Communication interface

A communication interface for the protocol of a layer 1 using an ISDN digital communication network, includes: a first communication controller to execute the communication control for a standard terminal; a second communication controller to execute the communications control for a network terminal device; and a switch between to switch the first and second communication controllers. The first communication controller executes the control as a layer 1 of the ISDN standard terminal. The second communication controller executed the control as a layer 1 of the network terminal device which is connected to the ISDN. The communication interface can execute the two-way connection test separately from the ISDN network and can be used in three modes of the ISDN layer 1 interface, TE layer 1 interface, and NT layer 1 interface.

BACKGROUND OR THE INVENTION 
1. Field of the Invention 
The present invention relates to a communication interface for the protocol 
of a layer 1 (electrophysical layer) of the communication using a digital 
communication network such as an ISDN (Integrated Services Digital 
Network) or the like. 
2. Related Background Art 
As shown in FIG. 6, in a standard construction of the ISDN subscribers, a 
terminal device (hereinafter, referred to as a TE) 50 such as a telephone, 
facsimile apparatus, or the like is connected to an ISDN network via a 
network terminal device (hereinafter, referred to as an NT) 51. 
The applicant of the present invention has already applied a communication 
system or communication apparatus using the ISDN disclosed in U.S. patent 
application Ser. Nos. 151,309 and 258,768. 
In FIG. 6, a transmission path between the TE and the NT is defined as an 
S/T point by the International Telegraph and Telephone Consultative 
Committee (CCITT). The interface specification called the layer 1 is 
recommended. 
As shown in FIG. 4, in each direction of the transmission between the TE 
and the NT, a bit train is constructed by a frame consisting of 48 bits. 
The frame construction in the case of transmitting from the TE to the NT 
is different from that in the case of transmitting from the NT to the TE. 
In the case where the TE transmits a bit train of a frame 41 to the NT or 
in the case where the TE receives a bit train of a frame 40 from the NT, 
the TE extracts and reproduces a clock signal from the bit train received 
from the NT and transmits or receives the bit train by using the timing of 
the clock signal. On the other hand, in the case where the NT transmits 
the frame 40 to the TE, the NT transmits the bit train by using a clock 
signal from the ISDN. 
In the case where the NT receives the frame 41 from the TE, the NT extracts 
and reproduces a clock signal from the bit train received from the TE and 
receives the bit train from the TE by using the clock signal. 
On the other hand, in the idling state, that is, in the use waiting state, 
the transmission path between the TE and the NT is set into the stop state 
to reduce the electric power consumption. Therefore, in order to correctly 
communicate, the layer 1 in the stop state is set into the start state. On 
the contrary, when the communication is completed, the layer 1 in the 
start state is set into the stop state. 
FIG. 7 shows start and stop procedures in such a case. FIG. 7A shows the 
start procedure in the case of performing a start request from the TE 
side. INF01 denotes a signal to indicate the start request to the NT. 
INF02 represents a signal which is necessary for the TE to perform the 
synchronization. That is, the INF02 is the signal in which the channel 
bits (B.sub.1, B.sub.2, D) are 0 in the frame 40. On the basis of the 
INF02 signal, the TE extracts a clock signal which is necessary to 
transmit data. INF03 denotes a signal indicating that the synchronization 
was obtained for the NT. INF04 is a signal to indicate the transmission of 
data to the TE. FIG. 7B shows the start procedure in the case where the 
start request is executed from the NT to the TE in response to the 
terminating from the ISDN network side. FIG. 7C shows the stop procedure 
from the network side. As mentioned above, in the S/T point interface of 
the ISDN, the bit construction of the frame differs dependence on the 
transmitting direction. In addition, the control functions of the NT and 
TE also differ. Therefore, in the control of the conventional ISDN layer 1 
, the control functions are respectively independently distinguished for 
the TE and NT. 
Therefore, for the S/T point, an interface for the NT layer 1 must be 
provided for the NT, while another interface for the TE layer 1 must be 
also provided for the TE. 
Further, in the interfaces for the TE layer 1 and NT layer 1 , since the 
clock signals which are used upon transmission and reception differ, there 
is a drawback such that even when two TE terminals are simply connected, 
the two-way connection test (maintenance and inspection services such as 
failure analysis and the like) of each TE terminal cannot be executed. 
Therefore, even in the case of executing the two-way connection test, the 
TE terminal must be also connected to the ISDN via the NT in a manner 
similar to the case where the actual data communication is performed, so 
that there is a drawback such that the two-way connection test of the TE 
terminal cannot be easily executed. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to eliminate the drawbacks in the 
conventional techniques as mentioned above. 
Another object of the invention is to provide a communication interface 
which can be used for any of a TE layer 1 interface and an NT layer 
interface. 
Still another object of the invention is to provide a communication 
interface in which the two-way connection tests can be executed among TE 
terminals without passing through a digital communication network such as 
an ISDN or the like. 
Still another object of the invention is to provide a communication 
interface which can easily execute the two-way connection test of a 
terminal without using a line. 
Still another object of the invention is to provide a communication 
interface which can be used as an interface of three modes by a one-chip 
LSI. 
Still another object of the invention is to provide a communication 
interface which can select either one of the communication control for a 
standard terminal of the ISDN and the communication control for a network 
terminal device of the ISDN. 
Still another object of the invention is to provide a communication 
interface comprising: first control means for transmitting and receiving 
data by using a clock signal which is extracted from reception data; and 
second control means for receiving data by using the extracted clock 
signal and for transmitting data by using another clock signal whose 
period is different from that of the above-mentioned clock signal. 
The above and other objects and features of the present invention will 
become apparent from the following detailed description and the appended 
claims with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A preferred embodiment cf the present invention will be described in detail 
hereinbelow with reference to the drawings. 
FIG. 1 is a block diagram showing a schematic construction of an ISDN layer 
1 interface (i/f) unit 20 as an embodiment of the present invention. FIG. 
2 is a diagram showing the connection of a terminal device using the ISDN 
i/f unit 20 in three cases of a reference model of an ISDN user network 
interface. 
FIG. 2A shows the case where the ISDN i/f unit 0 operates as an interface 
for a TE terminal. Reference numeral 21 denotes an ISDN network and 22 
indicates an ISDN standard terminal. Up to eight ISDN standard terminals 
22 can be connected to one NT. Reference numeral 23 denotes an NT (device) 
which functions as a terminal (Network terminal device) of the network and 
interface side transmission path, and 24 indicates a TE terminal including 
the ISDN i/f unit 20 and a terminal device 22' of, for instance, a 
facsimile apparatus or the like. 
FIG. 2B shows the case where the ISDN i/f unit 20 provided in a 
communication terminal 25 is directly connected to the ISDN network 21 and 
functions as a network terminal device (NT). Further, FIG. 2C shows a 
communication terminal 26 in an embodiment which functions as a pseudo NT 
terminal for the connection test of the TE terminal 22 for the S/T point 
side and in which the connection test can be executed between the TE 
terminal 22 and the terminal device 22'. In any of the above cases, 
reference numerals 24 to 26 denote the same communication terminals except 
that their operations differ. 
FIG. 1 is a block diagram showing a detailed construction of the ISDN i/f 
unit 20 of the communication terminals 24 and 26. The ISDN i/f unit 20 can 
operate in two modes. 
That is, the first mode relates to the case where the ISDN i/f unit 20 
functions as an interface for the TE layer 1 as shown in FIG. 2A and the 
second mode relates to the case where the ISDN i/f unit 20 functions as a 
pseudo NT layer 1 interface as shown in FIG. 2C. 
The ISDN i/f unit 20 is constructed as a one-chip LSI. 
In FIG. 1, R denotes a control input signal which is obtained by binarizing 
a signal (a signal from the NT in the case where the ISDN i/f unit 20 
operates as the TE layer 1 or a signal from the TE in the case where it 
operates as the pseudo NT layer 1) received from an S/T interface. XTAL 
denotes an original clock signal input from the outside (in the 
communication terminal 24 or 26). Reference numeral 101 denotes a 
frequency demultiplexer (DPLL) for extracting the transmission frequency 
component included in the control input signal R, for frequency 
demultiplexing the original clock signal XTAL in accordance with the 
frequency dividing ratio shown by the signal R, and for outputting as a 
bit rate timing signal (RCLK). Reference numeral 102 denotes a sampling 
circuit for sampling the control input signal R as the timing of the 
signal RCLK and for specifying as a reception bit train (R.sub.1). The 
reception bit train specified as mentioned above is as shown in, for 
instance, FIG. 4. 
Reference numeral 103 denotes a reception system circuit. A construction of 
the circuit 103 will now be described. Reference numeral 104 denotes a 
frame resolution circuit which receives a switch signal 121 and functions 
as the TE layer 1 when the switch signal 121 is at the high level, that 
is, when a switch 120 is set to OFF and functions as the NT layer 1 when 
the switch signal 121 is at the low level. When the switch signal 121 is 
at the high level, as shown in FIG. 2A, the frame resolution circuit 104 
functions as the TE layer 1 interface and resolves the bit train (40 in 
FIG. 4) of the frame construction which was sent from the NT through the 
sampling circuit 102 into the B.sub.1, B.sub.2, D, and E channels. The 
resolved signals are output to the terminal device 22' side through an 
interface unit 107. 
On the contrary, when the switch signal 121 is at the low level, the frame 
resolution circuit 104 functions as the pseudo NT layer 1 interface as 
shown in FIG. 2C. Therefore, the frame resolution circuit 104 resolves the 
frame data of the type as shown by 41 in FIG. 4 which was sent from the TE 
through the sampling circuit 102 into the B.sub.1, B.sub.2, and D channels 
so that such frame data can be received. The resolved signals are output 
to the terminal device 22' side. At this time, the terminal device 22' 
operates as a pseudo ISDN network. 
Reference numeral 105 denotes a frame sync detection circuit for detecting 
and monitoring the frame synchronization during the receiving process on 
the basis of a frame bit F obtained from the sampling circuit 102 and for 
outputting the result to a procedure control circuit 108. Reference 
numeral 106 denotes an info signal detection circuit to detect an info 
signal as shown in FIG. 7. When the info signal is detected, the detection 
circuit 106 outputs the result to the procedure control circuit 108. 
Reference numeral 114 denotes a transmission system circuit and its 
construction will now be described. The procedure control circuit 108 
receives output signals from the frame sync detection circuit 105 and info 
signal detection circuit 106 and also receives a start request signal 122 
which is input from the interface unit 107. When the switch signal 121 is 
at the high level, the transmission system circuit 114 discriminates by 
the procedure on the TE layer 1 side on the basis of the input signals and 
instructs an info signal generation circuit 109 to transmit the info 
signal so as to respond to the NT on the distant side. The circuit 114 
also turns on or off an indication signal 123 of the start state in 
accordance with the progress of the procedure and informs the start or 
stop state to the terminal device 22' side through the interface unit 107. 
On the other hand, when the switch signal 121 is at the low level, the 
transmission system circuit 114 discriminates by the procedure on the NT 
layer 1 side on the basis of the input signals and instructs the info 
signal generation circuit 109 to transmit the info signal so as to respond 
to the TE terminal on the distant side. Reference numeral 109 denotes the 
info signal generation circuit to generate the corresponding info signal 
in accordance with the instruction of the procedure control circuit 108. 
Reference numeral 110 denotes a frame composition circuit for receiving a 
transmission channel signal 124 and info signal from the terminal device 
22' and for making and outputting a frame signal. 
When the switch signal 121 is at the high level (when the switch 120 is set 
to OFF), the frame composition circuit 110 reconstructs the info signal 
from the info signal generation circuit 109 and the transmission channel 
signals 124 of the B.sub.1 channel, B.sub.2 channel, D channel, and the 
like which are input from the terminal device 22' into a frame for 
transmission of the TE layer 1 and outputs as an output signal S to the 
transmission path side (for instance, like the frame bit train 41 in FIG. 
4). At this time, the RCLK is used as a transmission clock signal. On the 
other hand, when the switch signal 121 is at the low level (when the 
switch 120 is set to ON), the bit train shown in the frame construction 40 
in FIG. 4 is made and the output signal S is output. In this case, the 
SCLK is used as a transmission clock signal. 
FIG. 3 is a diagram showing the difference of the frame data depending on 
the transmitting direction between the NT and TE terminals in the layer 1 
as mentioned above. 
Reference numeral 30 denotes a frame construction from the NT to the TE 
terminal and 31 indicates a frame construction from the TE terminal to the 
NT. B.sub.1 and B.sub.2 denote information channels of 64 kbits/sec; D 
indicates a common line signal channel of 16 kbits/sec; and E represents 
an echo bit. 
Returning to FIG. 1, reference numeral 112 denotes a switch to change the 
connection in correspondence to the level of the switch signal 121. When 
the switch signal 121 is at the high level, the switch 112 is connected to 
the side of a contact a as a TE layer 1 mode and the RCLK is input as a 
clock signal of the transmission system circuit 114. On the contrary, when 
the switch signal 121 is at the low level, the switch 112 is connected to 
the side of a contact b as a pseudo NT layer 1 mode and an SCLK, which 
will be explained hereinafter, is input as a clock signal of the 
transmission system circuit 114. Reference numeral 113 denotes a frequency 
demultiplexer for frequency demultiplexing the original clock signal XTAL 
by a preset frequency dividing ratio and for outputting a pseudo bit rate 
clock signal SCLK. Reference numeral 20 denotes the switch for switching 
the switch signal 21 as mentioned above and for indicating whether the 
communication terminal is allowed to function as the TE layer 1 or the NT 
layer 1. When the switch 120 is set to OFF, the TE layer 1 is indicated. 
When the switch 20 is set to ON, the NT layer 1 is indicated. 
With the above construction, when the switch 20 is set to OFF (TE layer 1), 
the RCLK is used as a clock signal of each of the reception system circuit 
103 and the transmission system circuit 114. The circuits 103 and 114 
operate in accordance with the RCLK. The frame resolution circuit 104 
receives the frame from the NT shown by 40 in FIG. 4 and outputs to the TE 
terminal 22' side by the reception channel. On the other hand, in the 
transmission mode, the procedure control circuit 108 allows the info 
signal to be generated in accordance with the control procedure for the TE 
layer 1 and responds to the NT side by the frame composition circuit 110. 
When the switch 120 is set to ON (NT layer 1), the RCLK is used as a clock 
signal of the reception system circuit 103 and the SCLK is used as a clock 
signal of the transmission system circuit 114. The frame resolution 
circuit 104 is set sc as to receive and resolve the frame shown by 41 in 
FIG. 4. The transmission system circuit 114 is driven by the clock signal 
(SCLK) of the NT layer 1 and the frame data shown by 40 in FIG. 4 to be 
transmitted to the TE side is made and output from the frame composition 
circuit 110. 
In the embodiment, the switching of the layer 1 in the ISDN i/f unit 20 has 
been indicated by the switch 120. However, the invention is not limited to 
such a method. The switching of the layer 1 can be also obviously 
designated by a signal, command, or the like from the TE terminal side. 
As described above, according to the embodiment, the interface for the ISDN 
layer 1 having the functions of both of the TE layer 1 and the pseudo NT 
terminal layer 1 can be constructed. Therefore, for instance, the two-way 
connection test of the TE terminal can be executed separately from the 
ISDN network. There is an advantage such that the working efficiencies for 
the maintenance and inspection services of the TE terminal and NT and the 
like can be remarkably improved. 
In the foregoing embodiment, the example in which the ISDN i/f unit 20 can 
be selected as the TE layer 1 interface and pseudo NT layer 1 interface 
has been described. As the second embodiment, an example in which the ISDN 
i/f unit 20 can be selected as the TE layer 1 interface, NT layer 1 
interface, and pseudo NT layer 1 interface will now be described. 
FIG. 5 is a block diagram showing a detailed construction of the ISDN i/f 
unit 20 of the communication terminals 24, 25, and 26 in FIG. 2. The ISDN 
i/f unit 20 can operate in the following three modes. 
That is, the first mode relates to the case where the ISDN i/f unit 20 
functions as the TE layer 1 interface as shown in FIG. 2A. The second mode 
relates to the case where the ISDN i/f unit 20 functions as the NT layer 1 
interface as shown in FIG. 2B. The third mode relates to the case where 
the ISDN i/f unit 20 functions as the pseudo NT layer 1 interface as shown 
in FIG. 2C. 
The ISDN i/f unit 20 shown in FIG. 5 is also constructed by a one-chip LSI 
in a manner similar to that shown in FIG. 1. 
In FIG. 5, R indicates the control input signal which is obtained by 
binarizing the signal (the signal from the NT in the case of operating as 
a TE layer 1 or the signal from the TE in the case of operating as an NT 
layer 1 or pseudo NT layer 1) received from the S/T interface. XTAL 
indicates the original clock signal input from the outside (in the 
communication device 24, 25, or 26). Reference numeral 201 denotes a 
frequency demultiplexer (DPLL) for extracting the transmission frequency 
component included in the control input signal R, for frequency 
demultiplexing the original clock signal XTAL in accordance with the 
frequency dividing ratio shown by R, and for outputting as a bit rate 
timing signal (RCLK). Reference numeral 202 denotes a sampling circuit for 
sampling the control input signal R at the timing of the RCLK and for 
specifying as a reception bit train (R.sub.1). The reception bit train 
specified as mentioned above is a bit train as shown in, e.g., FIG. 4. 
Reference numeral 203 denotes a reception system circuit and its 
construction will now be described. Reference numeral 204 denotes a frame 
resolution circuit for receiving a switch signal 221 and functions as a TE 
layer 1 when the switch signal 221 is at the low level, that is, when a 
switch 220 is set to OFF. The frame resolution circuit 204 functions as an 
NT layer 1 when the switch signal 220 is at the high level. Now, assuming 
that the switch signal 221 is at the low level, as shown in FIG. 2A, the 
frame resolution circuit 204 functions as a TE layer 1 interface and 
resolves the bit train (40 in FIG. 4) of the frame construction 
transmitted from the NT through the sampling circuit 202 into the B.sub.1, 
B.sub.2, D, and E channels and outputs to the terminal device 22' side via 
interface unit 207. 
On the contrary, when the switch signal 221 is at the high level, the frame 
resolution circuit 204 functions as a pseudo NT layer 1 or NT layer 1 in 
accordance with the switch signal 221. Therefore, the frame resolution 
circuit 204 resolves the frame data shown by 41 in FIG. 4 which was 
transmitted from the TE through the sampling circuit 202 into the B.sub.1, 
B.sub.2, and D channels so as to receive the frame data and outputs to the 
ISDN or terminal device 22' side. When the frame resolution circuit 204 
operates as a pseudo NT layer 1, the terminal device 22' operates as a 
pseudo ISDN network. 
Reference numeral 205 denotes a frame sync detection circuit for detecting 
and monitoring the synchronization of the frame during the receiving 
process on the basis of a frame bit F and outputting the result to a 
procedure control circuit 208. Reference numeral 206 indicates a detection 
circuit to detect an info signal as shown in FIG. 7. When the info signal 
is detected, the detection circuit 206 outputs the result to the procedure 
control circuit 208. 
Reference numeral 214 denotes a transmission system circuit and its 
construction will now be described. The procedure control circuit 208 
receives output signals of the frame sync detection circuit 205 and info 
signal detection circuit 206 and also receives a start request signal 222 
which is input from the interface unit 207. When the switch signal 221 is 
at the low level, the procedure control circuit 208 discriminates by the 
procedure on the TE layer 1 side on the basis of the input signals and 
indicates an info signal generation circuit 209 to transmit an info signal 
so as to respond to the distant side NT. The procedure control circuit 208 
also turns on or off a start state indication signal 223 in accordance 
with the progress of the procedure and informs the start or stop state to 
the terminal device 22' side through the interface unit 207. 
On the contrary, when the switch signal 221 is at the high level (NT layer 
1), the procedure control circuit 208 discriminates by the procedure on 
the NT layer 1 side on the basis of the input signals and indicates the 
info signal generation circuit 209 to transmit an info signal so as to 
respond to the distant side TE terminal (e.g., TE 22). Reference numeral 
209 denotes the info signal generation circuit to generate the 
corresponding info signal in accordance with the instruction from the 
procedure control circuit 208. Reference numeral 210 denotes a frame 
composition circuit for receiving a transmission channel signal 224 and 
info signal from the terminal device 22' and for making and outputting a 
frame signal. 
When the switch signal 221 is at the low level (when the switch 220 is set 
to OFF), the frame composition circuit 210 reconstructs the info signal 
from the info signal generation circuit 209 and the transmission channel 
signals 224 of the B.sub.1 channel, B.sub.2 channel, D channel, and the 
like which are input from the terminal device 22' into a frame for 
transmission of the TE layer 1 and outputs as an output signal S to the 
transmission path side (for instance, like the frame bit train 41 in FIG. 
4). At this time, the RCLK is used as a transmission clock signal. On the 
other hand, when the switch signal 221 is at the high level (when the 
switch 200 is set to ON), the frame composition circuit 210 makes the bit 
train shown by the frame construction 40 in FIG. 4 and outputs the output 
signal S. In this case, the SCLK is used as a transmission clock signal in 
the case of the pseudo NT layer 1 mode (FIG. 2C) or the MCLK is used as a 
transmission clock signal in the case of the NT layer 1 mode (FIG. 2B). 
FIG. 3 is a diagram showing the difference of the frame data depending on 
the transmitting direction between the NT device and the TE terminal as 
mentioned above in the layer 1. 
Reference numeral 30 denotes the frame construction from the NT device to 
the TE terminal and 31 indicates the frame construction from the TE 
terminal to the NT device. B.sub.1 and B.sub.2 denote information channels 
of 64 kbits/sec, D indicates a common line signal channel of 16 kbits/sec, 
and E represents an echo bit. 
Returning to FIG. 5, reference numeral 212 denotes a switch to change the 
connection in correspondence to the level of the switch signal 221. When 
the switch signal 221 is at the low level (when the switch 220 is set to 
OFF), the switch 220 is connected to a contact a side and the TE layer 1 
mode is set. The RCLK is input as a clock signal of the transmission 
system circuit 214. On the contrary, when the switch signal 221 is at the 
high level (when the switch 120 is set to ON), the switch 220 is connected 
to a contact b side. The SCLK, which will be explained hereinlater, is 
input as a clock signal of the transmission system circuit 114. At this 
time, a switch 230 is set to OFF. Therefore, the pseudo NT layer 1 mode is 
set. 
Reference numeral 213 denotes a frequency demultiplexer for frequency 
demultiplexing the original clock signal XTAL by a preset frequency 
dividing ratio and for outputting a pseudo bit rate clock signal SCLK. 
Reference numeral 220 denotes the switch for switching the switch signal 
221 as mentioned above, thereby indicating whether the communication 
terminal is allowed to function as a TE layer 1 or an NT layer 1. When the 
switch 220 is set to OFF, the TE layer 1 is indicated. When the switch 220 
is set to ON, the NT layer 1 is indicated. 
A switch signal 231 is set to the high level when the switch 230 is set to 
ON. The switch signal 31 is set to the low level when the switch 230 is 
set to OFF. When both of the switches 220 and 230 are set to ON, an AND 
circuit 215 is opened and a switch 216 is switched to a terminal b side. 
Thus, the master bit rate clock signal (MCLK) defined on the ISDN side is 
input to the transmission system circuit 114. Thus, a frame can be output 
to the ISDN network and the device can function as an NT device. 
With the above construction, when both of the switches 220 and 230 are set 
to OFF (TE layer 1), the RCLK is used as a clock signal of each of the 
reception system circuit 203 and the transmission system circuit 214. The 
circuits 203 and 214 operate in accordance with the RCLK. The frame 
resolution circuit 204 receives the frame shown by 40 in FIG. 4 from the 
NT device and outputs to the TE side by the reception channel. On the 
other hand, upon transmission, the procedure control circuit 208 generates 
an info signal in accordance with the control procedure for the TE layer 1 
and responds to the NT side by the frame composition circuit 210. 
When the switch 220 is set to ON and the switch 230 is set to OFF (pseudo 
NT layer 1), the RCLK is used as a clock signal of the reception system 
circuit 203 and the SCLK is used as a clock signal of the transmission 
system circuit 214. At this time, the frame resolution circuit 204 is set 
so as to receive and resolve the frame shown by 41 in FIG. 4. The 
transmission system circuit 214 is driven by the clock signal (SCLK) of 
the NT layer 1 and makes the frame data shown by 40 in FIG. 4 to be 
transmitted to the TE side and outputs from the frame composition circuit 
210. 
Next, when both of the switches 220 and 230 are set to ON, the RCLK is used 
as a clock signal of the reception system circuit 203 and the frame 
resolution circuit 204 is set to the NT layer 1. On the other hand, since 
the switch 216 is switched to the terminal b side, the MCLK is used as a 
clock signal of the transmission system circuit 214. Both of the procedure 
control circuit 208 and frame composition circuit 210 are switched to the 
NT layer 1. Thus, the transmission system circuit 214 functions as an NT 
device and the frame data can be transmitted and received between the TE 
terminal and the ISDN network. 
In the embodiment, the switching of the layer 1 in the communication 
terminal 20 has been indicated by the switches 220 and 230. However, the 
invention is not limited to such a method. The switching of the layer 1 
can be also obviously designated by a signal, command, or the like from 
the TE terminal side. 
As described above, according to the second embodiment, since the clock 
signal (RCLK) from the reception signal, clock signal (SCLK) based on the 
internal oscillator, and clock signal (MCLK) from the outside can be 
selectively used as a clock signal which is used for the ISDN layer 1 
interfaces the communication interface can be used as a TE layer 1 
interface and an NT layer 1 interface. 
On the other hand, by attaching the ISDN layer 1 interface to the TE 
terminal, the TE terminal can function not only as a TE terminal but also 
an NT device. Therefore, there are advantages such that, for instance, the 
two-way connection test of the TE terminal can be executed separately from 
the ISDN network and the working efficiencies of the maintenance and 
inspection services and the like of the TE terminal, NT device, and the 
like can be remarkably improved. 
Although the embodiment has been described with respect to the layer 1 
interface of the communication using the ISDN as an example, the invention 
can be also used as a layer 1 interface in the case of using another 
digital communication network. 
The present invention is not limited to the foregoing embodiments but many 
modifications and variations are possible within the spirit and scope of 
the appended claims of the invention.