 packet-switched data. Each B-channel offers 64Kbps, and Chapter 2 Traditional Transmission Media the D-channel provides 16Kbps. So, in total, 2B+D offers 144Kbps, deliv- ered over a single twisted-pair with a maximum loop length of about 3.5 miles (5.5 km). BRI is used in residences, in small businesses that need only a couple lines, and for centrex customers. (A centrex customer leases exten- sions from the local exchange rather than acquiring its own PBX for the customer premise. Thus, the local exchange pretends to be a private PBX that performs connections among the internal extensions and between the internal extensions and the outside network.) Primary Rate Interface (PRI)-Also referred to as Primary Rate Access (PRA), PRI is used for business systems. It terminates on an intelligent sys- tem (e.g., a PBX, a multiplexer, an automatic call distribution system such as those with menus). There are two different PRI standards, each deployed over two twisted-pair: The North American and Japanese infrastructure uses 23B+D (T-1), and other countries use 30B+D (E-1). As with BRI, in PRI each of the B-channels is 64Kbps. With PRI, the D-channel is 64Kbps. So, 23B+D provides 23 64Kbps B-channels for information and 1 64Kbps D-channel for signaling and additional packet data. And 30B+D provides 30 64Kbps channels and 1 64Kbps D-channel. Given today's interest in Internet access and Web surfing, as well as the avail- ability of other high-speed options, BRI is no longer the most appropriate specifica- tion. We all want quicker download times. Most people are willing to tolerate a 5-second download of a Web page, and just 1 second can make a difference in cus- tomer loyalty. As we experience more rapid information access, our brains become somewhat synchronized to that, and we want it faster and faster and faster. There- fore, N-ISDN has seen better days, and other broadband access solutions are gain- ing ground. (ISDN is discussed further in Chapter 7, "Wide Area Networking.") The DSL family includes the following: Hi