We consider the problem of slotted asynchronous ccoded communication, where in each time frame (slot), the ctransmitter is either silent or transmits a codeword from a given c(randomly selected) codebook. The task of the decoder is to decide cwhether transmission has taken place, and if so, to decode the cmessage. We derive the optimum detection/decoding rule in the csense of the best trade-off among the probabilities of decoding cerror, false alarm, and misdetection. For this detection/decoding crule, we then derive single-letter characterizations of the exact cexponential rates of these three probabilities for the average code cin the ensemble. It is shown that previously suggested decoders care in general strictly sub-optimal.