  Several communication models that are of relevance in practice are asymmetric
in the way they act on the transmitted "objects". Examples include channels in
which the amplitudes of the transmitted pulses can only be decreased, channels
in which the symbols can only be deleted, channels in which non-zero symbols
can only be shifted to the right (e.g., timing channels), subspace channels in
which the dimension of the transmitted vector space can only be reduced,
unordered storage channels in which the cardinality of the stored (multi)set
can only be reduced, etc. We introduce a formal definition of an asymmetric
channel as a channel whose action induces a partial order on the set of all
possible inputs, and show that this definition captures all the above examples.
Such a general approach allows one to treat all these different models in a
unified way, and to obtain a characterization of optimal error-detecting codes
for many interesting asymmetric channels by using Sperner theory.
