hat limited ability to con- nect those lines either to other lines or to other switches. Finding the right balance be- tween the number of lines and the internal switching resources is another problem for the art of traffic engineering. It is impractical to bring every telephone number's twisted pair all the way back to the local exchange office, especially for very large offices. In practice, the connections to the local exchange office are distributed by using remote switching modules and subscriber-loop carrier systems, essentially a form of multiplexer. The vast majority of lines will be installed and administered, provisioned in tele- phony jargon, as two-wire analog lines. The LEC also may offer two- or four-wire digi- tal service, called ISDN Basic Rate. This service provides two 64-kbps time slots and a separate 16-kbps time slot used for signaling. ISDN Basic Rate is widely deployed in Europe. It is not widely available, or used, in North America. Local exchange offices are also used to provide service to PBXs, and this is accom- plished with trunks. Trunks may be either analog or digital, as required by the customer contract. Digital trunks typically are provisioned as DS-1s. The number of trunks will be engineered for the traffic level and quality of service required by the customer. The trunks provisioned on digital facilities may emulate the trunk signaling used with analog trunks by embedding the signaling within the 8-kbps samples, a technique referred to as robbed-bit signaling. The trunks also may be provisioned with out-of- band signaling. This is accomplished by taking the twenty-fourth time slot of the DS-1 and dedicating it to a signaling channel for the remaining 23 time slots. DS-1 facili- ties provisioned in this manner are referred to as ISDN primary rate interface (PRI) facilities. The DS-1 facility is primarily available in North America. Europe and much of the rest of the world have standardized on a similar digital transmission technology, called an E-1. E-1 has 30 channels as compare