Performance models of software designs can give early warnings of problems such as resource saturation or excessive delays. However models are seldom used because of the considerable effort needed to construct them. The ANGIOTRACE/sup TM/ was developed to gather the necessary information from an executable design and develop a model in an automated fashion. It applies to distributed and concurrent software with synchronous (send-reply or RPC) communications, developing a layered queuing network model. The trace-based load characterization (TLC) technique presented here extends the ANGIOTRACE/sup TM/ to handle software with both synchronous and asynchronous interactions. TLC also detects interactions which are effectively synchronous or partly-synchronous (forwarding) but are built up from asynchronous messages. These patterns occur in telephony software and in other systems. The TLC technique can be applied throughout the software life-cycle, even after deployment.