systems. Line build-out is the adding of capacitance and/or resistance to a transmission line to look "electrically" longer or shorter than it actually is physically. DIGITAL NETWORKS Most regenerative repeaters transmit a bipolar (AMI) waveform (see Figure 6.10). Such signals can have one of three possible states in any instant in time-positive, zero, or negative (volts)-and are often designated +, 0, - The threshold circuits are gates to admit the signal at the middle of the pulse interval. For instance, if the signal is positive and exceeds a positive threshold, it is recognized as a positive pulse. If it is negative and exceeds a negative threshold, it is recognized as a negative pulse. If it has a (voltage) value between the positive and negative voltage thresholds, it is recognized as a 0 (no pulse). When either threshold is exceeded, the regenerator is triggered to produce a pulse of the appropriate duration, polarity, and amplitude. In this manner the distorted input signal is reconstructed as a new output signal for transmission to the next repeater or terminal facility. 6.7 PCM SYSTEM ENHANCEMENTS 6.7.1 Enhancements to DS1 The PCM frame rate is 8000 frames a second. With DS1, each frame has one framing bit. Thus 8000 framing bits are transmitted per second. With modern processor technology, all of the 8000 framing bits are not needed to keep the system frame-aligned. Only one- quarter of the 8000 framing bits per second are actually necessary for framing and the remainder of the bits, 6000 bits per second, can be used for other purposes such as on-line gross error detection and for a maintenance data link. To make good use of these overhead bits, DS1 frames are taken either 12 or 24 at a time. These groupings are called superframe and extended superframe, respectively. The extended superframe, in particular, provides excellent facilities for on-line error monitoring and troubleshooting. 6.7.2 Enhancements to E1 Remember that time slot 0 in the E1 format is the synchronization channel, with a channel bit