Private branch exchange networks

Customers that have groups of stations served from multiple digital switching systems, which frequently include mixtures of PBXs and Centrex, require capabilities that will allow those station groups to operate as one global group. This invention permits customer station groups, including PBXs and Centrexes interconnected by private facilities or by the public switched telephone network (including long distance carriers), to act as a single entity that supports integrated voice and data communications. Advantageously, Integrated Services Digital Network (ISDN) Primary Rate Interface (PRI) facilities are used to carry the communications and signaling to and from the PBXs.

TECHNICAL FIELD 
This invention relates to telecommunications networks for serving a single 
customer from a plurality of private branch exchanges (PBXs) and/or 
Centrexes using integrated voice and data signals. 
PROBLEM 
Among the most important customers of telecommunications common carders, 
such as telephone companies, are those customers whose operations are 
scattered in a number of physically separated locations but whose work is 
becoming increasingly more integrated. Typically, such customers have both 
a very high volume of intra-location traffic and a substantial volume of 
inter-location traffic. An ideal arrangement would provide for ease of 
dialing of inter-location calls; for ability to serve ordinary (analog) 
telephones as well as a few or many premium telephones, such as integrated 
services digital network (ISDN) telephones; and for minimization of 
charges for inter-location calls. A problem of the prior art is that there 
is no arrangement for providing low cost intra-location service and cost 
effective integrated voice and data inter-location service for a 
multilocation customer. 
SOLUTION 
The above problem is solved and an advance is made over the prior art in 
accordance with our invention wherein the telecommunications users of a 
large customer in one location are interconnected by a private branch 
exchange (PBX) and that PBX is connected with other locations and with the 
public switched telephone network by an integrated voice and data facility 
to a local switching system. Advantageously, such an arrangement provides 
for low cost intra-location telecommunications connections and, through 
the integrated voice and data connection, provides the capabilities for 
access to a broad range of services provided from the local switching 
system for interconnecting with other locations and with the public 
switched telephone network. 
In accordance with one feature of the invention, two PBXs of the customer 
are connected by one or more dedicated primary rate interface (PRI) 
facilities when the traffic between these PBXs warrants such a connection. 
Also, private PRI facilities are provided, where traffic warrants, to 
interconnect serving switches connected to different PBXs of the customer, 
to carry inter-PBX traffic for the customer. 
In accordance with one aspect of the invention, the integrated voice/data 
interface is an ISDN PRI. Advantageously, the PRI provides a cost 
effective arrangement for transporting station originated voice or data 
and station or switch originated signaling or control data between the 
local switching system and the PBX. 
In accordance with one aspect of the invention analog telephones, ISDN 
telephones, or a mixture are supported by the PBX. The system further 
supports inter-PBX and PBX to Centrex ISDN-to-ISDN connections. 
In accordance with another aspect of this invention, a uniform numbering 
plan is provided throughout the customer's private network. Information is 
available in the local switch to route calls to numbers not recognized by 
the PBX. 
In accordance with another aspect of the invention, calling number display 
for inter-location calls is implemented by sending the identification of 
the caller from the calling PBX over the PRI to the local switch. This 
information is then transmitted via a signaling system 7 network or a PRI 
interconnecting the local switches serving the originating and terminating 
PBX and then transmitted via the PRI connected to the terminating PBX for 
display at the customer's ISDN or analog telephone. 
Further, detailed signaling information such as a call forwarding 
indication and the identification of the name of the calling party 
recorded at the local switch connected to the originating PBX is similarly 
transmitted. 
In accordance with another aspect of the invention, a common voice 
messaging system is used by some or all of the PBXs of the customer 
network. The signaling arrangement discussed above is used to control the 
called stations message waiting lamp or other indicator. 
In accordance with another aspect of this invention, remote access to 
private facilities is made possible by accessing these private facilities 
via the PRI connection from the originating PBX to the connected local 
switch and using connections from the local switch to the switch or PBX 
connected to the private facility. 
In accordance with another aspect of the invention, common group 
restrictions are advantageously implemented at the local central office 
connected to the PBX. The group of the caller is identified from the 
caller's number transmitted over the PRI to the connected local switch.

DETAILED DESCRIPTION 
FIG. 1 is a block diagram of the hardware of applicants' invention. PBXs 1, 
3 and 4 are program controlled PBXs, such as the Definity.RTM. System 
manufactured by AT&T. For example, PBX 1 comprises a processor 5, 
including memory 6 and a control program 7. PBXs 1, 3, and 4 all serve the 
stations of a single business or other entity, and are called herein a 
common business customer group 20. PBX 1 is connected via primary rate 
interface (PRI) facility 21 to switching system 30 and PBX 3 is connected 
via primary rate interfaces 23 and 24 to switching system 31. PBX 1 serves 
ISDN station 11 and analog stations 12 and 18. Analog station 18 has a 
display. ISDN station 13, a Centrex line, is connected by a basic rate 
interface (BRI) 22 directly to switching system 30. Switching system 31 
serves ISDN stations 14 and 15 and analog stations 16 and 17. PBX 4 is 
part of the customer group but is accessible only via the public switched 
telephone network (PSTN) 40, and, from PBX 1 via the private PRI facility 
25. Switching systems 30 and 31 are program controlled switching systems 
such as the 5ESS.RTM. switch manufactured by AT&T. In switching system 30 
is shown processor 32 including memory 33 and program storage 34 for 
controlling switching system 30. Switching systems 30 and 31 are connected 
by a direct connection over facility 37, dedicated to the customer of PBX 
1, 3, and 4, carrying a PRI signal and are also both connected to the PSTN 
40 and a Signaling System 7 network 45. The PSTN network is also used for 
accessing any station outside the customer group, such as analog station 
46. 
An intra-PBX call, for example between stations 16 and 17, is set up within 
PBX 3. A call from station 12 to station 16 is set up via PBX 1 which 
transmits call control and signaling information over the D-channel of PRI 
21 to switch 30. Part of the information that is transmitted is the 
identity of the called station (station 16) and another part is the 
identity of the calling station (station 12). To access station 16, a 
signaling message is transmitted over PRI 37 to switching system 31 to 
signal the request to connect a call on one of the channels of the PRI to 
analog station 16. The identity of the calling station is also signaled 
over the PRI using the conventional ISDN signaling protocol. Switching 
system 31 receives this signaling message, sets up a connection from a 
B-channel of PRI 37 to a B-channel of PRI 23. Switching system 31 signals 
PBX 3 over the D-channel of PRI 23 to set up a connection from a channel 
of PRI 23 to analog station 16 and to display the identity of the caller 
(including the caller's name obtained from memory by processor 32 of 
switching system 30) at the called analog station 16. 
Analog stations 16 and 18 both have displays for displaying the calling 
number and, for example, for identifying whether a call is an incoming or 
an intra-customer group call. An example of such a telephone is the AT&T 
7404 telephone station. Such telephones are usually designed to work 
directly with a PBX and have different arrangements for gathering the 
information for a display than the incoming calling line identification 
(ICLID) arrangement that is used for residential customer service. 
Stations 12 and 17 are simple analog stations without the capabilities for 
displays. 
In this particular embodiment, switching system 31 is also connected to a 
number of specialized facilities. Switching system 31 is directly 
connected to a long distance carrier 52 to bypass the local exchange 
access network. Switching system 31 is also connected to a distant 
customer facility 54, for example, a remotely located customer branch via 
switched or unswitched private facility. Switching system 31 is also 
connected to voice message system 56. While blocks 52, 54, and 56 could 
also be connected to switching system 30, a connection from a single 
switching system is frequently more economical since such a connection 
allows all traffic from the customer group to be concentrated onto one 
transmission or access facility. 
The numbering plan of this particular customer group is typical. A 5-digit 
number is used for making intra-customer group calls and calls to the 
public switched telephone network are requested by dialing 9 followed by a 
public switched telephone network directory number such as 234-5712 for 
accessing analog station 46. The 5-digit telephone numbers which are 
assigned to each telephone station connected to the PBXs of the customer 
group are usually the last 5 digits of the directory number used for 
accessing that station from the outside via the PSTN. While, in some PBXs 
the numbers of the stations are in one or more blocks of numbers, such an 
arrangement limits the flexibility of individuals to move from one PBX to 
another of the same customer group without changing their directory 
numbers. For this specific embodiment, therefore, it is assumed that any 
of the 5-digit numbers of the customer group of PBXs can be assigned to 
telephone stations in any of the PBXs of the customer group. In the 
specific example, therefore, telephones 11, 12, 18, 13, 14, 15, 16, and 17 
have directory numbers 23456, 23457, 23405, 23471, 23400, 23401, 23402, 
and 23403. In this particular example, the user of telephone station 18 
recently moved from PBX 3 to PBX 1 and was able to do so without changing 
her telephone number. 
In this particular example, if station 11 dials 23457, the connection is 
set up within PBX 1 without involving switching system 30. If station 11 
dials station 18, PBX 1 has the information for recognizing that the 
number 23405 is attached to PBX 1 and will also be able to establish the 
call without involving switching system 30. If station 23456 dials station 
23471, PBX 1 recognizes that this is an intra-customer PBX group call to a 
station not served by PBX 1 and therefore passes the call to switching 
system 30 via PRI 21. Switching system 30 recognizes that this is a call 
to a station served directly as a Centrex line by switching system 30 and 
establishes a call via BRI 22 to station 23471. If station 23456 dials 
23400, switching system 30 recognizes that station 23400 is served by a 
PBX connected to switching system 31 and passes the call to that switching 
system over PRI 37. The signaling channel of PRI 37 is used to identify 
both the calling and the called numbers and to identify the fact that this 
is an intra-PBX group call. Finally, if station 23456 dials 9 234-5712, 
the initial 9 is an indication to the PBX 1 that this is a call outside 
the PBX group and PBX 1 passes the call to switching system 30 as a call 
to directory number 234-5712. Switching system 30 recognizes that this is 
not to a station which it serves and passes the call to the public 
switched telephone network 40 for completion to analog station 46. 
PRI 37 is a highly desirable facility for interconnecting switching systems 
30 and 31 for inter-PBX traffic. The PRI facility can be used to transmit 
not only voice but also data traffic at rates of up to 64 kilobits (or 
more if multiple channels are used) and offers the availability of a 64 
kilobit D-channel for transmitting control and signaling information 
concerning the calls between switching systems 30 and 31. 
The connection between the switching systems 30 and 31 to the public 
switched telephone network 40 also provides the capability for overflow 
traffic in case of a failure of PRI 37 or even normal traffic between 
switching systems 30 and 31 for traffic between two PBXs of the same 
customer group. While such a connection is more expensive than a heavily 
used connection over PRI facility 37, as an overflow connection in 
particular, this is an efficient arrangement. Of course, if there is 
little traffic for the customer between systems 30 and 31, PRI 37 is not 
provided. 
PBXs 1 and 4 are also interconnected by a private PRI facility 25. Overflow 
traffic between these two PBXs is carded via PSTN 40 and switching system 
31. 
FIGS. 2-5 are flow diagrams of actions carded out by switching systems such 
as switching system 30 or in the case of FIG. 5, PBXs such as PBX 1, to 
implement applicants' invention. FIG. 2 is a flow diagram of actions 
carded out by switching system 30 when it receives a call from a PBX such 
as PBX 1. The call is received (block 201) and the switching system 
translates the destination number received with the call (action block 
203). The result of the translation may be one of five outcomes described 
with respect to blocks 211, 221, 231, 241, 251, and their respective 
successor blocks. Block 211 is entered if the destination number is that 
of a telephone station served by a PBX served from this system. In that 
case, the call is completed to that PBX (action block 213 and if 
appropriate, i.e, if the destination telephone contains a display, the 
caller name and number is forwarded to the PBX for display on the called 
telephone station. Alternatively, the caller name and number can be 
transmitted from the switching system 30 regardless of the type of 
destination telephone station and the PBX will only operate displays for 
terminating telephone stations that have them. 
If the result of translation 203 is that the destination station is found 
to be on a PBX served from a connected system (action block 221), then a 
connection is set up to the connected system (action block 223). The 
caller name and number and the called number are transmitted to the 
connected system to assist that system in setting up the terminating 
connection. The latter process is described in FIG. 3. 
If the facilities to the connected system are busy or otherwise 
unavailable, then the actions initiated by block 231 are performed. 
If the result of the translation is that the called station is served by a 
PBX attached to an unconnected system such as PBX 4 (action block 231), 
then a connection is set up to the public switching telephone network 
(action block 233). If the PSTN is used then the full called number (i.e., 
the 7 or 10-digit version) and not simply the 5-digit number must be 
provided to the PSTN. In addition the facilities of signaling system 7 can 
be used to transmit the caller name, number, and a customer group 
indicator (action block 235). 
If the result of the translation of the destination number is that the 
switching system 30 recognizes that the call is to a number outside the 
customer group (action block 241), then a connection is set up to the 
public switched telephone network (action block 243) unless the call is to 
another telephone served by switching system 30 in which case the call can 
be directly completed. The caller number is transmitted (action block 245) 
in order to allow the terminating telephone station to receive an ICLID 
display. 
If the result of translation 203 is the recognition that the call is to a 
privately accessed facility (action block 251), then test 253 is performed 
to determine whether this facility is accessed from this switch. If so, 
then a connection is set up to the privately accessed facility (action 
block 255) and the caller name and number is transmitted if appropriate 
(action block 257). If the privately accessed facility is a voice message 
system, then the called number is also transmitted. If the privately 
accessed facility is not accessed from this switch as determined in test 
253, then a connection is set up to the connected system that does access 
this facility (action block 261) and the caller name and number as well as 
the called number are transmitted to the connected switch (action block 
263). Before setting up a connection to the facility, a check is made as 
part of block 255 whether the caller is authorized to access this 
facility. This check is made by checking the translation data of the 
caller (class of service); usually, stations of the customer group are 
grouped into classes, each with its own access authorization pattern. The 
same type of test is made as part of blocks 355 and 455. 
FIG. 3 is a flow chart of actions performed by a switch such as switching 
system 30 when receiving a call from another switching system such as 
switching system 31. The same basic flow diagram describes the actions 
except that many of the corresponding blocks have the same designation 
number except with the 3 in the first digit instead of a 2. Further, the 
blocks corresponding to blocks 221, 223, 225, 23 1,233,235, 241,243,245, 
253, 261,263, are not required in this flow chart because the call has 
already been received from another switching system. 
FIG. 4 is a flow chart of actions performed switching system 30 in response 
to calls received from the public switched telephone network. Again, most 
of the blocks of FIG. 4 are the same as corresponding blocks of FIG. 2 and 
some have been eliminated. The blocks that have been eliminated are blocks 
equivalent to blocks 241,243, and 245. In addition, a number of the other 
blocks have been changed and these changes will be described. 
Block 415 includes forwarding the caller identification and an outside call 
indicator unless a customer group indicator had been received as a result 
of the transmission of such an indicator in block 235. 
Block 425 differs from block 225 in that the caller identification is 
transmitted since it may not be possible to identify the name of the 
caller if the call is from a line and of course the indication signal 
would be that the call is an outside call. 
Similarly, action block 435 is similar to action block 235 except that only 
the caller identification is transmitted and the outside call indicator is 
transmitted. 
The actions associated with block 451 (corresponding to block 251) for 
accessing privately accessed facilities from the public switched telephone 
network differ in one respect. Before the caller can be given access to 
the facility, the caller's access fights must be checked (test 452). This 
may be by requesting that the caller dial some personal identification 
number, or by testing the caller's number identification, ascertained by 
Automatic Number Identification (ANI). If the caller does not have access 
rights, then the caller is connected to an announcement (action block 47 
1) announcing this denial. Otherwise, the actions starting from test 453 
(corresponding to test 253) are the same as the actions described with 
respect to blocks 253,255, 257,261, and 263 of FIG. 2. 
FIG. 5 is a flow diagram of actions performed within a PBX such as PBX 1. 
If the call is received from the switch (action block 501) then the called 
number is translated to find its location within the PBX and to find out 
the type of station involved (action block 503) prior to establishing a 
connection to the called station. If the call is received from a station 
within the PBX (action block 511) then the called number is translated 
(action block 513) and if the call is within the PBX (action block 515), 
steps starting with action block 521 are performed to establish a 
connection to the called station. If the result of translation 5 13 is 
that the call is not within the PBX, that the called number is not one 
served by the PBX, then the call is connected to the switching system and 
the calling and called number are forwarded to that system (action block 
517). Thereafter, the actions described with respect to FIG. 2 are 
performed by that switching system. 
After it has been established that the call is to a station within the PBX 
and the translation has been performed, a connection is established to the 
called station (action block 521). If the called station has a display, 
then the display for that called station is controlled on the basis of 
information received from the switch or on the basis of the identity of 
the caller if the call is an intra-PBX call. If the PBX group has 
facilities for voice messaging and the called station is busy or does not 
answer (action block 525) then the call is forwarded to the voice 
messaging system by requesting the connected switch to establish a 
connection such as the connection to block 56. The called number and 
calling number are transmitted to the voice messaging system when the 
connection to that system is established. 
It is to be understood that the above description is only of one preferred 
embodiment of the invention. Numerous other arrangements may be devised by 
one skilled in the art without departing from the scope of the invention. 
The invention is thus limited only as defined in the accompanying claims.