Virtual Null Modem will emulate couples of RS-232 ports connected via null-modem cable. Null-modem cable is basically used for communication of two RS-232 "DTE" devices with each other. To create null-modem connection between the devices you need to use two computers or two serial ports of a single computer connected with null-modem cable. Both methods described are expensive and difficult. The most reliable way to create null-modem connection is using of virtual serial ports emulated by virtual serial port driver and null-modem cable.