A computer-aided design tool has been developed to study the hardware/software structure of various types of data switching systems used in the local loop distribution of computer communication networks. A simulation package is used to evaluate the performance parameters (such as the system throughput, average message delay, and probability of data loss) of local access systems under different input traffic conditions. Two types of traffic commonly used in teleprocessing applications are considered: the inquiry/response mode and the file transfer mode. Design tradeoffs for a line concentrator and a message interswitch are discussed and their performance is compared. The message interswitch permits a number of low-speed terminals to share communication lines and also to gain access to local common resources such as line printers, databases, and optical character readers.