y sample for its own sig- naling purposes, which succeeded in reducing the user's overall through- out. In D4, that is no longer necessary: Instead, we signal less frequently, and only occasionally rob a bit from the user. In fact, because the system operates at a high transmission speed, network designers determined that signaling can occur relatively infrequently and still con- vey adequate information to the network. Consequently, bits are robbed from the sixth and eighth iteration of each channel's samples, and then only the least significant bit from each sample. The resulting change in voice quality is negligible. Figure 3-41 Superframe (D4) framing Superframe: 12x193 = 2,316 bits Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Telephony Telephony Back to the signal framing bits: Within a transmitted superframe, the second and fourth signal framing bits would be the same, but the sixth would toggle to the opposite value, indicating to the receiving equipment that the samples in that subframe of the superframe should be checked for signaling state changes. The eighth and tenth signal framing bits would stay the same as the sixth, but would toggle back to the opposite value once again in the twelfth, indicating once again that the samples in that subframe should be checked for signaling state changes. Today: Extended Superframe While superframe continues to be widely utilized, an improvement came about in the 1980s in the form of extended superframe (ESF), shown in Figure 3-42. ESF groups 24 frames into an entity instead of twelve and, like superframe, reuses some of the frame bits for other purposes. Bits 4, 8, 12, 16, 20, and 24 are used for framing and form a constantly repeat- ing pattern (001011 ). Bits 2, 6, 10, 14, 18, and 22 are used as a six-bit cyclic redundancy check (CRC) to check for bit errors on the facility. Finally, the 