Enhanced communications network testing and control system

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. In a network testing and control system for use with a digital communications network which provides digital communication between a plurality of sites, one of said sites being designated a central site and the other sites being designated remote sites and said digital communications between said central site and said remote sites being over a plurality of analog communications links, each remote site having means for generating a unique site address identifying said site, and wherein said remote sites can dynamically connect to and disconnect from said network, an improvement for reporting on the operational status of remote sites connected to said communication network communications links, said improvement comprising, at the central site

means for polling the remote sites connected to said communications links with an address inquiry signal to determine their addresses;

means for storing at the central site a record of the addresses determined by remote site responses to such polling, to provide a site location table containing the addresses of the remote sites;

means for updating said site location table as remote sites connect to and disconnect from said communications links;

means for transmitting, over said communications links in a shared and selectively concurrent arrangement with said digital communications network, a status request signal to each remote site address contained in the site location table, in response to a command, to evoke a status signal from each remote site indicative of the status of at least one selected variable associated with that remote site, said variable comprising a status condition, signal or operating parameter at the remote site; and

means for receiving remote site status signals transmitted from remote sites over said communication links in a shared and selectively concurrent arrangement with said digital communications network.

2. The improved network testing and control system of claim 1 wherein the means for transmitting further includes means for repeating automatically the transmission of status request signals to the remote sites at predefined intervals.

3. The network testing and control system of claim 1 or claim 2 wherein said remote sites include means for generating status signals, and further including, at the central site, means for detecting a change of status in a remote site variable and means for displaying an indication of such change of status.

4. The network testing and control system of claim 1 or claim 2 further including, at the central site,

means for storing permissible values for remote site variables,

means for accepting and storing permissible values for selected monitored remote site variables,

means for detecting variable deviation from its permissible values, and

means for signalling to a system operator when a selected monitored remote site variable deviates from its permissible value.

5. The network testing and control system of claim 4 further including at the central site means for generating and storing information pertaining to each remote site variable which deviates from its permissible value.

6. For use with a digital communications network which provides digital communication between a plurality of sites, one of said sites being designated a central site and the other sites being designated remote sites and said digital communications between said central site and said remote sites being over a plurality of analog communications links, each site being uniquely identified by a site address selected from a pool of available site addresses, an improved network testing and control system comprising, at the central site;

means for selectively and concurrently sharing said communication links with a host processor at said central site for reporting on the operational status of remote sites connected to said communication network communications links, said sharing and reporting means having

means for testing operating equipment at a remote site including means for transmitting a test signal sequence to the remote site, and for receiving a test signal response from the remote site, via the shared communications link associated with the remote site address;

means for accepting at the central site test-scheduling information including a dynamically changeable designation of a test sequence to be performed at a later time, the address of the remote site on which the test sequence is to be performed, and the time when the test sequence is to begin;

means for storing the test-scheduling information; and

means for providing the test-scheduling information to the test signal sequence transmitting means at the designated date and time, for automatically initiating the scheduled test sequence at the designated remote site.

7. The network testing and control system of claim 6 wherein the means for testing a remote site includes means for receiving a response to a test signal sequence and further including means for recording such response.

8. The network testing and control system of claim 7 wherein the means for recording includes means for providing a visual indication of the response.

9. The network testing and control system of claim 8 wherein the visual indication is in the form of a printed copy.

10. A method of monitoring the operative status of remote site equipment in a digital communications network from a central site of the network, wherein said network has plural remote sites each of which is operable asynchronously of other remote sites and of said central site, the network having analog communication links connecting said central site to said remote site equipment, said central site having a host data processing system, said method including the monitoring of a plurality of status variables for each remote site and comprising, for each remote site, the steps of

sharing said communication links between said host system and a central-site monitoring system;

generating, in response to selectively concurrently shared two-way signal communication between said central site and said remote site on said communication links, an initial status listing responsive to the status variables for the remote site;

generating a status change signal responsive to a change of at least one status variable from its value in the status listing; and

responsive to the status change signal, determining for the associated variable, the state change or changes.

11. The method of claim 10 wherein a status variable may be in one of two available states and the change signal is generated in response to a status variable changing from one of said states to the other of said states.

12. The method of claim 11 further including the step of verifying that at least one variable has changed state upon the generation of the status change signal.

13. A method of monitoring the operative status of remote site equipment in a digital communications network from a central site of the network, wherein said network has plural remote sites each of which is operable asynchronously of other remote sites and of said central site, the network having analog communications links connecting the central site to the remote site equipment, and over which a host processor at the central site communicates with the remote sites, and wherein the central site further includes first and second data processing systems, the first data processing system being connected directly to the second data processing system and the second data processing system being further connected to the remote sites over said links, said method comprising, for each remote site to be monitored, the steps of

selectively concurrently sharing communication of status signals between said central-site data processing systems and said remote sites over said analog communications links with the digital communication of said network;

generating, in response to said communication of status signals, a status listing containing the initial status of the status variables to be monitored;

the first data processing system sending to the second data processing system a status change inquiry signal;

responsive to the status change inquiry signal, the second data processing system sending a status change signal to the first data processing system if any of the monitored status variables have changed state since the generation of the status listing, and the second data processing system sending a no status change signal if none of the monitored status variables has changed state since the generation of the status listing;

responsive to receiving a status change signal, the first data processing system sending to the second data processing system another, second status change inquiry signal;

responsive to the second status change inquiry signal, the second data processing system sending a second status change signal to the first data processing system if any of the monitored status variables have changed state since the generation of the status listing and sending a said no status change signal if none of the monitored status variables has changed state since the generation of the status listing;

responsive to receiving a second status change signal, the first data processing system sending to the second data processing system a first status request signal; and

responsive to the first status request signal, the second data processing system sending to the first data processing system a status update signal indicating the status of each of the monitored status variables.

14. The method of claim 13 further including the step of updating the status listing to indicate correctly the status of each of the monitored status variables, according to the status update signal, and wherein the steps commencing with the step of the first data processing system sending to the second data processing system a status change inquiry signal are repeated at predefined intervals and after each repetition of such steps the status listing is updated to indicate correctly the status of each of the monitored status variables, according to the status update signal.

15. For use with a digital communications network which provides digital communication between a plurality of sites, one of said sites being designated a central site and the other sites being designated remote sites and said digital communications between said central site and said remote sites being over a plurality of analog communications links, said central site having a host processing system communicating with said remote sites over said communications links, an improved network testing and control system comprising, at the central site,

means for selectively concurrently sharing said communications links with said host processing systems for reporting on the operational status of remote sites connected to said communication network communications links, said sharing and reporting means having

a data processing system including a plurality of connection port means, each port means being connected to at least one communications link of the network for providing serial communications between the data processing system and each remote site on such link, said data processing system and said host processing system selectively concurrently sharing said communications links;

the data processing system further comprising

means for communicating substantially simultaneously, in parallel, through each port means over said communications links with the remote sites connected thereto and means, with respect to each port, responsive to a preselected signal, for monitoring, over said communications links, the remote sites connected thereto, and for ascertaining from data received over said shared links the status of at least one selected variable associated with each remote site, said variable comprising a status condition, signal or operating parameter.

16. Apparatus for monitoring the operative status of remote site equipment in a digital communications network from a central site of the network, wherein said network has plural remote sites each of which is operable asynchronously of other remote sites and of said central site, said network providing digital communications between said central site and said remote sites over a plurality of analog communication links, said central site having a host processing system communicating with said remote sites over said communication links, said monitoring apparatus comprising

memory means for storing status information,

means for generating and storing in said memory means, in response to status inquiry signals, information corresponding to the initial status of the status variables for each monitored remote site,

means for generating a status change signal responsive to a change of state of at least one status variable of a monitored remote site,

means at the remote site responsive to the status change signal for determining the variable state change or changes in response to which said status change signal was generated, said means for determining comprising means for comparing the states of status variables to the initial status information stored in the memory means, and

means for selectively concurrently sharing the communication of said status inquiry signals and said status variable signals and said status change signals between said central and said remote sites over said communications links with said digital communication.

17. For use with a digital communications network which provides digital communication between a plurality of sites, one of said sites being designated a central site and the other sites being designated remote sites and said digital communications between said central site and said remote sites being over a plurality of analog communications links, each site being uniquely identified by a site address selected from a pool of available site addresses, said central site having a host computer communicating over said communication links, an improved network testing and control system comprising, at the central site,

means for selectively concurrently sharing said communications links with said host processing systems for reporting on the operational status of remote sites connected to said communication network communications links,

a plurality of connection port means, each port means being connected to at least one communications link of the network for providing serial communications between the central site and each remote site on such link with said sharing;

means at the central site for accepting test-scheduling information including a dynamically changeable designation of a test to be performed at a later time, the port means for which a remote site test is to be performed, and the time of day when the test is to begin;

means for storing the test-scheduling information; and

means for initiating automatically the scheduled test, on the remote sites connected to the designated port means, at the designated time and on said links with said sharing.

18. For use with a digital communications network which provides digital communication between a plurality of sites, one of said sites being designated a central site and the other sites being designated remote sites and said digital communications between said central site and said remote sites being over a plurality of analog communications links, each site having means for generating a unique site address identifying said site, and wherein said remote site can dynamically connect and disconnect from said network, an improved network testing and control system, comprising, at the central site,

means for selectively concurrently sharing said communications links with a host processor at said central site for reporting on the operational status of remote sites connected to said communication network communications links,

means for polling the remote sites connected to said shared lines to determine their addresses,

means for storing a record of the addresses determined by such polling, to provide a site location table containing the addresses of the remote site, and

means for updating said site location table.

19. The improved network testing and control system of claim 18 further comprising means responsive to the site location table for organizing the remote site addresses according to at least its port and line connections for associating with each stored remote site address, its port and line connection for directing communications to said site.

20. The improved network testing and control system of claim 18 wherein said polling means further comprises means for determining which of the responding remote sites is a hot spare.

21. A method for monitoring the operative status of remote site equipment in a digital communications network from a central site of the network, wherein said network has plural remote sites each of which is operable asynchronously of other remote sites and of said central site, the network having analog communications links connecting said remote sites to said central site and over which a host processor at the central site communicates with the remote sites, said method including monitoring a plurality of status variables for each remote site for reporting on the operational status of remote sites connected to said communication network communications links, and comprising the steps of

selectively concurrently sharing communication of such status between the central site and the remote sites over the analog communications links used by said host processor with said digital communications of said network,

generating at and transmitting from the central site, and receiving at at least one remote site, over said analog communications links, time mask data,

monitoring and processing at said central site status variable changes at least at said one remote site and received over said analog communications links, and

inhibiting, at the central site, the processing of status variable changes received over said analog communications links from at least said one remote site during time periods determined by said time mask data.

22. The network testing and control system of claim 21 further comprising the step of

selectively disabling and enabling the transmission of status request signals to selected ones of said remote sites.

Description Submit all comments and votes

CROSS REFERENCE TO RELATED APPLICATION AND PATENTS

U.S. patent application Ser. No. 947,058 filed Sept. 29, 1978, for MODEM TESTING SYSTEM, which application is a continuation of application Ser. No. 503,625, deposited Sept. 6, 1974 and having a filing date of Nov. 4, 1974 for DATA PROCESSING SYSTEM TESTING CIRCUITS, and now abandoned.

U.S. Pat. No. 4,076,961, issued Feb. 28, 1978 for an AUTOMATIC SWITCHING UNIT FOR DATA COMMUNICATIONS NETWORK.

U.S. Pat. No. 4,055,808, issued Oct. 25, 1977, for a DATA COMMUNICATIONS NETWORK TESTING SYSTEM.

FIELD OF THE INVENTION

This invention relates to digital data communications networks and, more specifically, to diagnostic testing and control systems for use in those networks.

BACKGROUND OF THE INVENTION

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.

SUMMARY OF THE INVENTION

In accordance with this invention, diagnostic circuitry is provided which is compatible with prior diagnostic circuitry. This system is flexible and can be used with various types of digital data communications networks. A primary attribute of the system is a high degree of automation which permits the system both to learn the network configuration without manual intervention and continually to interrogate all drops in the system to determine the operational status of all system components; if any parameter or set of parameters changes state, the change is noted and the system automatically ascertains whether a fault condition exists.

A so-called "automatic preventive maintenance" capability is also provided, permitting the system to accept certain commands for execution at a later time so that an operator need not be present to initiate such tests. Typically the tests performed in the automatic preventive maintenance mode relate to the reliability of the communications system and obviate any other simultaneous use of the portions of the system under test. Thus it is highly desirable to be able to schedule them at times when the network is not normally active--e.g., outside normal business hours. The system permits such advance scheduling and obviates the need for an operator to be present at those times; the scheduled tests are automatically performed at their assigned times.

The system is controlled from a single central site in an economical, modular fashion, facilitating the addition of drops to the system as the size of the network increases. Flexibility is maintained through the use of reprogrammable microprocessors as the basic elements of the modules located at the central site. Command and test signals are generated under microprocessor control and thus can be changed with minimum hardware impact.

The aforesaid modular system design involves the use at the central site of a structure having a hierarchy of three "levels" or "tiers" of data processors which are tied together "serially". Each level of the structure has a particular responsibility and performs a unique, defined group of tasks.

The invention is capable both of digital status testing of the equipment in the network as well as analog testing and/or measurement of the communications links and analog communications equipment (e.g., in the modems).

In addition to having automated testing capability, the invention permits testing to be manually initiated as well. An operator at a central site can test any site in the system by selectively entering at a console the port line and drop address of a unit to be tested, together with command instructions regarding the test(s) or other control operations to be performed. The indicated drop site testing unit performs the operation commanded and generates a response which is relayed to the central site.

Operator interaction with the network testing and control system takes place at the central site by means of one or more consoles and printers or other input/output devices. The consoles are "intelligent" terminals which provide both input and output display capability. The printers simply provide hard copy output in a conventional manner. Test results may be displayed on a console or provided as a hard copy output from a printer, or both. Certain tests associated with the automatic preventive maintenance mode are usually performed at a time when an operator is not present, so that a printed copy output is routinely made of the results.

An audio monitor is also incorporated into the consoles. The audio signal on any of the links can be "patched" through the tiers of processors at the central site, to the audio monitor of one or more consoles. The audio monitors include an audio amplifier and loudspeaker, permitting the operator to listen in on the audio lines. An audio alarm signal may be annunciated through the monitor's loudspeaker when it is necessary to draw the attention of the operator--e.g., upon detection of a serious fault condition.

This invention is pointed out with particularity in the appended claims. The above and further objects and advantages of this invention may be readily and thoroughly understood by referring to the following detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the network testing and control system of the present invention;

FIG. 2 is a block diagram of the central processor 12 of FIG. 1;

FIG. 3 is a block diagram of a satellite processor 22.sub.i of FIG. 1;

FIG. 4 is a block diagram of an interface processor 24.sub.i of FIG. 1;

FIG. 5 is a schematic diagram of the analog signal level measurement circuit of the interface processor of FIG. 4;

FIG. 6 is an illustration of a display and keyboard arrangement for the consoles of FIG. 1;

FIG. 7 is a block diagram of a console 14 of FIG. 1;

FIGS. 8A-8C are a listing of command classes and commands;

FIG. 9 is an illustration of the structure of console-initiated commands which are sent to the CP;

FIG. 10 is an illustration of the structure of CP responses to console-initiated commands;

FIGS. 11-13 illustrate the outbound character protocols in learn, AM and NSF modes, respectively;

FIG. 14 illustrates the special translation sub-mode for the NSF mode;

FIG. 15 illustrates the translation of a short form sequence when the RTM is an MPEXT testing unit;

FIGS. 16A and 16B illustrate the translation of a short form sequence when the RTM is a CLEXT testing unit, under conditions detailed in the specification;

FIG. 17 illustrates codes for the translation of commands;

FIGS. 18A, 18B, and 18C are a summary tabulation of commands showing translation category and response format category; and

FIG. 19 is an illustration of the IP's modes of operation.

DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

A general system diagram of the present invention is shown in FIG. 1. Dashed line 1 separates the central site of network testing and control system 10 from the network 20.

The basic system components of the central site 10 include a central processor (CP) 12, one or more console work stations 14 and printers 16 which are connected to the CP for input/output, one or more satellite processors (SP's) 22a-22n which are connected to the central processor 12 and to each other via a system bus 18, and one or more interface processors (IP's) for each of the satellite processors, e.g., IP's 24a-24n for SP 22a. Each interface processor is associated with a "port" of an SP and with a modem, e.g., 26a-26n, respectively, through which the host data processing system (HDPS) connects to the respective communications network lines 28a-28n. The HDPS thus communicates with the hubbing and other remote sites in the network via the modems 26a-26n.

The primary functions of the CP 12 are to act as a scheduler and dispatcher of all activities of the testing system and to handle input and output from and to the consoles and printers. It accepts commands from, e.g., the console 14 and controls all data flow to and from SP's; it also dispatches information to printer 16 and to console 14.

Only one CP is needed for the system. The CP and the SP's are interconnected by a parallel bus 18, the "system bus;" the console(s) and printer(s), however, are serially interfaced to the CP. CP 12 may have several (e.g., eight) ports available for connection to printers and consoles. For example, four ports may be allocated to consoles and four to printers. One SP is used for approximately every eight central site modems, and one IP is used for each central site modem. The number of SP's will thus depend on the number of IP's employed. SP's 22a-22n each have several (e.g., eight) serial interfaces 23a-23n through which they connect to the central site IP's 24a-24n.

Each remote site in the network includes a modem 32 and a remote test module (RTM) 34. RTM addressing, according to the scheme used in the illustrated embodiment, allows up to 10 line numbers per port, and up to 40 drop numbers per line. Each SP may support up to 320 drops spread across its eight ports (due to memory limitations). Further, up to 20 SP's may attach to the system bus 18. Naturally, other implementations would be readily feasible, to permit different degrees of system expansion at each level.

The SP's receive sequences of control and command information from the CP and in response thereto issue direct commands to the IP's. The IP's both translate the SP commands into forms acceptable to the RTM's and transfer corresponding analog signals over the lines 28a-28n. Analog responses received over those lines are first demodulated and then mapped into digital signals by the IP's and then transmitted back to the SP's. The SP's provide the CP with information concerning the results of the tasks performed and/or data regarding system status and error conditions.

It should be noted that the RTM's are, or at least may be, prior art devices such as those described in U.S. Pat. No. 4,055,808 and application Ser. No. 503,625. Compatibility with the RTM signalling format is provided by the transition function of the IP, so the RTM's can have their own signalling format and instruction set. This permits the present invention to be compatible with prior art devices and adaptable for use with test modules. In fact, all the RTM's need not necessarily employ the same signalling format.

SYSTEM COMPONENTS

The CP

The CP, SP, IP and consoles are each microprocessor based subsystems. The microprocessors may, for example, be the type Z-80 microprocessor CPU manufactured by Zilog, Inc. The CP, SP, and IP (and the consoles), in the detailed embodiment illustrated herein by way of example, each contain a Z-80 microprocessor, some amount of read-only memory (ROM) and random-access read/write memory (RAM), and some kind of input/output (I/O) equipment to allow communication with other parts of the system. The CP, for example, is illustrated as having eight I/O ports suitable for the attachment of peripheral devices such as consoles and printers and an interconnection to the system bus 18 for communication with the SP's. In each of the CP, SP's and IP's, the Z-80 microprocessor constitutes the CPU (central processing unit) which is the heart of the particular data processor. The CPU's function is to obtain instructions (and data) from memory and perform the desired operations.

In its memory, the CP keeps several tables reflecting system status. In a "resource" table it keeps track of what devices are attached to the system bus, their type, and numerical designation or address. In a "connect" table it maintains a dynamic list showing the logical connection between an SP port and its designated logging device or console. An "active fault" table is also kept, to record a dynamic listing of all fault types previously identified during the AM mode and remaining unresolved; for practical reasons, a ceiling is placed on the number of such faults maintained in the table. Additionally, an APM schedule table is maintained to keep track of the user generated schedule of APM times and tests; each entry consists of port number, time of day and APM test sequence type to be performed.

The CP dispatches test commands and results and thus acts as a clearing house for the consoles and printers on the one hand and the SP's and RTM's on the other. During AM mode, when an SP elevates a CHANGE response to CP level, the CP directs its status request command to the RTM reporting. The results are analyzed to find the parameter which changed. The fault table is updated by the CP to reflect the result, and a message is sent to the console to indicate that the number of outstanding faults has changed.

The CP initiates APM testing on the ports selected at TOD's selected, as represented in the APM schedule table. When responses to the APM test commands come in, the CP must properly route them from SP to printer.

For manually driven tests, the CP relays the console-generated command to the proper SP. Results from the destination RTM, IP or SP are interpreted and relayed back to the console and/or printer for display to the operator.

The CP also controls the operation of self-diagnostics for itself and the SP's. It generates commands to the devices on the system bus and interprets responses. Self-test results are displayed on the console initiating the test. Components tested during a system self-test include the CP, all SP's and all consoles.

The SP's

The CP initiates all modes other than learn and APM, but the SP must support the action. It must follow proper line protocol by repeating the address/command sequence outbound (i.e., in the direction of the remote site) and must properly spot, qualify and relay the response inbound (i.e., toward the CP). Further, in APM mode the SP must control the test initiation as well as the use of its drop configuration table. The results of the tests run in APM are stored in a buffer for transmittal to CP for printing.

In addition to a Z-80 CPU, and RAM and ROM memories, each SP also includes a universal synchronous/asynchronous receiver/transmitter (i.e., USART or UART) interface for each of the connections possible between the SP and a central site IP. If an SP has eight output ports, for example, it will have eight USART interfaces--one for each port. Each IP interface has seven leads--two audio leads and leads labelled CA, CF, BA, BB and GND. In addition, the SP's interface directly to the system bus 18 and contain analog hardware for switching the selected analog signal to the audio-monitor leads.

The CA, CF, BA, and BB leads are standard; they are discussed, for example, in the above-identified application, Ser. No. 503,625, incorporated by reference herein. The CA lead carries a signal indicating that the SP is prepared to transmit data to the IP. The BA lead carries digital data from the SP to the IP. The BB lead carries digital data from the IP to the SP. The CF lead carries a signal transmitted from the IP to the SP to indicate the detection of a carrier signal.

Manually operated configuration switches (or "straps") in the SP are used to set the following parameters: SP device address (0 to 31), number of ports used (1 to 8), maximum serially-embedded hub level (0 to 7), highest line number (0 to 9), and highest drop number (00 to 39).

The IP's

As previously noted, there is a modem associated with each IP. Both the modem and the IP have a transmitter and a receiver. The modem enables the HDPS to communicate with the remote site terminals via their modems. Also, the IP and RTM's at remote sites can communicate with each other via the same link. The HDPS and remote site equipment transm