ISDN terminal adapter for access from analog signal equipment of four-wire full duplex type to ISDN

In order to back up by an ISDN line two analog lines connected to MODEM of a four wire full duplex type for data transmission and reception, a terminal adapter is connected to a pair of back-up ports. Dial pulse signals delivered through the back-up ports are detected and converted into an ISDN telephone number. A D-channel processor performs the call connection by use of the D-channel signal having the ISDN telephone number and an identifier of a particular one of B-channels. An outgoing analog data from the MODEM through one of the back-up ports is A/D converted and then transmitted on the particular B-channel to the ISDN line. An incoming digital data on the particular B-channel signal received at the terminal adapter is D/A converted and is supplied to the MODEM through the other of the back-up ports. Thus, the back-up is performed by use of a single B-channel on the ISDN line.

BACKGROUND OF THE INVENTION 
The present invention relates to an ISDN (Integrated Services of Digital 
Network System) terminal adapter and, in particular, to such a terminal 
adapter for connecting an analog signal equipment of a four-wire full 
duplex type to the ISDN line. 
In the ISDN system, a digital subscriber line (DSL) extends from an 
exchanging office to a user's area. The DSL terminates to a network 
terminal (N.T.). A plurality of terminal equipments are connected to the 
network terminal and are, therefore, connected to the DSL through the 
network terminal. 
According to CCITT (International Telegraph and Telephone Consultative 
Communication) recommendations I series, communication information and 
control signals are transmitted through different channels which are 
multiplexed in a time division fashion as a time division multiplex (TDM) 
signal. The TDM signal comprises two communication channels or B channels 
which are separately called B1 and B2 channels and a single control 
channel which is called D channel. 
In the ISDN system, a virtual circuit is set up on a particular one of B1 
and B2 channels through the exchanging office between a calling party of a 
terminal equipment and a called party of another terminal equipment by a 
data link establishment and a call control on the D channel in response to 
a call request from the calling party. 
As terminal equipments, there are known a digital telephone set or sets, a 
facsimile set or sets, a personal computer set or sets, and the like. 
In order to enable to access to the ISDN from existing terminal equipments 
not for ISDN, there are known terminal adapters. 
In the prior art, an ISDN terminal adapter of an analog type is used for 
connecting an existing analog terminal equipment such as a telephone set 
for an analog line to the ISDN line. The analog port type terminal adapter 
has one or more analog ports to be connected to one or more telephone 
sets. Since the analog telephone set is a two-wire type, the known 
terminal adapter is arranged in a form of the two-wire type for each 
analog port. 
In order to connect an analog signal equipment of a four-wire full duplex 
type to the ISDN line, two terminal adapters with a single analog port can 
be used. Alternatively, an adapter with two or more analog ports can be 
used with two ports being connected to the analog signal equipment. In 
both cases, two B-channels on the ISDN line can be used corresponding to 
the two terminal adapters or the two analog ports. Therefore, the analog 
signal terminal equipment unfortunately results in use of two B-channels 
for access to the ISDN line at a time. 
In a case, for example, a data terminal equipment such as a personal 
computer performs transmission and reception of data with another data 
terminal equipment through an analog network, the data terminal equipment 
is connected through a modulator and demodulator (which is called MODEM) 
to two analog lines each being made of two wires. One analog line of two 
wires is for data transmission and the other is for data reception. 
The MODEM usually has a line back-up function whereby occurrence of any 
fault in the currently used analog lines is detected and switching is 
performed from the fault lines to other analog lines for back-up. That is, 
the modem has a pair of current ports for data transmission and data 
reception and another pair of back-up ports for data transmission and data 
reception. 
In order to back up the fault analog lines by the ISDN line, the back-up 
ports can be connected to the ISDN line through the analog port type ISDN 
terminal adapter or adapters. Two B-channels on the ISDN line are used for 
data transmission and data reception, as described above. 
It is desired that only one of B-channels is used for the data transmission 
and data reception even for the analog signal equipment of the four wire 
full duplex type, the other B-channel can be left for use by other 
terminal equipments. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an ISDN 
terminal adapter which can connect the four wire full duplex analog signal 
equipment to the ISDN line with the data transmission and reception being 
performed through only one B-channel. 
It is another object of the present invention to realize back-up of analog 
lines of a modem through an ISDN line by use of the ISDN terminal adapter. 
A terminal adapter according to the present invention is for use in access 
from an analog signal equipment to ISDN (Integrated Services Digital 
Network), the analog signal equipment having a first and a second port for 
transmitting an outgoing analog signal and for receiving an incoming 
analog signal, respectively, and to be connected to a first and a second 
analog public telephone line, respectively, so as to use the first and the 
second analog public telephone lines as a transmitting line and a 
receiving line, respectively, and the ISDN including an ISDN subscriber's 
line to be connected to the terminal adapter, the ISDN subscriber's line 
transmitting a TDM (Time Division Multiplex) signal which comprises first 
and second communication information channels for conveying communication 
information and a control channel for conveying a control signal for data 
link establishment and call control. The terminal adapter comprises: input 
circuit means to be connected to the first port for receiving the outgoing 
analog signal as a received analog signal; analog-digital converting means 
coupled to the input circuit means for converting the received analog 
signal to an outgoing digital signal; interface circuit means to be 
connected to the ISDN subscriber's line for transmitting an outgoing TDM 
signal to and receiving an incoming TDM signal from the ISDN subscriber's 
line, the interface circuit means coupled to the analog-digital converting 
means for delivering the outgoing digital signal into a particular one of 
the first and the second communication information channels of the 
outgoing TDM signal, the interface circuit means deriving, as an incoming 
digital signal, a signal of the particular communication channel of the 
incoming TDM signal; digital-analog converting means coupled to the 
interface circuit means for converting the incoming digital signal into a 
converted incoming analog signal; and output circuit means to be connected 
to the second port of the analog signal equipment and coupled to the 
digital-analog converting means for delivering the converted analog signal 
as the incoming analog signal to the second port of the analog signal 
equipment. 
The analog signal equipment sends out, as a call request, a first and a 
second selection signal through the first and the second ports, 
respectively. The terminal adapter further comprises: first detecting 
means coupled to the input circuit means for detecting the first selection 
signal to produce a first detected selection signal; second detecting 
means coupled to the output circuit means for detecting the second 
selection signal to produce a second detected selection signal; address 
converting means coupled to the first and the second detecting means for 
producing an ISDN telephone number signal corresponding to the first and 
the second detected selection signal; and control channel processing means 
coupled to the address converting means and responsive to the ISDN 
telephone number signal for producing a control channel signal as a call 
connection request, the control channel signal comprising the ISDN 
telephone number signal and a channel indicator representative of a 
particular one of the first and the second communication information 
channels to be connected; the interface circuit means coupled to the 
control channel processing means and responsive to the control channel 
signal for delivering the control channel signal into the control channel 
of the outgoing TDM signal, to thereby establish a virtual communication 
line on the particular communication information channel. 
The interface circuit means derives, as an incoming control channel signal, 
the control channel from the incoming TDM signal to deliver the incoming 
control channel signal to the control channel processing means. The 
control channel processing means decides whether or not the incoming 
control channel signal has a call setup signal and an ISDN subscriber's 
number signal assigned thereto, the control channel processing means 
producing a first and a second driving signal when the incoming control 
channel signal has the call setup signal and the ISDN subscriber's number 
signal. The terminal adapter further comprises: first ringing signal 
generating means coupled to the control channel processing means and to 
the input circuit means and responsive to the first driving signal for 
generating a first ringing signal to deliver the first ringing signal to 
the analog signal equipment through the first port; and second ringing 
signal generating means coupled to the control channel processing means 
and to the output circuit means and responsive to the second driving 
signal for generating a second ringing signal to deliver the second 
ringing signal to the analog signal equipment through the second port.

DESCRIPTION OF PREFERRED EMBODIMENTS: 
Prior to description of preferred embodiments of the present invention, 
known back-up systems for analog lines by an ISDN line are described for 
the sake of better understanding of the present invention with reference 
to the drawings. 
Referring to FIG. 1, in order to perform data transmission and reception by 
a data terminal equipment (DTE) 11 through an analog line, a modulator and 
demodulator (MODEM) 12 is used between the data terminal equipment 11 and 
the analog line 13. The modem 12 modulates the data signal from the data 
terminal equipment 11 into the analog signal to deliver the analog signal 
to the analog line 13. The modem 12 also receives an analog signal 
incoming through the analog line 13 and demodulates the analog signal into 
a digital data signal which is delivered to the data terminal equipment 
11. The modem 12 usually uses the four wire full duplex system and 
therefore uses two analog lines, one 13a being for transmission of the 
analog data signal and the other 13b for reception of the analog data 
signal. 
The modem 12 has a line back-up function. That is, the modem 12 detects 
occurrence of any fault of the analog lines 13a and 13b and then connects 
the modem 12 to the back-up lines 14a and 14b of analog public telephone 
lines shown by dotted lines in the figure. The modem 12 has a pair of 
back-up ports 15a and 15b for connecting the back-up analog lines 14a and 
14b used for analog data signal transmission and reception, respectively. 
In detail, when the modem 12 detects any fault of the currently used analog 
lines 13a and 13b, the modem 12 delivers a fault detected signal to the 
data terminal equipment 11. Then, the modem 12 cooperates with the data 
terminal equipment 11 and switches from the fault analog lines 13a and 13b 
to the back-up lines 14a and 14b. Then, selection signals or dial signals 
of subscriber's lines to which a destination modem is connected are 
delivered to the back-up lines 14a and 14b by a network control unit (not 
shown) and the data transmission and reception is performed through the 
back-up lines 14a and 14b after completion of connection with the 
destination modem. The network control unit is provided separately from 
the modem 12 but can be combined within the modem 12. In the system shown 
in FIG. 1, the modem 12 is provided with the function of the network 
control unit. 
It is recently desired to use an ISDN line in place of the analog lines as 
the back-up line. 
One possible method is to use a switch 16 and a known ISDN terminal adapter 
(T.A.) 17 of a digital type. As shown in FIG. 1, the data terminal 
equipment 11 is usually connected to the modem 12 through the switch 16. 
When the fault is detected, the data terminal equipment 11 controls the 
switch 16 to connect with the ISDN terminal adapter 17. The terminal 
adapter 17 is connected to the network terminal (N.T.) 18 through a 
connector 19. Thus, the data terminal equipment 11 can be backed up by the 
ISDN line through the switch 16, the terminal adapter 17, the connector 19 
and the network terminal 18. 
The modem 12 is usually provided with various functions such as test 
function as well as the fault detection and the network control function. 
In back-up condition through the switch 16 and the terminal adapter 17 by 
the ISDN line, the modem 12 is disconnected from the data terminal 
equipment 11, and therefore, it is impossible to use the various functions 
of the modem 12. 
Referring to FIG. 2, another back-up system can be realized by use of the 
analog port type terminal adapter 20. In detail, the back-up ports 15a and 
15b are connected to two analog ports of the terminal adapter (T.A.) with 
analog ports 20 which is connected to the network terminal 18 through the 
connector 19. In this case, two B channels are used for data transmission 
and for data reception at a same time, as described in the preamble. 
Referring to FIG. 3, a terminal adapter for four wire full duplex analog 
signal 21 according to an embodiment of the present invention is connected 
to the pair of back-up ports 15a and 15b of the modem 12 in place of the 
terminal adapter with analog ports 20 in FIG. 2. The terminal adapter 21 
can make access from the pair of back-up ports 15a and 15b to a single 
B-channel on the ISDN subscriber line, as described below. 
Referring to FIG. 4, the terminal adapter 21 according to an embodiment of 
the present invention comprises an input circuit 22 to be connected to one 
15a of the back-up ports which is a first port for transmitting an 
outgoing analog signal. The input circuit 22 comprises a DC power source 
23 and a primary winding 24a of a transformer 24. The input circuit 22 
receives the outgoing analog signal as a received analog signal. 
A ringing signal generator 25 is connected to the input circuit 22 and is 
for generating a ringing signal to the modem 12 through the first port 15a 
when the terminal adapter 21 is called by a calling party, as described 
hereinafter. 
The input circuit 22 and the ringing signal generator 25 are equivalent to 
an analog public telephone line for the first port 15a of the modem 12. 
An analog-digital (A/D) converter 26 is coupled to a secondary winding 24b 
of the transformer 24. The analog-digital converter 26 converts the 
received analog signal to an outgoing digital signal. 
An interface circuit 27 is connected to an ISDN subscriber's line through 
the connector 19 for transmitting an outgoing TDM signal to and receiving 
an incoming TDM signal from the ISDN subscriber's line. The interface 
circuit 27 is also coupled to the analog-digital converter 26 and delivers 
the outgoing digital signal into a particular one of B1 and B2 channels of 
the outgoing TDM signal. 
The interface circuit 27 derives, as an incoming digital signal, a signal 
of the particular one of B1 and B2 channels of the incoming TDM signal. 
A digital-analog (D/A) converter 28 is coupled to the interface circuit 27 
and converts the incoming digital signal into a converted analog signal. 
An output circuit 29 is connected to the other port 15b of the back-up 
ports of the modem 12 which is a second port for receiving an incoming 
analog signal. 
The output circuit 29 comprises a DC power source 30 and a secondary 
winding 31a of a transformer 31. A ringing signal generator 32 is also 
connected to the output circuit 29 and is for generating a ringing signal 
to the modem 12 through the second port 15b when the terminal adapter 21 
is called by a calling party, as described hereinafter. 
The output circuit 29 and the ringing signal generator 32 are equivalent to 
an analog public telephone line for the second port 15b of the modem 12. 
The digital-analog converter 28 is connected to a primary winding 31b of 
the transformer 31 and is, therefore, coupled to the output circuit 29. 
The output circuit 29, therefore, delivers the converted analog signal as 
the incoming analog signal to the second port 15b of the modem 12. 
Accordingly, the modem 12 can perform transmission and reception of data 
signal through a particular one of B1 and B2 channels on the ISDN 
subscriber's line. 
At start of the back-up mode, the modem 12 sends out, as a call request, a 
first and a second selection signal through the first and the second ports 
15a and 15b, respectively. 
The terminal adapter 21 further comprises first dial pulse detector 33 
coupled to the input circuit 22 through the transformer 24. The first dial 
pulse detector 33 detects the first dial pulse or the first selection 
signal to produce a first detection selection signal. 
A second dial pulse detector 34 is also coupled to the output circuit 29 
through the transformer 31. The second dial pulse detector 34 detects the 
second dial pulse or the second selection signal to produce a second 
detected selection signal. 
An address converter 35 is coupled to the first and the second dial pulse 
detectors 33 and 34 and produces an ISDN telephone number signal 
corresponding to the first and the second detected selection signals. 
A control channel processor or a D-channel processor 36 is coupled to the 
address converter 35. Responsive to the ISDN telephone number signal, the 
D-channel processor 36 produces a control channel signal as a call 
connection request. The control channel signal comprises the ISDN 
telephone number signal and a channel indicator representative of a 
particular one of B1 and B2 channels to be connected. 
The interface circuit 27 is coupled to the D-channel processor 36. 
Responsive to the control channel signal, the interface circuit 27 
delivers the control channel signal into the D-channel of the outgoing TDM 
signal. So that a virtual communication line is established on the 
particular one of B1 and B2 channels. 
Thus, data transmission and reception is performed on the particular one of 
B1 and B2 channels, as described above. 
The interface circuit 27 derives, as an incoming D-channel signal, the 
D-channel from the incoming TDM signal and delivers the incoming D-channel 
signal to the D-channel processor 36. The D-channel processor 36 decides 
whether or not the incoming D-channel signal has a call setup signal and 
an ISDN subscriber's number signal assigned thereto. The D-channel 
processor 36 produces a first and a second driving signal when the 
incoming D-channel signal has the call setup signal and the ISDN 
subscriber's number signal. 
The first and the second driving signals are supplied to the ringing signal 
generators 25 and 32. Then, the ringing signal generators 25 and 32 are 
responsive to the first and the second driving signals and generate first 
and second ringing signals. 
The first and the second ringing signals are supplied to the modem 12 
through the first port 15a and the second port 15b, respectively. 
Thus, the modem 12 can answer the call setup signal.