 CCITT Recs. V.5 and V.6 (Refs. 15, 16) as well as EIA-269 (Ref. 17). These data rates are 600, 1200, 2400, 4800, 9600, 14,400, 19,200, 28,800, and 33,600 bps. They are not evenly divisible into 64,000 bps, the standard digital voice channel. Two methods are described in the following to interface these standard data rates with the 56/64 small kbps digital channel. The first is AT&T's Digital Data System (DDS) and the second is based on CCITT Rec. 7.110 (Ref. 18). 10.9.7.1 AT&T Digital Data System (DDS). The AT&T digital data system (DDS) provides duplex point-to-point and multipoint private line digital data transmission at a number of synchronous data rates. This system is based on the standard 1.544-Mbps DS1 PCM line rate, where individual bit streams have data rates that are submultiples of that line rate (i.e., based on 64 kbps). However, pulse slots are reversed for identification in the demultiplexing of individual user bit streams as well as for certain status and control signals and to ensure that sufficient line pulses are transmitted for receive clock recovery and pulse regeneration. The maximum data rate available to a subscriber to the system is 56 kbps, some 87.5% of the 64-kbps theoretical maximum. The 1.544-Mbps line signal as applied to DDS service consists of 24 sequential 8-bit words (i.e., channel time slots) plus one additional framing bit. This entire sequence is repeated 8000 times per second. Note that again we have (192 + 1)8000 = 1.544 Mbps, where the value 192 is 8 X 24 (see Chapter 6). Thus the line rate of a DDS facility is compatible with the DS1 (T1) PCM line rate and offers the advantage of allowing a mix of voice (PCM) and data where the full dedication of a DS1 facility to data transmission would be inefficient in most cases. AT&T calls the basic 8-bit word a byte. One bit of each 8-bit word is reserved for network control and for stuffing to meet nominal line bit rate requirements. This control bit is called a C-bit. With the C-bit removed we see where the standard channel bit r