Pre-Paid Cellular Telephone System

This application relates to U.S. application entitled "Prepaid Security Cellular Telecommunications System" filed Nov. 15, 1995 and assigned Ser. No. 08/559,283, which application is a continuation-in-part of an application entitled "Security Cellular Telecommunications System" filed Dec. 23, 1994 and assigned Ser. No. 08/364,479.

This invention relates to a cellular telephone system that allows only pre-paid subscribers to complete cellular telephone calls and, in particular, to a fraud resistant, pre-paid subscriber system that does not require special hardware connections to existing telephone networks.

A cellular telephone is a multi-frequency, portable transceiver, typically constructed with state of the art integrated circuits and capable of transmitting voice and digital data reliably almost anywhere in the world. Except for cost, a cellular telephone would likely be the appliance of choice for every person in the country. A cellular telephone costs at least three times as much, per month, as a "land line," i.e. as a telephone connected by wire to a public switching network. Despite the sophistication and quality of cellular telephones, the relatively high cost of a cellular telephone is not the telephone itself but the charges for the use of the telephone. Unfortunately, a significant portion of the cost of the system is due to theft of services and many techniques for detecting or thwarting theft have been proposed or implemented.

Some security systems rely on calling patterns to detect theft and such systems are prone to false positives, i.e. incorrectly identifying a legitimate call as an illegal call. Also, such systems require a number of calls to detect theft, which means that, if the calls were illegal, the cost of the calls must be absorbed by someone other than the caller. It is preferred to have a system that can detect theft of service as soon as the theft is attempted.

Another aspect of cost is the ability of a subscriber to pay for services compared to the subscriber's desire to use the services. Many subscribers sign up on the basis that the cellular telephone will only be used for emergencies and then find themselves defining "emergency" progressively more broadly. For many subscribers, it has been found beneficial to provide a pre-paid subscription that is self-limiting in the sense that the service is terminated when the payments are used up. U.S. Pat. No. 5,353,335 (D'Urso et al.) describes a modified network architecture in which a special platform for pre-paid calling service is connected to a node in a public, switched telephone network.

The above-identified, related application also describes a pre-paid, cellular system. The system disclosed in the application requires no modification to cellular telephones and, for outgoing calls, uses the automatic number identification (ANI) from each cellular telephone as a file link to identify and to authenticate the cellular telephone. Incoming calls are transferred to the mobile telephone switching office (MTSO) in which a number of lines are reserved for pre-paid subscribers. An incoming call is screened for a valid number and a positive, pre-paid balance by the subscription service and is then passed on to the subscriber. If the MTSO will not dedicate lines to pre-paid subscribers, a significant geographic area is unavailable to a specific but viable market.

A system requiring no modification to the cellular telephone is most easily implemented but the security of such a system is not as great as when the cellular telephone is modified. A problem with a system requiring modification to the cellular telephone is that physical access to the telephone is required.

In view of the foregoing, it is therefore an object of the invention to provide a cellular telephone system for pre-paid subscribers in which the system is transparent to existing telephone networks.

Another object of the invention is to provide a prepaid subscription service that detects theft as it occurs and terminates the call.

A further object of the invention is to provide a prepaid cellular telephone system that monitors both outgoing and incoming calls without the need for dedicated lines at an MTSO.

Another object of the invention is to provide a secure, pre-paid cellular telephone system requiring only software modification to existing cellular telephones to adapt the telephones to the system.

A further object of the invention is to provide a secure cellular telephone system in which modifications to a cellular telephone do not require physical access to the telephone.

The foregoing objects are achieved in this invention in which a pre-pay service unit uses pseudo-numbers to identify subscribers and substitutes actual numbers to complete a call after verification of authenticity and verification of a positive balance in the subscriber's account. The service unit and the cellular telephone exchange data in the process of authentication. The pseudo-numbers are combined with other data to provide a secure check of the identity of the cellular telephone and the authenticity of the call. It has been found that modifying a cellular telephone to share identification between the system and the telephone increases the security of the system and enables the operation of the system to be transparent to an MTSO.

In particular, for outgoing calls, the cellular telephone first transmits an "800" number and the ANI. The "800" number is the telephone number of a pre-pay service unit which receives the call. A switching network in the service unit then requests the dialed number, which the cellular telephone transmits with a security code and status data. If the data is authentic, the dialed digits are then passed to a local exchange carrier to complete the call. For incoming calls, a pseudo-number directs the call to a pre-pay service unit, which converts the pseudo-number to the actual telephone number of a subscriber's cellular telephone. After exchanging data with the subscriber's telephone, and after verifying authenticity and credit balance, the call is completed. Both incoming and outgoing calls can be monitored for charges and a call is terminated if a subscriber's balance reaches zero.

A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates an outgoing call from a cellular telephone in accordance with the prior art;

FIG. 2 illustrates an outgoing call from a cellular telephone in accordance with the invention;

FIG. 3 illustrates an incoming call to a cellular telephone in accordance with the prior art;

FIG. 4 illustrates an incoming call to a cellular telephone in accordance with the invention;

FIG. 5 is a flow chart of an outgoing call in accordance with a preferred embodiment of the invention;

FIG. 6 is a flow chart of an incoming call in accordance with a preferred embodiment of the invention;

FIG. 7 is a block diagram of a cellular telephone suitable for use in the invention; and

FIG. 8 is a block diagram of the apparatus used by the pre-pay service unit .

FIG. 1 illustrates an outgoing call from a cellular telephone to a conventional telephone. Cellular telephone 10 transmits the dialed number followed by the automatic number identification code that uniquely identifies telephone 10. This data is received by mobile telephone switching office 12, wherein the number is verified as a valid subscriber and, if so, the call is transferred to local exchange carrier 14. Local exchange carrier 14 switches the call to the appropriate land line connecting carrier 14 with telephone 16.

FIG. 2 illustrates the path of an outgoing call from a pre-pay subscriber through a pre-pay service unit. The subscriber enters the desired destination telephone number on cellular telephone 20 and then presses the "send" button. Telephone 20 displays the dialed number but actually transmits a pseudo-number, viz. an "800" number to mobile telephone switching office 12, followed by the automatic number identification code of cellular telephone 20. The pseudo-number is the number of a pre-pay service unit. An "800" number is used to obtain the automatic number identification code of telephone 20.

Switching office 12 passes the call through local exchange carrier 14 and tandem switch 22 to pre-pay service unit 24. Service unit 24 recognizes the automatic number identification as a pre-pay subscriber and transmits back to cellular telephone 20 a request for the number actually dialed. Cellular telephone 20 responds with the dialed number, which passes through switching office 12, carrier 14, switch 22, and service unit 24 to local exchange carrier 26, which switches the call to the appropriate land line for connection to telephone 16. Thus, the number initially transmitted by a pre-paid subscriber is the number of a pre-pay service unit, which then completes the call with the dialed number. In the process, the pre-pay service unit monitors the calls and the account of the subscriber to be sure that the call is genuine and that the account of the subscriber is has a positive balance.

In FIGS. 1-4, a pre-pay subscriber is indicated by a cellular telephone symbol and a non pre-pay subscriber is represented by a symbol of a conventional telephone. This is solely for the sake of clarity. A pre-pay subscriber can call anyone, including another subscriber. An outgoing call from a subscriber's telephone is always a call to an "800" number and, therefore, always includes the dialed number and the automatic number identification code. This characteristic of cellular telephone service is used to direct all outgoing calls to the pre-pay service unit, enabling the pre-pay service unit to monitor all subscriber calls for authenticity and appropriate charges.

FIG. 3 illustrates an incoming call to a cellular telephone in accordance with the prior art. In this case, the user dials the destination number on telephone 16 and the number is recognized as a cellular telephone number by local exchange carrier 14. Carrier 14 transfers the call to mobile telephone switching office 12, which then authenticates the call and pages cellular telephone 10.

In accordance with the invention, the actual telephone number of the cellular phone is known only by the pre-pay service unit and a pseudo-number is provided to the public, including the subscriber. The pseudo-number is actually a number belonging the pre-pay service unit. Thus, as illustrated in FIG. 4, a user dials the pseudo-number on telephone 16 and local exchange carrier 26 recognizes the number as belonging to pre-pay service unit 24. Upon receipt of the incoming call, pre-pay service unit 24 looks up the actual number of cellular telephone 20 in a conversion table and transfers the number to local exchange carrier 14. Carrier 14 recognizes the number as a cellular number and passes the call to mobile telephone switching office 12 which authenticates the number and pages cellular telephone 20.

For both incoming and outgoing calls, the pre-pay service unit is interposed in the path of a call and converts the dialed number into a second number for completing the telephone call. The pre-pay service unit is independent of the mobile telephone switching office and is connected into the system by way of land lines or its own cellular connections.

While intercepting telephone calls to or from a subscriber, the pre-pay service unit exchanges data with the subscriber cellular telephone prior to completing the call. The data exchange between the pre-pay service unit and a subscriber's cellular telephone is illustrated in FIGS. 5 and 6. In FIGS. 5 and 6, a small dot in the lower left hand corner of a rectangle indicates an action by the subscriber's cellular telephone. A small dot in the lower right hand corner of a rectangle indicates an action taken by the pre-pay service unit. A rectangle with a dot in neither location represents an action taken by a public switching network.

FIG. 5 is a flow chart of an outgoing call from a subscriber's cellular telephone. In step 51, the subscriber dials the desired phone number. If the cellular telephone has a display, the dialed number is displayed. In step 52, the subscriber's cellular telephone actually transmits an "800" number followed by the automatic number identification code. The telephone call is passed to the pre-pay service unit, which stores the automatic number identification and opens a channel to the subscriber's telephone. The pre-pay service unit then requests the dialed number (step 54) and the subscriber's telephone responds (step 55) with the number actually dialed and other data, such as roaming information, and a security code. "Roaming" is the situation in which the cellular telephone is outside of the service area of the subscriber. Service within the area is considered a local call and service outside the area is subject to additional charges for "roaming" and long distance.

After receiving the data from the subscriber's telephone, the pre-pay service unit uses the automatic number identification to check the subscriber's account for a positive balance and verifies the automatic number identification code with the security code transmitted by the subscriber's telephone. If there is a positive balance and the codes are authentic, the pre-pay service unit sends an "off hook" signal to the mobile telephone switching office, step 57, and transfers the call to the local exchange carrier, step 58, completing the call.

FIG. 6 illustrates the exchange of information between the pre-pay service unit and a subscriber's cellular telephone for an incoming call. In step 61, a party dials what he believes is the actual telephone number of a subscriber's cellular telephone. In step 62, the local exchange carrier recognizes the number as belonging to the pre-pay service unit and transfers the call accordingly. In step 63, the pre-pay service unit looks up the pseudo-number in a table and replaces the pseudo-number with the actual number of the subscriber's telephone. In step 64, the pre-pay service unit checks the subscriber's account for a positive balance, based upon the actual number of the subscriber, also known as the mobile identification number (MIN).

In step 65, the pre-pay service unit retrieves a security code for the subscriber's account and then passes the actual telephone number to a local exchange carrier (step 66). The carrier passes the call to a mobile telephone switching office for paging the subscriber's cellular telephone. Upon receipt of the page, the subscriber's telephone goes off-hook, in what is known as auto-answer mode, but does not ring. In other words, the subscriber's cellular telephone must have the features of auto answer and silent ring. These features are used in many, but not all, commercially available cellular telephones. To be compatible with the invention, a cellular telephone must have both features.

After the subscriber's telephone goes off-hook, the pre-pay service unit sends a security code to the subscriber's telephone, step 73. The subscriber's telephone verifies the code, step 74, sends a verification code to the pre-pay service unit and provides an audible ring, step 75. The pre-pay service unit verifies the code and monitors the account of the subscriber for call duration and billing. The subscriber "answers" the telephone call by pressing "send", step 77.

The exchange of data between the subscriber's telephone and the pre-pay service unit makes it very difficult to steal services. If desired, the data transferred between the subscriber's telephone and the pre-pay service unit can be scrambled or encrypted. For example, in step 55 (FIG. 5) the DID number, roam code, and security code need not be sent consecutively but can be mixed in a predetermined sequence and sent as a single block of data. Similarly, the subscriber's telephone can verify the code (step 74, FIG. 6) as it is received from the pre-pay service unit or it can re-order the digits in a predetermined manner prior to verification. Alternatively, one can use a checksum to verify the digits or a checksum of selected digits for verification. The particular manner for scrambling or encrypting data is a matter of choice.

FIG. 7 illustrates a cellular telephone suitable for use in the invention. In particular, cellular telephone 80 includes microprocessor 81 coupled to memory 82 and to an I/O device such as encoder 83 and transceiver 84. Antenna 85 is coupled to transceiver 84 for emitting the high frequency radio waves that couple cellular telephone to the mobile telephone switching office. Most cellular telephones transmit data in what is known as multiple frequency (MF) or dual tone multiple frequency (DTMF) form. In accordance with the invention, a cellular telephone, in addition to providing silent ring and auto answer, includes at least a decoder for MF/DTMF signals.

A DTMF encoder/decoder handles numbers in base 10, i.e. the digits 0-9, as opposed to binary numbers, the digits 0-1. In a preferred embodiment of the invention, a cellular telephone includes a modem for transmitting binary data to and from microprocessor 81 through transceiver 84. Some commercially available cellular telephones include an encoder, a decoder, and a modem. For such telephones, only the software and the data stored in memory 82 need be modified to implement the invention. Transmitting a pseudo-number, waiting for a request for data, comparing data with stored data, and sending a code stored in memory are tasks that are easily implemented in software whether microprocessor 81 has a rich instruction set or a reduced instruction set.

An advantage of the invention over the prior art is the ability to encode a subscriber's cellular telephone remotely. Because the telephone includes a modem, data can be transferred over telephone lines to the subscriber's telephone, for example to change security codes or to modify the programming within the telephone. Not only is physical access not required but this capability enables one to sell telephones through a national chain of stores and activate the telephones remotely. Similarly, if a subscriber moves, the subscriber's "home" cell can be changed remotely to the new location. The memory in the cellular telephone need only include an EAROM (electrically alterable, read-only memory).

FIG. 8 is a block diagram of a pre-pay service unit. Unit 90 includes microprocessor 91 connected to memory/mass storage 92 and line interface 93. Memory/mass storage 92 contains the codes, account information, and tables for translating incoming calls to the appropriate destination telephone number and for monitoring the accounts of subscribers. Line interface 93 includes appropriate circuitry for coupling land lines to microprocessor 91, encoding and decoding MF/DTMF signals, and modems for transferring data to and from subscribers. Printed circuit boards or cards are commercially available for performing these functions and a array of cards is used in unit 90 for connecting lines 96 to a public switching network. Lines 96 can be implemented by wire, optical fiber, microwave links, or a cellular network. Line 94 is a "WATS" or "800" line. More than one "800" number can be used, depending upon loading.

A system incorporating the invention operates conventionally for billing, busy signals, inadequate prepaid balance, and the like. A distinct advantage of the invention is that attempted theft of services is immediately detected because of the data exchange between a subscriber's cellular telephone and the pre-pay service unit. Referring to FIG. 4, for example, if someone obtained the actual dial-in number is for telephone 20, the pre-pay service unit cannot be by-passed because telephone 20 awaits a security code before proceeding. Without the code, the call is terminated and is counted as suspect in telephone 20. In a preferred embodiment of the invention a fixed number of suspect calls, e.g. three, disables telephone 20 until the telephone is physically returned to the pre-pay service provider for correction. Similarly, without the security code, a call cannot be placed from telephone 20.

The invention thus provides a cellular telephone system for pre-paid subscribers in which the system is transparent to existing telephone networks and can monitor both outgoing and incoming calls without the need for dedicated lines at an MTSO. Theft is detected as soon as an attempt is made and the call is terminated. The invention can be implemented with commercially available cellular telephones and switching equipment, requiring only software modification.

Having thus described the invention, it will be apparent to those of skill in the art that various modifications can be made within the scope of the invention. For example, a rotating coding system can be used within the cellular telephone. That is, for step 55 (FIG. 5), one of several possible codes is transmitted for each outgoing call. Both the telephone and the pre-pay service unit have the same table of codes and increment through the table in step. Attempted thefts of service will very likely send the wrong code. The tables and codes are different for each subscriber.