The DOS-based software running on the PC consists of device drivers and a TCP/IP stack combined into a single executable; higher level software performs the real-time flow analysis. DOS was chosen as operating system, because Unix has a higher interrupt latency. Since the Texas Instruments cards in the original OC3MON design required polling at 1/128 the cell rate in order to obtain accurate timestamp granularity at full OC3 rate (the card itself did not timestamp the cell), a very fast response to interrupts was necessary. Even if the two cards would have been put into two very fast Pentium PCs, timing would still have been critical on Unix Systems which cannot guarantee realtime response.
The latest OC3MON design uses Fore cards that can attach timestamps to the cells on their own; the host no longer needs to poll the card at all. Interrupts only occur at most every 1/40 second (e.g., if both links received 40 byte packets simultaneously), so low latency is no longer a constraint. Therefore now there is a first effort to port the OC3MON software to a UNIX environment.