|  | 5.4 | ISDN Services: BRI and PRI
| 5.4.2 | How BRI connectivity is established
Based on application need and traffic engineering, BRI or PRI services are selected for ISDN connectivity from each site. Traffic engineering may require multiple BRI services or multiple PRIs at some sites. Once connected to the ISDN fabric by BRI or PRI interfaces, design of ISDN end-to-end services must be implemented. The BRI local loop is terminated at the customer premise at an NT1. The interface of the local loop at the NT1 is called the U reference point. On the customer premise side of the NT1 is the S/T reference point. The figure shows a typical BRI installation. Two common types of ISDN CPE are available for BRI services: LAN routers and PC TAs. Some BRI devices offer integrated NT1s and integrated TAs for analog telephones. ISDN LAN routers provide routing between ISDN BRI and the LAN by using dial-on-demand routing (DDR). DDR automatically establishes and releases circuit-switched calls, providing transparent connectivity to remote sites based on networking traffic. DDR also controls establishment and release of secondary B channels based on load thresholds. Multilink PPP is used to provide bandwidth aggregation when using multiple B channels. Some ISDN applications may require the SOHO user to take direct control over ISDN calls. PC TAs connect to PC workstations either by the PC bus or externally through the communications ports (such as RS-232) and can be used similarly to analog (such as V.34) internal and external modems. PC TAs can provide a single PC user with direct control over ISDN session initiation and release, similar to using an analog modem. Automated mechanisms must be provided to support the addition and removal of the secondary B channel. Cisco 200 Series PC cards can provide ISDN services to a PC.