y digital data ser- vice (DDS) does not use modems, but receives the customer's digital data directly into a data service unit (DSU). The system handles data at rates from 2400 bps to 1.544 Mbps. DDS presently services about 100 cities nationally. Digital services currently being offered are: DDS=DATAPHONE DIGITAL SERVICE S=TERRESTRIAL DIGITAL SERVICE (1.544 MBIT/S) SDS=SATELLITE DIGITAL SERVICE (1 544 MBIT/S) HSSDS=HIGH-SPEED SWITCHED DIGITAL SERVICE 1.544 MBIT/S SWITCHED) CSDS=CIRCUIT-SWITCHED DIGITAL SERVICE (56 KBIT/S SWITCHED) 7.2.1 Private Alternatives Private wire is usually a 20 gauge twisted pair cable that has been installed and is owned by the user. Since there are no restrictions on the signal strengths or voltages run through your own cable, modems can be used which are designed to transmit "brute force" signals requiring very simple transmitter and receiver circuitry in the mo- 114 TELECOMMUNICATIONS dem. These simple units are classed as "line drivers", and typically sell in the under $200 range for surprisingly good performance from 0 to 19.2 KBS. The line cost is about 25 cents per foot for a four- wire twisted pair line; to this you have to add labor cost for instal- lation time and termination connectors. Fiber-optic local data links consisting of optical modems, multi- plexers, and cable are now available at reasonable cost from Canoga Systems, RCA, Valtec, and others. As with customer owned copper cable, fiber optic cable may be installed only on your own premises, or with the permission of a utility if public thoroughfares are ac- cessed. Some benefits of fiber optics systems are data rates in excess of 20 MHz and immunity to electrical interference as discussed in Chapter 1. Fiber-optic cable is as low as 95 cents/foot and distances of 3000 feet are practical and with repeaters much longer distances can be realized. A major consideraton is the number of data channels that can be accommodated in one 1/8" diameter cable as compared to copper, particularly if cable ducts are crowded. Because f