An integral part of ISDN (integrated services digital network) is the provision of full-duplex digital transmission capability over voice-grade metallic subscriber lines with all the associated disturbances from echo, intersymbol interference, and crosstalk, among other things. We here present a theory for analyzing the optimal performance, in MMSE (minimum mean-squared error) sense, of full-duplex transceiver structures incorporating echo cancellers and decision-feedback equalizers. This theory augments previously published results by allowing a colored input signal and a fractionally spaced multitap MMSE forward filter in the decision-feedback equalizer. Computational considerations of the theoretical results are also addressed, where we investigate the properties of and efficient ways of calculating the MMSE solutions for various types of line codes, including the precoded partial-response codes and the block codes. We also discuss on how the MMSE is related to the ubiquitous SNR (signal-to-noise ratio) measure and the concerns associated with using it to gauge the transmission performance.