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Premise Equipment versus
Central Office Equipment

by Don Hurter

In the Frame Relay equipment room, there were some Cascade switches mounted in the racks doing Frame and ATM duty. These are boxes about two feet high by a foot deep, filling a standard 19" rack, and they sport modular plug-in cards with many connectors on each card. An unusual feature of their installation at 611 Folsom was a Plexiglas shield wrapping around the back to protect the connectors and their associated cables. The PacBell technician told us that these switches were not originally intended as CO equipment by the manufacturer, but instead were sold as "premise" equipment. Even  though each unit cost over $100,000 fully loaded, they weren't considered by the phone company technicians to be in the same class as the fiber muxes or channel banks elsewhere in the building. Why did they make that distinction?

All professions rely on certain tools and equipment as an essential part of their business. The more critical the job at hand, the more important it is to have dependable equipment. Determining the specifications for such equipment often requires considerable experience in that field, and what looks good to one industry may turn out to be inadequate for another. For example, almost everyone at one time or another has needed a ladder for a particular task, and there are a number of outlets available to purchase them, such as garden shops, hardware stores, or even contractor supply houses.

There are numerous grades of ladders, from lightweight models for occasional home-owner use to heavy-duty models which a roofer might specify, and the prices range accordingly. However, if one sought the "best extension ladder money could buy," they'd likely end up with a penultimate ladder, unless they considered going straight to a manufacturer who builds custom ladders for fire-fighters, as theirs are necessarily in a different league than what most people envision.

In a similar vein, when the Boulder Dam was being excavated (that's Hoover to us), Mack Truck built a number of "severe service" dump trucks to carry away the rocks, which needed to operate in a harsh environment where breakdowns could have dire consequences. Among other features, the trucks had chain-drive rear ends, which not only yielded a tremendous gear reduction, but also eliminated the likelihood of a snapped axle, which could potentially set back the work for days if the disabled truck clogged up one of the few available passages while it awaited repairs.

The Phone Company's specifications for their equipment is likewise demanding. Everything must be able to be mounted on the equipment racks, and all the cabling attaches at the rear in a serviceable manner. Most of the boxes we saw employed hot-swappable cards and chassis, and redundant components were designed-in where appropriate. 48 volt operation ensured that the equipment could tap into the CO's battery banks, and critical equipment usually features dual hot-swappable power supplies. Even the physical construction of the boxes must be rugged enough to take the repeated insertions and removals that a unit may see over its decade or more life span. And it goes without saying that everything in a CO is seismically engineered to survive an earthquake.

Most of the newer Frame Relay and ATM boxes are still in the early years of their development, and the manufacturers generally sell most of their products to private firms, not phone companies. In the eyes of the Telco community, any equipment not explicitly engineered for Central Office duty falls under the lesser category of premise equipment, meaning a customer's premise, where the equipment doesn't need to service a quarter-million users 24 hours a day, 365 days a year.

--Don Hurter

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