Using suitable modems and multiplexers it is possible to provide for a combination of data channels multiplexed together to form an aggregate bit rate for transmission purposes.The principle shown in Annex A and Figure 1/M.1320 may be applied to higher bit rates as modems,etc.,are developed and deployed.The numbering of data channels is obtained by indicating the multiplex channel followed by the sub-channel data rate assigned number in accordance with the scheme contained in Table A-1/M.1320.