In general each Call fm MSC is carried in 64kbps.This bitrate is too high to be carried feasible over a Radio Link earlier.So it was decided used a suitable coding method like RPE-LPC(Regular Pulse excited – linear predictive coding) so that the same speech call is coded in 16kbps.(13kbps for speech + 3kbps for signalling).
Thats why the Transcoder converts a 4E1 link to 1E1 link.