Instead of vague "send something, wait something" the responses are now
analyzed by (overridable) parsers. Makes all the modem implementations much
easier since each caller knows without any code whether the call succeeded
or failed.
Another thing that makes modem code simpler (and the whole thing more robust),
is the queueing of sent commands. Each queued command has a command and a
callback which is quaranteed to get called, even if sending failed.
Define and implement error reporting.
