Method and system for selectively accessing multiplexed data transmission network for monitoring and testing of the network

What is claimed is:

1. In a multiplexed data transmission system receiving data from a plurality of data channels and time division multiplexing the received data for transmission over a transmission path as a multiplexed data stream containing a predetermined pattern of framing signals together with the data from the plurality of data channels positioned in time at predetermined data channel locations relative to the pattern of framing signals, a system for accessing data of a desired data channel comprising:

line access means connected in series with the transmission path for accessing the multiplexed data stream without disturbing the transmission of the multiplexed data stream along the transmission path;

means responsive to the multiplexed data stream from said line access means for detecting the framing signals in the multiplexed data stream;

means for generating a control signal designating one of the plurality of data channels as the desired data channel;

means receiving said multiplexed data stream and, responsive to said control signal and to the detected framing signals, for locating and providing access to the designated one of the plurality of data channels in the multiplexed data stream without disturbing the transmission of the multiplexed data stream over the transmission path.

2. The system of claim 1 wherein data is received from a second plurality of data channels and is time division multiplexed to form a second multiplexed data stream for transmission over a second transmission path, the second data stream containing the predetermined pattern of framing signals and the data from the second plurality of data channels at predetermined data channel locations relative to the pattern of framing signals in the second multiplexed data stream, the system including:

second line access means in series with the second transmission path for accessing the second multiplexed data stream without disturbing the transmission of the second multiplexed data stream; and,

means for selectively connecting one of said two line access means to said detecting means and to said locating and access providing means.

3. The system of claim 1 wherein said line access means includes input and output terminals and comprises:

circuit means connected between siad input and output terminals in series with the transmission path for the multiplexed data stream for presenting an impedance to the passage of the multiplexed data stream to thereby develop, at said intput terminals, voltage signal levels corresponding to data signal levels in the multiplexed data stream; and,

switching means for selectively applying said voltage signal levels to said detecting means and to said locating and access providing means.

4. The system of claim 1 including:

means for generating a framing pattern corresponding to an expected pattern of framing signals in the multiplexed data stream; and

means responsive to detected framing signals in the multiplexed data stream for detecting errors in the pattern of framing signals in the multiplexed data stream.

5. The system of claim 4 including:

means for establishing a predetermined time interval; and,

means for counting the errors in the pattern of framing signals detected by said error detecting means during the established time interval to thereby provide an error rate of the pattern of framing signals in the multiplexed data stream.

6. The system of claim 1 wherein said access means includes means for selectively opening the transmission path, the system further including:

means responsive to said locating means for removing the data from the designated one of the data channels in the multiplexed data stream and inserting different data in the designated one of the data channels without disturbing the transmission of the remaining data channels of the multiplexed data stream.

7. The system of claim 1 including means responsive to said locating means for monitoring data in the designated one of the subscriber channels.

8. A method for accessing data of a desired data channel in a multiplexed data stream transmitted over a predetermined transmission path and containing a predetermined pattern of framing signals and data from a plurality of data channels at predetermined data channel locations relative to the pattern of framing signals in the multiplexed data stream, the method comprising the steps of:

providing a line access module in series with the transmission path for the multiplexed data stream;

developing in the line access module, a data signal identical in data content to the data content of the multiplexed data stream without disturbing the transmission of the multiplexed data stream through the line access module;

detecting the framing signals in the developed data signal;

generating a control signal designating one of the plurality of data channels as the desired data channel;

receiving the developed data signal and locating the designated one of the plurality of data channels in the developed data signal in response to the control signal and to the detected framing signals without disturbing the transmissionof the multiplexed data stream through the line access module;

selectively opening the transmission path through the line access module and rerouting the multiplexed data stream around the line access module from a receiving side to a transmitting side thereof and through a gating cirucit connected in parallel with the line access module; and,

selectively controlling the gating means to remove the data in the desired, located data channel without disturbing transmission of the remaining data channels in the multiplexed data stream.

9. The method of claim 8 wherein there is transmitted, over a second transmission path, a second multiplexed data stream containing the predetermined pattern of framing signals and data from a second plurality of data channels at predetermined data channel locations relative to the pattern of framing signals in the second multiplexed data stream, the method including the steps of:

providing a second line access module in series with the second transmission path;

developing, in the second line access module, a second data signal identical in data content to the data content of the second multiplexed data stream without disturbing the transmission of the second multiplexed data stream through the second line access module; and,

selecting one of said two data signals for detecting framing signals and locating the designated subscriber channel therein, and for rerouting the one of the two multiplexed data streams corresponding to the selected one of the two data signals throughout the gating circuit for removal of the desired data channel.

10. Apparatus for testing a desired channel of a plural channel time division multiplexed data transmission system in which a plurality of channels containing data are multiplexed to form a multiplexed data stream which is transmitted over a multiplexed data transmission path, the apparatus comprising:

gating circuit means having a multiplexed data input terminal, a multiplexed data output terminal and a control terminal, the gating means being responsive to a gating signal applied to the control terminal to selectively block passage to the multiplexed data output terminal of a signal applied to the multiplexed data input terminal;

line access means having receiving and transmitting terminals connected in series with the multiplexed data transmission path to open the transmission path between the receiving and transmitting terminals and route the data stream from the receiving terminals to the input terminal of said gating circuit means, the multiplexed data output terminal of the gating circuit means being connected to the transmitting terminals of the line access means;

means for generating a control signal designating one of the plurality of channels as the desired channel;

means for detecting the framing signals in the multiplexed data stream;

means responsive to the control signal and the detected framing signals for generating a gating signal specifying the location of the desired channel in the multiplexed data stream;

means for applying said gating signal to the control terminal of said gating circuit means to block passage of the data in the desired channel from the multiplexed data input terminal to the multiplexed data output terminal of the gating circuit mean, whereby transmission of the desired channel can be selectively blocked without disturbing transmission of the remainder of the data stream.

11. The apparatus of claim 10 wherein said gating circuit means includes an insert data input terminal and means for selectively inserting a data signal in the desired channel when passage of the data signal in the desired channel of the multiplexed data stream is blocked.

12. The apparatus of claim 11 wherein said inserted data signal comprises a control code that actuates a device in the multiplexed data transmission system remote from the line access means to form a closed loop path in the transmission system between said line access means and said device, said inserted data also including a predetermined signal inserted in the desired channel for transmission of said closed loop path.

13. The apparatus of claim 12 wherein said line access means includes impedance matching means for terminating the multiplexed data transmission path in an impedance matched with the transmission path impedance when the line access means opens the transmission path.

14. The apparatus of claim 13 further including means for selectively monitoring predetermined channels of the multiplexed data stream containing predetermined signal patterns, and means for determining a bit error rate of the multiplexed data stream in response to the predetermined signal pattern.

15. The apparatus of claim 10 wherein said line access means includes impedance matching means for terminating the multiplexed data transmission path in an impedance matched with the transmission path impedance when the line access means opens the transmission path.

16. The apparatus of claim 10 further including means for selectively monitoring predetermined channels of the multiplexed data stream containing predetermined signal patterns, and means for determining a bit error rate of the multiplexed data stream in response to the predetermined signal pattern.

17. In a time division multiplex signal transmission system for transmitting, over a signal transmission path, a multiplexed signal stream containing a predetermined pattern of framing signals interleaved with information bearing signals from a plurality of sources to thereby form a plurality of time multiplexed signal channels, a system for selectively blocking transmission of the signals in a desired one of the plurality of signal channels comprising:

means for generating a control signal designating one of the plurality of signal channels as the desired one of the channels;

means connected to the signal transmission path for detecting the framing signals in the multiplexed data stream;

control circuit means responsive to the control signal and the detected framing signals for generating a gating signal specifying the location of the desired one of the plurality of signal channels in the multiplexed signal stream;

gating circuit means connected in the signal transmission path and having a multiplexed data input terminal connected to receive the multiplexed signal stream from the signal transmission path, a multiplexed data output terminal connected to transmit signals passed by the gating circuit means onto the signal transmission path, and a control input terminal connected to receive the gating signal from the control circuit means, the gating circuit means selectively passing and blocking the passage of the multiplexed signal stream from the multiplexed signal input terminal to the multiplexed signal output terminal in response to the gating signal from the control circuit means, whereby passage of the signal in the desired one of the plurality of signalchannels is selectively blocked.

18. The system of claim 17 wherein said gating circuit means includes an insert signal input terminal and means for selectively inserting a desired signal in the desired one of the plurality of signal channels that is blocked.

19. The system of claim 17 wherein said gating circuit means includes means for selectively opening the multiplexed signal transmission path and diverting the signal stream to said multiplexed signal input terminal.

20. The system of claim 19 wherein said selective opening means includes impedance matching means for terminating the multiplexed signal path with an impedance matched to the signal transmission path when the signal path is opened.

Description Submit all comments and votes

BACKGROUND OF THE INVENTION

The present invention relates to monitoring and testing of multiplexed communication networks and, more particularly, to a method and system for selectively accessing desired information in a multiplexed data stream without interrupting the transmission of the data stream to thereby provide for non-disruptive monitoring and testing of transmission paths and equipment serving subscribers in a time division multiplexed data transmission system.

Various systems have been proposed and are presently in use for transmitting data produced by a large number of subscribers from one location to another distant location. These systems almost invariably employ time division multiplexing to combine the data into one multiplexed data stream. The multiplexed data stream is then typically transmitted over transmission networks which include telephone lines, telephone switching equipment and radio transmission (microwave) links.

One system presently in use is referred to as the Dataphone Digital Service (DDS). In such a system, a central office typically serves a large number of local data subscribers. Equipment at the subscribers' stations transmits and receives data over lines connecting the subscribers' stations to the central office. At the central office, the data received from the subscribers' stations is multiplexed and the multiplexed data stream is then transmitted from the central office to a distant central office or, in some situations, to a nearby location, where the data is eventually demultiplexed and transmitted to its ultimate destination.

In one typical common carrier system, the multiplexed data stream transmitted from the subscriber's central office is transmitted at a bit rate of 1.544 megabits per second and the data stream may contain data from up to 460 subscribers. The multiplexing is typically accomplished in two stages with a subrate data multiplexer (SRDM) accepting and multiplexing data from up to 20 subscribers and with a T1 rate data multiplexer (T1DM) accepting and multiplexing up to 23 SRDM channels.

As with data transmitted from the central office serving the local data subscribers, the data received by the central office from other central offices is received in a multiplexed format and is demultiplexed for local transmission to the local subscribers. In this manner, the numbers of very long transmission links between two central offices are minimized since as many as 460 subscribers may be served in a single multiplex of which many can be carried on a transmission link.

Present procedures for testing the data links or communication paths serving each individual subsciber are time consuming and require considerably equipment at each central office. The prevalent manner of testing data links requires access to the network at points preceding the multiplexing of the subscriber data and therefore requires test jacks for each individual subscriber and a pair of test units which can be moved from jack to jack to test the subscriber data links individually. Thus, in a central office serving thousands of subscribers, a test panel having thousands of jacks must be provided. The space consumed by the test jack panel alone can therefore be considerably in one of the larger central offices of a typical common carrier system. Moreover, since this prevalent testing technique requires an operator in insert the test unit connectors in the test jacks on an individual basis and operate the test sets, the testing of all of the subscriber lines may be an enormously time consuming task.

OBJECTS AND BRIEF SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a novel method and system for monitoring and testing data networks which carry individual subscriber data and multiplexed data.

It is another object of the present invention to provide a novel method and system for selectively accessing a desired channel or time slot of information in a multiplexed data stream without disturbing the transmission of the data stream and without demultiplexing the data stream.

It is yet another object of the present invention to provide a novel method and system for monitoring and testing data networks serving a large number of subscribers through multiplexing of subscriber data wherein access to each individual subscriber data link is provided automatically and without the need for test jacks.

It is still another object of the present invention to provide a novel test set and method for accessing any subscriber data link in a multiplexed dat network for either monitoring or testing purposes at a point in the network carrying multiplexed data and therefore requiring access to only a single line.

It is a further object of the present invention to provide a novel method and system for monitoring and testing a multiplexed data transmission network at a single point in the system carrying multiplexed data through the monitoring and/or insertion of data in a selected subscriber's channel in the multiplexed data stream without disturbing any other data in the data stream.

In accordance with the present invention, data in the communication network is accessed at the multiplexed level. The multiplexed data stream containing a predetermined pattern of framing signals and subscriber data from a plurality of subscriber channels is accessed directly by line accessing means located in the transmission path of the data stream. The line accessing means does not disturb the transmission of the multiplexed data stream, i.e., allows the multiplexed data stream to pass through the line accessing means, but provides a monitored data signal having the same data content as the multiplexed data stream, thereby essentially providing undisturbed direct access to the multiplexed data stream. A control signal designating one of the plurality of subscriber channels as the desired subscriber channel is generated and framing signals in the multiplexed data stream are detected. The detected framing signals and the generated control signal are utilized to locate the designated one of the plurality of subscriber channels in the multiplexed data stream for monitoring, testing or for other purposes.

In accordance with one embodiment of the invention, the located one of the plurality of subscriber channels is monitored so its data framing and content can be observed. For testing purposes, various codes such as a loopback code or a multipoint junction code may be inserted in the designated one of the plurality of subscriber channels in the multiplexed data stream without demultiplexing the data stream of otherwise disturbing the transmission of the stream. In this manner, loopbacks at various points in the system can be effected and test codes such as a pseudo-random code may be transmitted through the loopback and monitored for errors. The multiplexed data transmission network may therefore be tested for transmission quality and faults at the multiplexed level without disturbing transmission of subscriber data through the network. An entire multiplexed data stream comprising data from a large number of subscribers can thereby be tested from a central location without the need for time consuming operator removal and insertion of test jacks.

These and other objects and advantages of the present invention will become apparent to one skilled in the art to which the invention pertains from the following detailed description when read in conjunction with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a functional block diagram of a prior art data transmission network of the type with which the present invention is utilized;

FIG. 1B is a graphical representation of the multiplexed data stream illustrating the data format of the network of FIG. 1A;

FIG. 2 is a block diagram functionally illustrating the present invention as utilized in connection with the system of FIG. 1;

FIG. 3 is a functional block diagram illustrating the line access unit of FIg. 2 in greater detail;

FIG. 4 is a schematic circuit diagram illustrating the line access module of FIG. 3 in greater detail;

FIG. 5 is a schematic circuit diagram illustrating the LAM select switches of FIG. 3 in greater detail;

FIG. 6A is a functional block diagram illustrating the LAM select switch control unit of FIG. 3 in greater detail;

FIG. 6B is a functional block diagram illustrating the unit select circuit of FIG. 6A in greater detail;

FIG. 6C is a functional block diagram illustrating the monitor/test select circuit of FIG. 6A is greater detail;

FIG. 6D is a functional block diagram illustrating the LAM switch/terminate circuit of FIG. 6A in greater detail;

FIG. 7A is a functional block diagram illustrating the analog interface unit of FIG. 3 in greater detail;

FIG. 7B is a functional block diagram illustrating the line receiver of FIG. 7A in greater detail;

FIG. 7C is a functional block diagram illustrating the level control circuit of FIG. 7A in greater detail;

FIG. 7D is a functional block diagram illustrating the insert data circuit of FIG. 7A in greater detail;

FIG. 7E is a functional block diagram illustrating the timing extractor of FIG. 7A in greater detail;

FIG. 8A is a pictorial representation of the control and monitoring panel of FIG. 3;

FIG. 8B is a functional block diagram illustrating the control and monitoring panel of FIG. 3 in greater detail;

FIGS. 9A and 9B are functional block diagrams illustrating the control logic circuits of FIG. 3 in greater detail;

FIG. 10A is a functional block diagram illustrating the T1 and SRDM framing detectors and comparators of FIG. 9A in greater detail;

FIG. 10B is a schematic circuit diagram illustrating the T1 frame detector and comparator of FIG. 10A in greater detail;

FIG. 10C is a schematic circuit diagram illustrating the T1 framing pattern generator and detector of FIG. 10A in greater detail;

FIG. 10D is a schematic circuit diagram illustrating the frame detector and comparator of FIG. 10A in greater detail;

FIG. 10E is a schematic circuit diagram illustrating the Near/Far tri-state output buffer of FIG. 10A in greater detail;

FIG. 11A is a functional block diagram illustrating the data decoder of FIG. 9A in greater detail;

FIG. 11B is a schematic circuit diagram illustrating the rate control circuit of FIG. 11A in greater detail;

FIG. 11C is a schematic circuit diagram illustrating the byte store and decode circuit of FIG. 11A in greater detail;

FIG. 12A is a functional block diagram illustrating the error rate circuit of FIG. 9A in greater detail;

FIG. 12B is a schematic circuit diagram illustrating the time interval counter of FIG. 12A in greater detail;

FIG. 12C is a schematic circuit diagram illustrating the T1 error rate counter of FIG. 12A in greater detail;

FIG. 12D is a schematic circuit diagram illustrating the SRDM error rate counter of FIG. 12A in greater detail;

FIG. 12E is a schematic circuit diagram illustrating the sync error selector of FIg. 12A in greater detail;

FIG. 13 is a sehematic circuit diagram illustrating the sync and error data output circuit of FIG. 9A in greater detail;

FIG. 14 is a functional block diagram illustrating the control data demux and select circuit of FIG. 9B in greater detail;

FIG. 15A is a functional block diagram illustrating the T1 line subscriber channel latches and comparators of FIG. 9B in greater detail;

FIG. 15B is a schematic circuit diagram illustrating the T1 channel latch and comparator of FIG. 15A in greater detail;

FIG. 15C is a schematic circuit diagram illustrating the SRDM channel latch and comparator of FIG. 15A in greater detail;

FIG. 15D is a schematic circuit diagram illustrating the T1 line latch and comparator of FIG. 15A in greater detail;

FIG. 15E is a schematic circuit diagram illustrating the near/far test selector of 15A in greater detail;

FIG. 16A is a functional block diagram illustrating the sequence control circuit of FIG. 9B in greater detail;

FIG. 16B is a schematic circuit diagram illustrating the mode latch circuit of FIG. 16B in greater detail;

FIG. 16C is a schematic circuit diagram illustrating the test sequence control circuit of FIG. 16B in greater detail;

FIG. 16D is a schematic circuit diagram illustrating the tristate buffer of FIG. 16B in greater detail;

FIg. 17A is a functional block diagram illustrating the MJU and panel display data control circuit of FIG. 9B in greater detail;

FIG. 17B is a schematic circuit diagram illustrating the MJU command generator of FIG. 17A in greater detail;

FIG. 17C is a schematic circuit diagram illustrating the MJU message decode circuit of FIG. 17A in greater detail;

FIG. 17D is a schematic circuit diagram illustrating the panel display data output circuit of FIG. 17A in greater detail;

FIG. 18A is a functional block diagram illustrating the data output and correlator circuits of FIG. 9B in greater detail;

FIG. 18B is a schematic circuit diagram illustrating the pseudorandom test code generator of FIG. 18A in greater detail;

FIG. 18C is a schematic circuit diagram illustrating the loopback change detector of FIG. 18A in greater detail;

FIG. 18D is a schematic circuit diagram illustrating the code selector and transmitter of FIG. 18A in greater detail;

FIG. 18E is a schematic circuit diagram illustrating the slow rate clock generator of FIG. 18A in greater detail; and

FIG. 18F is a schematic circuit diagram illustrating the data correlator of FIG. 18A in greater detail.

DETAILED DESCRIPTION

The invention is described herein in connection with one embodiment, the description being organized in accordance with the following Table of Contents:

TABLE OF CONTENTS

I. general System Description (FIGS. 1 and 2)

Ii. line Access Unit (FIG. 3)

A, line Access Module (FIG. 4)

B. lam select Switch (FIG. 5)

C. lam select Switch Control Unit (FIG. 6A)

1. unit Select Circuit (FIG. 6B)

2. monitor/Test Select Circuit (FIG. 6C)

3. lam switch/Terminate Circuit (FIG. 6D)

D. analog Interface Unit (FIG. 7A)

1. line Receiver (FIG. 7B)

2. level Control Circuit (FIG. 7C)

3. insert Data Circuit (FIG. 7D)

4. timing Extractor (FIG. 7E)

Iii. control and Monitoring Panel (FIGS. 8A, 8B)

Iv. control Logic Circuits (FIGS. 9A, 9B)

A. t1 and SRDM Framing Detectors and Comparators (FIGS. 10A - 10E)

B. data Decoder (FIGS. 11A - 11C)

C. error Rate Circuit (FIGS. 12A - 12E)

D. sync and Error Data Output Circuit (FIG. 13)

E. control Data Demux and Select Circuit (FIG. 14)

F. t1 line-Subscriber Channel Latches and Comparators (FIGS. 15A - 15E)

G. sequence control circuit (FIGS. 16A - 16D)

H. mju and Panel Display Data Control Circuit (FIGS. 17A - 17D)

I. data Output and Correlator Circuits (FIGS. 18A - 18F)

I. General System Description

FIG. 1A illustrates a typical prior art multiplexed data transmission system service network of the type provided by the telephone companies and other common carriers. Referring now to FIG. 1A, a typical common carrier data transmission system includes subscriber data equipment 550 connected to a subscriber line unit (SLU) 52 located at the subscriber premises. The subscriber line unit 52 is typically connected over a four-wire loop circuit (usually in a multipair cable) to an office line unit 54 at a central telephone switching office. Each subscriber station is connected to the central office in this manner and there may be thousands of office line units at the central office, each receiving data from individual subscribers and supplying data to the individual subscribers over the four-wire loop circuit.

Each of the office line units 54 is connected to a data mulitplexer at the central office. To facilitate the description, it will be assumed that all of the subscribers transmit data to the office line units at compatible bit rates such as bit rates which are an even multiple of each other (e.g., 2.4 kilobits per second, 4.8 kilobits per second and 9.6 kilobits per second). In an actual operating system, other bit rates may also be employed and the successive multiplexing steps (illustrated by T1 hereinafter) actually employ various rates; therefore the multiplexing scheme may be slightly different from that illustrated. However, it should be understood that these different types of multiplexing schemes do not alter the basic operating principles of the present invention in any substantial manner.

With the system as described above, the office line units 54 may be connected to subrate data multiplexers (SRDM) 56 which may in turn be connected to higher T1 rate data multiplexers (T1DM) 58. It should be noted that some of the office line units 54 may be connected to the multiplexers 56 through multipoint junction units (MJU) 60 as illustrated. The multipoint junction units 60 are essentially code controllable or addressable switches which receive encoded signals from multiple office line units and selectively apply these signals to the data multiplexer 56.

In a prior art system such as that illustrated in FIG. 1A, test jacks 62 are provided in each of the four-wire loop circuits between the office line units 54 and the first multiplexer stage 56. The test jacks 62 are arranged so that a test unit can be plugged into the transmitting or receiving lines for test access to the individual subscriber lines. It should be noted that the test jacks 62 are located at points in the system at which data to or from a single subscriber is present. It will therefore be appreciated that testing of customer data channels is accomplished in the prior art systems on a customer by customer basis at the individual jacks 62.

The operation of the prior art system may be more fully appreciated with reference to the functional block diagram of FIG. 1A and to the data format diagram of FIG. 1B. Referring to FIGS. 1A and 1B, data from the subscriber data equipment 50 is transmitted through the subscriber line unit 52 to a corresponding office line unit 54 over the four-wire loop circuit connecting these units. The data is transmitted by a predetermined bit rate, e.g., 2.4 kilobits per second, and the office line units 54 presents the data in an appropriate form to the first multiplexing stage 56. For example, in one known system, the subrate data multiplexer 56 receives bytes of data over 20 different customer channels. The office line units 54 sequentially repeat each byte of data received over a 2.4 kilobit/second customer channel 20 times. Accordingly, a byte of customer data is presented 20 times at each of the input lines connected to the multiplexer 56, and the multiplexer 56 sequentially scans the input lines, taking one byte of data from each input line, adding an appropriate framing and/or status signal and transmitting a 20 byte frame of multiplexed data to the second stage multiplexer 58 at the SRDM rate.

The second or T1 rate multiplexer 58 is connected to as many as 23 substrate data multiplexers in the known system illustrated in FIG. 1A. The frames of multiplexed data received from the multiplexers 56 are conventionally multiplexed by the T1 rate data multiplexer 58 and a T1 multiplexed signal is transmitted by the T1 rate data multiplexer 58 to a distant central office. It will be appreciated by one skilled in the art that the receiving channel is essentially the same except that the received T1 information is demultiplexed to separate the 460 customer channels from the multiplexed T1 data signal.

Specifically with reference to FIG. 1B which illustrates a typical dta format for the known system of the type illustrated in FIG. 1A, the T1 rate data multiplexer 58 combines the multiplexed data from the 23 subrate multiplexers 56 to form sequential 193 bit frames of multiplexed data. The substrate multiplexers 56 organize the subscriber channels into eight bit bytes of data containing six subscriber data bits D, an SRDM frame bit F, and a status bit S. The T1DM scans the 23 SRDM's and takes one of these eight bit bytes from each of the 23 SRDM's and then adds eight houskeeping overhead bits and a T1 framing bit to form each 193 bit T1DM frame. A total of 20 of the T1DM frames together form one major data frame containing data from each of the 460 subscribers.

Of course, the above described example assumes that there are 460 subscribers in the system all operating at 2.4 kilobit per second data rate. For subscribers operating at a 4.8 kilobit per second rate, the data is similarly multiplexed but a single T1DM serves a maximum of 230 subscribers. At a data rate of 9.6 kilobits per second, the capacity of one T1DM is 115 subscribers. In addition, the higher bit rate subscribers can be combined with data from the lower bit rate subcribers but such higher bit rate subscribers will occupy more than one channel in a major data frame. For example, a 56 kilobit per second data rate subscriber will occupy one channel in each T1 frame, i.e., 20 channels in each major data frame, whereas a 2.4 kilobit per second data rate subscriber will only occupy one channel in each major data frame.

The prevalent method of testing the system illustrated in FIG. 1A is to manually plug a test set into the jacks 62 associated with a particular subscriber channel. The test set transmits special codes and various test signals over the four-wire loop circuits to prepare signal transmission quality testing and to isolate faults within the transmission system. Typically, the common carrier equipment such as the subscriber and office line units are provided with code operated switching devices for the purpose of loopback testing. A particular code transmitted from the test set will actuate the switching devices in the remote equipment (in the office line unit 54, for example) causing the remote equipment to form a closed loop ahead of and/or behind the unit. A test code may then be transmitted over this closed loop nd monitored for signal transmission quality. Other different codes may be transmitted by the test set to actuate code responsive switches in other remote equipment, e.g., in the subscriber line units. In this manner, each individual piece of common carrier supplied equipment as well as the lines connecting the equipment may be tested for faults and for signal transmission quality.

It can be seen from the foregoing that present test methods require physical access to each customer channel so that a large number of test jacks or access points are required. This may be very costly both in terms of equipment and space. Moreover, only one line at a time can be tested by this technique and a full time manual participation of at least one trained operator is required. It will also be appreciated that testing by the foregoing technique on a state or regional basis requires further operator intervention.

In contrast to test system described in connection with FIG. 1A, the present invention illustrated functionally in FIG. 2 provides automatic test access to any one of a large number of subscriber or customer channels and provides access for test over an entire city, state or region from one location. Referring now to FIG. 2, a line access unit 64 is connected in each of the four-wire loop circuits carrying T1 level multiplexed data between central offices. The T1 lines designated T1TX and T1RCV are connected to each of the T1 rate data multiplexers 58 (T1DM 1 to T1DM 16 corresponding to like mumbered LAM's) and enter the line access unit 64. These T1 lines are connected through associated line access modules (LAM) 66 described hereinafter in greater detail and exit the line access module to complete their normal paths as illustrated. To facilitate a description of the invention, the side of the line access unit 64 nearest the central office is designated the NEAR side and the other side is designated FAR. The multiplexed data received from the NEAR side is designated the NRCV data signal and this data is designated NXMT as it exits the line access unit. The multiplexed data received from the FAR side of the line access unit 64 is similarly designated the FRCV signal and this data is designated FXMT as it exits the line access unit.

Each of the line access modules 66 is connected to LAM select and interface circuits 68 which receive control signals from and supply data signals to control logic circuits 70 as is subsequentially described in greater detail. A control and monitoring panel 72 connected to the control logic circuit 70 provides operator control over the testing and monitoring of the subscriber channels, and a microcomputer 74 may be connected to the control logic circuit 70 for programmed testing and monitoring of the subscriber channels.

As will be appreciated from the description hereinafter, the line access module provides the testing and monitoring system with access to the multiplexed data stream on the transmit and receive T1 lines. The line access module 66 is in series with the multiplexed data stream and presents an impedance to the passage of the multiplexed data stream therethrough. An output signal having voltage levels corresponding to the data signal level in the multiplexed data stream is thereby developed at the transmit and receive data input sides of the line access module 66, and the developed signal is available for application to the LAM select and interface circuit 68 as a monitor signal for use as is described hereinafter.

The line access module 66 contains various switches described hereinafter in greater detail. These switches may be selectively actuated by control signals from the LAM select and interface circuits 68 to permit the system to perform various monitoring and testing functions. For example, in a monitor mode of operation, the multiplexed data stream passes through the line access module 66 unaltered insofar as data content is concerned. It should be understood that the multiplexed data stream may be attenuated or amplified depending upon whether or not the line access module 66 contains active amplification circuits. However, this does not affect the data content of the multiplexed data stream and therefore transmission of the multiplexed data stream is undisturbed.

In the monitor mode of operation, the data signals developed across the line access module 66 at the near and far ends are applied to the LAM select and interface circuits 68. The framing signals in the multiplexed data stream are detected so that the location of each subscriber channel in the multiplexed data stream can be ascertained. Either the microcomputer 74 of the control and monitoring panel 72 provides a signal designating one of the subscriber channels as the channel to be monitored. The control logic circuits 70 locate the designated one of the subscriber channels in the multiplexed data stream and the data contained in that channel is supplied to either the control and monitoring panel 72 or the microcomputer 74 for monitoring purposes. It should be noted that during this entire monitoring procedure the multiplexed data stream passes through the line access module 66 without disturbance of the data content.

In a test mode of operation, appropriate switches in any selected one of the line access modules 66 open the data path through the selected line access module 6 and reroute the multiplexed data stream through a gating circuit in the LAM select and interface circuit 68. For example, the T1TX line through the line access module 66 is opened and appropriately terminated by switches responding to the control signals from the LAM select and interface circuits 68. The multiplexed data stream NRCV entering the line access unit on the near T1TX line is routed to the gating circuit in the LAM select and interface circuits 68 and this multiplexed data stream is then returned as the NXMT signal to the far T1TX line by the gating circuit in the LAM select and interface circuits 68.

The control logic circuits 70 locate the desired subscriber channel in response to the framing signals and the channel designating signals as was previously mentioned. The multiplexed data stream passes through the gating circuit in the LAM select and interface circuits 68 without disturbance until a desired test is initiated. When, for example, a test requiring a loopback is initiated at the control and monitoring panel 72 or by the microcomputer 74, the gating circuit in the LAM select and interface circuit 68 blocks the passage of the byte of data in the designated subscriber channel of the multiplexed data stream. A loopback code or other desired test code is inserted in place of the subscriber data in the subscriber channel being tested. This insertion of a test code in the multiplexed data stream does not disturb the data stream except at the location of the designated subscriber channel in the data stream. In the next major frame of multiplexed data, another test code such as a pseudo-random code may be inserted in the same manner as the loopback code and in subsequent data frames the loopback and psuedo-random codes may be alternatively inserted at the location of the designated customer channel in the multiplexed data stream.

The insertion of the loopback code in the subscriber channel will cause a loopback at a desired location in the designated subscriber's transmission network. The transmitted psuedo-random test code will follow this loopback path and return to the line access unit 64. The test code, being in the multiplexed data stream at the location of the designated subscriber channel, can thus be monitored to determine if the transmission network is operating properly throughout the closed loop formed in the network.

Again, it can be seen that the multiplexed data stream is accessed and a designated subscriber channel is located without disturbing the transmission of the multiplexed data stream in the test mode of operation, as in the monitor mode of operation, even through the data stream is diverted through the LAM select and interface circuits 68. The data content of the multiplexed data stream also remains undisturbed except for the designated subscriber channel. As will be seen hereinafter, the system may provide for the monitoring of the status of a subscriber channel prior to initiating any testing procedure to insure that the tested subscriber channel is not carrying subscriber data when a test is initiated, thereby further insuring that subscriber data is not disturbed by the testing system.

II. Line Access Unit

As was previously mentioned, each of the line access modules 66 is connected in one of the T1 level data