DIGITAL NETWORK SYSTEM HAVING ARRANGEMENT FOR TESTING DIGITAL SUBSCRIBER LINE

Abstract: A digital network system having an arrangement for test­ ing a digital subscriber line includes a switching device (1), an exchange terminal device as a subscriber circuit connected with the switching device (1), a subscriber line (4) connected with the exchange terminal device, a network terminal device (5) at a subscriber's side connected between the subscriber line (4) and terminal equipment devices (61, ..., 6n). The net­ work terminal device includes a turn-around portion (56) for forming a turn-around loop through which the turning-­ around of a signal to the subscriber line (4) connected with the network termination device (5) is carried out, and a discrimi­ nation portion (57) for detecting that data transmission can be carried out and cancelling the formation of the turn-around loop. The turn-around loop is formed when a frame synchro­ nization bit signal from the exchange terminal device is re­ ceived by the network terminal device (5) and cancelled when the discrimination portion (57) detects that data transmission can be carried out. Communication for either a monitoring of transmission or a bit error testing can be carried out between the network terminal device (5) and the switching device (1) while the turn-around loop is formed, and data transmission between the exchange terminal device and the terminal equipment devices (61, ..., 6n) can be carried out after the turn-around loop is cancelled.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital network system having an arrangement for testing a digital subscriber line. The digital network system provides a digital subscriber line testing system for carrying out a transmission test of digital subscriber lines between a subscriber circuit of an exchange station in an integrated services digital network (ISDN) or the like and terminal equipment at customer station lines. 2. Description of the Related Arts In general, in the integrated services digital network, several kinds of bus type terminal equipment are connected to the terminal equipment of customer station lines, and digital subscriber lines are connected between the line terminal equipment and the subscriber circuit of a subscriber switch. It is necessary to check these digital subscriber lines to determine whether or not the transmission quality is that desired, as in the case of ordinary transmission lines. The integrated services digital network (ISDN) are being standardized under the direction of the International Telegraph and Telephone Consultative Committee or CCITT, as described above, and accordingly, the line between a subscriber circuit of a switch device and terminal equipment of customer station lines is connected by a digital subscriber line. As a form of transmission data, for instance, a (2B + D) format is selected that is composed of a channel D of 16 Kb/s and a channel B of 64 Kb/s and both a bit of a frame synchronization signal and a control bit (or a housekeeping bit) are added to constitute one frame. If the control bit is used only for starting the line terminal equipment, only one bit will be sufficient for such a control bit. Where a turn-around loop is formed with the line terminal equipment, and a transmission test of the digital subscriber line is carried out, a control bit is necessitated to instruct the formation and removal of the above-described turn-around loop, and thus there is a demand for at least two bits as the control bit. Such a system is described in the Article "Maintenance System for Digital Local Network" by R. Shimakage et. al. Review of the Electrical Communication Laboratories, Vol. 32, No. 2, Part 2, March 1984, pp 307-315. In the digital subscriber line, desirably the effect of crosstalk upon other lines is greatly reduced, relying upon the decline of a transmission bit rate as much as possible, and the transmission line section is lengthened. For this reason, the amount of all control data other than the inherent data must be minimized. As described before in connection with the prior art digital subscriber line transmission, there is thought to be a need for at least two bits as a control bit, but in accordance with the present invention, it is possible to carry out the formation and elimination of a turn-around loop by means of just one bit for the control bits, and to execute a transmission test of the digital subscriber line. SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved digital network system having an arrangement for testing a digital subscriber line in which the formation and cancellation of a turn-around loop by a control bit of one bit are carried out so that the transmission test of the digital subscriber line can be achieved. The object of the present invention is solved by a digital network system having an arrangement for testing a digital subscriber line, said system comprising a switching device, an exchange terminal device as a subscriber circuit connected with said switching device, a subscriber line connected with said exchange terminal device, a network terminal device at a subscriber's side connected between said subscriber line and terminal equipment devices; said network terminal device comprising a turn-around portion for forming a turn-around loop through which the turning-around of a signal to the subscriber line connected with said network terminal device is carried out; whereby communication for either a monitoring of transmission or a bit error testing can be carried out between the network terminal device and the switching device while said turn-around loop is formed and data transmission between the exchange terminal device and the terminal equipment devices can be carried out after the turn-around loop is cancelled; characterized by said network terminal device including a discrimination portion for detecting that data transmission can be carried out and cancelling the formation of said turn-around loop; said turn-around loop being formed when a frame synchronisation bit signal from the exchange terminal device is received by the network terminal device and cancelled when the discrimination portion detects a control bit signal from the exchange terminal device. In accordance with the present invention, there is provided a turn-around portion in a network terminal equipment of a customer station, by which a turn-around loop is formed on the subscriber circuit side of a switching device until data transmission can be carried out, to loop back the testing data from the subscriber circuit. When a state wherein data transmission is enabled exists, the turn-around loop is eliminated to carry out data transmission, thus enabling a transmission test of the digital subscriber line without employing a particular control signal. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of the digital network system to which the present invention is applied; Fig. 2 is a block diagram showing the structure of the network terminal equipment in the system of Fig. 1; Fig. 3 shows a structure of one frame for explaining a transmission format; Fig. 4 shows a sequence of signal transmission, particularly a sequence of terminating and clear-back; and Fig. 5 shows a sequence of the bit error ratio test. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 is a schematic diagram of the digital network system to which the present invention is applied. To the right of a subscriber line in Fig. 1, there is arranged an exchange station, and to the left, a customer station. 1 denotes a switching device, 2 on exchange terminal (ET), and 3 a subscriber circuit. A bit error ratio (BER) tester 11 and a central controller 12 are connected to the switching device 1. The subscriber circuit 3 is connected to a network terminal equipment 5 via a subscriber line 4. The network terminal equipment 5 is composed of a TE reception and transmission portion 51, an ET reception and transmission portion 52, a data stream conversion portion 53, a sequence control circuit 54, a SYN bit detection circuit 55, a turn-around portion 56, and a discrimination portion 57. Details of the network terminal equipment are described in Fig. 2. A plurality of terminal equipment (TB) 61 to 6n is connected with the network terminal equipment 5. The network terminal equipment 5 connected to the subscriber circuit 3 and an exchange terminal 2 of the switching device or an exchange 1 via the digital subscriber line 4, is provided with a turn-around portion 56 which forms a turn-around loop to the subscriber circuit 3 side and a discrimination portion 57 which discriminates the line status to determine whether or not data transmission can be carried out, thus eliminating the turn-around loop. Until the data transmission can be carried out, the testing data from the bit error ratio tester 11 or the like is turned around via the turn-around loop, which is constituted at the turn-around portion 56 to carry out the transmission test of digital subscriber lines 4. When the discrimination portion 57 determines that data transmission can be carried out, the turn-around loop is removed to enable a regular data transmission by means of the terminal equipment 61 to 6n. A description of the mode of operation is given as follows. The network terminal equipment 5 is activated by one bit of frame synchronization bits transmitted from the subscriber circuit 3 via the digital subscriber line 4 to the network terminal equipment 5, thus enabling a display of the capability of data transmission by means of one bit of the housekeeping bits. Therefore, when the discrimination portion 57 of the network terminal equipment 5 discriminates that data transmission can be carried out, the turn-around loop formed by the turn-around portion 56 is removed to enable a regular data transmission, and during a period when the turn-around loop is being formed, the transmission test of the digital subscriber line 4 is carried out. The invention will now be described in detail below with reference to the accompanying drawing. Figure 2 is a block diagram showing the structure of the line terminal equipment in the system of Fig. 1. In Fig. 2, TE denotes terminal equipment and ET (exchange terminal) on a subscriber circuit side. Reference 511 denotes a TE receiving circuit, 531 a data stream conversion circuit, 561 a selector, 521 an ET transmitting circuit for transmitting data to the subscriber circuit ET side, 523 a hybrid circuit, 522 an ET receiving circuit, 532 a data stream conversion circuit, 512 a TE transmitting circuit, 571 an HK bit detection circuit for detecting a control bit (or a housekeeping bit HK), 55 a SYN bit detection circuit for detecting a frame synchronization bit SYN, 54 a sequence control circuit, and 562 a data buffer. The turn-around portion 56 in Fig. 1 is constituted by the above-described data buffer 562 and the selector 561, and the discrimination portion 57 is constituted by means of an HK bit detection circuit 571. To the TE receiving circuit 511 and the TE transmitting circuit 512 are connected the bus type terminal equipment TE (not shown in the figure). The digital subscriber line is arranged between the hybrid circuit 523 and the subscriber circuit 3. Data is transmitted from the ET transmitting circuit 521 via the hybrid circuit 523 to the digital subscriber line, and data transmitted via the digital subscriber line is received by the ET receiving circuit 522 through the hybrid circuit 523. A frame synchronization bit which is transmitted from the subscriber circuit 3 of an exchange via the digital subscriber line is detected by a SYN bit detection circuit 55 to carry out a frame synchronization. In response to the frame synchronization, the sequence control circuit 54 controls all separate portions. The HK bit detection circuit 571 detects a housekeeping bit HK next to the frame synchronization bit SYN and adds the detected signal to the sequence control circuit 54, and also controls the selector 561. When the selector 561 is switched over to select the output of the data buffer 562, a turn-around loop is formed, and when the selector 561 is switched over to select the output of the data stream conversion circuit 531, normal communication state is established. In this case, it is possible to control the selector 561 so that the turn-around loop is always formed, or the turn-around loop may be formed by activation from the turn-around loop may be formed by activation from the subscriber line ET. When the HK bit detection circuit 571 detects that data transmission can be carried out, a control is made to remove the turn-around loop, that is, to select the output of the data stream conversion circuit 531. The data stream conversion circuits 531 and 532 convert data from the terminal equipment TE into the data format, which is delivered to the digital subscriber line side, and convert data received via the digital subscriber line into the data format, which is delivered to the terminal TE side. Figure 3 shows a structure of one frame for explaining a transmission format. One frame is composed of a first frame synchronization bit SYN, a housekeeping bit HK, two bits D channel data D0 and D1, eight B1 channel data bits B10 to B17, and eight B2 channel data bits B20 to B27. Accordingly, the data transmission is carried out depending on the "2B + D" type, which is composed of 16Kb/s of one D channel and 64Kb/s of two B channels. Figure 4 shows a sequence of signal transmission, particularly a sequence of terminating and clear-back. In Fig. 4, TE denotes a terminal equipment, NT a network terminal equipment, and ET an exchange terminal. When a frame synchronization bit SYN from the subscriber circuit ET is "1" and a housekeeping bit HK is "0", the selector 561 in Fig. 2 selects the output of the data buffer 562 to add the selected output to the ET transmitting circuit 521, thus indicating the formation of a turn-around loop. When a terminating call is sent to the terminal TE, which is accommodated in the network terminal equipment NT, the subscriber circuit ET of an exchange, shown as SYN=1, delivers a frame synchronization bit SYN in the transmission frame to the digital subscriber line as "1". In this case, the housekeeping bit HK is "0". In the network terminal equipment NT, frame synchronization is carried out via a hybrid circuit 523 and ET receiving circuit 522 in the SYN bit detection circuit 55 to detect a frame synchronization bit SYN, and the detected signal is applied to a sequence control circuit 54 and an HK bit detection circuit 571. In this case, as the frame synchronization bit SYN from the subscriber circuit ET is "1" and the housekeeping bit HK is "0", the selector 561 is controlled to form a turn-around loop. When activation from the subscriber circuit ET side is once discriminated, the sequence control circuit 54 delivers the frame synchronization bit SYN as "1" in the transmission frame to the subscriber circuit ET side from the ET transmitting circuit 521 to the digital subscriber line. Meanwhile, the housekeeping bit HK is "0", and the network terminal equipment NT delivers a control information data INFO 2, which indicates the terminating state according to the recommendation of CCITT, from the TE transmitting circuit 512 to the terminal equipment (TE) side. The terminal equipment (TE) receives a control information data INFO 2 to prepare for data receive and once ready for receiving, a control information data INFO 3, which shows that preparation for receiving is ready, is transmitted. When the network terminal equipment NT receives this control information data INFO 3, a housekeeping bit HK, which is now "0", is delivered as "1" to the subscriber circuit ET, shown as HK = 1. On the subscriber circuit (ET) side, since the housekeeping bit HK from the network terminal equipment NT has become "1", the housekeeping bit HK in the transmission frame which delivers it to the network terminal equipment NT is regarded as "1". In the network terminal equipment NT, a housekeeping bit HK of "1" from the subscriber circuit ET is detected by a HK bit detection circuit 571, and since SYN = 1 and HK = 1, it is discriminated that the data transmission is finished and thus the selector 561 is controlled and the removal of the turn-around loop carried out. That is, the selector 561 is switched over from the data buffer 562 to the side of the data stream conversion, and a control information data INFO 4, which shows the data transmission is completed for starting, is delivered to the terminal equipment (TE) side to become the state of data transmission (during conversation). As soon as data transmission is finished, the house-keeping bit HK is made "0" on the subscriber circuit (ET) side. In the network terminal equipment NT, when the housekeeping bit HK "0" is detected by the HK bit detection circuit 571, the selector 561 is controlled to form a turn-around loop. The housekeeping bit HK is delivered as "0" to the subscriber circuit ET and a control information data INFO 0 is also sent to the terminal equipment TE, which shows that the data transmission is completed. When the terminal equipment TE receives the control information data INFO 0, the terminal equipment TE also sends the control information data INFO 0, which denotes that the data transmission is completed. When the subscriber circuit ET receives the housekeeping bit HK "0" from the network terminal equipment NT, it delivers a frame synchronization bit SYN as "0". As both the housekeeping bit HK and the frame synchronization bit SYN are "0", the network terminal equipment NT controls the selector 561 to remove the turn-around loop. As described before, the formation and removal of the turn-around loop is carried out by one bit the housekeeping bits HK, and the transmission test of a digital subscriber line can be carried out from the subscriber circuit ET side during the formation period of the turn-around loop in the network terminal equipment NT. That is, when testing data is delivered from the subscriber circuit ET side, the data is received via the hybrid circuit 523 by the ET receiving circuit 522. The received data is transferred to the data buffer 562. As the content of the data buffer 562 is applied to the ET transmitting circuit 521 via the selector 561, the testing data is looped back from the ET transmitting circuit 521 via the hybrid circuit 523 to the subscriber circuit side. Figure 5 shows a sequence of the bit error ratio test. When a frame synchronization bit SYN is made "1", a test activation can be carried out from the subscriber circuit ET side. Similar to the on-hook activation, the network terminal equipment NT sends a control information data INFO 2 to the terminal TE to deliver a frame synchronization bit SYN "1" to the subscriber circuit ET. Corresponding to the control information INFO 2, the terminal equipment TE delivers a control information INFO 3 and the network terminal equipment NT receives this control information INFO 3 to deliver the housekeeping bit HK as "1" to the subscriber circuit ET. On the contrary, for the test activation, the subscriber circuit ET leaves the housekeeping bit HK of the transmission frame which is sent to the network terminal equipment NT as "0". The terminal equipment TE receives the control information INFO 2 which shows the on-hook state, and after the lapse of a definite time delivers the control information INFO 3 which shows the response thereto. Since the terminal equipment TE cannot receive the control information INFO 4, it delivers the control information data INFO 0 in the time-out state. As both the frame synchronization bit SYN is "1" and the housekeeping bit HK is "0" in the network terminal equipment NT, the state wherein a turn-around loop is being formed is maintained, thus turning around the testing data from the subscriber circuit ET side. Therefore, the transmission test for the digital subscriber line can be carried out. The subscriber circuit ET delivers the frame synchronization bit SYN as "0" upon termination of the test, while the network terminal equipment NT delivers the control information INFO 0, which represents the termination of data transmission, to the terminal equipment TE and delivers the frame synchronization bit SYN of "0" to the subscriber circuit, the sequence for the transmission test is then terminated.

Claims: A digital network system having an arrangement for testing a digital subscriber line, said system comprising a switching device (1), an exchange terminal device as a subscriber circuit connected with said switching device (1), a subscriber line (4) connected with said exchange terminal device, a network terminal device (5) at a subscriber's side connected between said subscriber line (4) and terminal equipment devices (61, ..., 6n); said network terminal device (5) comprising a turn-around portion (56) for forming a turn-around loop through which the turning-around of a signal to the subscriber line (4) connected with said network terminal device (5) is carried out; whereby communication for either a monitoring of transmission or a bit error testing can be carried out between the network terminal device (5) and the switching device (1) while said turn-around loop is formed and data transmission between the exchange terminal device and the terminal equipment devices (61, ..., 6n) can be carried out after the turn-around loop is cancelled; characterized by said network terminal device (5) including a discrimination portion (57) for detecting that data transmission can be carried out and cancelling the formation of said turn-around loop; said turn-around loop being formed when a frame synchronisation bit signal from the exchange terminal device (5) is received by the network terminal device (5) and cancelled when the discrimination portion (57) detects a control bit signal from the exchange terminal device (5).

Inventors: OGAWA, YASUNORI, SHIMOZONO, RYOJI, TANAKA, YASUO, TSUTSUMI, KENJI
Applicants: FUJITSU LIMITED
Application year: 1986
Main classification: H04M3
IPC codes: H04M3, H04Q11, H04L12