ly more rudimentary in function than modern systems (see Figure 3-40). In D1, every eight-bit sample carried seven bits of user information (bits 1 through 7) and one bit for signaling (bit 8). The signaling bits were used for exactly that: indications of the status of the line (on-hook, off- hook, busy, high and dry, etc.), while the seven user bits carried encoded voice information. Since only seven of the eight bits were available to the user, the result was considered to be less than toll quality (128 possible values, rather than 256). The frame bits, which in modern systems indicate the beginning of the next 192-bit frame of data, toggled back and forth between zero and one. Figure 3-40 Frame Bits D1 framing User 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 Chapter 3 Evolution: D4 As time went on and the stability of network components improved, an improvement on D1 was sought after and found. Several options were developed, but the winner emerged in the form of the D4 or superframe format. Rather than treat a single 193-bit frame as the transmission entity, superframe "gangs together" twelve 193-bit frames into a 2,316- bit entity-shown in Figure 3-41-that obviously includes twelve frame bits. Please note that the bit rate has not changed: We have simply changed our view of what constitutes a frame. Since we now have a single (albeit large) frame, we clearly don't need twelve frame bits to frame it; consequently, some of them can be redeployed for other functions. In superframe, the six odd-numbered frame bits are referred to as "terminal framing bits," and are used to syn- chronize the channel bank equipment. The even-numbered framing bits, on the other hand, are called "signal framing bits" and are used to indi- cate the receiving device where robbed-bit signaling occurs. In D1, the system reserved one bit from ever