rovider is com- monly used to refer generically to providers of different types of services. Network Access Figure 4.1 is a simple diagram that shows network access. On the left-hand side is the customer environment, which includes residences (single-line instruments being served by an access line) and business premises (with onsite telephone sys- tems such as private branch exchange [PBX] or key telephone systems-smaller site systems for installations where there are 50 or fewer employees). Those in the customer environment are connected to the PSTN via access lines. The access net- Local Exchange Customers Interexchange Carriers Access Network, or Local Loop International Local Carriers Exchange Network Cellular Providers ILEC or CLEC (e.g., ISP, wireless provider, cable TV Internet operator, power utility) Service Providers Transport Network Figure 4.1 Network access The PSTN Infrastructure work, also called the local loop, includes the network interface at the premises (also referred to as the demarcation point), the access line leading to the local exchange, the outside plant (e.g., telephone poles, drop lines), the components at the local exchange on which those access lines terminate (e.g., the line cards with a port associated with each telephone line), and the logic used to help control the flow of traffic over the access lines. Where competition is allowed in the local loop, a myr- iad of players may be interested in owning the local loop (e.g., ISPs, wireless oper- ators, cable TV companies, power utilities). However, worldwide, the incumbent local providers continue to dominate the local loop, and, as usual, politics and eco- nomics are principal factors in delaying the mass deployment of high-speed resi- dential access. The local exchange, in the center of Figure 4.1, provides access to the back- bone, or the core, of the network, which begins with the tandem switch and includes toll and international exchanges. From the local exchange, connections are established with the other providers, such as IX