t up to 6 sub-channels at 9600bit/s. tion, the CSU was provided by the communications carrier, while The NTU provides a CCITT interface for customer data at 2.4, the DSU could be obtained from third party sources. After deregu- 4.8, 9.6 or 48kbit/s to include performing data control and supervi- lation, several third party vendors started manufacturing devices sion, which is known as structured data. At 64kbit/s, the NTU that combined the functions of the DSU and the CSU. provides a CCITT interface for customer data without performing data control and supervision, which is known as unstructured data. The NTU controls the interface via CCITT Recommendation 21.8.2 BT's KiloStream X.21, which is the standard interface for synchronous operation on public data networks. An optional V.24 interface is available at KiloStream is a point to point leased line digital service that was 2400, 4800 and 9600bit/s, while an optional CCITT V.35 interface first offered commercially in January 1983. Like DDS, KiloStream can be obtained at 48kbit/s. With the X.21 interface, the control is an all synchronous facility. BT provides a Network Terminating circuit (C) indicates the status of the transmitted information, data Unit (NTU) which is similar to a DSU/CSU to terminate the sub- or signalling, while the indication circuit (I) signals the status of scriber's line. The NTU encodes customer data for transmission via information received from the line. The control and indication 21/16 Time division multiplexing DTE/NTU Network Multiplex 2Mbit/s Interface Terminating Unit Equipment Equipment Digital Path 2.4kbit/s 4.8kbit/s 9.6kbit/s 48.0kbit/s Bipolar Tributary (64.0kbit/s) Interface 12.8kbit/s 64.0kbit/s G.703 Interface X.21bis Bipolar 2.048Mbit/s 48. Okbit/s Interface 64.0kbit/s 2.4kbit/s 4.8kbit/s 9.6kbit/s X.21bis Bipolar Interface 12.8kbit/s Digital Local Line System Customer's Premises Digital Local Telephone Exchange or Customer's Premises Figure 21.14 Basic KiloStream structure circuits control or check the status bit of the 8