Arrangement for serving a telephone office code from two switching systems

This invention relates to a method and apparatus for establishing connections over a public switched telephone network to a customer whose telephone number is in a block associated, for routing purposes, with a first telephone switching system, but who is actually served by a second telephone switching system. The facility for carrying out this method is desirable in order to serve customers who need the advanced functions of a digital switching system, who are currently served by an analog switching system, and do not wish to have their telephone number changed when they switch to being served by the digital switching system. In accordance with the invention, a centralized database is queried to find the identity of the switch serving the called customer. The database returns a routing index for routing the call to that switch.

CROSS-REFERENCE TO RELATED APPLICATION 
This application relates to an application by William J. Bushnell entitled 
"Method of Rerouting Telecommunications Traffic" filed Apr. 24, 1991, Ser. 
No. 691,578, and assigned to the same assignee as this application. 
TECHNICAL FIELD 
This invention relates to telecommunications networks and, more 
specifically, to routing calls in such networks. 
PROBLEM 
During the last decade, a large number of digital switching systems 
(switches) have been introduced into the local and toll telephone 
switching plant in the United States. At the same time, a large number of 
customers continued to be served by the older analog switches, such as the 
1A ESS.TM. switches manufactured by AT&T Network Systems. In the past when 
customers have wanted to have features offered only by digital switches, 
such as the 5ESS.RTM. switch manufactured by AT&T Network Systems, they 
have been transferred to such switches. At present, such a transfer is 
inevitably accompanied by a change of the customer's telephone number, a 
public routing number for routing calls to the customer's telephone(s), 
since each switch serves one or more blocks of such numbers and when a 
customer with a particular number is transferred out of one switch to be 
served by a second switch, his number is not in one of the blocks of 
numbers of that second switch. It would be possible to route all traffic 
to that customer through the first switch, but such an arrangement would 
be very expensive, since it would continue indefinitely to require a 
double switching operation in order to be connected to that customer and 
would require the use of additional plant facilities interconnecting the 
two switches. On the other hand, many customers are very unhappy about 
having their telephone number changed because their number is widely 
known, is on their stationery, and all of their customers and other 
contacts must be notified of the change. Furthermore, a number change is 
expensive for a telephone operating company, since calls to the old number 
must be intercepted and the callers be provided with the new telephone 
number. 
A number of solutions have been used in the past for avoiding a number 
change while serving the customer from a different switch. One such 
solution is to route the call to the original switch and pass the call 
forward to the new switch via a set of foreign exchange lines. This 
solution is very expensive because of the large number of foreign exchange 
lines that are required and the high cost for each. A second solution is 
to use permanent call forwarding from the original switch to the new 
switch. The calls are routed to the original switch which has a call 
forwarding indication to a pseudo number in the new switch and forwards 
the call to that pseudo number. The callers are not aware of this pseudo 
number. Again, this solution is expensive because the call must be 
processed in the orignal switch as well as the new switch and because 
extensive interoffice facilities between the two switches must be 
provided. Further, there may be problems of interaction among different 
features of the called customer if the call is a forwarded call; for 
example, the customer's real number, not the pseudo number should be 
provided to others if the customer makes an outgoing call to someone 
having incoming call line identification, and the second switch must be 
adapted to transmit that real number. A third arrangement is to provide 
routing on the thousands digit as well as the ten thousands group. If the 
customer who is being moved to the new switch fully occupies one or more 
thousands groups of public routing numbers, then this solution is possible 
if the subtending offices are equipped to perform their routing on the 
thousands digit. This arrangement is very inflexible and especially cannot 
be used to serve smaller customers who do not occupy a full thousands 
group. 
Accordingly, a problem of the prior art is that there is no satisfactory 
arrangement for transferring customers being served by a first switch to a 
second switch for service by that switch without undesirably changing 
their telephone number. 
SOLUTION 
In accordance with the principles of this invention, an advance is made 
over the prior art through the use of a new method and apparatus for 
establishing calls to a customer having a public routing number, the 
public routing number having an office code assigned to a first switch, 
but who is served from a second switch; a centralized database, shared by 
a plurality of switches, stores information for the public routing numbers 
served by the first switch and the second switch, and provides routing 
information, for example, a routing index to switches subtending the first 
switch. Prior to the completion of a call, a subtending switch which has 
received the call accesses this database using the called public routing 
number, and the database returns a routing index. Routing indexes are well 
known in the prior art and are described, for example, in W. Ulrich et 
al.: "Translations in the No. 1 Electronic Switching System", Bell System 
Technical Journal, Sep. 1964, pp. 2533-2574. The subtending switch then 
routes the call to the proper destination office by using this routing 
index. 
In accordance with one aspect of the invention, if any subtending office is 
not equipped to query the database, the first office comprises translation 
data for routing the call for completion to the second office. 
Advantageously, not every subtending office needs to be equipped to query 
the database. 
In accordance with one feature of this invention, customers from a 
particular first switch may be transferred to one of a plurality of second 
switches. The identification of that second switch is stored via the 
routing index in the translation data in the first office.

DETAILED DESCRIPTION 
FIG. 1 is a diagram of an arrangement for implementing applicant's 
invention. A call from a caller connected to originating switch 5 to a 
called customer is connected via public switched network 20 to subtending 
switch 10, comprising processor 12 comprising data 14. The called customer 
is served by digital switch 50. However, the customer's public routing 
number is part of a block that is served by analog switch 40. 
A database 90 shared by a plurality of switching systems, is provided. This 
database is accessed over a signaling network 30 by a database query 
message 92 which includes the called customer telephone (public routing) 
number 93. The database 90 responds to this query by returning database 
response message 94 which includes a route index 95. The routing index is 
found in an entry, 97, of a table 96 for switch 10 stored in database 90. 
(The route index is shown as being 50, to indicate that is a route index 
for routing to digital switch 50.) Subtending switch 10 uses route index 
95 to select a trunk group for routing the call to the appropriate one of 
switches 40 or 50, in this case, to digital switch 50. The call is then 
established by sending a common channel signaling (CCS) message 60, an 
Initial Address Message (IAM), or by transmitting the public routing 
number by multi-frequency (MF) signaling, to request establishment of the 
call by digital switch 50. The call is routed over a trunk such as trunk 
16 connected to digital switch 50. The database query and response 
messages, and an IAM message, are transmitted over common channel 
signaling network 30, which comprises a plurality of signal transfer 
points. Such networks are described, for example, in The Bell System 
Technical Journal, vol. 16, no. 7, part 3, Sep. 1982, pages 1573-1816, 
especially pages 1579-1654. 
While FIG. 1 illustrates two switches one of which is a digital switch and 
the other an analog switch, any pair or group of switches serving a common 
office code can be served using the principles of this invention. 
The method is illustrated in FIGS. 2 and 3. FIG. 2 describes the actions 
performed in the subtending switch. The subtending switch receives an 
incoming or originating call (action block 204). The subtending switch 10 
only performs a database query for certain office codes. The office code 
translations stored in data 14 of the processor of subtending office 10 
are used to identify the office codes requiring the database query. A test 
201 determines whether a query is necessary. If not, the call is completed 
conventionally (block 203). If so, the subtending switch sends a query 
including the called number to the database (action block 202). In 
response, the subtending switch receives a response message including a 
route index from the database (action block 204). The call is then routed 
by the subtending switch using the received route index (action block 
206). 
FIG. 3 illustrates the actions performed in the database. The database 
receives a query message including the called customer telephone number 
(action block 300) from the subtending switch. The database performs a 
translation using the called customer public routing number and the 
identification of the querying switch (action block 302). It then returns 
a response message including a routing index for routing the call (action 
block 304) to the subtending switch. 
The route index is a particularly desirable quantity to be stored in the 
database. By supplying the subtending switch with a route index, the 
database permits that switch to perform normal routing using the dialed 
public routing number and merely substituting the route index obtained by 
the database for a route index from the data 14 of switch 10 that could 
otherwise be used for routing calls for public routing numbers of a given 
office code to the analog switch 40. Thus, no additional office codes or 
pseudo directory numbers are required for routing. Since routing indexes 
are individual to each switch, a separate table, such as table 96, is 
stored in the database 90 for each subtending switch. 
For those cases in which the subtending switches are not equipped to query 
a database, the call is initially attempted over a trunk such as trunk 15 
subtending switch to the analog switch. In the analog switch, a 
translation is made and if the call is to a customer served by the digital 
switch 50, the call is extended over a trunk such as trunk 45 
interconnecting the analog and digital switch. The call is then completed 
in the digital switch. This method, as well as the method using a database 
query, is illustrated in FIG. 4. The end switch either receives an 
incoming call Initial Address (IAM) message (action block 400) from a 
switch which does not query the database or, if either the end switch or 
subtending switch is not equipped to use common channel signaling, then 
the incoming call is received without a common channel signaling message 
(action block 401). In either case the translation is obtained in the end 
switch for the terminating public routing number (action block 402). Test 
404 is used to determine whether the terminating customer is served by 
this switch. If so, then a connection is set up to the called customer 
(action block 406). If not, the call is routed from this switch to the 
switch serving the called customer (action block 408). This latter action 
can be performed, for example, through the use of a translation in data 42 
of switch 40 of the received public routing number to a route index for 
routing the call to switch 50. 
The arrangement of FIG. 4 can be used in conjunction with the methods of 
FIGS. 2 and 3 in networks which are in a transitional phase of modernizing 
so that some of their switches are equipped with common channel signaling 
and/or arrangements for accessing databases while others of their switches 
may not be similarly equipped. Advantageously, the entire network need not 
be upgraded simultaneously while permitting major economic benefits for 
traffic from switches that have been upgraded. 
The database method is useful for those cases in which the switch from 
which the customer has been transferred is an especially old switch not 
equipped for common channel signaling while many or most of the subtending 
switches are equipped to access a database. The use of a centralized 
database to store routing information has a major administrative advantage 
to a Telephone Administration. Once a particular office code is split 
across two switches, such as an analog switch and a digital switch, the 
instruction to query the database can be given to all the subtending 
switches. After that, data concerning individual lines need only be stored 
in the central database, and the data in the subtending switches need not 
be changed as lines are moved to the digital switch. 
The concept of using a routing index instead of a pseudo directory number 
has the advantage of conserving directory numbers, which are a valuable 
commodity to a telephone administration. By using the original called 
directory number, a public routing number, instead of translating it to a 
pseudo directory number, correct operation of many switching features is 
assured. The routing index is a convenient method for a subtending switch 
to choose the correct route to the destination switch without altering the 
called partiy's public routing number. A routing index supplies data for 
selecting a trunk group, and, directly or indirectly, the data to be sent 
with a CCS message when using that trunk group for a call to a given 
destination, and also supplies an identification of an alternate route 
index in case all trunks of the trunk group of the original route index 
are unavailable. Thus, a route index directly or indirectly supplies full 
call routing information. 
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.