Digital communication apparatus and method

What is claimed is:

1. A communications system having a master terminal, a remote terminal, and at least one remote repeater, the master terminal transmitting along a transmit data path information serially through said repeater to said remote terminal and receiving along a receive data path information serially through said repeater from said remote terminal, such information comprising a plurality of successive frames, each frame having a framing bit, the communication system comprising:

means disposed within said remote repeater, coupled to said transmit data path for receiving a plurality of said framing bits;

means, disposed within said remote repeater, for transferring information from said master terminal transmitted on said transmit data path back to said master terminal on said receive data path when said received plurality of framing bits has a predetermined pattern relative to a prestored reference pattern.

2. The communications system as recited in claim 1 further comprising:

means disposed within said master terminal and coupled to said transmit path for transmitting a plurality of test information to said remote repeater transmitting said test information back to said master terminal via the receive data path.

3. A communication system having a master terminal, a remote terminal, and at least one remote repeater, the master terminal transmitting along a transmit data path information serially through said repeater to said remote terminal and receiving along a receive data path information serially through said repeater from said remote terminal, such information comprising a plurality of successive frames, each frame having a framing bit, the communication system comprising:

means disposed within said remote repeater and coupled to said transmit data path for receiving a plurality of said framing bits, said plurality of framing bits produce a predetermined sequence;

means, disposed within said remote repeater, for comparing said predetermined sequence of framing bits to a stored reference pattern and for producing a control signal when said reference pattern and said plurality of framing bits that produce a predetermined sequence have a predetermined relation with the other; and

means disposed within said repeater and responsive to said control signal for transferring information, transmitted on said transmit data path from said master terminal, back on said receive data path to said master terminal.

4. The communication system as recited in claim 3 further comprising:

means disposed within said master terminal and coupled to said transmit data path for transmitting along said transmit data path said plurality of framing bits that produce said predetermined sequence along said transmit data path to said remote terminal; and

means disposed within said master terminal and coupled to said transmit path for transmitting a plurality of test information to said remote repeater, said repeater transmitting said test information back to said master terminal via the receive data path.

5. The communication system as recited in claim 4 further comprising:

means disposed within said master terminal, for receiving said test information along said receive data path transmitted by said test information transmitting means to isolate faults along said transmit and receive data paths.

6. A digital carrier communication system having a master terminal, remote terminal, and at least one serially coupled remote repeater, said master terminal being adapted to transmit along a transmit data path through said repeater to said remote terminal and receive along a receive data path through said repeater from said remote terminal time multiplexed information in a T1 digital bit stream format, the format of the T1 digital bit stream comprising a plurality of frames, each frame comprising a plurality of time slots and one framing bit, such system comprising:

means disposed within said repeater and coupled to said transmit path for receiving a plurality of said framing bits;

means disposed within said repeater for producing a control signal when said plurality of framing bits bears a predetermined relationship to a reference pattern stored within said repeater; and

means disposed within said repeater and responsive to said control signal for transferring information transmitted on said transmit data path from said master terminal back to said master terminal on said receive data path.

7. The digital carrier communications system recited in claim 6 wherein said master terminal further comprises:

means for transmitting said plurality of framing bits in a T1 format along said transmit path serially to said repeater when said plurality of framing bits bear a predetermined relationship to said reference pattern;

means coupled to said transmit path for transmitting a plurality of test information in a T1 data format to said repeater, said repeater transmitting said test signal back to said master terminal via the receive data path.

8. The digital carrier communications system as recited in claim 5 wherein said master terminal comprises:

means for receiving said test information, transmitted by said test information transmitting means along said receive path to isolate system faults.

9. The method of testing a communications link having respectively separated uni-directional data paths, to localize the section of said link having transmission problems, said communications link comprising a master terminal, a remote terminal and at least one remote repeater, the master terminal transmitting along a transmit data path information serially through said repeater to said remote terminal and receiving along a receive data path information serially through said repeater from said remote terminal, comprising the steps of:

transmitting, by said master terminal along said transmit path, a plurality of data frames serially to said repeater, said data frame comprises a framing bit and a plurality of data channels, a plurality of said framing bits produce a predetermined sequence;

receiving said predetermined sequence of framing bits by said remote repeater along said transmit path;

comparing said received predetermined sequence of framing bits to a stored reference pattern;

producing a control signal by said remote repeater when said stored reference pattern and said predetermined sequence of framing bits have a predetermined relation with the other; and

transmitting by said remote repeater all signals transmitted along said transmit path to said remote repeater from said master terminal, back to said master terminal via the receive data path in response to the produced control signal.

10. The method of testing a communication link as recited in claim 9 further comprising the steps of:

transmitting test information by said master terminal along said transmit path to said remote repeater;

receiving along said transmit data path by said remote repeater said test information transmitted along said transmit path by said master terminal;

transmitting along said receive path said test information received by said remote terminal to said master terminal;

receiving along said receive path said test information transmitted by said remote repeater to said master terminal;

comparing said received test information by said master terminal with said test information transmitted by said master terminal to isolate faults along said communications link;

transmitting a second predetermined sequence of framing bits from said master terminal along said transmit path to said remote repeater;

receiving said second predetermined sequence of framing bits by a means disposed within said remote repeater coupled to said transmit data path;

comparing said received second predetermined sequence of framing bits to a predetermined reference pattern;

producing a second control signal when said reference pattern and said second predetermined sequence of framing bits have predetermined relation with the other; and

transmitting by said remote repeater to said master terminal all information transmitted to the remote terminal from said remote terminal, in response to said second produced control signal.

11. The method of testing a communications link as recited in claim 9 comprising the additional step of:

transmitting all data frames by said master terminal in a time multiplexed T1 digital bit stream.

12. The method of testing a communication link as having respectively separated uni-directional transmit and receive paths, to localize communications link comprising a master terminal communicating to a remote terminal through one or more serial coupled remote repeaters interposed in and between said paths, comprising the steps of:

transmitting by said master terminal a plurality of superframes of data in a T1 format serially to the remote terminal along said transmit path, said superframes of data comprise a plurality of data frames, said data frame comprises a plurality of data channels and a framing bit, said framing bits disposed within said superframes of data combine to form a plurality of sync bits and a predetermined sequence;

detecting said predetermined sequence of framing bits by said receiver section disposed within said remote repeater;

changing said remote repeater to a loopback condition in response to said predetermined sequence of framing bits bearing a predetermined relationship to a reference pattern stored within said remote repeater;

transmitting a test information from said master terminal along said transmit path through said link to said remote repeater and back to said master terminal along said receive path to verify said communications link is operational;

transmitting a second plurality of superframes of data in a T1 format from said master terminal through said remote repeater along said transmit path, said second plurality of superframes of data contains a predetermined sequence of framing bits;

detecting said predetermined sequence of framing bits by said receiver section disposed within said remote repeater; and

charging said remote repeater to a reset condition in response to said predetermined sequence of framing bits bearing a predetermined relationship to a reference pattern stored within said remote repeater.

13. A repeater station apparatus for use in a transmission system passing time multiplexed information, the apparatus comprising:

first and second regenerators for receiving and transmitting time multiplexed information on respectively separate unidirectional paths of the system, said information having a plurality of frames, each frame having a framing bit;

means responsive to said time multiplexed information for comparing a plurality of said framing bits of said time multiplexed information received on one of said unidirectional paths with a predetermined stored reference pattern and for generating a control signal in accordance with such comparison; and

means responsive to said control signal for transferring information on said one unidirectional path onto the other unidirectional path of the system.

14. A repeater station apparatus for use in a digital transmission system passing time multiplexed information, comprising a plurality of successive superframes, each superframe having a plurality of frames, each frame having a framing bit, the plurality of said framing bits producing said address, the apparatus comprising:

two digital regenerators for respectively separate unidirection paths of the system;

means for comparing the address portion of said time multiplexed information sent along one of the unidirectional paths with a predetermined stored reference pattern and for generating a control signal in accordance with such comparison; and

means responsive to said control signal for enabling a loopback condition between the unidirectional paths of the system.

15. The apparatus as recited in claim 14 wherein said time multiplexed information is sent in a T1 format.

16. The apparatus as recited in claim 15 wherein a combination of said framing bits disposed within said time multiplexed information produces a plurality of sync bits.

17. A repeater station apparatus for use in a digital transmission system passing time multiplexed information, where a combination of even framing bits disposed within said time multiplexed information produces a plurality of sync bits and a combination of odd framing bits disposed within said time multiplexed information produces said address, the apparatus comprising:

two digital regenerators for respectively separate unidirection paths of the system;

means for comparing the address portion of said time multiplexed information sent along one of the unidirectional paths with a predetermined stored reference pattern and for generating a control signal in accordance with such comparison; and

means responsive to said control signal for enabling a loopback condition between the unidirectional paths of the system.

18. The apparatus as recited in claim 14 wherein said framing bits that produce said address occur in predetermined framing bit locations of successive superframes.

19. The apparatus as recited in claim 13 comprising means for comparing a control pattern of said time multiplexed information with said predetermined stored reference pattern and for generating said control signal in accordance with such comparison.

20. A repeater station comprising:

means for receiving multiplex information from a source for retransmission, said information having a plurality of sequential frames, each having a framing bit; and

means responsive to a predetermined bit pattern of said received framing bits for transmitting received multiplex information back to said source.

Description Submit all comments and votes

Background of the Invention

This invention relates generally to digital communication apparatus and methods and more particularly to digital communication apparatus and methods for testing repeaters used in such apparatus.

As is known in the art, a communications system generally includes a central office (master terminal), remote office (remote terminal), and one or more repeaters disposed between the central office and the remote office to regenerate signals passing therethrough and to thereby extend the transmission distance between the offices. Typically, the distance between a pair of repeaters is about one mile while the distance between the central and remote offices is typically 10 to 12 miles.

As is also known in the art, one type of digital communication system is a T1 communication system. A typical T1 system has a pair of transmit lines through which the master terminal transmits information to the remote terminal and a pair of receive lines by which the remote terminal transmits to the master terminal. A T1 system also has an additional pair of test wires that parallel the transmit/receive lines. The purpose of the test wires will be explained later. Suffice it to say here, however, that with such a system, information is transmitted as a series of superframes. Each superframe is made up of 12 frames, F.sub.1 -F.sub.12 as shown in FIG. 1. (Each frame has a time duration of 125 microseconds). Each one of the frames F.sub.1 -F.sub.12 in turn is made up of 24 time slots or channels, C.sub.1 -C.sub.24. Each time slot C.sub.1 -C.sub.24 has up to 8 bits of data or voice information, as shown for exemplary time slot C.sub.2 as C.sub.2 b.sub.1 -C.sub.2 b.sub.8. Each one of the frames, F.sub.1 -F.sub.12, also includes a framing bit FB.sub.1 -FB.sub.12, respectively, as shown in FIG. 2. Thus, exemplary frame F.sub.2 is shown to have a framing bit FB.sub.2 as shown in FIGS. 1 and 2. Thus, each one of the 12 frames F.sub.1 -F.sub.12 is made up of 193 bits; 192 bits of data or voice information and one framing bit. The framing bits FB.sub.1 -FB.sub.12 are used for synchronization at a receiver located at a remote terminal. More particularly, the 12 framing bits FB.sub.1 -FB.sub.12 transmitted in each superframe for a standard T1 system make up a predetermined 12 bit binary word or sychronization pattern. The order of the sequence of the framing bits FB.sub.1 -FB.sub.12 in a standard T1 system which forms the synchronization pattern is as follows: For the odd numbered framing bits FB.sub.1, FB.sub.3, FB.sub.5 , FB.sub.7, FB.sub.9, the sequence is alternating 1, 0, 1, 0, 1, 0, respectively. For the even numbered framing bits FB.sub.2, FB.sub.4, FB.sub.6, FB.sub.8, FB.sub.10, FB.sub.12, the sequence of framing bits is 0, 0, 1, 1, 1, 0, respectively. When the receiver detects this 12 bit synchronization pattern, the receiver is placed in synchronization with each superframe.

As is also known, some T1 systems use an extended superframe, frame, as shown in FIG. 3. Each extended superframe is made up of 24 frames, EF.sub.1 -EF.sub.24, each one of the frames EF.sub.1 -EF.sub.24 is made up of 24 time slots or channels and a framing bit. Each time slot has up to 8 bits of data or voice information. Thus, each one of the frames EF.sub.1 -EF.sub.24 in an extended superframe includes 192 bits of data or voice information and one framing bit, EFB.sub.1 -EFB.sub.24. The 12 framing bits in the even numbered frames in each extended superframe, i.e. ESF.sub.N FB.sub.2, ESF.sub.N FB.sub.4, ESF.sub.N FB.sub.6...ESF.sub.N FB.sub.24 make up a predetermined 12 bit binary word or synchronization pattern of an extended standard superframe with the following exceptions: First, an extended superframe contains 24 frames EF.sub.1 -EF.sub.24, and second, the sequence of even numbered framing bits ESF.sub.N FB.sub.2, ESF.sub.N FB.sub.4, ESF.sub.N FB.sub.6...ESF.sub.N FB.sub. 24 provide a 12 bit binary word or synchronization pattern and thus provide an equivalent function as the total 12 framing bits FB.sub.1 -FB.sub.12 as in the standard T1 system as shown in FIG. 2. Thus, the sequence for framing bits ESF.sub.N FB.sub.4, ESF.sub.N FB.sub.8, ESF.sub.N FB.sub.12, ESF.sub.N FB.sub.16, ESF.sub.N FB.sub.20, ESF.sub.N FB.sub.24 will be 0, 0, 1, 0, 1, 1. When the receiver detects this 6 bit word or pattern, the receiver is placed in synchronization with each extended superframe.

Thus, in a T1 system using data transmitted in either the standard superframe format or the extended superframe format, the receiver is interrogating sequence of framing bits to identify a pattern to enable synchronization of each superframe or extended superframe, respectively.

As further known, bipolar formatted data is transmitted at a fixed rate in a T1 system. That is, in a bipolar system, each time a logical 1 information bit is transmitted, the voltage polarity of the successive logic 1 information bit is reversed from the polarity of the preceding logic 1 information bit, as shown for a typical pattern of 0, 1, 1, 0, 0, 1, 0 in FIG. 4. A bipolar violation consists of a signal sent on the two consecutive pulses of the same polarity.

One technique used to test a selected one of 12 repeaters in a T1 system is a tone monitoring method using a test set disposed within the central office. As described in reference Rural Electrification Administration, Telecommunications Engineering & Construction Manual, Section 956, Vol. No. 1, Sept. 1982 (REA manual), a T1 system has only 12 variable bi-polar violation frequencies. Consequently, the tone monitoring method allows only a maximum of 12 repeaters to be tested, one repeater per violation frequency.

As further described in the REA manual, the test set consists of a pulse generator and a voice frequency selective voltmeter (receiver). The pulse generator output is connected to the transmit lines, and provides the transmit line driving signal. The test set receiver is connected to test wires; this is a voice frequency cable pair. The pulse generator output is 1.544 Mb/s line signal consisting of trios of pulses with a large quantity of bipolar violations. These are transmitted in specific patterns of positive trios (positive-negative-positive) and negative trios (negative-positive-negative) as illustrated by the brackets in FIGS. 4A and 4B. Thus, FIG. 4A illustrates one positive violation between a pair of trios followed by one negative violation between a pair of trios, while FIG. 4B shows three positive violations followed by three negative violations between four trios. The low frequency characteristics of these signals are contained in the rate by which the pattern alternates from positive trios to negative trios. If this signal is passed through a voice frequency filter within each repeater, the 1.544 Mb/s bit stream would be eliminated and a voice frequency signal would remain on the test wires.

For example, FIG. 4A shows a test pattern waveform having a fundamental frequency component f.sub.1 and FIG. 4B shows a waveform having a fundamental frequency component f.sub.3. Further, each one of the 12 repeaters has a narrowband filter tuned to a corresponding one of the 12 fundamental frequency components produceable with the 12 possible violation patterns. Thus, one repeater of the 12 repeaters is tuned to frequency f.sub.1 and another is tuned to frequency f.sub.3. The filter passes the fundamental frequency to an amplifier, and the amplified signal thus provides a test tone to the central office test set on the pair of test wires.

During test operation, the test set measures the test tone. When the test set measures the test tone and detects the proper frequency, the repeater under test is known to be correctly operating.

While the technique is useful in some applications, it has certain problems. First, a tuned filter must be installed in each repeater. The added filter increases repeater cost. Second, the coupled tone from each filter must be carried on an extra pair of test wires that parallel the transmit and receive lines. Extra wires add additional expense to the system. Third, the amplitude cf the tone is attenuated through the cable. At long cable lengths, the tone is attenuated sufficiently to be highly susceptible to distortion, making it harder to be detected. This detection is more difficult with long cable lengths. Therefore, the further the repeater is from the test set, the lower the reliablility of a repeater test. Additionally, the tone is also susceptible to noise induced from the environment. This noise alters the characteristics of the tone by causing a faulty test tone having a large amount of noise riding on top of the test tone. A greatly noisy test tone received by the test set may make the test invalid.

As is also known in the art, in digital communication systems, a repeater can be tested by being placed in loop-back mode as described in U.S. Pat. Nos. 4,564,933 and 4,630,268. As described in U.S. Pat. No. 4,564,933, in response to a signal from a central office, a loop is established at a repeater, which returns signals sent on the "send" path back to the same station on the "receive" path. Thus, by looping back at various repeaters, the location of faults may be established. Such loop-back may be used not only to establish, for example, cable breaks or serious faults in repeaters, but may preferably also be used to monitor remotely the quality of the system performance and thereby give early warning of system degradation. To use this method in a T1 system, however, requires imbedding a predetermined sequence of characters in a data stream to set the system to a loop-back condition with the concommitant disadvantage that an unwarranted user may be able to put the system into a loopback condition by inserting the predetermined sequence of characters into the data stream, or more likely, the system may be mistakenly or inadvertently placed in a "loop-back" condition if this predetermined sequence of characters occurs randomly in a data stream in normal operation.

A second method to set a repeater to a loop-back mode is described in U.S. Pat. No. 4,630,268. This method sets the repeater to a loop-back condition upon detecting a change in frequency of the transmission rate. This method is not however suitable for use in a T1 system because a T1 system requires that information be transmitted at a fixed transmission rate. Existing repeater and remote terminal equipments in a T1 system contain internal oscillators which are tuned to and synchronized at 1.544 MHz. When a T1 repeater or remote terminal receives data, it detects each information bit and regenerates the received information bit at a preset time and shape. If the information were transmitted at a different bit rate, the electronics within the repeater could not regenerate the information.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved communications system.

It is an object of the present invention to provide a communications system that allows loop-back of all test signals without the requirement for extra test lines.

It is another object of the invention to provide testing of the communications system without requiring a multifrequency test signal to operate in existing T1 communication systems.

It is an object of this invention to provide a T1 communications system that allows for testing of greater than 12 repeaters.

It is another object of the invention to provide testing of the communications system without the system inadvertantly going into a loop-back condition.

In accordance with the present invention, a communications system is provided having a master terminal, a remote terminal, and at least one remote repeater, the master terminal transmitting along a transmit data path information serially through the repeater to the remote terminal and receiving along a receive data path information serially through the repeater from the remote terminal. The information comprises a plurality of successive frames, each frame having a framing bit. The communications system comprises a means disposed within the remote repeater and ccupled to the transmit data path for receiving a plurality of the framing bits. A means is disposed within the remote repeater, for transferring information from the master terminal transmitted on the transmit data path back to the master terminal on the receive data path in response to a predetermined pattern of the received framing bits. With such arrangement, the available transmit receive lines may be used during the test mode. The system can be used to test a T1 system using the fixed T1 system data rate, and because the test pattern code placing the system in a loop-back mode is in the framing bits rather than in the data bits, inadvertent placement of a repeater in the test mode is eliminated.

Also provided is the method of testing a communication link as having respectively separated uni-directional transmit and receive paths, to localize ccmmunications link comprising a master terminal communicating to a remote terminal through one or more serial coupled remote repeaters interposed in and between the paths. The method is practiced by transmitting by the master terminal a plurality of frames of data in a T1 format serially to the remote terminal along the transmit path, the data frame comprises a plurality of data channels and a framing bit, the framing bits disposed within the frames of data combine to form a plurality of sync bits and a predetermined sequence. The next step is detecting the predetermined sequence of framing bits by the receiver section disposed within the remote repeater. Subsequently, changing the remote repeater to a loopback condition in response to the predetermined sequence of framing bits bearing a predetermined relationship to a reference pattern stored within the remote repeater. Further, transmitting test information from the master terminal along the transmit path through the link to the remote repeater and back to the master terminal along the receive path to verify the communications link is operational. Then transmitting a second plurality of frames of data in a T1 format from the master terminal through the remote repeater along the transmit path wherein the second plurality of frames of data contains a predetermined sequence of framing bits detecting the predetermined sequence of framing bits by the receiver section disposed within the remote repeater. Finally, changing the remote repeater to a reset condition in response to the predetermined sequence of framing bits bearing a predetermined relationship to a reference pattern stored within the remote repeater.

This invention allows the following features over the prior art. First, no additional pairs of test wires are required because there is no need to have an additional line to monitor a test signal. Second, the test signals are digital; therefore there is no degradation of the test signals due to extra cabling. Third, to test the system no frequency shifting of the digital signal is required. Data is transmitted at a fixed bit rate. Fourth, a large quantitv of repeaters may be tested; the only limit to the number of repeaters to be tested is the number of digital bits encoded in a predetermined pattern which initiates or disconnects the loop-back. Fifth, the equipment initiating the predetermined pattern may be remotely located from the physical line and repeaters being tested. The invention allows greater than 27 lines or repeaters to be tested from a remote test site. Sixth, the predetermined pattern can be encoded in framing bits and, for all practical purposes, not accessible to the user, thereby preventing inadvertent loop-back of a repeater by the normal bit stream data.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of this invention, as well as the invention itself, may be more fully understood from the following detailed description, read together with the accompanying drawings, in which:

FIG. 1 is a diagram of the data and framing bits of a T1 superframe;

FIG. 2 is a diagram of the framing bits of a standard T1 superframe;

FIG. 3 is a diagram of an extended T1 superframe and its components;

FIG. 4 is a seven bit T1 pulse stream without any bipolar violations;

FIG. 4A is a T1 pulse stream having a sequence of positive and negative bi-polar violations, a test tone having a waveform produced as a result of filtering the fundamental frequency, the test tone frequency being f.sub.1 ;

FIG. 4B is a T1 test pulse stream having a sequence of positive and negative bi-polar violations, a test tone having a waveform produced as a result of filtering the fundamental frequency, the test tone frequency being f.sub.3 ;

FIG. 5 is a system diagram of a communication system in accordance with the invention;

FIG. 6 is a block diagram cf an Enhanced T1 repeater;

FIG. 7 is a block diagram cf the loop-back detection circuitry with FIG. 6; and

FIG. 8 is a flow chart of the loop-back detection software.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiment of the invention as disclosed comprises a T1 digital communications system wherein a plurality of bi-directional, time division multiplexed information signals, each having a series of frames and predetermined number of binary digits per frame for transmission of voice and data services through repeaters between a central office and a remote terminal location are modified so that a repeater will loop back data to the source of the transmissions to test the communications system. Although a T1 system is discussed, the invention can be used in systems with faster bit rates such as T2, T3, CCITT ICM (2.048 MHz), etc.

Referring to FIG. 5, a block diagram of a T1 communications system is shown. The communications system has a master terminal 10 serially connected through repeaters 16a-16n to remote terminal 18. Typically, the master terminal 10 is a central office or other location where the information signal originates. The remote terminal 18 is located close to customer sites. Disposed within or remotely connected to the master terminal 10 is a test set 24. The test set 24 is used when verifying the operation of the communications system in a manner to be described.

The master terminal 10 communicates with remote terminal 18 through a plurality of repeaters 16a-16n and a plurality of data lines (i.e. transmit data lines 12a-12n, respectively, and receive lines 14a-14n, respectively). Each data line, for example, data line 12a, has two wires, 12a' and 12a" as shown in FIG. 6. The span of data lines 12a-12n and data lines 14a-14n is nominally one mile. A typical distance between the master terminal 10 and the remote terminal 18 is ten to twelve miles. The voltage on one wire 12a' in a pair 12a always compliments the other wire 12a" in the pair 12.

The internal structure of each of repeaters 16a-16n between master terminal 10 and remote terminal 18 are identical. An exemplary one thereof, here repeater 16a, is shown in detail to include a repeater transmit buffer 30a, repeater receive buffer 32a and loop-back detector 34a. Repeater 16a also includes internal data line 36a, internal line 38a, loopback enable line 40a and internal data line 42a. Internal data line 36a connects repeater transmit buffer 30a to loopback detector 34a. Internal data line 38a connects repeater transmit buffer 30a to repeater receive buffer 32a. Loop-back enable line 40a and internal data line 42a connect loop-back detector 34a to repeater receive buffer 32a.

In normal operation, repeater transmit buffer 30a receives information from master terminal 10 via transmit data line 12a. Repeater transmit buffer 30a then regenerates the received information, and then retransmits the information on transmit data line 12b through repeaters 16b-16n to remote terminal 18. The remote terminal 18 receives information along transmit data line 12n and does the following: First, the remote terminal 18 stores the information. Second, upon storing information, the remote terminal 18 scans the stored information to detect the sync pattern disposed within framing bits FB.sub.1 -FB.sub.12 or ESF.sub.N FB.sub.2, ESF.sub.N FB.sub.4...ESF.sub.N FB.sub.24. Third, upon detecting the sync pattern, the remote terminal 18 determines the beginning of the superframe. Finally, once the superframe beginning has been determined, the remote terminal 18 processes the information received on line 12n.

In normal operation, the remote terminal 18 transmits information to master terminal 10 through repeaters 16n-16a via data lines 14n-14a. Information transmitted by remote terminal 18, after leaving repeater 16b, enters repeater 16a via line 14b. In repeater 16a, the information is sent into repeater receive buffer 32a. Repeater receive buffer 32a regenerates the information, and then transmits it onto line 14a to master terminal 10.

In repeater 16a, loop-back detector 34a monitors and receives information on line 36a passing through repeater transmit buffer 30a. When loop-back detector 34a receives a predetermined sequence of information from consecutive sets of framing bits SF.sub.N FB.sub.1 -SF.sub.N FB.sub.12 or EF.sub.2, EF.sub.4...EF.sub.24, the loop-back detector 34a passes information on line 12a through repeater transmit buffer 30a to repeater receive buffer 32a via lines 38a, 42a. Repeater receive buffer 32a prevents the information on line 14b from going into repeater receive buffer 32a. The details of the loop-back detector 34a operation and the predetermined sequence of framing bits will be explained later in connection with FIG. 7.

When testing the communications link that uses a standard T1 superframe, a test set 24 is placed within the master terminal 10 or is located at a remote location and electrically connected to master terminal 10. The test set 24 is coupled to the transmit data line 12a and the receive data line 14a. Test set 24 sends information along the transmit data line 12a to repeaters 16a-16n. This information on line 12a contains a normal sequence of framing bits to assure that all loop-back detectors 34 (FIG. 6) establish T1 line sychronization. Upon initiation of the test, the t