be equally problematic regardless of which cable is on the front of the patch panel.

If the length of your cable from the work area through the TR to the MC would exceed 90 meters, and you are contractually required to meet the TIA/EIA-568-B.1, then splice the horizontal to the backbone in a splice tray.

Centralized optical-fiber cabling systems are intended for single-tenant users who do not want to locate the horizontal crossconnection in the TR. But if your goal is to centralize the network equipment, centralized cabling is not your only- and likely, not your best-option in this case.

You can also use the typical telecommunications cabling system described in TIA/EIA-568-B.1 and size the backbone strand count to equal or exceed the horizontal strand count on each floor. Simply terminate both the horizontal and the backbone cables on patch panels and patch between the two-a crossconnection. But at what cost? Twice the hardware in the TR. But you are less likely to have strands broken in your backbone cable during routine moves, adds and changes.

If I were going to dedicate the resources in the TR for deploying the network equipment in the future, I would not wait until later. I would install the equipment as close to the user as possible and reduce the strand count in the backbone and the size of the MC.

I will attempt to explain. Four thousand work areas, each with four strands of fiber, terminated on two MT-RJ connectors in the work area, and terminated on four MT-RJ connectors in the TR (two on the horizontal cable and two on the backbone cable) and terminated on two MT-RJ connectors at the MC, equals roughly 32,000 two-fiber connectors to terminate.

If you installed pull-thru cables, that number would be reduced to 16,000 two-fiber connectors to terminate. But you would still have a huge amount of cable to install and manage in your backbone pathways, and 16,000 strands of fiber to terminate, patch and manage within one telecommunications space-your MC.

Assuming termination and cable management of 32 strands per rack unit, you will need 500 rack units just for the fiber. And several times that number of rack units for the network equipment. I contend that you no longer have a MC, but that you now would have something that more closely resembles a data center-and all the interesting challenges that such an implementation brings to the project.

Concentrating all the cabling and network equipment in the MC, while also planning to disperse the network equipment into the TRs in the future, requires you to design the MC to accommodate the cabling and network equipment (square meters on the floor plate, electrical power, cooling, etc.) on day one. At the same time, you need to consider the square meters on the floor plate, electrical power, cooling, etc. in the TRs to accommodate the same network equipment.

You now have dedicated twice the space, electrical power, and cooling in the project that distributed the network equipment to the TRs on day one, plus a one-for-one match between the number of strands from the work area to the TR and between the TR and the MC.

What if you installed a 24-strand backbone cable from the MC on the eighth floor to the TR on each of the other floors, and horizontal cables from the TRs to the work areas? Now you are looking at 16,552 two-fiber connectors to terminate, with a lot less fiber cable to purchase, install, and manage.

The closer that you install the network equipment to the work area, the less cable you will be adding to the building pathways and the more space you will have for the next cabling system.


Donna Ballast is a communications analyst at The University of Texas at Austin and a BICSI registered communications distribution designer (RCDD). Questions can be sent to her at Cabling Installation & Maintenance or at PO Drawer 7580, The University of Texas, Austin, TX 78713; e-mail: ballast@utexas.edu.

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