A system for optimally mapping circuits into packets based on round trip delay (RTD), and a system for measuring RTD for use in packet communications systems such as circuit emulation (CEM) systems is disclosed. The measured RTD value can be used in a system that adjusts packet size to reduce capture delay to partially offset an increase in RTD. As the use of smaller packets increases the overhead burden on the packet communication system, the packet size can be increased to reduce the overhead burden when the size of the current RTD becomes appropriately short. The disclosure also teaches the placement of data from two or more circuits destined for the same emulation endpoint into the same transmission packet in order to improve system performance. The abstract is a tool for finding relevant disclosures and not a limitation on the scope of the claims.