Digital service unit

Network termination equipment including structure for transmitting signals back for loopback test. This can test all the functions of the unit by executing the loopback test only after having verified the normal operation of the circuits related to a call origination.

FIELD OF THE INVENTION 
This invention relates to an integrated services digital network (ISDN), 
and more particularly to a network termination equipment (NT1), which 
terminates a subscriber line to provide an ISDN basic user-network 
interface for terminal equipment. Further in particular, this invention 
relates to a loopback test for NT1. 
BACKGROUND OF THE INVENTION 
FIG. 1 is a block diagram which shows a conventional NT1 (network 
termination equipment). 
The equipment comprises a transmitter circuit 11 and a receiver circuit 12 
on a terminal equipment side, a receiver circuit 13 and a transmitter 
circuit 14 on a subscriber line side, a circuit termination 15, a call 
origination signal detector 16, a call origination signal transmitter 17, 
a power feeder 18 and a power unit 19. 
FIG. 2 is a sequence chart to show operations when the equipment is 
activated. 
When an activation request for NT1 is made at a terminal equipment (TE), 
the equipment transmits a call origination signal to the NT1. The call 
origination signal detector 16 in the NT1 detects the signal and feeds a 
detection output to the call origination signal transmitter 17. The 
transmitter 17 transmits a call origination signal to the line termination 
equipment at the telephone office via a subscriber line. The call 
origination may be notified by closing the loop on a subscriber line or by 
transmitting a tone signal. 
The line termination equipment at the telephone office originates an 
activation request for a subscriber line when it receives a call 
origination signal, and transmits training pulses to the NT1. The receiver 
13 receives the training pulses and conducts adaptive line equalization 
when it receives them. When equalization ends and when frame 
synchronization is established by the circuit 15, the transmitter 14 
transmits training pulses to the equipment at the telephone office. When 
adaptive line equalization is completed at the office equipment and the 
frame synchronization is established, the activation of subscriber line is 
completed to start communication. 
FIG. 2 shows the case of activation by an incoming call of a caller. FIG. 3 
shows the activation sequence at the time of a loopback test. 
The line termination equipment at the telephone office transmits training 
pulses to the NT1 upon request of activation for a subscriber line, and 
the NT1 in turn transmits training pulses to the equipment at the office 
after having established frame synchronization. 
The equipment at the office transmits a control signal for loopback test to 
the NT1 by using control bits within the frame structure, on the 
subscriber line after having established the frame synchronization. 
When the circuit termination 15 detects a control signal indicative of 
loopback test, it transmits an SW1 driving signal to switch SW1 for the 
loopback to close the loop by turning on the switch SW1. Then, the circuit 
15 sends to the equipment at the office an indication signal of loopback 
test by using control bits within the frame structure on the subscriber 
line. 
The equipment at the office recognizes completion of loop closure for 
loopback at the NT1 when it receives the indication signal, and measures a 
bit error rate on the loopback path. 
As stated above, the conventional NT1 closes a loop under control of the 
equipment at the central office, to test the loopback path. Such a test 
verifies the operations of the transmitting/receiving lines, both at the 
subscriber line side and at the terminal equipment side of the NT1. It 
also tests the circuit terminations, and the power unit which supplies 
power to the above circuits. 
These operations are controlled by the circuit termination 15. In order to 
facilitate the understanding, description will now be given of the 
operation of the circuit 15. 
The circuit termination 15 has four modes of operation, an initial mode, an 
activation mode, a communication mode, and a loopback test mode. FIG. 4 
shows the relation among these modes, and FIGS. 5 through 7 show 
respectively the operation flows of the above four modes. 
In the initial mode (FIG. 5), the circuit termination 15 does not send out 
signals to the circuits 11 and 14, but rather waits for training pulses 
from the line termination equipment at the telephone office. When the 
training pulses arrive from the equipment at the office, in the initiation 
sequence as shown in FIG. 2, the operation of the circuit 15 is shifted 
from the initial mode to the activation mode (FIG. 5), and then to the 
communication mode (FIG. 7). 
In the communication mode, the circuit 15 executes a call between the 
terminal equipment and the line termination equipment at the telephone 
office. When the call is completed, the mode of the circuit 15 is shifted 
from the communication mode back to the initial mode. Step 600 detects a 
control signal indicative of loopback test having arrived from receiver 13 
during the communication mode, and then step 602 shifts the circuit 15 to 
the loopback test mode. When the test ends, circuit 15 is returned to the 
communication mode at step 604. 
The loopback test of the NT1 is intended for testing all the functions and 
performance of the NT1. The conventional system is defective, however, in 
that it cannot test the call origination signal detector or the call 
origination transmitter which operates when activated by a call from the 
terminal equipment as the subscriber line is activated by the equipment at 
the telephone office at the loopback test. Moreover, it cannot verify the 
operation of the power feeder which supplies the power to the terminal 
equipment by such loopback test. 
This invention was conceived to overcome such defects encountered in the 
prior art and aims at providing a NT1 which can test a circuit which 
operates when the unit is activated by an incoming call from the terminal 
equipment and/or the power feeder which supplies power to the terminal 
equipment. 
SUMMARY OF THE INVENTION 
The NT1 according to this invention has a means which originates a 
pseudo-call origination signal to a call origination signal detector and a 
call origination transmitter, and has a loopback means includes a means 
which executes a loopback test only on the condition that the pseudo-call 
origination signal has been normally transmitted. 
It is preferable that the DSU further includes a means which can verify the 
normal operation of the power feeder. 
At the loopback test, the operations of the call activation and the power 
feeder are first tested, and if they are normal, the loopback test is then 
conducted. Therefore, all the functions of a NT1, including those for 
operation of activation by an incoming call, and the feeding of power can 
be tested by a loopback test.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 8 shows a block diagram of the first embodiment of an NT1 according to 
this invention. 
The embodiment includes a call origination detector 16 which detects a call 
origination transmitter 17 which transmits the information on the call 
origination detected by the detector 16 to the line termination equipment 
at the telephone office of an ISDN, and a power feeder 18 which supplies 
power to the terminal equipment at the customer premises A circuit 
termination 150 is also provided. A switch SW1 forms a means for sending 
back a transmitted signal, to respond a loopback control signal from the 
office. The embodiment includes a transmitter 11 which transmits signals 
to the terminal equipment, a receiver 12 which receives signals from the 
terminal equipment, a receiver 13 which receives signals from the 
subscriber lines, and a transmitter 14 which transmits signals to the 
subscriber line. It also includes a power unit 19 which feeds power to the 
respective circuits. 
This NT1 also has a call origination signal generator 21 as a means which 
generates pseudo-call origination signals to the detector 16 and the 
transmitter 17. This causes the output from the power feeder 18 to be 
connected to the call origination detector 16 in order to verify that the 
power unit 18, is operating normally. Means is also provided for executing 
a loopback test only on the condition that the pseudo-call origination 
signal has been duly transmitted. 
In operation, generator 21 outputs a call origination signal to the 
detector 16. The detector 16 is then supplied with the output from the 
power feeder 18. 
The operation at the time of activation of the NT1 is identical to that 
described in relation to FIG. 2. However, FIG. 9 shows an activation 
sequence for the loopback test, which is superior to the prior art 
technique. 
The equipment at the telephone office sends out training pulses to the NT1 
upon detecting a request for subscriber line activation. The NT1 transmits 
the training pulses to the equipment at the office after having 
established a frame synchronization. 
The equipment at the office transmits a call origination unction test 
control signal to the NT1 by using control bits within the frame of the 
subscriber line, after having established frame synchronization. The 
circuit termination 150 resets all circuits of the NT1 to an initialized 
state when it detects a call origination function test control signal and 
then suspends all operations. Simultaneously, the circuit 150 opens the 
switch SW2 by outputting an SW2 driving signal so as to remove the 
connection to the terminal equipment and thereby prevent the power feeder 
18 from being affected by any failure within the terminal equipment. 
Subsequently, the generator 21 outputs a pseudo call origination signal 
which is coupled to the detector 16. When the detector 16 receives the 
signal, it executes the activation sequence shown in FIG. 2. In other 
words, the detector 16 transmits the detection output to the transmitter 
17, and the transmitter 17 transmits a call origination signal to the 
equipment at the office via a subscriber line. This transmission is not an 
authentic call origination signal, because it is not responsive to a real 
call signal. This signal is referred to as a pseudo signal. It may be 
executed by closing a loop on the subscriber line or sending out a tone 
signal. 
When the equipment at the telephone office receives the transmitted call 
origination signal, it sends out training pulses to the NT1 upon request 
for activation of the subscriber line. The receiver 13 conducts an 
adaptive line equalization when it receives the training pulses. Upon 
completion of the adaptive line equalization and establishment of a frame 
synchronization, the transmitter 14 transmits training pulses to the 
equipment at the office. 
The subscriber line is fully activated by the completion of adaptive line 
equalization and frame synchronization establishment at the equipment at 
the office. Then, the equipment transmits a loopback control signal to the 
NT1 by using control bits within the downstream subscriber line frame. 
The circuit termination 150 of the NT1 detects the control signal, and 
transmits an SW1 driving signal to SW1, thereby closing a loop for 
loopback across the contacts of SW1. 
When the loop is formed, the circuit 150 transmits the indication signal 
for loopback to the equipment at the office by using control bits out of 
the subscriber line frame. By receiving the indication signal, the 
equipment at the office recognizes the completion of loop formation, and 
measures a bit error rate by using the loopback path. 
In the sequence described above, when either the detector 16 or the 
transmitter 17 fails, the NT1 cannot notify the equipment at the office of 
a call origination when it has received a call origination test control 
signal from the equipment. Therefore, this failure at the NT1 can be 
detected by the equipment at the office when the call origination is not 
received. 
When the power feeder 18 fails, it causes a corresponding failure of 
detector 16, since it is supplied with power from the output a of the 
circuit 18. Therefore, when the NT1 receives a call origination control 
signal from the equipment at the office, the NT1 remains suspended. As a 
result, this abnormality of the NT1 can also be detected. A similar effect 
may be achieved by constructing the unit in a manner that the transmitter 
17 is operated by the output of the power feeder 18. 
The operation of the circuit termination 150 will now be described. The 
circuit termination 150 has all modes of operation of circuit termination 
15, and also has a fifth operation mode for a call origination function 
test mode in addition to these four modes. FIG. 10 shows the relation 
among those modes. 
FIG. 11 shows the operation flow in the communication mode. FIG. 12 shows 
the operation flow in the call origination function test mode. The other 
three modes, the initial, activation and loopback test, are identical to 
those in the prior art, and the two modes of activation and loopback test 
are respectively shown in FIGS. 5 and 7. 
The call origination function test mode is the operation mode to which the 
circuit is shifted from the communication mode when it receives a call 
origination function test control signal from the receiver 13. The mode is 
reset to the prior activation mode, and frame synchronization is made at 
step 1200. Switch SW2 is turned off at step 1202, and a driving signal is 
transmitted to the generator 21 at step 1204. When the call origination 
function test mode ends, the circuit 150 enters the activation mode at 
step 1206. 
In the operation flow shown in FIG. 9, an example is shown to first 
transmit a pseudo-call origination signal to the equipment at the 
telephone office and then to execute the loopback test, which constitutes 
an essential part of this invention. FIG. 11 shows receiving a call 
origination function test control signal at step 1102 and executing a call 
origination function test mode in response thereto at step 1104 similarly, 
the loopback test mode is executed at step 1108 when it receives the 
loopback control signal at step 1106. Therefore, the tests can be 
continuously conducted, since an NT1 loopback test may be conducted at the 
second test and thereafter without generating a pseudo-call origination 
transmission each time. 
FIG. 13 shows an embodiment of a call origination signal generator together 
with a call origination signal detector. 
The detector 16 is connected to the terminal equipment via a transformer 
1300 and receives a call origination signal as input in pulses from the 
terminal equipment. The detector 160 is a circuit which is used to detect 
pulse power. When voltage pulses are provided externally to an input from 
outside, it recognizes them as a call origination signal. In the 
embodiment shown in FIG. 13, a voltage V (e.g. V=5 volts) is applied at a 
capacitor 211 by closing a switch 213 within a predetermined period and 
the capacitor 211 is connected to an input of the detector 160 by turning 
on the switch 212 using a driving signal from the circuit termination 150. 
Changes in voltage caused by discharging of the capacitor 211 are detected 
as a call origination signal by the detector 160. If the voltage V is 
supplied from the feeder 18, the operation of the circuit may be checked 
simultaneously. 
FIG. 14 is a block diagram to show the second embodiment of the NT1 of this 
invention. 
The second embodiment differs from the first one in that it outputs a call 
origination signal directly from the circuit termination 151 instead of 
employing the generator 21 used in the first embodiment. The second 
embodiment is simpler in the structure than the first embodiment. 
The circuit termination 151 is generally formed of a plurality of ICs which 
embody the functions thereof. This allows an addition of extra functions 
either by additional ICs or otherwise, for outputting a call origination 
signal. The activation sequence at the loopback test and the operation 
modes of this embodiment are identical to those in the first embodiment. 
FIG. 15 is a block diagram to show the third embodiment of the NT1 of this 
invention. 
The circuit structure in the third embodiment is further simplified as it 
can execute loopback tests without using the call origination signal 
generator 21 used in the first embodiment. 
FIG. 16 shows the activation sequence of the loopback test in this 
embodiment which differs from the first embodiment shown in FIG. 9 in the 
operation to be taken when it receives a call origination function test 
control signal from the equipment at the office. 
More particularly, this embodiment temporarily resets all the circuits of 
the NT1 after receiving the call origination function test control signal, 
turns off the switch SW2 and turns on the switch SW1. Then the circuit 
termination 15 outputs a call origination signal which is supplied to the 
call origination signal detector 16 via the transmitter 11 and the switch 
SW1. The sequence in the subsequent procedure is identical to those in the 
first embodiment. 
FIG. 17 shows the operation flow in the call origination function test mode 
at the circuit termination 158 to execute the activation sequence. The 
flow differs from the first embodiment in that it outputs a SW1 driving 
signal after having outputted a SW2 driving signal. Other flow and the 
operation modes, other than the call origination function test mode are 
identical to those in the first embodiment. 
FIG. 18 is a block diagram to show the fourth embodiment of the NT1 
according to this invention. 
This embodiment differs from the first embodiment in that the output b of 
the call of the call origination transmitter 17 is supplied to the circuit 
termination 150. 
FIG. 19 shows the activation sequence at the loopback test in this 
embodiment which differs from the sequence shown in FIG. 9 by the 
operation to be taken upon receipt of a call origination function test 
control signal from the equipment at the office. 
More specifically, when this embodiment receives a call origination 
function test control signal, it feeds the call origination signal from 
the generator 21 to the detector 16, instead of resetting the NT. As a 
result, the circuit 150 can transmit a signal indicating that the result 
of the call origination test was good using status indication bits at an 
allocated position within the upstream subscriber line frame, after a call 
origination signal is output at the output b of the transmitter 17. This 
provides the information that all the functions of the detector 16 and of 
transmitter 17 which operate when activated by an incoming call and of the 
power feeder 18 which supplies power to the terminal equipment, are 
normal. Then, all the circuits of the NT1 are reset, the switch SW2 is 
turned off, and a call origination signal is fed from the generator 21 to 
the detector 16. The subsequent sequence is identical to the rest of the 
first embodiment. 
FIG. 20 shows the operation flow of the circuit termination in the call 
origination function test mode for executing the sequence. It differs from 
the first embodiment in that the transmission of a driving signal to the 
generator 21 and the receipt of a call origination transmissions signal 
from the line b are verified, but other flow and operation modes than the 
call origination test mode are identical to those in the first embodiment. 
FIG. 21 is a block diagram to show a fifth embodiment of the NT1 according 
to this invention. 
The power feeder circuit 180 of this embodiment is provided with a short 
circuit protector 22, which protects the feeder 18 when the output is 
short-circuited. A power generator 23 is also provided, which generates 
the power. The short-circuit protector 22 can electrically disconnect the 
power output. 
This embodiment is simplified in that the switch SW2 can be omitted. More 
particularly, the protector 22 is controlled by the control signal of 
power feeder disconnection from the circuit termination 150. The 
activation sequence is identical to the one in the first embodiment. 
In the above embodiments, description has been made of the case where the 
operation of the power feeder 18 is verified by the voltage at the output 
a. However, this invention may be realized by structuring the circuit so 
that a load is inserted at the output a for testing power. In this case, 
it can detect a failure which lowers the power, even if the output voltage 
from the circuit 18 remains within the normal scope. 
As is described in detail in the foregoing statement, the NT1 of this 
invention first verifies if the circuits which should operate at the time 
of activation by an incoming call from the terminal equipment and a power 
feeder which supplies power to the equipment are operating normally or 
not, and then executes a loopback test. This allows the NT1 according to 
this invention to test all the functions thereof including those of 
activation by an incoming call and of the power feeder simply by 
conducting a loopback test. 
Although only a few embodiments have been described in detail above, those 
having ordinary skill in the art will certainly understand that many 
modifications are possible in the preferred embodiment without departing 
from the teachings thereof. 
All such modifications are intended to be encompassed within the following 
claims.