FX, not to be confused with FAX, refers to a foreign exchange circuit. In this case, foreign refers to a CO other than one’s own local CO, not to a location outside the country.
Consider the case of an airline that wishes to locate all of its reservations clerks in Atlanta. It cannot expect all of its customers to pay long-distance charges to make reservations. What are its alternatives? One possibility is a group of 800 circuits. Indeed, it will probably have a large number of those, but 800 trunks cover large areas (and are priced accordingly). What about service for customers calling from large, high-density metropolitan centers, such as Chicago? Perhaps a more focused service might be more cost-effective.
Think of an FX line (or trunk) as two-thirds of a dedicated point-to-point (or tie) connection. It starts at the customer’s location, connects to the local CO, and extends from there to another foreign CO anywhere in the country. There is a fixed monthly charge for all that mileage; but there are no usage-sensitive charges for these miles. At the foreign CO, it is open. It has a telephone number associated with that foreign CO. Calls made to that number ring at the customer’s location. Calls made from the customer’s location over the FX line emanate from the foreign CO, incurring only local charges for the call from the foreign CO to the called location.
FX lines are often used by companies to provide a local number that customers can call in cities where those companies do not in fact have offices. In the airline’s case, it could arrange a group of FX lines from its Atlanta offices to a Chicago CO. All of the lines could share one Chicago local telephone number. People from anywhere could call the number, but normally only Chicagoans would, because it would appear only in their telephone book—and it would be a local call only for them. If the airline wished to allow it, service representatives could also place calls from Atlanta to Chicago over the FX lines. The calls would be billed as though they were placed from within Chicago. Perhaps calls notifying customers of changed flight information might be placed this way
This introduction is meant to teach you about the functions and technology of a Central Office.
Analog to Digital Bandwidth
The Telephone Network
A Topology of Connection
Network Hierarchy (pre 1984)
Network Hierarchy (post 1984)
North American Numbering Plan
The Subscriber Extension
Local Access and Transport Areas
Wiring Connections: Hooking Things Up
Types of Communication
Lines Vs. Trunks
Foreign Exchange Signal