�1���� Introduction
�2����
Signalling rates
������� 2.1���� Data signalling rates
������� 2.2���� Signalling rates on line
�3����
Differentiation between user and network data signals
������� 3.1���� Scrambler (64 kbit/s only)
������� 3.2���� Descrambler (64 kbit/s only)
�4����
Interfaces
������� 4.1���� V.24-type interface
������� 4.2���� X.24-type interface
�5����
Rate adaptation
�6����
Testing facilities
������� 6.1���� Test loops
������� 6.2���� Self tests
�7����
Multiplexing
�8����
Internal management
Appendix I � Functional block diagram
Appendix II � Connection schematic
Appendix III � Rate adaptation of data signalling rates below 48 kbit/s
����� III.1���� Data signalling rates
����� III.2���� Rate adaptation
����� III.3���� Interfaces
Appendix IV � Example of implementation of the transmission unit
������ IV.1���� General
������ IV.2���� Physical model of the line transmission system
������ IV.3���� Functional description of the DCE
������ IV.4���� Functions of TU functional block
������ IV.5���� Requirements for a line transmission system using 2B1Q line
code
������ IV.6���� DCE management channel
������ IV.7���� Equipment management function