Telephone interexchange carrier selection

Claims

What is claimed is:

1. A method of selecting a carrier for a telephone call being made over an originating terminal link, comprising the steps of:

examining signals associated with the call and received over the originating terminal link to determine whether they include signals other than call destination-identifying signals that identify a particular one of a plurality of carriers to carry the call;

examining contents of a memory associated with the terminal link to determine whether they identify a carrier predesignated to carry calls made over the originating terminal link, when the signals received over the terminal link are determined not to include the signals that identify a carrier to carry the call; and

connecting the call to the identified carrier when a carrier is determined to be identified by either the received signals or the memory contents.

2. A method of selecting a carrier for a telephone call being made over an originating terminal link, comprising the steps of:

examining signals associated with the call and received over the originating terminal link to determine whether they include signals that identify a carrier for the call;

examining contents of a memory associated with the terminal link to determine whether they identify a carrier for the call, when the signals received over the terminal link are determined not to include signals that identify a carrier for the call;

connecting the call to the identified carrier when a carrier for the call is determined to be identified by either the received signals or the memory contents; and

requesting over the originating terminal link that a carrier for the call be identified when a carrier is determined not to be identified by the received signals and by the memory contents.

3. The method of claim 1 wherein the step of examining contents of a memory associated with the terminal link is followed by the steps of:

examining contents of a second memory to determine whether they identify a carrier designated to carry calls for which no other carrier is identified, when the contents of the memory associated with the terminal link are determined not to identify a carrier; and

connecting the call to the identified carrier when a carrier is determined to be identified by the second memory contents.

4. The method of claim 3 wherein the step of examining contents of a second memory is preceded by the further step of determining that the call may be connected to a carrier identified by the second memory when the contents of the memory associated with the terminal link are determined not to identify a carrier.

5. The method of claim 1 wherein the step of examining received signals is preceded by the step of:

storing in the memory associated with the terminal link information identifying a carrier predesignated by a subscriber associated with the terminal link to carry calls made over the terminal link.

6. The method of claim 1 wherein the step of examining received signals is accompanied by the step of:

examining the received signals to determine whether the call requires selection of a carrier.

7. A method of automatically selecting an interexchange carrier for a telephone call incoming over a terminal link to a switching office, comprising the steps of:

examining digits associated with the call and received over the terminal link to determine whether they include digits that identify a carrier for the call;

examining contents of a memory associated with the terminal link to determine whether they identify a carrier for the call when the received digits are determined not to include digits that identify a carrier for the call;

determining whether the identified carrier is appropriate for a call type that includes the call, when a carrier is determined to be identified; and

connecting the call from the switching office to the identified carrier when the identified carrier is determined to be appropriate.

8. The method of claim 7 further comprising the step of:

requesting over the terminal link that an appropriate carrier be identified over the terminal link when an appropriate carrier is determined not to be identified by the received digits and by the memory contents.

9. The method of claim 7 wherein the step of examining received digits is accompanied by the step of:

examining the received digits to determine whether the call is of a type requiring selection of a carrier.

10. The method of claim 7 wherein the step of examining contents of a memory associated with the terminal link comprises the steps of:

examining contents of the memory that are exclusively associated with the terminal link to determine whether they identify a carrier when the received digits are determined not to identify a carrier; and

examining contents of the memory that are associated with a terminal link group of which the terminal link is a member to determine whether they identify a carrier when the memory contents that are exclusively associated with the terminal link are determined not to identify a carrier.

11. The method of claim 7 wherein the step of examining contents of a memory associated with the terminal link is followed by the step of:

examining contents of a memory associated with the switching office to determine whether they identify a default carrier when the contents of the memory associated with the terminal link are determined not to identify a carrier.

12. The method of claim 11 wherein the step of examining contents of a memory associated with the switching office is preceded by the step of:

examining the contents of the memory associated with the terminal link to determine whether the call may be connected to a carrier identified by the contents of the memory associated with the switching office when the contents of the memory associated with the terminal link are determined not to identify a carrier.

13. The method of claim 7 wherein the step of examining received digits is preceded by the step of:

storing in the memory associated with the terminal link information identifying a carrier predesignated to carry calls that originate on the terminal link and are to be carried by a carrier.

14. The method of claim 11 wherein the step of examining contents of a memory associated with the switching office is preceded by the step of:

storing in the memory associated with the switching office information identifying at least one default carrier designated to carry calls incoming to the switching office over originating terminal links and required to be carried by a carrier and for which a carrier is determined not to be identified.

15. The method of claim 7 wherein the step of examining received digits comprises the step of:

examining the received digits, comprising digits dialed to make the call, to determine whether they include at least two digits that identify a carrier for the call and precede digits of a called number.

16. The method of claim 7 wherein the step of examining received digits comprises the step of:

examining the received digits, comprising digits dialed to make the call, to determine whether they include digits one plus zero followed by at least two digits that identify a carrier for the call and precede digits of a called number.

17. A method of automatically selecting an interexchange carrier at a switching office for a telephone call incoming thereto over a terminal link, comprising the steps of:

examining digits associated with the call and received over the terminal link to determine therefrom whether the call is of a type requiring selection of a carrier for the call;

examining the digits received from the terminal link to determine whether they include digits that identify a carrier for the call;

examining contents of memories associated with at least some of the received digits and with a class-of-service of the terminal link to determine whether they identify a carrier for the call when the received digits are determined not to include digits that identify a carrier for the call;

examining contents of a memory associated with the terminal link to determine whether they identify a carrier for the call when the memories associated with at least some of the received digits and with the class-of-service are determined not to identify a carrier for the call;

determining whether the identified carrier is appropriate for a call type that includes the call, when a carrier for the call is determined to be identified;

connecting the call from the switching office to the identified carrier when the identified carrier is determined to be appropriate for the call type; and

requesting over the terminal link that an appropriate carrier for the call be identified over the terminal link when an appropriate carrier for the call is determined not to be identified.

18. The method of claim 17 further comprising the steps of:

examining the contents of the memory associated with the terminal link to determine whether the call may be connected to a default carrier when the contents of the memory associated with the terminal link are determined not to identify an appropriate carrier for the call;

examining contents of a memory associated with the switching office to determine whether they identify a default carrier when it is determined that the call may be connected to a default carrier; and

examining the contents of the memory associated with the switching office to determine whether the identified default carrier is an appropriate default carrier for the call when a default carrier is determined to be identified.

19. The method of claim 17 wherein the step of examining contents of a memory associated with the terminal link comprises the steps of:

examining contents of the memory that are exclusively associated with the terminal link to determine whether they identify a carrier designated to carry calls incoming from the terminal link, when the received digits are determined not to include digits that identify a carrier for the call; and

examining contents of the memory that are associated with a terminal link group of which the terminal link is a member to determine whether they identify a carrier designated to carry calls incoming from terminal links of the terminal group, when the memory contents that are exclusively associated with the terminal link are determined not to identify a carrier.

20. A switching system for selecting a carrier for a telephone call being made over an originating terminal link, comprising:

first operational means, for storing information associated with the terminal link;

second operational means, for receiving signals over the originating terminal link;

third operational means, cooperative with the second means, for examining signals associated with the call and received by the second means to determine whether they include signals other than call destination-identifying signals that identify a particular one of a plurality of carriers to carry the call;

fourth operational means, responsive to the third means and cooperative with the first means, for examining contents of the first means to determine whether they identify a carrier predesignated to carry calls made over the originating terminal link, when the third means determine that the received signals do not include the signals that identify a carrier to carry the call; and

fifth operational means, responsive to the third and the fourth means, for connecting the call to the identified carrier when the third means or the fourth means determine that a carrier to carry the call is identified.

21. A switching system for selecting a carrier for a telephone call being made over an originating terminal link, comprising:

first operational means, for storing information associated with the terminal link;

second operational means, for receiving signals over the originating terminal link;

third operational means, cooperative with the second means, for examining signals associated with the call and received by the second means to determine whether they include signals that identify a carrier for the call;

fourth operational means, responsive to the third means and cooperative with the first means, for examining contents of the first means to determine whether they identify a carrier for the call, when the third means determine that the received signals do not include signals that identify a carrier for the call;

fifth operational means, responsive to the third and the fourth means, for connecting the call to the identified carrier when the third means or the fourth means determine that a carrier is identified; and

sixth operational means, responsive to the third and the fourth means, for requesting over the originating terminal link that a carrier for the call be identified when the third and the fourth means determine that a carrier is not identified.

22. The system of claim 20 further comprising:

sixth operational means, for storing information; and

seventh operational means, responsive to the fourth means and cooperative with the sixth means, for examining contents of the sixth means to determine whether they identify a carrier designated to carry calls for which no other carrier is identified, when the fourth means determine that a carrier is not identified; and wherein

the fifth means are further responsive to the seventh means and cooperative with the sixth means for connecting the call to the identified carrier when the seventh means determine that a carrier is identified.

23. The system of claim 22 wherein the seventh means include:

eighth operational means, responsive to the third means, for determining whether the call may be connected to a carrier whose identity may be stored by the sixth means, when the fourth means determine that a carrier is not identified; and

ninth operational means, responsive to the eighth means and cooperative with the sixth means, for examining contents of the sixth means to determine whether they identify a carrier when the eighth means determine that the call may be connected to that carrier.

24. The system of claim 20 further comprising:

sixth operational means, cooperative with the first means, for storing in the first means information identifying a carrier predesignated by a subscriber associated with the terminal link to carry calls made over the terminal link.

25. The system of claim 20 further comprising sixth operational means, responsive to the second means, for examining signals received by the second means to determine whether the call requires selection of a carrier;

and wherein the third means include seventh operational means, responsive to the sixth means and cooperative with the second means, for examining signals received by the second means to determine whether they identify a carrier when the sixth means determine that carrier selection is required for the call.

26. A switching system for automatically selecting an interexchange carrier for a telephone call incoming over a terminal link to the switching system, comprising:

first operational means, for storing information associated with the terminal link;

second operational means, for receiving digits sent over the terminal link;

third operational means, cooperative with the second means, for examining digits associated with the call and received by the second means to determine whether they include digits that identify a carrier for the call;

fourth operational means, responsive to the third means and cooperative with the first means, for examining contents of the first means to determine whether they identify a carrier for the call, when the third means determine that the received digits do not include digits that identify a carrier for the call;

fifth operational means, responsive to the third and the fourth means, for determining whether the identified carrier is appropriate for a call type that includes the call, when a carrier for the call is determined to be identified; and

sixth operational means, responsive to the fifth means, for connecting the call from the switching system to the identified carrier, when the identified carrier is determined to be appropriate for the call type.

27. The system of claim 26 further comprising:

means, responsive to the fourth and the fifth means, for requesting over the terminal link that an appropriate carrier be identified over the terminal link, when the fourth means or the fifth means determine that an appropriate carrier is not identified.

28. The system of claim 26 further comprising seventh operational means, responsive to the second means, for examining digits received by the second means to determine whether the call is of a type requiring selection of a carrier; and

wherein the third means include eighth operational means, responsive to the seventh means, for examining digits received by the second means to determine whether they include digits that identify a carrier, when the seventh means determine that selection of a carrier is required.

29. The system of claim 26 wherein the first means include

seventh operational means, for storing information exclusively associated with the terminal link, and

eighth operational means, for storing information associated with a terminal link group of which the terminal link is a member; and

wherein the fourth means include

ninth operational means, responsive to the third means and cooperative with the seventh means, for examining contents of the seventh means to determine whether they identify a carrier for the call, when the third means determine that the received digits do not include digits that identify a carrier for the call, and

tenth operational means, responsive to the ninth means and cooperative with the eighth means, for examining contents of the eighth means to determine whether they identify a carrier for the call, when the ninth means determine that the contents of the seventh means do not identify a carrier for the call.

30. The system of claim 26 further comprising:

seventh operational means, for storing information associated with the switching system; and

eighth operational means, responsive to the fourth means and cooperative with the seventh means, for examining contents of the seventh means to determine whether they identify a default carrier for the call, when the fourth means determine that the contents of the third means do not identify a carrier for the call.

31. The system of claim 30 wherein the eighth means comprise:

ninth operational means, responsive to the fourth means and cooperative with the first means, for examining the contents of the first means to determine whether the call may be connected to a carrier whose identity may be stored in the seventh means, when the fourth means determine that the contents of the third means do not identify a carrier for the call; and

tenth operational means, responsive to the ninth means and cooperative with the seventh means, for examining contents of the seventh means to determine whether they identify a default carrier for the call, when the ninth means determine that the call may be connected to that carrier.

32. The system of claim 26 further comprising:

means, cooperative with the first means, for storing in the first means information identifying a carrier predesignated to carry calls that originate on the terminal link and are to be carried by a carrier.

33. The system of claim 30 further comprising:

means, cooperative with the seventh means, for storing in the seventh means information identifying at least one default carrier designated to carry calls that are to be carried by a carrier for which a carrier is determined not to be identified.

34. The system of claim 26 wherein the second means include seventh operational means, for receiving at least two digits that identify a carrier for the call and that precede digits of a called number.

35. The system of claim 26 wherein the second means include seventh operational means for receiving digits dialed to make the call and including digits one plus zero followed by at least two digits that identify a carrier for the call and precede digits of a called number.

36. A switching system for automatically selecting an interexchange carrier at the switching system for a telephone call incoming thereto over a terminal link, comprising:

first operational means, for storing information associated with at least some received digits dialed to make the call and with a class-of-service of the terminal link;

second operational means, for storing information associated with the terminal link;

third operational means, for receiving the digits dialed to make the call over the terminal link;

fourth operational means, cooperative with the third means, for examining the digits received by the third means to determine whether the call is of a type requiring selection of a carrier;

fifth operational means, responsive to the fourth means and cooperative with the third means, for examining the digits received by the third means to determine whether they include digits that identify a carrier for the call, when the fourth means determine that carrier selection is required;

sixth operational means, responsive to the fifth means and cooperative with the first means, for examining contents of the first means to determine whether they identify a carrier for the call, when the fifth means determine that the received digits do not identify a carrier for the call;

seventh operational means, responsive to the sixth means and cooperative with the second means, for examining contents of the second means to determine whether they identify a carrier for the call, when the sixth means determine that the contents of the first means do not identify a carrier for the call;

eighth operational means, responsive to the fifth, the sixth, and the seventh means, for determining whether the identified carrier is appropriate for a call type that includes the call, when a carrier for the call is determined to be identified;

ninth operational means, responsive to the eighth means, for connecting the call from the switching system to the identified carrier, when the identified carrier is determined to be appropriate for the call type; and

tenth operational means, responsive to the eighth means, for requesting over the terminal link that an appropriate carrier for the call be identified over the terminal link, when an appropriate carrier is determined not to be identified.

37. The system of claim 36 further comprising:

eleventh operational means, for storing information associated with the switching system;

twelfth operational means, responsive to the seventh means and cooperative with the second means, for examining contents of the second means to determine whether the call may be connected to a default carrier, when the seventh means determine that the contents of the second means do not identify a carrier for the call;

thirteenth operational means, responsive to the twelfth means and cooperative with the eleventh means, for examining contents of the eleventh means to determine whether they identify a default carrier, when the twelfth means determine that the call may be connected to a default carrier; and

fourteenth operational means, responsive to the thirteenth means and cooperative with the eleventh means, for examining the contents of the eleventh means to determine whether the identified default carrier is an appropriate default carrier for the call, when a default carrier for the call is determined by the thirteenth means to be identified.

38. The system of claim 36 wherein the second means comprise

eleventh operational means, for storing information exclusively associated with the terminal link, and

twelfth operational means, for storing information associated with a terminal link group of which the terminal link is a member; and

wherein the sixth means comprise

thirteenth operational means, responsive to the fifth means and cooperative with the eleventh means, for examining contents of the eleventh means to determine whether they identify a carrier for the call, when the fifth means determine that the received digits do not identify a carrier for the call; and

fourteenth operational means, responsive to the thirteenth means and cooperative with the twelfth means, for examining contents of the twelfth means to determine whether they identify a carrier for the call, when the thirteenth means determine that the contents of the eleventh means do not identify a carrier for the call.

CROSS-REFERENCE TO RELATED APPLICATIONS

Concurrently filed herewith and assigned to the same assignee as this application are: application of B. R. Bimonte, M. A. Gauldin, and D. H. Riley, entitled "Telephone Interexchange Call Routing", Ser. No. 519,841; and application of A. Friedes, T. V. Greene, F. W. Idenden, K. L. Moeller, R. M. Musen, and J. F. O'Hara, Jr., entitled "Telephone Interexchange Signaling Protocol", Ser. No. 520,112. The related applications share a substantially common disclosure with this application, but their claims are directed to different aspects of that disclosure.

TECHNICAL FIELD

This invention relates to communication call routing in a multi-carrier environment in general, and to the selection of an interexchange call carrier and the routing and connection of a call to the selected carrier in particular.

BACKGROUND OF THE INVENTION

For purposes of telephone communications, a geopolitical area, such as the United States, is divided into a plurality of contiguous, non-overlapping districts, called exchanges, each of which is served by a local telephone company. In telephony parlance, the exchanges are also referred to as Local Access Transport Areas (LATAs). Telephone calls originating and terminating within the same exchange, referred to as intraexchange calls, are generally handled end-to-end by a local, intraexchange, telephone company. Calls originating within one exchange or in a foreign country and terminating in a different exchange or in a foreign country, referred to as interexchange calls, are handled at each end by the intraexchange company that services the originating or the terminating exchange, and the calls are carried between the intraexchange companies by one or more interexchange carriers.

A plurality of alternative interexchange carriers have come to be available to callers to carry their interexchange calls. It is therefore necessary to provide callers with the capability of selecting interexchange carriers, and to provide interconnection between callers and the selected interexchange carriers. Furthermore, it is desirable to provide interconnection between any caller and any interexchange carrier on the same type of basis.

The conventional scheme of providing interconnection between callers and carriers has not completely satisfied the objective of providing interconnection to all carriers on an equal basis. Traditionally, to have their interexchange calls completed and carried by one interexchange carrier, callers have merely had to dial the number of the called party. However, to have their interexchange calls carried by other interexchange carriers, the callers have had to dial access codes assigned to those carriers. Dialing of the access code has provided the callers with connection to the selected carrier, and the callers then have had to dial a personal identification code and only then the number of the called party, to have the call completed.

Additionally, an intraexchange telephone company has typically been able to transfer interexchange calls to one interexchange carrier over various routes, as a function of certain parameters that are associated with each call, such as the class of service of the call-originating equipment and the called number. Alternative interexchange carriers have generally not been provided with this capability in an efficient manner and they have had to receive calls of all types over a single type of route.

Furthermore, the intraexchange telephone companies have conventionally been able to keep various records, and to perform various servicing activities, on calls routed via an interexchange carrier, and therefore the protocol for communications between switching offices of the local telephone companies and of the carrier has not had to provide for transfer of information necessary to make these records and to perform these activities. But the protocol has been found lacking in capabilities to communicate the requisite information from the local telephone companies to the carriers to enable the carriers to keep their own records on, and to provide services for, calls routed to them. And while conventionally a switching office could assume that information sent by it to a carrier office was properly received, a need has arisen for determining that sent information has actually been received. But the conventional interoffice communication protocol has been found lacking in capabilities for determining whether the carrier office has received the communicated information.

SUMMARY OF THE INVENTION

The invention is directed toward eliminating such and other inadequacies of the prior art.

According to an aspect of the invention, a carrier for a originating terminal link's telephone call is selected by examining signals associated with the call and received over the terminal link to determine whether they include signals other than call destination-identifying signals that identify a carrier, and if a carrier cannot be so identified, by examining contents of a memory associated with the terminal link to determine whether they identify a carrier. Optionally, provision may be made for examining contents of a second memory, generally associated with the switching office at which carrier selection is taking place, to determine whether they identify a carrier when even the memory associated with the terminal link does not identify a carrier. When a carrier cannot be identified by the above-described means, a request is made over the terminal link that a carrier be identified. For example, an announcement may be returned to the caller requesting that the call be placed again, this time identifying a carrier. Once a carrier is identified, connection of the call to that carrier is attempted. Carrier selection is preferably performed only for those calls for which examination of the signals received over the terminal link indicates that carrier selection is required.

Advantageously, in accordance with the invention, a subscriber is allowed to presubscribe to, i.e., specify, any carrier equally easily, by having information identifying that carrier entered in memory associated with the subscriber's terminal link. The subscriber can then have interexchange calls routed to that carrier without taking any additional steps to select or specify the carrier. The conventional, simple, and convenient way of routing calls through a carrier is thus preserved, yet at the same time carrier selection is accommodated.

Furthermore, the scheme allows a caller to select any available carrier on a per-call basis by transmitting, e.g., dialing, signals identifying the desired carrier. Thus great versatility is added to carrier selection. For example, a subscriber may prefer one carrier for certain calls and another for other calls, and the scheme accommodates the subscriber's preferences by allowing the subscriber to override previous presubscription to a carrier on a per-call basis. Or it allows a subscriber who has not presubscribed to a carrier to select any available carrier on an equal basis. Additionally, a caller is enabled to select any carrier when making a call over a terminal link shared with others, such as a public telephone, in a manner no different than if the call were being made over a terminal link dedicated to that caller's use.

Optionally, the scheme allows a caller to complete interexchange calls without having to select a carrier, by identifying in a second memory a default carrier for calls for which no carrier is specified. A subscriber has the option of indicating whether or not a default carrier may be used for calls originating at the subscriber's terminal link. Any available carrier may be designated equally well as a default carrier at each office acting in the capacity of an end office.

And lastly, the caller need not be aware of whether or not carrier selection is required for a particular call. Should one be required yet no carrier be identifiable by any of the available means, a request is made of the caller to identify a carrier for the call.

These and other advantages and features of the invention will become apparent from the following description of an illustrative embodiment of the invention, taken together with the drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a simplified block diagram of a telephone network;

FIG. 2 is a block diagram of a switching system portion of the telephone network of FIG. 1;

FIG. 3 presents a logical-flow diagram of end office call processing;

FIGS. 4-7 are logical-layout diagrams of data structures of the switching system of FIG. 2;

FIGS. 8A through 8E present a logical-flow diagram of the route-selection blocks of FIGS. 3 and 12;

FIGS. 9A and 9B are logical-layout diagrams of the scratch pad of FIG. 5;

FIGS. 10A and 10B present a logical-flow diagram of the route-establishment block of FIG. 3;

FIG. 11 present a logical-flow diagram of carrier-office call processing; and

FIGS. 12A and 12B present a logical-flow diagram of access tandem call processing.

DETAILED DESCRIPTION

The invention is situated within the environment of a telephone network. FIG. 1 illustrates a simplified version of a representative telephone network. The details of conventional configuration and operation of an illustrative network of this nature are described in Notes on the Network, Issue 2, published in December 1980, by American Telephone and Telegraph Company. Reference may be made to this publication for a detailed understanding of the conventional features of the telephone network.

For purposes of such a network, a geopolitical area served by the network, comprising, for example, the United States and perhaps other countries, is divided into a plurality of contiguous, non-overlapping geographical areas called exchanges, represented in FIG. 1 by the exchanges 80 and 81. The telephone communication needs of each exchange are served by a local, intraexchange, telephone operating company. Within each exchange, the intraexchange company provides switching offices for routing calls to their destinations. For example, in exchange 80, one intraexchange company provides switching offices 98, 99, and 100, while in exchange 81 another intraexchange company provides switching offices 95, 96, and 97.

Interoffice communication routes are provided by trunks 132. While for clarity only individual trunks 132 are indicated in FIG. 1, typically two switching offices are interconnected by one or more groups of trunks 132 aptly referred to as trunk groups. The trunks 132 are assigned to trunk groups on the basis of their characteristics, such that the trunks 132 in a trunk group share some common characteristic, for example, a similar signaling scheme.

Subscribers to an intraexchange company's services have their stations connected to the various switching offices of the operating company. For example, subscriber stations 103 in the exchange 80 are connected to the offices 98, 99, and 100, while subscriber stations 103 in the exchange 81 are connected to the offices 95, 96, and 97. The subscriber stations 103 may comprise a wide variety of equipment. A station 103 may be, for example, a conventional telephone set, a pay telephone, a private branch exchange (PBX), or a Centrex telephone. Station-to-office connection is via lines and trunks, which will be commonly referred to herein as terminal links 127 to distinguish them from interoffice trunks 132.

Intraexchange calls in each exchange are commonly handled end-to-end by the switching offices of the intraexchange company. Interexchange telephone calls are handled at each end by the intraexchange operating companies of the respective exchanges, but they are carried from one exchange to the other by an interexchange carrier (IEC). A plurality of interexchange carriers may exist and each may provide its own switching office and transmission facilities to interconnect the exchanges. In the example of FIG. 1, one interexchange carrier provides switching office 101 to interconnect the exchanges 80 and 81, while a second interexchange carrier provides a switching office 102 for that purpose. Furthermore, each interexchange carrier may provide more than one switching office to service a given exchange. Connection of intraexchange company offices to the interexchange carrier offices is likewise via trunks 132.

If the exchanges 80 and 81 lie in different countries, the interexchange carriers that interconnect them are more specifically referred to as international carriers (INC). Because the following discussion applies to both domestic interexchange and international carriers and because a single carrier can be both a domestic and an international carrier, both will be understood to be included in the term "interexchange carrier", or just "carrier" (IC). Where discussion applies to international carriers specifically, the term "international carrier" will be used.

Intraexchange switching offices that merely switch communication paths between terminal links 127, and between trunks 132 and terminal links 127, are referred to as end offices (EO). In the example of FIG. 1, offices 96, 97, 99, and 100 are end offices. Certain end offices, such as the offices 97 and 100, have direct trunk 132 connections to carrier offices 101 and 102. Other end offices, such as the offices 96 and 99, only have indirect connections to carrier offices 101 and 102 via other offices such as the offices 95 and 98, respectively, of the respective intraexchange company. The offices such as the offices 95 and 98 that interface, i.e., provide access between, end offices and carrier offices are referred to as access tandems (AT). Access tandems may likewise service subscriber stations as do the end offices, and hence discussion of end offices will be generally understood to pertain as well to the access tandems acting in an end office capacity.

Because there may be more than one carrier available to a telephone subscriber to carry his interexchange calls, each subscriber can, but is not required to, identify a carrier as that subscriber's primary interexchange carrier (PIC). The primary carrier may be an interexchange carrier only, an international carrier only, or a combined interexchange and international carrier. Presubscription to, i.e., predesignation of, a primary carrier may be done by a subscriber on a per-line basis, for example when the subscriber's station 103 is a conventional telephone or an individual Centrex telephone; on a per-line group basis, for example when a subscriber's group of stations 103 comprise Centrex telephones; or on a per-trunk group basis, for example when the subscriber's station is a PBX.

Though identifying a primary carrier is not mandatory, such designation is necessary if the customer wishes to use conventional dialing, for example the 7 or 10 digit dialing of a domestic called number on a conventional telephone, to reach a carrier, other than a default carrier, if optional provision for designating a default carrier has been made and a default carrier has been defined. A default carrier can be one that has optionally been designated by an intraexchange office to carry an interexchange call for which no carrier has been designated. An office can designate a default interexchange carrier and a separate default international