 they could then engage in circuit-switched, leased-line, or packet-switched options. Although all these options were available before ISDN, each one generally required its own special access line and device, which meant extra costs and administrative responsibilities because of the large number of options. The goal of Circuit-Switched Networks ISDN was to provide one plug into the network, from which you could then go out over multiple alternatives. ISDN Networks A couple key elements are required to form an ISDN network (see Figure 7.10). First, you must have a digital local exchange, and that digital exchange must be loaded with ISDN software. This is an expensive proposition. The ISDN software alone costs around US$1 million, and each exchange costs in the neighborhood of US$3 million to US$5 million. Along with a digital exchange and the proper ISDN software, you also need a Signaling System 7 (SS7) network and CPE that is com- patible with the ISDN network. (SS7 is discussed in detail in Chapter 4.) As discussed in Chapter 2, "Traditional Transmission Media," N-ISDN has two interfaces (see Figure 7.11): BRI-BRI is primarily used for residential service, small businesses, and centrex environments. BRI is offered only by local telcos, and how they offer it varies greatly. It can be configured as either 1B, 2B, 1B+D, or the full 2B+D; 128Kbps transmission requires that two B-channels be used. One application for this is Internet access, SO you might have a BRI device, such as an ISDN modem, that would bond two B-channels (and possibly the D-channel) to provide the 128Kbps (or 144Kbps) access to the ISP. Digital Local Digital Local Exchange Exchange 64Kbps Circuit Switched Basic Basic 64Kbps Circuit Switched Dedicated Services Primary Primary Packet Switched SS7 Network Digital Digital Subscriber Services Subscriber Services Figure 7.10 ISDN components Chapter 7 Wide Area Networking Circuit- Switched BRI: 2B+D Network B-channel Terminal B-channel Local D-channel Exchange Packet- Switched Network PRI: 23B+D/30B+