the local serving switch such that subscriber loops terminate through a hybrid directly to a PCM channel bank. Hybrid return losses could now be notably improved by adjusting the balancing network for its dedicated subscriber loop. 5.5 ms 5.5 ms Figure 8.20 Example of echo round-trip delay (5.5 + 5.5 = 11-msec round-trip delay). REVIEW EXERCISES 8.5.7 Loss Plan for Digital Networks (United States) For digital connections terminated in analog access lines, the required loss values are dependent on the connection architecture: For interLATA or interconnecting network connections, the requirement is 6 dB. For intraLATA connections involving different LECs (local exchange carriers), 6 dB is the preferred value, although 3 dB may apply to connections not involving a tandem switch. For intraLATA connections involving the same LEC, the guidelines are 0-6 dB (typically 0 dB, 3 dB, or 6 dB). The choice of network loss value depends on performance considerations, administrative simplicity, and current network design (Ref. 4). Loss can be inserted in a digital bit stream by using a digital signal processor involving a lookup table. By doing this, the bit sequence integrity is broken for each digital 8-bit time slot. Some of these time slots may be carrying data bit sequences. For this reason we cannot break up this bit integrity. To avoid this intermediate digital processing (which destroys bit integrity), loss is inserted on all-digital connections on the receiving end only, where the digital-to-analog conversion occurs (i.e., after the signal has been returned to its analog equivalent). Devices such as echo cancelers, which utilize digital signal processing, need to have the capability of being disabled when necessary, to preserve bit integrity. In Section 8.5.6 round-trip delay was brought about solely by propagation delay. In digital networks there is a small incremental delay due to digital switching and digital multiplexing. This is due to buffer storage delay, more than anything else. REVIEW EXERCISES 1. Name at leas