number this is on the basis that the transport channels are numbered in order of importance thus the most important channels are processed first and inserted into the frame first in a step 320 the converter inserts the tfci and other control bits into the frame it should be noted that this step is not dependent on the transport channels having already been inserted the location of all the control bits is preferably reserved and therefore the control bits may be inserted before during or after insertion of the transport channel bits the characteristic of the generic trau frame is that its format will be adapted at call set up for each transport format combination preferably using the messages that are used to configure the flexible layer one these messages provide for each transport format combination the number of active transport channels the number of bits that are sent over each active transport channel and an indication of which bits require error checking e g crc checking referring to fig6 the generic trau frame for a transport format combination comprising two transport channels is illustrated in this example the first transport channel carrying class a bits requires crc protection and the second transport channel carrying class b bits which does not require crc protection the generic trau.