274Mbps and 4,032 channels. The Transport Network Infrastructure Table 4.1 The T-Carrier Digital Signal Hierarchy Digital Signal Level Bit Rate DS-0 Channel Number of T-1 Lines DS-0 (T-0) 64Kbps DS-1 (T-1) 1.544Mbps DS-2 (T-2) 6.312Mbps DS-3 (T-3) 44.736Mbps DS-4 (T-4) 274.176Mbps 4,032 T-Carrier and J-Carrier: Robbed Bit Signaling In the T-carrier and J-carrier systems, the signaling for each conversation flows in the same channel as the conversation. Thus, for voice purposes, we have the full 64Kbps channel, and every now and then, we steal one of the bits of digital voice and replace it with the proper signaling bit. To do this, signaling equipment steals the eighth bit of each channel on every sixth frame. Called Robbed Bit Signaling (RBS), this is a type of channel- associated signaling used in North America on T-1 trunks. Voice is not very sensitive to losing this data, so it doesn't cause much degradation of voice quality, and it does not affect the understandability or voice quality of the message. However, it does reduce the available bandwidth by 12.5%. If a data stream were traveling through that channel and we went about stealing bits, we would obviously change the meaning of the content. Therefore, we accommodate 56Kbps of data within each channel, leaving a little bit of room for the signaling bits to be inserted as needed. Each of the DS or T, E, or J levels requires a separate multiplexing level. Because each of the muxes is driven by a different clocking source, they each bun- dle their channels in a slightly different framework. In building the 64Kbps chan- nels up to a T-1 and then building those T-1s up to T-2s and those T-2s up to T-3s, everything is fine unless somewhere along the way one customer decides to extract some capacity to drop off that allocation midway. Say, for example, that you're in Washington, DC, and you need to connect to San Francisco. In Washington, DC, you'd have a T-1 coming into the local exchange, along with multiple other cus- tomers in the neighborhood. The local ex