Method and system for arranging common channel signal network through

A method and system for arranging a common channel signal through an ISDN in a switching network. In the switching network, a signal is transferred between the switching stations in a common channel signalling manner. This method and system serves to allocate a signal path between the stations through the ISDN without having to provide a dedicated channel between the stations as a common channel signal path. Then, the method and system establishes a temporary connection between the stations through the ISDN using a channel in the ISDN (for example, D channel) used for controlling the call setup between the ISDN and a terminal and transfers a common channel signal between the stations by transferring transparent user information to the ISDN. In the common channel signal network in which a common channel signal is transferred through a leased line, the signal transfer is switched from the leased line to the ISDN when failure takes place in the common channel signal path.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and system for arranging a common 
line signal network through an ISDN (Integrated Service Digital Network) 
in a switching network in which a signal is transferred between switching 
stations through a leased line in a common channel signalling manner. More 
particularly, the invention relates to a method and a system for 
alternating a common channel signal path using the ISDN in a common 
channel signal network in which a signal is transferred through a leased 
line. 
The conventional local switching network includes local switching stations 
and leased lines connecting the local switching stations. For using the 
leased lines, the contract is made on a line number and a using period. 
Recently, there has been a strong desire for expanding the service inside 
of one switching station to that between the switching stations. For 
achieving the service between the switching stations, a common channel 
signalling system has been used. The common channel signalling system is 
designed to transfer a signal between switching stations through a common 
signal line provided independently of a speech channel. The system thus 
makes it possible to transfer a signal during speech as well as without 
any concern with a call. If the switching network employs the conventional 
leased lines and a common channel signalling system for transferring a 
signal between switching stations, the network is designed to transfer a 
signal between switching stations by allocating a specific channel of the 
leased lines to a common channel signalling path. This kind of common 
channel signalling system has been disclosed in Japanese patent 
application laid-open No. JP-A-61-10347. 
As described above, the foregoing conventional technology is designed so 
that a switching station allocates a specific channel of the leased lines 
to an object switching station in a fixed manner. Hence, if the switching 
station has only a small number of lines, the conventional technology 
lowers the efficiency of use of the leased lines. 
SUMMARY OF THE INVENTION 
In order to overcome the shortcoming, it is therefore an object of the 
present invention to provide a method and a system for arranging a common 
channel signal network through an ISDN which method and system are capable 
of making the most use of lines. 
It is another object of the present invention to provide a method and a 
system for arranging a common channel signal network which method and 
system have a reduced number of signal devices used in the arrangement. 
To achieve the foregoing objects, according to an aspect of the invention, 
the method and system for arranging a common channel signal network 
through an ISDN is designed to allocate a signal path between the 
switching stations through an ISDN if necessary without providing a leased 
signal channel served as a common channel signal path between both 
stations, establish a temporary connection between both switching stations 
through the ISDN using a channel (for example, D channel) employed for 
setting up a call between the ISDN and the terminal, and transfer 
transparent user information (for example, UUI or D channel packet) to the 
ISDN for the purpose of realizing signal transfer between the switching 
stations. Further, for the common channel signal network in which a signal 
is transferred through a leased line, the method is used for switching the 
common channel signal network to the ISDN if failure takes place in the 
common channel signal network. 
The foregoing method and system for arranging a common channel signalling 
network through an ISDN is capable of transferring a signal between the 
switching stations by, if necessary, establishing a connection between 
both of the stations using a channel (D channel) employed for setting up a 
call between the ISDN and a terminal. Hence, the method makes it possible 
to improve the efficiency of use of the lines, because it requires no 
leased signal device and does not exclusively use a leased-line channel as 
a signal path. This function can reduce the cost of the signalling device 
and improve the efficiency of use of the lines. In the common channel 
signalling network in which a common channel signal is transferred through 
a leased line, the signalling device and the signal path may often employ 
redundant arrangements as a result of considering the operation of them at 
the failure time. If, therefore, failure takes place in the signal device 
and the signalling path, the ISDN signal path may be used as an alternate 
path, thereby making efficient use of the lines and reducing the cost of 
the signalling device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Herein, a description will be directed to an embodiment of the invention 
with reference to FIGS. 1 to 5. 
FIG. 1 shows a common channel signal relaying system, which is used for 
illustrating a method and system for arranging a common channel signal 
network through an ISDN designed according to an embodiment of the 
invention. As shown in FIG. 1, 1 and 1' denote switching stations, which 
respectively includes switches 2, 2', central control devices 3, 3', 
speech trunks 4, 4', ISDN trunks 5, 5' used for connecting an ISDN, bus 
interface units 6, 6' through which the central control devices 3, 3' 
control the ISDN trunks 5, 5', a relay line 7 used by the speech trunks 4, 
4', an ISDN 8 connected to the ISDN trunks 5, 5', and terminals 9, 9'. 
FIG. 1 shows logical arrangement of sections for transferring a common 
line signal through the ISDN 8, which is closely concerned with the 
invention. That is, the arrangement requires no sections and devices other 
than those the switching station normally requires. 
In FIG. 1, the common channel signalling system employs the relay line 7 as 
a speech path for call setup between the terminals 9 and 9', while it 
employs a leased signal path provided independently of the speech path 
when transferring call setup information between the switching stations 1 
and 1'. This design allows control information to be transferred between 
the switching stations 1 and 1' during the speech time as well as 
independently of a call. It results in enabling the system to perform 
various kinds of services. In this embodiment, the description will be 
expanded on the basis of the office network built by applying the common 
channel signalling system to a between-stations signalling system provided 
between local switching stations in the office. For building the office 
network, the relay line 7 normally employs a leased line. In case of 
applying the between-stations signalling system to the common channel 
signalling system, the conventional technology serves to allocate a 
specific channel of the leased lines to a common channel signal path. The 
present embodiment, on the other hand, serves to transfer the common 
channel signal between the switching stations 1 and 1' through a D 
channel. The D channel is normally used for transferring setup information 
between the network and the user in the ISDN 8. 
FIG. 2 is a functional block diagram showing how to transfer a common 
channel signal through the D channel shown in FIG. 1. In FIG. 2, the 
function arrangement (mainly, central control devices 3, 3') about the 
transfer of a common channel signal between the switching stations 1, 1' 
consists of setup control sections 10, 10' and signal packet handling 
sections 11, 11'. The setup control sections 10, 10' serve to perform the 
setup controls such as interpretation of dialing digits sent from a 
terminal or another station, selection of the speech trunks 4, 4', and 
control of a speech path between the terminal and the speech trunk. The 
common channel signal designed according to the format regulated on the 
CCITT recommendation is transferred between the setup control sections 10 
and 10' of the switching stations 1 and 1' in a manner to control the call 
setup between the terminals 9 and 9'. The signal packet handling sections 
11, 11' have a function of transferring common channel signals to be sent 
to the call setup control sections 10, 10' between the switching stations 
1 and 1' through the ISDN 8. Further, these sections 11, 11' perform 
edition of signal packets, routing of signal packets, I/O control of the 
common channel signal to or from the D channel, and transfer control of 
the common channel signal to the call setup control sections 10, 10' so 
that a temporary connection for transferring signal packets is established 
between the signal packet handling sections 11, 11' of the switching 
stations 1, 1'. The temporary connection denotes such a connection between 
the terminals 9 and 9' as enabling a user signal to be transferred without 
establishing a circuit-switching call through the ISDN 8. FIG. 3 shows a 
call setup control sequence using the common channel signal shown in FIG. 
2. In FIG. 3, when the switching station 1 calls the terminal 9, the setup 
control section 10 serves to interpret dialing information (100) of 
another station. If another station is the terminal 9' of the switching 
station 1' having a route connected by the common line, the setup control 
section 10 selects the speech trunk 4 for the switching station 1' and 
sends out a start signal 1A1 (101) to the switching station 1' through the 
common channel signal. 
FIG. 4 shows a common channel signal format used in the call setup control 
sections 10, 10' employing the common channel signalling system shown in 
FIG. 2. The shown format is a CCITT-recommended normal signal format used 
for controlling the call setup between the stations and consists of a 
label 201, a header code 202 and additional information 203. The label 201 
consists of a receive station number DPC, a send station number OPC and a 
speech line number CIC. The call setup control sections 10, 10' of the 
switching stations 1, 1' use the label 201 for identifying a call. The 
header code 202 indicates a type of a signal to be transferred by the call 
setup control sections 10, 10'. The additional information 203 means data 
defined at each signal. For example, the start signal IAI contains 
additional information such as a selecting number and a call type of 
another terminal distributed according to the prescribed allocation. 
The signal packet handling section 11 receives a transfer request of the 
common channel signal employing the signal format shown in FIG. 4 edited 
in the setup control section 10 shown in FIG. 3. It serves to select the 
ISDN trunk 5 as an output path for the purpose of transferring the common 
channel signal to another switching station 1' through the D channel 
containing the ISDN 8. The common channel signal transferred from the call 
setup control section 10 is edited to a signal packet format and then 
output to the D channel. Unless a temporary connection for transferring 
signal packets is established between both the signal packet handling 
sections 11, 11' and the switching stations 1, 1', it is necessary to 
establish the connection according to the following procedure. The 
temporary connection for transferring signal packets is established on the 
procedure about a circuit-switching call setup between the ISDN and a 
user. 
FIGS. 5A and 5B show signal packet formats provided in the signal packet 
handling sections 11, 11' shown in FIG. 3. FIG. 5A shows a signal packet 
format (see 102 to 107 shown in FIG. 3) designed according to a 
network-user interface regulation for establishing a temporary connection 
through the ISDN 8. FIG. 5B shows a signal packet format (see 108 to 131 
shown in FIG. 3) used in a data transfer mode 140 of common channel signal 
information about the setup call to be transferred from the call setup 
control sections 10, 10'. After establishing the temporary connection, the 
signal packet shown in FIG. 5B, that is, the packet used in the data 
transfer mode transfers the transparent user information indicated therein 
to another terminal (the signal packet handling section 11' of another 
station designed according to the embodiment) through the ISDN 8 if the 
signal type of the common portion concerns with the user information. 
In response to a transfer request of the start signal IAI (101) of another 
station from the call setup control section 10, the signal packet handling 
section 11 shown in FIG. 3 established a temporary connection. Then, it 
serves to select the receive number of the ISDN trunk 5' based on a 
receiving station number DPC contained in the common channel signal 
information transferred from the call setup control section 10. And, it 
selects a transmit route to the ISDN 8 according to the normal 
route-selecting process. After establishing a temporary connection for the 
signal packet transfer, the signal packet handling sections 11, 11' enter 
into the data transfer mode 140 and send the signal to be transferred from 
the call setup control sections 10, 10' to the D channel of the ISDN 8 or 
transfer the signal to be input from the D channel to the call setup 
control sections 10, 10'. And, in the data transfer mode 140, the signal 
packet handling sections 11, 11' send back an acknowledge signal RR (110, 
116, 121, 126, 130) to the sender for acknowledging the transfer of signal 
packets when they receive the signal packets (108, 113, 119, 124, 128) as 
indicated in the call setup sequence example. If the sender cannot obtain 
any acknowledge, the signal packet handling sections 11, 11' perform the 
re-transmitting process. 
In the data transfer mode 140 shown in FIG. 3, the call setup control 
sections 10, 10' of the switching stations 1, 1' serve to establish calls 
of the terminals 9, 9' based on the setup procedure designed according to 
the normal CCITT recommendation. When the terminals 9, 9' finish each 
speech, the signal packet handling sections 11, 11' have already 
established the temporary connection for transferring the signal packets. 
Hence, the setup control section 10 of the switching station 1 is capable 
of transferring a release signal CLF (123) to the call connection control 
section 10' of the switching station 1' in the data transfer mode (140). 
On the other hand, the switching station 1' serves to transfer a release 
signal RLG (127) to the switching station 1 in the data transfer mode 
(140). In response to the release signal RLG (129), the switching station 
1 makes sure that the resource is released in the call setup control 
section 10 and sends out a request of releasing the temporary connection 
for transferring the signal packets to the signal packet handling section 
11. In response to the request of releasing, the signal packet handling 
section 11 sends out a request (132) of releasing the signal packet format 
indicated in FIG. 5A to the ISDN 8 for the purpose of releasing the 
connection with another station. 
The present embodiment makes it possible to establish a common channel 
signal path through the D channel of the ISDN without having to hold a 
leased signal path required for transferring call setup information 
between the switching stations in a common channel signalling manner. 
FIG. 6 is a block diagram showing a common channel signal relay system, 
which indicates the other embodiment of a method and system for arranging 
a common channel signal network through the ISDN. In FIG. 6, the switching 
stations 1, 1' respectively include switches 2, 2', central control 
devices 3, 3', leased line trunks 4, 4', ISDN trunks 5, 5', bus interface 
units 6, 6', a relay line 7 used by the leased trunks 4, 4', an ISDN 8 
connecting the ISDN trunks 5, 5', terminals 9, 9', leased line trunks 12, 
12', a relay line 7' connected to the leased line trunks 12, 12', and 
signalling devices 13, 13'. The relay lines 7, 7' connected between the 
switching stations 1 and 1' shown in FIG. 6 employ leased lines. The 
signalling system between the switching stations 1 and 1' is a common line 
signalling system. It employs redundant arrangement, because failure may 
take place in the common channel signal path. One channel is allocated to 
the common channel signal path (1) through which a common channel signal 
can be transferred between the switching stations 1 and 1' using the 
dedicated signalling devices 13, 13'. And, the common channel signal path 
(2) is used for establishing a common channel signal path for a call 
through the ISDN 8 shown in FIG. 1. 
The office network normally use the leased lines. The conventional 
technology has been designed to allocate a specific channel of the leased 
lines to the common channel signal path if the signalling system between 
the PBXs employs a common channel signalling system. On the other hand, 
the embodiment shown in FIG. 1 employs a method for allocating it to the 
common channel signal path through the ISDN. For the foregoing prior art, 
the charging unit is how long the leased line is leased, while for the 
embodiment shown in FIG. 1, the charge unit is the number of packets so 
that the charge increases in proportion to the increase of packets. The 
embodiment is thus effective as a common channel signal path between the 
stations in which path the traffic intensity is not so large. In the path 
where the traffic intensity is large, however, it may be more effective to 
allocate the common channel signal path to a specific channel of the 
leased lines. If the common channel signal path is allocated to the leased 
line in the office network, the embodiment shown in FIG. 6 is a method and 
system for alternating the common channel signal path arranged by the 
embodiment shown in FIG. 1 using the ISDN as an alternating circuit used 
when failure takes place in the common channel signal path. 
The common channel signal path having the redundant arrangement shown in 
FIG. 6 will be operated as follows. The signal path (1) is used if no 
failure takes place in the signal path (1) to which one channel of the 
leased line 7' is allocated between the switching stations 1 and 1'. If 
failure takes place in the signal path (1), the signal path (2) containing 
the ISDN 8 designed according to the embodiment shown in FIG. 1 is 
employed for transferring a signal line signal. For the purpose, the 
signalling devices 13, 13' of the switching stations 1 and 1' monitor if 
the common channel signal path (1) is normal and notifies the central 
control devices 3, 3' of failure caused in the signal path (1). When 
failure is detected in the common channel signal path (1), the central 
control devices 3, 3' serve to initialize the signalling devices 13, 13' 
and the signalling devices 13, 13' have a function of notifying the 
central control devices 3, 3' of recovery of the signal path (1). Hence, 
the central control devices 3, 3' serve to select the signal path (1) if 
it is normal and realize a function of alternating the signal path (2) 
using the ISDN 8 if failure takes place in the signal path (1). 
The embodiment shown in FIG. 1 has been designed to make a charge depending 
on the number of signal packets, that is, the number of calls, unlike the 
arrangement designed to transfer a common channel signal through the 
leased line. Hence, if the large number of calls are set up between the 
switching stations, the use of the leased lines may be more economical 
than the use of the ISDN designed in the embodiment. Even if, however, the 
use of the lease lines is more economical, the signalling device and the 
signal path may often employ redundant arrangement, because they have to 
be operated if failure takes place therein. Hence, if the signal path 
containing the ISDN is applied as an alternate path to be used when 
failure takes place in the signal path or the signalling device, it is 
possible to make more effective use of the line and reduce the cost of the 
signalling device. 
According to the invention, the common channel signalling system makes it 
possible to establish the common channel signal path through the ISDN 
without having to use the dedicated signal path in transferring setup 
information between the switching stations. It is especially effective for 
the arrangement of the common channel signalling network between 
small-scaled switching stations each having small traffic intensity. 
If the leased line is used between the switching stations having large 
traffic intensity, the signal path containing the ISDN can be applied as 
an alternate circuit for the signal path and the signalling device 
employing redundant arrangement as a result of considering failure caused 
therein. This results in making more effective use of the line and 
reducing the cost of the signalling device.