ferent operators in an area have often bought equipment from different manufacturers, which means there are a number of standards to deal with. Even if a country once followed ITU standards, new companies may have entered the country with North American-standard equipment and interfaced with the existing ITU-based equipment. Thus, it is important to be familiar with all the standards. The following sections cover the signal hierarchy for both T-carrier and E-carrier standards. DS-x Versus T-x and CEPT-x Versus E-x Technically, the DS-x and CEPT-x terminology (DS-1, DS-3, CEPT-1, CEPT-3, and so on) indicates a specific signal level (and thus usable bandwidth), as well as the electrical inter- face specification. T-x and E-x terminology (T-1, T-3, E-1, E-3, and so on) indicates the type of carrier-a specific physical implementation of a DS-x/CEPT-x. Today, however, the terms DS-x and T-x are often used interchangeably. For example, someone might use the term DS-1 and another person might use the term T-1 to refer to the same thing-a dig- ital transport that can carry 1.544Mbps over a total of 24 channels. The same applies to the use of the European designation: E-1 is the same as CEPT-1, and so on. The T-Carrier Digital Signal Hierarchy Table 4.1 lists the levels in the T-carrier digital signal hierarchy, the basic building block of which, DS-0, is the 64Kbps channel. The first subscriber level, Digital Signal level 1 (DS-1), provides 1.544Mbps and a total of 24 channels. The DS-2 level is not a subscriber level, nor is it used very frequently in the PSTN. You might see it installed on some campus networks, or perhaps to bundle some DS-1s out of a tandem exchange to a toll exchange. DS-3 is a high-bandwidth alternative for subscribers, and it is used for inter- exchange trunks. Both users and carriers get 44.736Mbps with DS-3, which is a total of 672 channels that can carry combined voice, data, fax, and image traffic. The DS-4 level is used only within the telco, again on interexchange trunks. DS-4 offers roughly 