We present a framework for maximizing the capacity (or bit rate) of symmetric bit rate communication services dominated by crosstalk, in particular digital subscriber line (DSL) services. We assume the DSL channel is a Gaussian channel, so transmit power spectral density (PSD) design is sufficient to maximize bit rates. Using the channel, noise, and crosstalk transfer functions, we set up an optimization problem and solve for transmit PSDs that maximize the joint capacity of same service users. Joint signaling techniques and optimal power distribution yield significant gains in bit rates (or performance margins) over current schemes. Furthermore, the designed transmit PSDs are inherently spectrally compatible with existing services on neighboring lines. We purposefully design a general framework; it does not depend on the exact choice of modulation scheme, for example. It is also extremely simple and of low computational complexity. The framework can also apply to other channels besides DSLs, such as wireless channels, coaxial cables, power lines, and telemetry cables used in geophysical well-logging tools.