 can operate independently for up to 2 64kbit/s channel is only available with an X.21 interface. At all data weeks without excessive slippage during outages. In Figure 21.13 rates, except at 64kbit/s, the NTU encodes the user data into a 6+2 the hierarchy of timing supplies as linked to AT&T's master refer- envelope structure as described in CCITT Recommendation X.50. ence clock is illustrated. As shown, the subsystem is a tree-like Hence, the line data rate is higher than the customer data rate. For network containing no closed loops. DTE datarates of 2400, 4800, and 9600bit/s the line data rate equals Within a DDS office, a composite timing signal is distributed over 12.8kbit/s, while a 48kbit/s user channel is transported at a 64kbit/s balanced pairs to each bay of equipment. This timing signal is a line rate. bipolar 64 kHz waveform having a 5/8 duty cycle. Each eighth pulse Customer data is framed into a 6+2 format to provide the signall- violates the bipolar rule. Hence, the basic waveform provides the bit ing and control information required by the network for mainten- clock information while the bipolar violation provides the byte ance assistance. This is known as envelope encoding and is clock information. illustrated in Figure 21.15. The rate of 8000 octets/s, imposed by the Connection to the DDS service originally required a channel 8kHz sampling frequency of PCM systems, allows a total net rate service unit (CSU) and a data service unit (DSU) to terminate a DDS of 48kbit/s per digital channel. One such channel could, for line. The DSU converts the signal from the terminal equipment into example, transport 5 sub-channels at 9600 bit/s, 10 sub-channels at the bipolar format used with DDS and T1 facilities. The CSU 4800bit/s, or 20 sub-channels at 2400bit/s. CCITT Recommenda- performs line conditioning functions to include equalisation and tion X.58 optimises this structure by minimising the signalling signal reshaping as well as line loopback testing. Prior to deregula- overhead and is able to transpor