  Problem of classification of parallel quantum channels for information
transfer is studied by method of ladder operators.
  Detailed compared to http://arxiv.org/abs/quant-ph/0702076 presentation of
method of ladder operators is given. Examples of paired channel and qubit-qunit
pair are added. Relations between eigenvalues of density matrices of composite
channel and its subchannels determining presence/absence of entanglement of
arbitrary mixed state of channel are obtained.
