Networks of workstations (NOWS) are cost-effective platforms to perform parallel computation. Usually, a NOW is not dedicated to parallel jobs. Local users may run some applications in their workstations which involve communications as well. This paper examines the effects of communication interactions of parallel and local jobs on a nondedicated NOW. Three representative communication patterns of parallel jobs are considered. A quantitative model to characterize the interactions is proposed. Measurement results on a NOW support the analytical model and indicate that the network interface in the TCP/IP protocol forms a communication bottleneck during interactions because a standard network interface with a single input/output queue is not able to distinguish communication requests from parallel and local jobs. Therefore, small but important communication messages of a parallel job, such as a barrier synchronization, could be easily blocked by a communication request of a local job, which would degrade the performance of the parallel job significantly. A double queue scheme in the network interface is proposed. Using available information from the protocol layer, the scheme is able to distinguish the two types of communication requests and give a higher priority to parallel jobs' communication requests. The simulation results show that the scheme could improve the performance of parallel jobs without significantly affecting the performance of local jobs.