A 1 ms frame carrying 144 bits of 2B+D data is mapped to 108 ternary symbols. These symbols are scrambled, with different scrambling codes for the two transmission directions, in order reduce correlation between transmitted and received signal. To this frame, an 11-symbol preamble and a symbol from the CL channel are added, yielding a frame size of 120 ternary symbols and a symbol rate of 120 kilobaud. The CL channel is used to request activation or deactivation of a loopback in either the NT1 or a line regenerator.