rs that filter out all of the channels except the one selected by the viewer. Because each tuner filters only one channel at a time, set-top boxes used for more than one television simultaneously will require multiple tuners. Personal digital video recorders such as TiVo also have two tuners SO that one tuner filters a channel that is being recorded and the other tuner filters the channel currently being viewed. Other tuners in end users' televisions filter signals for interactive ser- vices such as programming guides, VOD, and pay-per-view. Carrier Networks Cable operators remotely administer filters and traps in set-top boxes to allow subscribers access to basic cable TV or premium channels. Set-top boxes also have advanced security functions and contain links to billing systems. The security func- tion scrambles and unscrambles TV signals based on the information provided by the billing system as to which channels the subscriber is permitted to receive. Security is higher and TV theft is lower on digital service because of improved scrambling (encryption). The High Capital Cost of Set-top Box Upgrades On a cost-per-subscriber basis, when cable TV networks are upgraded to all digital service, the new set-top boxes required cost more than it costs to upgrade any other part of the network. This is because upgrades to the headend or core network are amortized over the entire network. By contrast, each set-top boxes is amortized from payments received from the individual customer. New set-top boxes add up to an enor- mous amount of money for cable providers with millions of customers. In addition, set-top box upgrades require more administrative time for programming the set-top boxes and answering customer inquiries. These costs are a major factor in many providers' decision to gradually implement digital service in their coverage areas. This spreads out the costs over a few years rather than during a single year. Legacy Circuit-Switching Service Switched services operate on landline and cellular networks. When a p