Method of and apparatus for networking a plurality of private branch exchange systems using

A method for networking a plurality of the private branch exchange systems is disclosed in which it is possible to both basically exchange information for the networking by using an ISDN line as a private wire in order to connect mutually a plurality of the private branch exchange systems and transmit/receive necessary information between systems by employing D channel with a line signal and an ISDN protocol and designating the specific channel of B channel to a networking private channel for the networking operation. In addition, one or two channels of several channels between a plurality of the private branch exchange systems can be used for exchanging information, so that the exchange of data between systems operating on a network enables expansion of a system and share of common resources, thereby to improve the efficiency of a system.

CLAIM FOR PRIORITY 
This application makes reference to, incorporates the same herein, and 
claims all benefits accruing under 35 U.S.C. .sctn.119 from an application 
for METHOD FOR NETWORKING A PLURALITY OF PRIVATE BRANCH EXCHANGE SYSTEMS 
earlier filed in the Korean Industrial Property Office on Jul. 31, 1996, 
and there duly assigned Serial No. 32018/1996, a copy of which application 
is annexed hereto. 
BACKGROUND OF THE INVENTION 
1. Technical Field 
The present invention relates to a network of private branch exchange 
systems using ISDN lines, and more particularly, relates to a method for 
networking a plurality of the private branch exchange systems for sharing 
common resources by connecting mutually the private branch exchange 
systems using ISDN lines. 
2. Related Art 
Generally, a conventional telephone network is a dedicated service network 
intended to provide only specific communication service to terminal 
subscribers. Typically, private branch exchange systems are connected to a 
public switched telephone network (PSTN) via private wires to provide 
voice communication services. In contrast to a conventional telephone 
network, integrated services digital network (ISDN) offers a variety of 
new features including multimedia communication service such as voice, 
high speed data and image communication services and other additional 
non-voice communication services via network interfaces on the basis of 
digitization of the telephone network. These new features have attracted 
contemporary private branch exchange systems for integration to benefit 
from ISDN services such as disclosed, for example, in U.S. Pat. No. 
5,305,313 for Electronic Switching System For Use In Connection To An ISDN 
And Method Of Setting Communication Disconnection Reasons issued to Katoh, 
U.S. Pat. No. 5,450,396 for Communication System And A Private Branch 
Exchange To Be Used In Such A Communication System issued to Havermsan, 
U.S. Pat. No. 5,481,605 for Private Branch Exchange Capable Of Analyzing 
Information Received From ISDN issued to Sakurai et al., and U.S. Pat. No. 
5,621,731 for Private Exchange For ISDN issued to Dale et al. 
As the ISDN services become widely in use, the private exchange and its 
terminal stations connected to the ISDN must effectively accommodate 
multimedia communication services and a variety of other additional 
services through the ISDN. ISDN interfaces must be provided in the private 
branch exchange system on the basis of network interface standards. 
Exemplary ISDN interfaces for private branch exchange system are disclosed 
in U.S. Pat. No. 5,305,312 for Apparatus For Interfacing Analog Telephones 
And Digital Data Terminals To An ISDN Line issued to Fornek et al., U.S. 
Pat. No. 5,450,486 for Telecommunication System And A Linecard issued to 
Maas et al., U.S. Pat. No. 5,319,640 for Transmitting/Receiving System 
Having Digital Switching Network issued to Yamasaki et al., and U.S. Pat. 
No. 5,550,822 for Private Branch Exchange issued to Ikeda. ISDN interface 
facilities may be used to facilitate communications and signaling between 
a plurality of private branch exchange systems in a network as disclosed, 
for example, in U.S. Pat. No. 5,422,943 for Private Branch Exchange 
Networks issued to Cooney et al., and U.S. Pat. No. 5,455,855 for System 
For Connecting Public Network Subscriber And Private Network Subscriber 
issued to Hokari. While these ISDN interfaces are implemented in 
accordance with network interface standards and networking between private 
branch exchange systems are possible, I have noted that further 
improvement in a method for enabling networking between a plurality of 
private branch exchange systems each using an ISDN line as a private wire 
can still be contemplated. 
SUMMARY OF THE INVENTION 
Accordingly, it is therefore an object of the present invention to provide 
a network of private branch exchange systems mutually connected together 
using ISDN Primary Rate Interface (PRI) as a private line for enabling the 
basic exchange of information and sharing common resources. 
It is also an object of the present invention to provide a method for 
enabling the basic exchange of information for the networking of private 
branch exchange systems using an ISDN Primary Rate Interface as a private 
wire for mutual connection. 
It is another object of the present invention to provide a method for 
enabling the transmission/reception of necessary information between 
private branch exchange systems by employing D channel with a line signal 
and ISDN protocol and designating the specific channel of B channel as a 
networking private channel for the networking operation. 
These and other objects of the present invention can be achieved by a 
method for connecting mutually a first PBX system A and a second PBX 
system B, comprising the steps of: (a) analyzing the called telephone 
number through the first PBX system A to determine in which system the 
number exists and determining whether or not said call is a networking 
call when dialing at a telephone A of the first PBX system A in the case 
of telephoning over the telephone in the system A connected to the system 
B; (b) connecting a call if X channel is active after determining whether 
X channel is in an active state and sending a call to the system B by 
calling said X channel and the call simultaneously and if not, prior to 
the formation of a call using a PRI line; and (c) sending and receiving 
information between two systems A and B through the X channel, which is 
generated while trying to make a telephone call, the line being busy or a 
telephone call being over and maintaining the information during a regular 
time although there is no related call for the X channel as required. 
The present invention is more specifically described in the following 
paragraphs by reference to the drawings attached only by way of example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings and particularly to FIG. 1, which illustrates 
a contemporary private branch exchange (PBX) system. Generally, the 
private branch exchange (PBX) system is connected using mainly a private 
wire which is regarded as a private central office. For purposes of 
networking, at least two private branch exchange (PBX) systems are 
mutually connected via private wires. However, there is no special method 
for networking a plurality of the private branch exchange systems. 
As shown in FIG. 1, the typical private branch exchange (PBX) system 
includes a central processing unit (CPU) 10 for controlling not only a 
telephone call switching by accessing a predetermined program but also all 
kinds of services and functions according to the present invention. A 
read-only-memory (ROM) 20 stores an application program for performing a 
call and a variety of functions including initial service data. A 
random-access-memory (RAM) 30 is used as a working memory of the CPU 10 
having a program and data processing area. An extension circuit 40 
provides an extension telephone of a extension subscriber with the 
telephone call current of a voice band and carrying out the interfacing 
operation between extension devices. A network 50 controls the switching 
of various tones and voice data under the control of the CPU 10. A tone 
supplier 60 generates a variety of signals under the control of the CPU 10 
and supplies the network 50 with the generated signals. A ring generator 
70 supplies the network 50 with a rectangularly shaped ring signal under 
the control of the CPU 10. A DTMF receiver 80 analyzes a multi-frequency 
code signal generated from a subscriber of the extension circuit 40 and 
applies the relevant digital data corresponding to the analysis to the CPU 
10. A DTMF transmitter 90 transmits the multi-frequency code signal under 
the control of the CPU 10, and a central office line circuit 100 for 
interfacing a signal incoming from a central office line by forming a 
central office line loop under the control of the CPU 10. 
A typical network connection between a plurality of the private branch 
exchange (PBX) systems is generally made using non-ISDN lines through the 
central office line circuit 100 for communications services. Non-ISDN 
lines are, however, incapable of transmitting and receiving networking 
information between PBX systems. In particular, there is no method which 
can make use of common resources which often are available for networking. 
As a result, networking operations are not supported during a basic 
telephone call. In addition, the connection between a plurality of PBX 
systems through a private wire imposes a variety of restrictions to the 
type of communication services available. 
Turning now to FIG. 2 which illustrates a network connection between, for 
example, three private branch exchange (PBX) systems 201-203 and the range 
of a calling number is 300 units for a first private branch exchange 
system A 201, the range thereof is 400 units for a second private branch 
exchange system B 203 and the range thereof is 500 units for a third 
private branch exchange system C 204. FIG. 3A illustrates a call outgoing 
table of X channel (i.e. B channel for networking purposes) according to 
an embodiment of the present invention, and FIG. 3B illustrates a call 
incoming table of X channel (i.e. B channel for networking purposes) 
according to an embodiment of the present invention. 
Now, an example of the operational process of executing the operation 
according to an embodiment of the present invention is set forth with 
reference to FIG. 4. 
At step 4a, the dialing operation of an extension number of a second PBX 
system B at an extension of a first PBX system A is performed. Then, the 
program proceeds to step 4b at which point it is determined whether or not 
the call is a networking call within the range of a calling number after 
analyzing a calling number table. If the call is not a network call within 
the range of a calling number, a common call is sent at step 4i. If, on 
the other hand, the call is a network call within the range of a calling 
number, the program proceeds to step 4c at which point the status of X 
channel (i.e., B channel for networking purposes) is read after checking 
that the call is a node of which system by referring to a table of X 
channel. Then, at subsequent step 4d, it is determined whether or not the 
status of X channel is active. If YES, i.e. if state of X channel is 
active, the program proceeds to step 4g at which point the network call is 
transmitted normally and networking message generated while connecting a 
call is transmitted and received through X channel at step 4h. That is, 
program permits networking between the first PBX system A and the second 
PBX system B during the network call through X channel. 
If the status of X channel is not active, however, the program proceeds to 
step 4e at which point a call is connected so as to set up the channel 
designated to X channel at the same time, the call is sent as an original 
call. Then, the message between the two PBX systems are transmitted and 
received through X channel after comparing a X channel number with a call 
incoming table of X channel if the X channel number is called and 
connecting X channel if the call is called in telephone number for X 
channel at step 4f. That is, the program permits networking between the 
first PBX system and the second PBX system during the network call through 
the channel designated as X channel. Then, the program is terminated. 
An explanation on the operation according to an embodiment of the present 
invention will be described in detail with reference to FIGS. 1 to 4 
hereinbelow. 
As shown in FIG. 1, the ROM 20 or the RAM 30 of each node exchange system 
stores a call incoming table of X channel and a call outgoing table of X 
channel. For example, in the case of trying to call an extension number of 
the second PBX system B 203 at an extension of the first PBX system A 201, 
since the range of calling number is set to 401-499 for the table of X 
channel of the second PBX system B 203, the CPU 10 reads the number table 
as shown in FIG. 3A from the ROM 20 and analyzes the same, and then checks 
that the call is a network call at step 4b. If the call is not a network 
call within the range of a calling number, a common call is sent at step 
4i. On the other hand, at step 4b, i.e. if the call is a network call 
within the relevant range of a calling number as shown in FIG. 3A, the 
program proceeds to step 4c at which point the status of X channel is read 
after checking that the call is a node of which system by referring to a 
table of X channel. The X channel is designated to a specific channel in a 
system and if a call is called, it is confirmed through a calling party 
number and the calling party sends a call by the telephone number for X 
channel, i.e. a calling party number. 
If it is determined that X channel calling party number within the relevant 
range of a calling number as described is active at step 4d, the call is 
transmitted normally at step 4g and networking message generated while 
connecting a call is transmitted and received through X channel at step 
4h. On the other hand, if state of X channel is in an idle state at step 
4d, i.e. if it is not active, the program proceeds to step 4e at which a 
call is connected so as to set up the channel designated to X channel, at 
the same time, the call is sent as an original call. At next step 4f; if 
the X channel number is called at the called party, it is connected to the 
telephone number for the X channel by referring to a call incoming table 
of X channel as shown in FIG. 3B. At this time, the message between the 
two PBX systems are transmitted and received through X channel. Then, the 
program is terminated. 
As described above, in the present invention, one or two channels of 
several channels between a plurality of the private branch exchange PBX 
systems can be used for exchanging information, so that the exchange of 
data between PBX systems connected in a network enables expansion of a PBX 
system and effectively shares common resources, thereby to improve the 
efficiency of a system. In addition, the present invention advantageously 
seeks to connect mutually and closely a plurality of PBX systems in such a 
way that it is now possible to use many PBX systems as if they were one 
single large communication system. 
While there have been illustrated and described what are considered to be 
preferred embodiments of the present invention, it will be understood by 
those skilled in the art that various changes and modifications may be 
made, and equivalents may be substituted for elements thereof without 
departing from the true scope of the present invention. In addition, many 
modifications may be made to adapt a particular situation to the teaching 
of the present invention without departing from the central scope thereof. 
Therefore, it is intended that the present invention not be limited to the 
particular embodiment disclosed as the best mode contemplated for carrying 
out the present invention, but that the present invention includes all 
embodiments falling within the scope of the appended claims.