D Common- Channel B-channel Signaling Terminal B-channel Network Local Exchange Channel- B-channel Switching Terminal D-channel Network Figure 7.11 N-ISDN network interfaces PRI-PRI is primarily used for business applications, and both local and interexchange carriers offer it. In support of the voice environment, PRI applications would include access to PBX and call center networks, replace- ment of existing analog trunk groups, and configuration of PBX tie-lines. Q-Sig, a standard that's an enhanced version of the D-channel signaling pro- tocol, supports feature transparency between different vendors' PBXs. A key data application of PRI is LAN/WAN integration. ISDN Applications ISDN can be used in a number of different situations. The following are the main applications for N-ISDN: Internet access-You could use N-ISDN to increase the speeds otherwise supported by your analog voice-grade line when you do not have other broadband access options, such as DSL, cable modems, or broadband wireless. Remote access-You could use N-ISDN to give teleworkers or telecommut- ers access to corporate resources. LAN/WAN connections-As mentioned earlier, N-ISDN is a technique for LAN interconnection, so you could bridge multiple LANs across a WAN over ISDN connections. Packet-Switched Networks High-capacity access-You could use N-ISDN if you needed to increase your capacity for things such as graphics, file transfer, video, and multi- media networking. N-ISDN does not provide motion as good as what you would get with the higher-capacity services, but it is much better than what you get with an analog facility. Private-line backup-You could use N-ISDN as a backup to the private-line services discussed earlier (e.g., DDS). Dialup Frame Relay access-Frame Relay is a very popular data network- ing option, particularly for LAN-to-LAN interconnection. You could use ISDN to provide measured-use dialup access to Frame Relay services in which the user dials in to a remote access port on the carrier's Frame Relay switch at either 64Kbps o