N CIRCUITS These are circuits that permanently connect two points when instantaneous communication is important, such as in the case of two brokerage traders needing to speak immediately to execute a trade. The person at one end presses a button on his telephone associated with the specific line; another button at the distant end flashes or rings, indicating that the line is to be answered. If the person originating the call needs only to depress the button associated with the line to signal the other end, then the signaling is automatic and the line is called an automatic ringdown line. If the person must depress the line button and then press a separate signal button to signal the other end, the line is known as a manual ringdown line. When a point-to-point line is rented from the lo- cal telephone company, the connecting point between the two locations will still be at the local central office rather than run- ning directly between the two end points. The cost of these lines is based upon mileage, which considers the distance from each point to the central office. Different parts of the U.S. have differ- ent methods for billing for point-to-point lines. There may also be long distance, formerly called long haul, ringdown private lines. Many brokerage firms on Wall Street in New York have lines going to Chicago and the West coast. In this case, the lines are rented from one of the long distance carri- ers, although the local part of the circuit may still be delivered to your premises on cable rented from the local telephone company. Centrex (Figure 10.9) Centrex is a type of telephone service offered by the local telephone company in most areas of the U.S. It goes by different marketing names in different areas including Centrum, Essex and Intellipath (in NYC), to name a few. The Telecom Handbook in 227 Figure 10.9 Centrex Ring Ring! EXT. 2411 YOUR COMPANY Local Telephone Co. Central Office EXT. 2480 With Centrex, there is no PBX on site. All connections are made through the switching equipment at the local tel