TDM
Analog signals from modems etc. are sampled in round robin fashion and then fed to the codec (rather than having 24 different codecs)
Each of the 24 channels puts 8 bits into the output stream. 7 are data bits, 1 is a control bit. (7x8000 + 1x8000) = 64Kbps per channel.
One frame = 24x8 + 1framing bit = 193bits