Typical data communications networks include a central site and one or more remote sites, or "drops."  Each such remote site connects to the central site by means of telephone or other equivalent communications links.  The equipment at each of the central and remote sites connects to a modulator-demodulator unit (i.e., a modem) which provides an interface between the communications link and the equipment which communicates with the network. 
The aforementioned U.S. Application Ser.  No.  503,625 discloses diagnostic circuitry that is used in such digital data communications networks.  This circuitry connects to the modem at each site and allows an operator at a central site to address the diagnostic circuitry at any one of several remote sites, and then to test or control the corresponding modem.  Such a modem is a "host" modem and the diagnostic circuitry is used to ascertain the operative status of the host modem, in many cases without interrupting normal communications over the communications link.  This circuitry, however, is limited to a network in which there are no intervening modems between the central site and each designated remote site. 
In some networks, however, the remote sites are widely separated geographically.  Yet within such networks, several remote sites might be clustered geographically.  A digital data communications network to which the system described in the aforementioned application Ser.  No.  503,625 is applied, requires redundant telephone lines between the various clustered sites.  To obviate the need for such redundancy, a more advanced digital data communications network design interposes a "hubbing" site, when possible, so that long distance communications links connect a central site or remote, hubbing sites with other remote, hubbing sites and then only local links are required between the hubbing sites and the remote, non-hubbing sites located near the hubbing sites.  The links from a hubbing site to the associated non-hubbing remote drops spread out like spokes from a hub, hence the terminology.  The hubbing site usually contains either a time-division or frequency-division multiplexer and demultiplexer to properly switch communications between the long distance link and the local links. 
Due to the characteristics of various multiplexers, time-division multiplexing (TDM) is preferred in these systems.  Time-division multiplexing allows greater amounts of data to pass through the hubbing site.  However, this method requires binary signals, so it is necessary to convert the incoming analog signal into binary signals and then to reconvert the binary signals into analog signals as data passes through the hubbing site.  Even so, time-division multiplexers are simpler to construct and operate then are frequency-division multiplexers.  The diagnostic circuitry described in application Ser.  No.  503,625, however, requires that the remote site and central testing unit be coupled by a "continuous" analog path (e.g., a telephone line); therefore, this diagnostic circuitry cannot be used in these newer types of digital data communications networks which use TDM techniques.  Basically, the conversion to a digital form at each hubbing site constitutes a barrier through which analog testing messages cannot pass.  A network testing system capable of operation across this digital barrier is shown in commonly assigned U.S. Pat.  No.  4,055,808.  This latter testing system also permits the diagnosis of problems in a data communications network including multiple computers and their respective communications facilities, from a single site. 
The present invention relates generally to the type of network testing and control system shown in U.S. Pat.  No.  4,055,808.  However, it provides substantial improvements thereover, to accomplish automatic status monitoring of the network components and automatic preventive maintenance (i.e., network performance) testing and to perform additional test measurements and combinations of measurements which cannot be performed with prior art systems.