s and E-3s. You can switch between DS-1s and DS-1s or between E-1s and E-1s. You can switch between DS-0s and E-Os, and you can also Line Side Digital Cross-Connect Trunk Side DS-0/E-0 #7 DS-1/E-1 #1 DS-0/E-O #7 DS-1/E-1 #1 DS-0/E-O #8 DS-0/E-0 #8 Line 1 Trunk 1 DS-0/E-0 #1 DS-3/E-3 DS-1/E-1 #2 DS-0/E-O #1 DS-1/E-1 #2 DS-0/E-0 #2 DS-0/E-0 #2 DS-0/E-0 #4 DS-1/E-1 #1 DS-1/E-1 #1 DS-0/E-O #4 DS-0/E-0 #5 DS-0/E-O #5 Line 2 Trunk 2 DS-0/E-0 #1 0 DS-3/E-3 DS-0/E-O #1 DS-0/E-0 #2 DS-1/E-1 #2 DS-1/E-1 #2 DS-0/E-0 #2 Port In Port Out Line 1 Trunk 2 Line 1 Trunk 1 Line 1 Trunk 1 Line 1 Trunk 2 Line 2 Trunk 2 Line 2 Trunk 2 Line 2 Trunk 1 Line 2 Trunk 1 Figure 4.7 An example of a DCS The Transport Network Infrastructure potentially switch below that level by using submultiplexed data streams within the DS-0 channel. Some of the individual intelligent multiplexers, such as T-1/E-1 muxes and T-3/E-3 muxes, also offer this capability. The fact that reconfigurations can be implemented in a matter of minutes-and that customers can implement this capability on their own private networks-is the most important and favorable characteristic of DCSs. The main applications for cross-connects are to provide disaster recovery, to bypass a system during mainte- nance without affecting traffic flows, to reconfigure the network to address peak traffic demands, and to implement a temporary application that can be reconfig- ured as needed. Voice Compression Standards The following sections describe how voice was digitized to carry over the PSTN under the original sampling theorem. PCM PCM was developed in an environment that was largely analog. Therefore, in designing a digitization scheme, it was important to consider that voice would have to undergo many conversions between analog and digital as it was transmit- ted through the numerous switching nodes and components of a network. And if it had to go through a number of conversions, it could withstand only SO many con- versions before it began to lose toll quality. Therefore, the sampling t