1. A communications network testing and control system for use with a digital communications network which provides digital communications between a dynamically changing plurality of sites via analog communications links, one of said sites being designated a central site and each other site being designated a remote site, and each site having a modem connected to at least one such analog link for communicating therethrough with at least one other site, said communications network testing and control system, at said central site, comprising: first, second and third data processing systems; means for entering commands to said first data processing system for real time control of the operation of the network testing and control system; means for displaying information representing real time data from the first data processing system; the third data processing system communicating with a remote site over said communications links in a shared and selectively concurrent arrangement with said communications network; the first and second data processing systems having means for communicating therebetween byte-wise in parallel; the second and third data processing systems having means for communicating therebetween bit-wise serially; the second data processing system thereby communicating with both the first and third data processing systems and including means for converting said byte-wise data from the first processing system to serial bit-wise data for the third data processing system; the third data processing system having means for communicating serially with the remote site in said shared arrangement with said digital communications network; and communications signals between the second data processing system and the remote site over said communications links being translated by the third data processing system into a format compatible with the destination of the signals.