ephone switches. It is best to require the supplier to provide the NT-1 and take end-to-end responsibility. Provide the supplier with the LEC contact's telephone number and rely on them to resolve all issues related to ISDN connections. Before buying ISDN-oriented interfaces, one should be certain that the LEC supports it. Allow a month or SO to get ISDN service installed because some LECs are notoriously slow. Depending upon pricing and bandwidth, organizations may opt to use ISDN PRI. ISDN PRI terminates in an NT-2 interface, a PBX, or a specially- equipped network access device. Customers who buy PRI from an IXC in the U.S. can often use HO and H11 bandwidth-on-demand service. This is true in some European Implementing Group-Oriented Systems countries as well; overseas high-speed dial up is offered as HO and H12 and is obtained from a PTT. Some organizations use PBX stations to provide network support for videoconferencing systems. Generally, ISDN BRI stations are used in this case. On the trunk side of the PBX, connections are made to carriers or other corporate sites using T-1, fractional T-1, or ISDN PRI channels. Channels not used for video can carry inbound and outbound voice traffic. Putting videoconferencing behind a PBX allows an organization to move the system between conference rooms as desired (presuming the rooms are equipped with the proper station jacks). Finally, a videoconferencing codec may interface to the network via T-1 or fractional T-1. Connections of this type are made through a T-1 multiplexer (T-1 MUX) or inverse multiplexer (I-MUX). Multiplexers connect to a codec using either a V.35 or RS-449/422 interface (again, V.35 is most common). Carriers bring T-1 service into the building demarc, where it is connected to a CSU. In review, the CSU is used for signal shaping and equalization, longitudinal balance, voltage isolation, and loopback testing. More than one installation has been halted because a customer forgot to order the CSU (and all the required cables) before starting the job. I