1. Field of the Invention
The present invention relates to cellular communication systems, and more particularly, to a method and apparatus for the routing of auxiliary communication services, such as voice mail, within such a system.
2. History of the Prior Art
Broadly speaking, the present invention involves an interrelationship between two telephone technologies, namely, cellular radio telephony and voice mail systems. The general state of the art of each of these technologies is discussed briefly below.
Cellular radio communications is, perhaps, the fastest growing field in the world-wide telecommunications industry. Although cellular radio communication systems comprise only a small fraction of the telecommunications systems presently in operation, it is widely believed that this fraction will steadily increase and will represent a major portion of the entire telecommunications market in the not too distant future. This belief is grounded in the inherent limitations of conventional telephone communications networks which rely primarily on wire technology to connect subscribers within the network. A standard household or office telephone, for example, is connected to a wall outlet, or phone jack, by a telephone cord of a certain maximum length Similarly, wires connect the telephone outlet with a local switching office of the telephone company. A telephone user's movement is thus restricted not only by the length of the telephone cord, but also by the availability of an operative telephone outlet, i.e. an outlet which has been connected with the local switching office. Indeed, the genesis of cellular radio systems can be attributed, in large part, to the desire to overcome these restrictions and to afford the telephone user the freedom to move about or to travel away from his home or office without sacrificing his ability to communicate effectively with others.
In a typical cellular radio system, the user, or the user s vehicle, carries a relatively small, wireless device which communicates with a base station and connects the user to a mobile switching center or exchange. The exchange facilitates communications between the user, other mobile stations in the system and landline parties in the public switched telephone network ("PSTN"). The interconnection of mobile telephone users ("mobile subscribers") with the PSTN requires that each mobile subscriber in the system be made available to anyone who has a telephone, whether fixed or mobile. Hence, the problem of locating a mobile subscriber moving from one area to another (a "roaming subscriber") within a wide geographic area has become of primary importance. A known solution to this problem is based on the concept of mobile registration.
Mobile registration is the process by which a mobile telephone unit becomes listed as being present in the service area of one of the mobile exchanges in a mobile telephone service network. In accordance with one early solution to the problem of locating roaming mobiles, as each mobile telephone unit enters a new area within the network, it transmits a unique identity signal which is detected by the mobile exchange associated with that area. This exchange records an indication of the presence of the mobile subscriber in its memory and then informs all of the other exchanges of the presence of the mobile within its coverage area at that particular moment. When the mobile subscriber crosses over into another area, the exchange associated with that area, upon receiving an identity signal from the mobile telephone unit, will record an indication of the mobile subscriber's presence there and then transmit the identity to all of the other exchanges together with its own identity signal, for the purpose of updating the mobile subscriber's position.
In other known solutions, a mobile subscriber s identity and position messages are sent by each exchange, whose respective areas are crossed by such mobile subscriber, to a specific center. Any exchange in the mobile network which contacts this center may receive all the information necessary for locating and making a connection to the mobile subscriber. This solution eliminates the need to advise one or more of the other mobile exchanges each time a mobile subscriber enters a new area without making or receiving a call there and thereby reduces the amount of mobile subscriber location data that must be processed by each of the mobile exchanges within the network.
In some systems, the aforementioned center may be a common national center such as that used in the mobile telephone location system disclosed in U.S. Pat. No. 4,700,374 issued to Bini. In other systems, the center may be the exchange to which a mobile subscriber is assigned ("home exchange") or it may be a home location register ("HLR") which stores the location information of mobile subscribers. In such other systems, the mobile subscriber may preregister in an area other than the normal service and billing area ("home area") for service to be provided in the other area ("visited area") by the exchange associated with the visited area ("visited exchange"). When a roaming subscriber arrives in the visited area, the mobile subscriber is qualified to make telephone calls from there and calls which are received in the mobile subscriber's home area are forwarded to the visited area for transmission to the mobile subscriber.
Qualification of a mobile subscriber in a visited area may be automatically performed when the roaming subscriber appears in the visited area and the mobile station is switched on, e.g. when the user initiates a first telephone call. The roaming mobile subscriber automatically transmits its identification number to the visited exchange and requests roamer service. If the roaming subscriber is a visitor from a cooperating exchange, the visited exchange provides service to the roaming subscriber by allocating a temporary roamer number to it. The visited exchange also notifies the roaming subscriber's home exchange of the roaming subscriber s location in the coverage area of the visited exchange. The roaming subscriber s identification number is then entered into a list of roamers so that incoming calls to the home exchange of the roaming subscriber are forwarded to the visited exchange where the roaming subscriber is then located.
With this overview of cellular radio technology in mind, the other parent technology of the present invention, i.e., the rendition of auxiliary communication services such as voice mail, fax mail, video mail, teletex or other services will now be briefly discussed. While it should be understood that the invention contemplates the routing of various auxiliary communication services within a cellular network, the most presently common of such services, voice mail, will be used hereinafter as exemplary of the principles of the present invention. Voice mail, as is commonly understood in the art, refers to an automatic answering and message storing and forwarding system which allows a caller to leave a stored message for a specific telephone extension or to forward a message to an intended recipient. The voice mail system in essence, offers two distinct features: call answering and message forwarding. The call answering feature is typically invoked when a caller directly dials a telephone number or requests to be connected to a specific extension which is found to be busy, inoperative, nonanswering, etc. The unavailability of the called party results in the telephone call being redirected to the voice mail system call answering feature. At this juncture the call is routed to a voice mail receiving station and the caller is greeted by a prerecorded announcement message or a live telephone operator and prompted to leave a message which is stored in a "voice mailbox" assigned to the called party.
While the call answering feature of a voice mail system is geared to the convenience of the message recipient, the message forwarding feature is geared towards aiding the message originator. The message forwarding feature may be involved, for example, when a message originator calls the voice mail system by dialing an access code using a telephone station set touch-tone pad. The message originator designates one or more message recipients, who are also served by the telephone switching system, by dialing their station or directory numbers, and then dictates a voice message for delivery by the voice mail system to the designated message recipients. The voice mail system records the dialed directory numbers and places the message in the voice mailbox assigned to each of the designated message recipients. Each of the message recipients may then access the voice mail system to listen to the message, save the message for subsequent action, delete it, append comments and route the message to other message recipients, or reply and return a message to the message originator.
Several voice mail systems are commercially available. While such systems vary in features and capacity, all share, as a common core of functions, the ability to store messages for later retrieval by system users. Each user is assigned a voice mailbox and may retrieve his messages by accessing the system and then identifying his voice mailbox. In general, anyone having knowledge of the telephone numbers for the system, may leave messages, but only users having a voice mailbox may retrieve messages. To retrieve messages a user of a typical voice mail system accesses the system over the telephone lines and, after identifying himself by mailbox number, and possibly a password, hears a recorded message produced by the system advising him of the status of his mailbox, e.g., number of messages waiting, etc. The user may then listen to brief prerecorded portions of each message, commonly referred to as "headers" which further describe the message, for example, by identifying the originator. The user may then decide whether or not to listen to each message.
The desirability of providing voice mail services to mobile radiotelephone system subscribers has not been lost on cellular system operators. The economic incentives for incorporating a voice mail system into a cellular radio network are, at least, twofold. First, the cellular operator can charge each participating mobile subscriber a monthly fee for voice mail services. Second, the ability of a mobile subscriber to retrieve messages left in his absence increases the likelihood that the mobile subscriber will, after listening to a particular message, call the message originator and thereby generate additional revenues for the cellular system operator.
The use of voice mail systems in cellular radio networks has developed in line with the growth of cellular radio communications generally. Initially, a single mobile switching center in each of several large metropolitan cities provided communications services to mobile subscribers in the local area. The national cellular radio landscape, at this stage of development, consisted of dispersed pockets or islands of cellular radio networks with little or no roaming therebetween. Where desired, a voice mail system was connected to the mobile switching center which handled all the incoming and outgoing calls to and from the mobile subscribers in the local area. The voice mail equipment could be located in, or within a short distance from, the mobile switching center facilities.
As each of the local cellular radio networks expanded, one or more mobile switching centers were, of necessity, added to the local area. Provision of voice mail services to all of the mobile subscribers in the local area required either the connection of a voice mail system to each of the mobile switching centers or, alternatively, the sharing of a common voice mail system among the mobile switching centers. In the former instance, voice mail equipment was located in, or within a short distance from, each of the mobile switching centers while, in the latter instance, the voice mail equipment was centralized and dedicated trunks were used to connect the voice mail system to each of the mobile switching centers. Since the network coverage area rarely extended beyond the limits of a metropolitan area, the use of dedicated trunks did not pose any special problems.
However, with the advent of roaming service for mobile subscribers travelling between cities served by cooperating exchanges, the cost inefficiencies inherent in the traditional designs of cellular voice mail systems became pronounced. To illustrate, suppose that a mobile subscriber assigned to a mobile exchange (home exchange) in Los Angeles travels to San Francisco and is qualified to receive calls there, e.g., is registered in an exchange (visited exchange) in San Francisco. Suppose, further, that a call is placed to the mobile subscriber and received at the home exchange in Los Angeles. The home exchange determines from its data records, or from the data records of the HLR of the mobile, that the mobile subscriber has roamed and, therefore, routes the call to San Francisco. The visited exchange receives the call and pages the roaming subscriber who does not return a page response became, for example, he has turned off the mobile telephone. At this point, the call is either terminated or, if the roaming subscriber has voice mail service, the call is transferred back to the roaming subscriber's mailbox in Los Angeles.
As may be seen from this illustration, two long distance links or the equivalent of two toll calls, e.g., Los Angeles to San Francisco and San Francisco to Los Angeles, were necessary in order to connect the caller with the voice mail box of the roaming subscriber. This result is known in the art as the "trombone effect." Mobile subscribers are loathe to pay for the double charges incurred because of the trombone effect only to hear a voice mail message, for example, that the caller will call back later. Hence, a mechanism for countering the trombone effect is needed and, as discussed more fully below, is provided by the present invention. One technique for avoiding these disadvantages is the use of roamer port access for a roaming cellular subscriber in which a local number in the visited city is dialed to gain access to the cellular network there and enable the dialing of the mobile subscriber directly. However, the disadvantage of this procedure is that it requires the calling party to know precisely in which city the roaming mobile subscriber is currently roaming.
Another shortcoming of existing cellular voice mail systems relates to the interface between the mobile switching centers, on the one hand, and the voice mail equipment on the other. Typically, the voice mailboxes of mobile subscribers are connected to their home exchange, not via the PSTN, but through dedicated trunks, and are accessed by dedicated numbers rather than PSTN-recognized telephone numbers. This is primarily for reasons of economy, including the savings on the lease of telephone numbers from the local phone company. Specifically, instead of leasing two telephone numbers from the phone company, one for the mobile subscriber and another for his voice mailbox, only the mobile subscriber number is leased and a dedicated number is used for his voice mailbox.
To illustrate the operation of the typical voice mail interface, assume that a call is placed to a mobile subscriber and received at his home exchange. Assume, further, that the mobile subscriber is unavailable and that the call is directly routed to his voice mailbox which is often in the same building as the home exchange. It is common practice to define a dedicated voice mail number which consists of the mobile subscriber number and a prefix such as "*99." Thus, if the dialed mobile subscriber number is 876-5432, for example, the dedicated voice mail number will be *99/876-5432. The voice mailbox of the mobile subscriber can then be accessed by the home exchange simply by using the designation *99/876-5432.
Returning to the case of the mobile subscriber who roams from Los Angeles to San Francisco, and assuming that the mobile subscriber has voice mail service, the call will have to be routed from San Francisco back to Lee Angeles where the voice mailbox of the mobile subscriber is located. However, because, as discussed above, his voice mailbox number is a non-PSTN-recognized number, such as *99/876-5432, PSTN facilities cannot be used to transfer the call. It is possible, of course, to use dedicated trunks between Los Angeles and San Francisco and route the call therethrough, but the cost in this case would be prohibitive. Further, it may also be illegal for a cellular operator which is not duly licensed as a long distance carrier to transfer the call in this manner.
Some systems have attempted to solve the problems highlighted in the above illustrative Los Angeles-San Francisco scenario by adopting a protocol whereby calls are always setup through the home exchange and, instead of assigning the task of detecting that the roaming subscriber has not answered to the visited exchange, a timer is activated in the home exchange upon call set up and, if the roaming subscriber does not answer within a predetermined length of time, the home exchange will route the call to the voice mailbox assigned to the roaming subscriber. As explained below, however, this solution may be implemented only in a mobile switching center ("MSC") environment, but not efficiently in a home location register ("HLR") environment.
Most cellular radio networks today operate in an MSC environment wherein location and service profile data for each mobile subscriber are stored in his home exchange and updated from time to time as required. By contrast, in an HLR environment, location and service profile data are recorded and updated in the home HLR which is connected, and may serve the mobile subscribers assigned, to several exchanges. Unlike an exchange, however, an HLR has no voice transmission, reception or switching facilities, but is essentially a database from and to which information can be read and written. Possible reasons for a shift from an MSC environment to an HLR environment include limitations in the capacity of an exchange and optimization of administrative procedures for handling calls. For example, while only 64,000 mobile subscribers may be connected to a typical exchange at one time, an HLR may have the capacity to serve over 500,000 mobile subscribers.
Implementation of an HLR environment in cellular radio networks, either exclusively or in conjunction with an MSC environment, is expected to become widespread in the future. However, integrating a voice mail system in an HLR environment poses certain difficulties which do not arise in an MSC environment. In particular, because the HLR has no voice facilities, the voice mail equipment cannot be directly connected to the HLR. One solution to these difficulties is to connect the voice mail equipment to one exchange in the cellular network dedicated as a "pseudo-home" exchange and route all calls through the pseudo-home exchange. However, as will be readily understood by person skilled in the art, this solution breeds an inefficient and inflexible network. As discussed more fully below, the present invention provides for efficient delivery of calls to the voice mailboxes of mobile subscribers operating in both an MSC and an HLR environment.