change might bundle those T-1s onto a T-2 to pass off to the tandem, and the tandem would bundle them up into T-3s to Chapter 4 The PSTN send to the toll center. The toll center would bundle them up for T-4s to pass across the long haul to San Francisco. This works great until you need to add an office in Kansas City. You need to then add a T-4 mux to break it down to all the respective T-3s. Then you need to break down the T-3s into their T-2s, and then break down the T-2s into all their T-1s, and then find your T-1 SO that you can extract the channels you want to drop off. Then you need to bundle them all back up onto the T-1 and go back up the scale again. This strict hierarchy (thus the use of the term hierarchy in the names of these technologies) requires you to go through all the changes-you can't jump steps as you bundle and unbundle traffic. Therefore, the PDH hierarchy is characterized by a lot of back-to-back multiplexing and demultiplexing in order to drop and add payload. That is one of the highest- cost components of this generation of the PSTN. The E-Carrier Digital Signal Hierarchy As shown in Table 4.2, E-carrier sig- nals are often called CEPT levels (for the European Conference of Postal and Tele- communications Administrations); 64Kbps is the basic increment in E-carrier. CEPT-1 (or E-1) operates at 2.048Mbps and is delivered over 32 channels. Of these 32 channels, 30 carry information, and the remaining 2 carry control information (1 for framing, and 1 for signaling information). CEPT-2, like T-2, is not used much. CEPT-3, the high-bandwidth alternative, offers 34Mbps and 512 channels. CEPT-4 and CEPT-5 are largely used within telco networks, again for their interexchange trunks. Like T-carrier, E-carrier has a strict hierarchy of multiplexers. Table 4.2 The E-Carrier Digital Signal Hierarchy CEPT Signal Level Bit Rate E-O Channel Number of E-1 Lines CEPT-0 (E-0) 64Kbps CEPT-1 (E-1) 2.048Mbps CEPT-2 (E-2) 8.488Mbps CEPT-3 (E-3) 34.368Mbps CEPT-4 (E-4) 139.246Mbps 2,048 CEPT-5 (E-5) 565.148Mbps