Composite concentration system

In a composite concentration system of the type utilizing a time division system in which voice signals are digitized and then concentrated in a traffic, there are provided a first subscriber line interface means of a first type which connects subscriber lines directly to subscriber line interface circuits, a second subscriber line interface means of a second type which connects subscriber lines to subscriber line interface circuits via a space division concentration switch and a time division concentration switch connected to the first and second subscriber line interface means via switching interfaces. The number of output terminals of the first subscriber line interface means is equal to that of the output terminals of the second subscriber line interface means when a concentration ratio of the space division concentration switch to the second subscriber line interface means is 1:1 and corresponding output terminals are constructed such that the same channel is assigned to the time division concentration switch. The composite concentration system of this invention is inexpensive and can readily expand a telephone system according to a variation in percentage of calls or change in a service performance.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a composite concentration system, and more 
particularly a concentration system for concentrating subscriber lines of 
a digital local switching system. 
2. Description of the Prior Art 
As is well known in the art, the size and scale of a local switching system 
are determined by taking into consideration such factors as the number of 
subscriber lines, the content of the service to be provided (percentage of 
call falls, for example), the types of subscribers and the type and scale 
of the system. 
The local switching systems generally used are classified into two types, 
one a space division system, and the other, a time division system. 
In a space division system the subscriber lines connected to telephone 
terminal sets of individual subscribers are firstly accommodated in a 
space division concentration stage. In this stage, the lines are collected 
at different concentration ratios for the different types of subscribers, 
such as residential subscribers or lines as well as for public uses. For 
example, since the frequency use of common residential subscriber lines is 
generally low, the line concentration ratio is 10:1, whereas in the public 
subscriber lines, where frequency of utilization is high, the lines are 
concentrated at a concentration ratio of 2:1. In this manner, the 
subscriber lines are concentrated into fewer signal lines than the 
quantity of subscriber lines. The signals of these signal lines are 
multiplexed through A-D and D-A converters constituting the subscriber 
line interface circuit and then are accommodated in a distribution stage. 
In a space division system, as described above, the subscriber lines are 
concentrated at the space division concentration stage and then are 
connected to the subscriber line interface circuits so that, when 
considered in the units of the subscribers, the number of expensive 
subscriber line interface circuits can be reduced, thus decreasing the 
installation cost of the whole system. The space division concentration 
circuits utilized in this stage, however, must have a different 
construction for different types of subscribers and different types of 
subscriber line interface circuits. This not only complicates the overall 
system but also increases the cost. Moreover, since the controls of the 
space division concentration stage should be different depending upon the 
types of the subscribers and subscriber line interface circuits, it is 
difficult to adequately and economically improve the service performance 
by adding a new subscriber line interface circuit in response to an 
increase in the quantity of subscriber lines, and an increase in the 
number of calls due to increases in the frequency of use. 
On the other hand, in a time division system, each subscriber line is 
accommodated in a time division concentration stage through A-D and D-A 
converters consisting of a single subscriber line interface circuit. At 
the concentration stage, the traffic in which calls and pagings for m 
subscriber lines are made is limited to n channels by a time switch, and 
the signals are then accommodated in a distribution stage through a 
digital highway. In a time division system, since it is necessary to 
provide a subscriber line interface circuit for each subscriber line, and 
since it is necessary to provide a buffer memory device for each 
subscriber line in order to execute a time switch processing, a time 
division system is more expensive than a space division system at a low 
percentage of calls. It is not necessary, however, to provide a space 
division concentration stage consisting of various combinations of circuit 
elements at the preceding stage of the subscriber line interface circuit. 
Accordingly, the construction of the time division system is definite or 
fixed and simple, irrespective of high or lower percentage of the calls. 
For this reason, a time division or concentration system becomes less 
expensive than a space division system at high percentage of calls. Since 
the time division has a simple construction, addition of the service and 
new capabilities to the subscriber line interface circuit can be made more 
easily than the space division system without increasing the installation 
cost. 
As above described, the space division system and the time division system 
have inherent advantages and disadvantages so that, when installing a 
telephone exchange in a district, the system to be adapted and the 
construction of the subscriber lines are determined by considering the 
type of subscriber, the number of the subscriber lines necessary in that 
district, the service required, and the percentage of calls. 
Generally, however, the number of the subscribers, the type thereof, and 
service vary with time so that it is necessary to change the concentration 
system of the subscriber lines in accordance with such variation. In the 
prior art, although the subscriber lines have been designed by forecasting 
the future increase in the number of subscriber lines, their design is 
fixed once the telephone system is installed and, therefore, it is not 
possible to modify the system in the future. Accordingly, when changing 
the concentration system of the existing subscriber lines it is necessary 
not only to exchange substrates but also to change wirings resulting from 
the exchange of the substrates. Such operations require much labor and 
time and also must be tested after modification. 
SUMMARY OF THE INVENTION 
Accordingly, it is a principal object of this invention to provide a novel 
line concentration system having a versatility for variations in the 
percentage of calls and service and economy. 
Another object of this invention is to provide a composite line 
concentration system capable of readily expanding the system by using 
common component parts. 
To accomplish these and further objects, the invention contemplates 
utilization of respective advantages of a space division system and a time 
division system. Basically, the invention is a time division system in 
which voice signals are digitized and then concentrated in a traffic. A 
subscriber line interface means of a first type which connects the 
subscriber line directly to the subscriber line interface circuits and a 
subscriber line interface means of a second type in which the subscriber 
lines are connected to the subscriber line interface circuit, are 
provided, the first and second subscriber line interface means having the 
same number of output terminals for assigning the same channel to a time 
division concentration switch installed in a succeeding stage. 
According to this invention, there is provided a composite concentration 
system of the type utilizing a time division system in which voice signals 
are digitized and then concentrated in a traffic, characterized in that 
there are provided a first subscriber line interface means of a first type 
which connects subscriber lines directly to the subscriber line interface 
circuits; a second subscriber line interface means of a second type which 
connects subscriber lines to subscriber interface circuits via a space 
division concentration switch; and a time division concentration switch 
connected to the first and second subscriber line interface means via 
switching interfaces. The number of output terminals of the first 
subscriber line interface means is equal to that of output terminals of 
the second subscriber line interface means when a concentration ratio of 
the space division concentration switch to the second subscriber line 
interface means is 1:1, the corresponding output terminals being 
constructed such that the same channel is assigned to the time division 
concentration switch.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A composite concentration system 10 of this invention shown in FIG. 1 
consists of a plurality of subscriber line interface circuits (SLIE) 20a 
and 20b connected to respective subscriber terminals 111-11.sub.m, 
121-12.sub.m, 131-13.sub.m, 141-14.sub.m, 151-15.sub.m, and a time 
division concentration unit 50, commonly connected to respective SLIEs. 
The common unit 50 is connected to a digital highway 60 which functions as 
time division talking channels. 
The SLIEs 20a and 20b will now be described by taking the SLIE 20a as an 
example. The SLIE 20a characterizing the invention is constructed by two 
types of subscriber line interface means 21A and 21B and an interface 23. 
In this example, the SLIE 20a is constructed by two subscriber line 
interface means 21A of the first type and one subscriber line interface 
means 21B of the second type, while the interface 23 is commonly used for 
all of these subscriber line interface means 21A and 21b. Each of the 
subscriber line interface means 21A and 21B have common input/ouput 
conditions. More particularly, the number of subscriber terminals 
connected to the input side of each of the subscriber line interface means 
21A and 21B is the same (in this example m=8). The subscriber line 
interface means 21A of the first type does not have concentrating 
capabilities, and has m output terminals, that is, the same number of 
output terminals as input terminals; and, the output terminals being 
connected to the interface according to an aligned order. The subscriber 
line interface means 21A sequentially assigns signals on respective output 
lines to one of a plurality of channels constituting one frame, or 
sequentially receives the signals of the plurality of channels on the 
output lines. 
The subscriber line interface means 21B of the second type has a 
concentration capability so that its output terminals are concentrated to 
n output terminal. In this example, n=2. In general, n is less than m. 
However, these output terminals are designed such that when they are 
connected to the interface 23, they are assigned to n of the time channels 
of the same timings as those of the respective time channels of the 
subscriber line interface means 21A of the first type. 
The subscriber line interface means 21A and 21B will now be described in 
greater detail. 
The subscriber line interface means 21A is provided with subscriber line 
interface circuits (SLIC) 221-22.sub.m, respectively, each directly 
connected to one of the m subscriber lines or subscriber terminals 
111-11.sub.m or 121-12.sub.m. In this case, the subscriber terminals 
include terminals for usual residence, public telephone sets, common 
telephone sets, PBX, key telephone sets, subscribers having common 
telephone numbers and a high percentage of calls, digital facsimiles, 
digital data interdigital telephone sets, and emergency telephone sets. 
Each of the subscriber line interface circuits 221-22.sub.m functions to 
assign to a predetermined traffic connected to respective subscriber 
lines, and vice versa. Consequently, each of these subscriber line 
interface circuits 221-22.sub.m has the features and capabilities of 
subscriber line signal transmitting and receiving trunk devices used for 
usual space division type telephone set, well known in the art, and a PCM 
channel encoding capability that converts such analog signals as voice 
signals into digital signals. That is, each of the subscriber line 
interface circuits 221-22.sub.m has so-called BORSCHT capabilities, which 
is an acronym for the combined features of battery (source supply), 
overload protection, ringing (ringing signal supply), supervision, codes 
(encoding and decoding performances), hybrid (two wire- four wire 
transformer) and test (subscriber's line testing performance). The use of 
such a term is disclosed in I.E.E.E. Spectrum, 1977, February, at page 46, 
for example. The component parts for realizing such a BORSCHT capability 
are required to have strict electrical characteristics. For example, due 
to leakage, a voltage of 350 V, and a current of 200 mA are applied to 
component parts. For this reason, component elements having high voltage 
durability and large power ratings are required. With recent developments 
in electronics, however, miniaturized and integrated thyristor switch 
elements satisfying the above described requirements have been developed. 
The output terminals of the subscriber line interface circuits 221-22.sub.m 
having the BORSCHT capabilities described above are connected to the 
interface 23 that multiplexes and demultiplexes voice signals and control 
signals. In this case, a predetermined channel is used between respective 
output terminals of the subscriber line interface circuits 221-22.sub.m 
and the interface 23 for transmitting and receiving the voice signals and 
the control signals. 
FIG. 2 illustrates channel assignment relations between the output 
terminals of respective subscriber line interface circuits 221-22.sub.m 
and the interface 23, in which letters V.sub.1, V.sub.2 . . . applied to 
the output sides of respective subscriber line interface circuits 
221-22.sub.m represent voice signals, and S.sub.1, S.sub.2 . . . represent 
control signals. These signals are assigned to respective channels, 
according to the timing shown in FIG. 3, to be sent to a common unit 50 in 
a succeeding stage. As can be noted from FIG. 3, the fact that the output 
conditions of respective subscriber line interface means are the same 
means than in each subscriber line interface means, the number of channels 
constituting one frame are the same. The output terminal of each 
subscriber line interface means is sequentially assigned to a 
corresponding channel according to an aligned order. Furthermore, as FIG. 
3 shows, a voice signal and a control signal corresponding to one 
subscriber line interface circuit are assigned to adjacent channels. For 
example, the channel to which the voice signal V.sub.1 is assigned at an 
output terminal of the subscriber line interface circuit 221, is adjacent 
to a channel for its control signal S.sub.1. 
The subscriber line interface means 21B of the second type consists of a 
one stage space division type concentration switch having a concentration 
ratio of n/m and having an input side connected to m subscriber lines, 
that is to the subscriber terminals 131-13.sub.m, and n (in this example 
n=2) subscriber line interface circuits 331 and 33.sub.n connected to the 
output side of the concentration switch. 
These subscriber line interface circuits 331 through 33.sub.n are identical 
or substantially identical to the subscriber line interface circuits 
221-22.sub.m described above and have aforementioned BORSCHT capabilities. 
The output terminals of the subscriber line interface circuits 331 through 
33.sub.n are connected to the interface 23 which multiplexes and 
demultiplexes the voice signal and the control signal. In this case, a 
predetermined channel is connected between the output terminals of the 
subscriber line interface circuits and the interface 23 for transmitting 
and receiving the voice signal and the control signal. 
In the system having the construction described above, when a terminal 111, 
for example of a telephone set, is in use, a voice signal V.sub.1 and a 
control signal S.sub.1 are sent to the interface 23 acting as the 
multiplexing and demultiplexing circuit. The multiplexed signals are sent 
to an interface 52 acting as a multiplexing and demultiplexing circuit 
where the signals are further multiplexed together with signals from SLIE 
20b and other data or signals. Among the multiplexed signals sent out from 
the interface 52, the voice signal is sent to a time division 
concentration switch 54, while the control signal is sent to a control 
circuit 55 and to a central control device CC, not shown. The voice signal 
collected or distributed by the time division concentration switch 54 is 
sent out to the digital highway 60 via the interface 56 acting as a 
multiplexing and demultiplexing circuit. 
Conversely, the voice signal V.sub.1 directed to the telephone set terminal 
111 is sent to the subscriber line interface means 21A of the first type 
via the digital highway 60, interface 56, time division concentration 
switch 54 and interfaces 52 and 23. The control signal S.sub.1 
corresponding to the voice signal V.sub.1 is sent from the central control 
device CC to the subscriber line interface circuit 221 via the control 
circuit 55 and interface 52 and 23 so as to control the subscriber line 
interface circuit 221 for sending the voice signal to the telephone set 
terminal 111. Since the method of processing the voice signal sent from 
the telephone set terminal 111 to the digital highway 60 or vice versa is 
well known, it will not be described herein. 
In this system, it is to be particularly noted that the subscriber line 
interface means 21A has no concentration performance but directly connects 
the subscriber terminal to the subscriber line interface circuit for 
forming a time divisioned signal to be assigned to a predetermined time 
slot or channel, and that whereas the subscriber line interface means 21B 
operates such that after concentrating the lines with a one stage space 
division concentration switch, it connects the line to the subscriber line 
interface circuit to obtain a time divisioned signal assigned to a 
predetermined time slot or channel. The use of two such types of 
subscriber line interface means permits standardization of the 
construction of the subscriber line interface circuit so as to meet 
various requirements, thus eliminating any special change in the design 
and wiring. 
It is also to be noted that the input/output conditions for the subscriber 
line interface means 21A and 21B are uniform. More particularly, each of 
these subscriber line interface means is constructed such that m 
subscriber terminals are connected to its input terminal. 
Since the subscriber line interface means 21A of the first type is 
connected directly to a subscriber line interface circuit, the number of 
its output terminals is the same as that of the input terminals, that is 
m. At these output terminals, voice and control signals are generated at 
predetermined timings which are assigned to predetermined adjacent 
channels when these signals are multiplexed in pairs. Accordingly, when 
viewed from the input side of the time division concentration switch 54 of 
the common time division concentration unit 50 it will be clear that 
signals at the output terminals of the subscriber line interface means 21A 
of the first type are assigned to predetermined channels according to the 
same order as the signals appearing at the output terminals of the other 
subscriber line interface means 21B. 
The same is true for the subscriber line interface means 21B of the second 
type. More particularly, the number of subscriber terminals is the same as 
that of the subscriber line interface means 21A, that is m. After being 
concentrated to a number n (in this example 2) by a space division 
concentration switch 32, the terminals are connected to the subscriber 
line interface circuits 331 through 33.sub.n. Consequently, although the 
number of the output terminals of this subscriber line interface means 21B 
is n, the voice and control signals appearing thereon are assigned to the 
same channels to which signals of the subscriber line interface means 21A 
have been assigned. As a consequence, where the space division 
concentration switch 32 has a concentration ratio of 1:1, the signals at 
respective output terminals are assigned to the same channel according to 
their order as that to which the signals of the subscriber line interface 
means 21A have been assigned. 
Consequently, the subscriber line interface means 21A and 21B are 
exchangeable. Thus, when these circuit means are formed into plug-in type 
packages in which each subscriber line interface means is fabricated on a 
single printed substrate, the concentration system can be changed by 
merely substituting one package with the other. 
This will be described with reference to FIG. 2. For example, when the 
percentage of call loss of subscriber terminals on which the subscriber 
line interface means 21B has been mounted exceeds a predetermined ratio 
and the concentration ratio of the space division concentration switch 32 
has decreased, but still a large percentage of call loss appears, the 
subscriber line interface means 21B of the second type is replaced by the 
subscriber line interface means 21a of the first type. In this case, to 
the input lines l.sub.1B of the interface 23 to which signals (V.sub.1, 
S.sub.1) at the output terminals of the present subscriber line interface 
means 21B have been applied, will be connected to the output terminals 
which transmit and receive the signals (V.sub.1, S.sub.1) of the 
subscriber line interface means 21A which are sent out at the same 
timings. As a consequence, even when the subscriber line interface means 
are exchanged, signals appearing on the multiplexed output side of the 
interface 23 would not change. 
FIG. 4 shows the detail of the subscriber line interface means 21A in which 
the interface 23 is divided into a plurality of groups and one of them is 
incorporated into the subscriber line interface means 21A. In FIG. 4, 
respective subscriber line interface circuits 221-22.sub.m are provided 
with independent control circuits 251-25.sub.m which extract only a 
control signal S supplied from an interface 23A at a predetermined timing. 
Based on the control signal S.sub.1 to S.sub.m, each control circuit 
performs such control functions as sending a call signal to a 
corresponding subscriber line, detection of a response signal, 
transmission of a reverse signal, etc. The control signal S.sub.1 to 
S.sub.m is sent to the interface 23A from the common time division 
concentration unit 50 via a multiplexed line. 
A state signal regarding each subscriber line, for example a signal 
obtained by scanning the state of each subscriber line at a predetermined 
timing, is sent to the interface 23A through opposite routing at a timing 
at which the signal is inserted into a specifically assigned channel. This 
signal is sent to the common unit 50 through the interface 23A to be used 
as a control signal. The extraction, and transmission of these control 
signals, as well as the control operations of the telephone exchange based 
on these signals will not be described herein since they are well known in 
the art. 
Regarding FIG. 4, it should be particularly noted that the interface 23A 
has been incorporated into the subscriber line interface means 21A. Then, 
when compared with the example shown in FIG. 1, it is possible to decrease 
the number of the external terminals of the package, thus greatly 
simplifying the peripheral circuits. 
FIG. 5 shows in more detail the subscriber line interface means 21B in 
which the interface 23B is incorporated into the subscriber line interface 
means 21B. In FIG. 5 a one stage space division concentration switch 32 
concentrates lines from the subscriber terminals on the space division 
basis and selectively connects the lines to the subscriber line interface 
circuits 331 to 33.sub.n. The space division concentration switch 32 has a 
matrix construction and at the cross-points of the matrix is connected to 
well known high voltage IC switches which are selectively rendered on by 
the outputs of decoders 351 to 35n so as to interconnect one of the 
subscriber terminals and one of the subscriber lines. 
For the purpose of checking the states of respective subscriber terminals, 
sensors XPS are provided for the space division concentration switch 32. 
By controlling a decoder 42 from an auxiliary controller 40 through a 
counter 37, supervisory signals can be obtained. These signals are sent to 
an interface 23B, via a route of a subscriber line scanning circuit 
36-*2-, the counter 37 where they are multiplexed and then sent to the 
common time division concentration unit 50. 
When supplied with a control signal from the common time division 
concentration unit 50 via the interface 23B, the auxiliary controller 40 
transfers the control signal to decoders 351 to 35n. The decoders 351 to 
35n turn on IC switch XP respectively in accordance with the control 
signal, when signals are supplied to the decoder from the control circuits 
341 to 34n. In addition to the control of the IC switch XP, the auxiliary 
controller 40 drives the counter 37 designating the order of scanning of 
the subscriber line to send the output of the counter to the decoder 42 so 
as to control the operation of the subscriber line scanning circuit 36. 
The subscriber line interface circuits 331 through 33.sub.n have equal or 
substantially equal BORSCHT capabilities as the subscriber interface 
circuits 221-22.sub.m utilized in the subscriber line interface means 21A 
described above, and control circuits 341 through 34.sub.n so as to 
control the subscriber circuits 331 through 33.sub.n based on the control 
signals supplied from the interface 23B and to send out a state signal 
regarding a subscriber line to the interface 23B. The subscriber line 
interface circuits have the same construction and operation as those of 
the subscriber line interface circuits already described. 
Regarding FIG. 5 it should be particularly noted that the interface 23B has 
been incorporated into the subscriber line interface means 21B. Then, in 
the same manner as in the subscriber line interface means 21A shown in 
FIG. 4, it is possible to decrease the number of the outside terminals of 
the package, thus simplifying the peripheral circuits. 
FIGS. 6a and 6b show examples in which the subscriber line interface means 
21A of the first type, and a plurality of subscriber line interface means 
21B of the second type are assembled into a one-stage unit and the number 
of units mounted on supporting frames are varied according to the number 
of traffic calls of a telephone exchange. In FIGS. 6a and 6b, COM 
designates a common control unit, HYB.LC a unit including the subscriber 
line interface means 21A and 21B of the first and the second type (usually 
about 256 lines/units), TD.LC(A) a unit for subscribers having high 
percentage of calls which includes the subscriber line interface means 21A 
of the first type (usually about 128 lines per unit), and TD.LC(B) a unit 
for such special subscribers as public telephone sets which includes the 
subscriber line interface means 21A of the first type (usually, about 64 
lines per unit). In a telephone exchange office in which percentage of the 
subscriber calls is relatively low, a number of HYB.LC units are mounted 
as shown in FIG. 6a, whereas in a telephone office in which the percentage 
of subscriber calls is high, the mixing ratio of the interface means 21A 
and 21B of the HYB.LC unit is changed and the numbers of TD.LC(A) units 
and TD.LC(B) units mounted are increased. In addition, residence telephone 
sets having low percentage of calls are accommodated in the HYB.LC unit 
while other subscribers having high percentage of calls such as companies, 
offices, PBX, etc., are accommodated in the TD.LC(A) unit, while such 
special subscribers as public telephone sets are accommodated in the 
TD.LC(B) unit. 
After installation, if it becomes necessary to change the degree of 
concentration as a result of measurement of the traffic data of a 
telephone office, an existing circuit means is dismounted or plugged out 
and then subscriber line interface means 21A and 21B are mounted or 
plugged in. Thus, it is possible to change the degree of concentration by 
such simple operation. Consequently, the versatility of the system is much 
higher than that of a prior art fixed system. 
FIG. 7 illustrates a modification of the composite concentration system 
embodying the invention in which subscriber line interface means 21A and 
21B are controlled by standardized software, thereby simplifying the 
expansion, maintenance and supervision of the software. 
A telephone exchange for public telephones utilized in this system and 
controlled by a stored program is required to operate and maintain the 
software during a long life of the telephone exchange, so that it is 
advantageous to connect respective terminals to the digital highway under 
the control of standardized software. For this reason, in this 
modification, instead of the space division concentration switch 32 
contained in the subscriber line interface means 21B of the second type 
and in the subscriber line interface means 21A of the first type, there is 
provided a fixed time division switch matrix 28 having a concentration 
ratio of 1:1 corresponding to the space division concentration switch 32. 
With this construction, the central control device CC reads subscriber data 
(usually stored in a main memory device of the central control device CC) 
corresponding to a terminal 131 and based on the address number storing 
the data, it is judged that the data concerns a subscriber line interface 
means 21B having the space concentration switch 32, whereby a control 
signal for driving the space division concentration switch 32 is sent to 
the controllers 40 and 341 via the time division control circuit 55 of the 
common unit 50, time division concentration switch 54 and interfaces 52 
and 23B. Then an IC switch at a cross-point XP of a matrix of the space 
division concentration switch 32 is closed to interconnect terminal 131 
and a specific channel of the digital highway 60. 
When the data read out by the central control device CC belongs to a 
subscriber line interface means 21A not provided with a space division 
concentration switch, the central control device CC sends a signal only to 
the controller 251 via the control circuit 55, time division concentration 
switch 54 and interfaces 52 and 23A, and, therefore, it is unnecessary to 
send out a signal corresponding to a signal sent to the controller 40 of 
the subscriber line interface means 21B. Although in the subscriber line 
interface means 21A a switch matrix 28 was provided corresponding to the 
space division concentration switch 32, actually, it is not necessary to 
provide such a switch matrix. Theoretically, it is considered that 
semi-fixed connections are made. Thus, the fundamental concept of this 
invention lies in that a standardized design is made with a link matching 
program including a space division concentration switch 32 accommodating 
subscribers having a low percentage of calls, that terminals not requiring 
the space division concentration switch 32 are considered to be connected 
semi-fixedly, that such concentration switch and its drive circuit are 
eliminated and that a cross-point drive instruction is not sent, or even 
when it is sent it is discarded. 
As above described according to this embodiment, signals from subscribers 
having low percentage of calls are passed through the space division 
concentration switch 32 and then concentrated, whereas signals from 
special terminals or subscribers having a high percentage of calls are 
directly concentrated, on the time division basis, without passing them 
through the concentration switch. Moreover, since the system is 
constructed to simultaneously control these signals, the system can be 
economically applied to traffic conditions of a wide range and the 
maintenance and supervision of the software are simple. 
In still another modification of this invention shown in FIG. 8 a telephone 
set P or Q are important public subscriber and it is supposed that the 
telephone set P is inherently accommodated by subscriber line interface 
means 21A. The subscriber line interface means 21A package accommodating 
the subscriber P also accommodates other subscribers. Should one of the 
packages become faulty, it is necessary to replace it with a new one, thus 
resulting in an interruption of the service. To prevent this, in the 
modification shown in FIG. 8, the talking line of the subscriber P is 
connected to the main subscriber line interface circuit 221 via a line 113 
and is also connected to one input line of a subscriber line interface 
means 21B. With this construction, so long as the subscriber line 
interface circuit 331 is normal, such services as connections and speech 
are made through this subscriber line interface circuit and until a 
subscriber line interface means 21A to which the subscriber line interface 
circuit 221 belong becomes faulty or exchanged, the service can be 
maintained through a routine including the subscriber line interface means 
21B. The detail of the connection of the elements bounded by dot and dash 
lines is shown in FIGS. 9a and 9b. In a case shown in FIG. 9a, when the 
subscriber line interface means 21A is inserted the contacts 115 and 116 
are separated, and the lines 112 and 114 are disconnected. The contact 116 
is connected to a floated or grounded terminal 117 formed on one surface 
of the board 100 of the subscriber line interface means 21A, whereas the 
contact 115 is connected to a contact 113 so that the software can process 
calls without considering a dual reversion under normal state. For 
exchanging a faulty subscriber line interface means 21A, when it is drawn 
out, the contacts 115 and 116 contact each other so as to connect the 
terminal 111 to the subscriber interface means 21B. At the same time, the 
change in the position of an accommodated subscriber and continuation of 
the service are made possible by sending to the central control device CC 
a command signal by an operator or detection of the address position of a 
withdrawn subscriber line interface means (an output Z shown in FIG. 9a 
becomes "1") when the subscriber line interface means is withdrawn. In the 
case shown in FIG. 9b, since the condition is a dual reversion, call 
detection outputs of a subscriber are produced from two portions. As a 
consequence, a program is prepared which makes effective only a detected 
output on the side of the main accommodating position under normal 
condition whereas the detected output on the side of an auxiliary 
accommodating position is made not effective, and when the main side 
becomes faulty, the program is switched by software. 
In the foregoing, an important subscriber P in which a subscriber line 
interface means 21A and a subscriber line interface means 21B are 
apparently connected in multiple, the other importanct subscriber Q in 
which two interface means are connected in multiple can be treated 
similarly for increasing reliability. 
It should be understood that the invention is not limited to the foregoing 
embodiments, and that many changes and modifications can readily be made 
by one skilled in the art without departing from the scope of the claims 
appended hereto.