d more often called simply long-distance carriers. These are the former AT&T Long Lines organization into channels by the carrier who then takes care of and other carriers, such as MCI and Sprint. multiplexing multiple DSO channels onto it. If unchannelized, the subscriber is responsible for The United States was partitioned into Local Access and multiplexing. Transport Areas (LATAs). LATA boundaries conform more or less to the standard metropolitan statistical areas 5. T3 Service is like T1 service but at DS3 speed. It is defined by the U.S. Department of Commerce. Origi- generally unchannelized, though some vendors do nally, they conformed to the boundaries of the areas offer channelized service. served by the BOCs. 6. Some carriers to international locations through LECs are prohibited from carrying inter-LATA calls certain gateway cities provide international private and IECs are prohibited from carrying intra-LATA calls. lines of various types. The subscriber must lease two Even when an LEC covers two LATAs, it must route the lines, one to the gateway and one from the gateway to call through an IEC. An IEC is represented in a LATA the international location. by a Point of Presence (POP). A subscriber can connect 7. Switched Data Services are often referred to as to the IEC's facilities only at a POP. Typically, this con- nection is made by a line provided by the LEC from the software-defined networks or virtual private subscriber's premises to the POP (A in the IEC diagram). networks (VPNs). They allow the subscriber to make Alternately, the subscriber can bypass the LEC and use of the circuit switching capabilities of the connect directly to the POP. For example, the subscriber carrier's long-distance facilities to control and moni- could install a microwave link to the POP (B). tor their private network. Typically, the carrier The providers of inter-LATA services fall into two provides the subscriber with terminals that interface classes, as follows: with the carrier's network control facility. The sub- Lo