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.
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