dwidth. Where deployment cost is justified by service opportunity, fiber optic cable is being deployed in the last mile from a telephone central office to the sub- scriber. This approach is known as fiber-to-the-home (FTTH) or fiber-to-the- office (FTTO). To divide fiber cable investments between multiple customers, a fiber connection can be built from the central office to a multiplexing point from which copper cable access is provided to multiple customers. This access method is known as fiber-to-the-building or basement (FTTB) [5]. 6.4.7 Leased Lines and WANs An enterprise consisting of multiple offices in an area usually needs continu- ous information access among sites. For this purpose a public network opera- tor leases cable pairs or optical fibers for the connection between offices (see Figure 6.24). This is often the most economical way to interconnect LANs when the distance is of the order of a few kilometers. The line terminals shown in Figure 6.24 may be HDSL terminals for copper cable or optical terminals for optical fiber depending on the required data rate and distance. In the case of a long-distance connection, it is not economically feasible to build for each customer to build its own dedicated physical connection. This would require repeaters and separate cable pairs or fibers throughout the country. Instead the required end-to-end transmission capacity is leased from the core network of the long-distance network operator. For long-distance connections the operator uses the same high-capacity optical transmission systems that are used for the interconnections of public exchanges in the net- work (see Figure 6.24). The basic data rate unit of the provided transmission rate through the network is 64 Kbps corresponding to the capacity of one time slot in the PCM frame (see Section 4.5). This is why the telecommuni- cations carriers provide leased-line services with data rates in multiples of 64 Kbps. The four-wire baseband connection and leased-line long-distance con- nections just explained are co