Patent Abstract:
Methods and apparatus are presented for providing local authentication of subscribers travelling outside their home systems. A subscriber identification token  230  provides authentication support by generating a signature  370  based upon a key that is held secret from a mobile unit  220.  A mobile unit  220  that is programmed to wrongfully retain keys from a subscriber identification token  230  after a subscriber has removed his or her token is prevented from subsequently accessing the subscriber&#39;s account.

Full Description:
BACKGROUND  
         [0001]    I. Field of the Invention  
           [0002]    The present invention relates to communication systems, more particularly, to local authentication of a communication system subscriber.  
           [0003]    II. Background  
           [0004]    The field of wireless communications has many applications including, e.g., cordless telephones, paging, wireless local loops, personal digital assistants (PDAs), Internet telephony, and satellite communication systems. A particularly important application is cellular telephone systems for mobile subscribers. (As used herein, the term “cellular” systems encompasses both cellular and personal communications services (PCS) frequencies.) Various over-the-air interfaces have been developed for such cellular telephone systems including, e.g., frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In connection therewith, various domestic and international standards have been established including, e.g., Advanced Mobile Phone Service (AMPS), Global System for Mobile (GSM), and Interim Standard 95 (IS-95). In particular, IS-95 and its derivatives, IS-95A, IS-95B, ANSI J-STD-008 (often referred to collectively herein as IS-95), and proposed high-data-rate systems for data, etc. are promulgated by the Telecommunication Industry Association (TIA) and other well known standards bodies.  
           [0005]    Cellular telephone systems configured in accordance with the use of the IS-95 standard employ CDMA signal processing techniques to provide highly efficient and robust cellular telephone service. Exemplary cellular telephone systems configured substantially in accordance with the use of the IS-95 standard are described in U.S. Pat. Nos. 5,103,459 and 4,901,307, which are assigned to the assignee of the present invention and fully incorporated herein by reference. An exemplary described system utilizing CDMA techniques is the cdma2000 ITU-R Radio Transmission Technology (RTT) Candidate Submission (referred to herein as cdma2000), issued by the TIA. The standard for cdma2000 is given in draft versions of IS-2000 and has been approved by the TIA. The cdma2000 proposal is compatible with IS-95 systems in many ways. Another CDMA standard is the W-CDMA standard, as embodied in 3 rd    Generation Partnership Project “ 3 GPP ”, Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214.  
           [0006]    Given the ubiquitous proliferation of telecommunications services in most parts of the world and the increased mobility of the general populace, it is desirable to provide communication services to a subscriber while he or she is travelling outside the range of the subscriber&#39;s home system. One method of satisfying this need is the use of an identification token, such as the Subscriber Identity Module (SIM) in GSM systems, wherein a subscriber is assigned a SIM card that can be inserted into a GSM phone. The SIM card carries information that is used to identify the billing information of the party inserting the SIM card into a mobile phone. Next generation SIM cards have been renamed as USIM (UTMS SIM) cards. In a CDMA system, the identification token is referred to as a Removable User Interface Module (R-UIM) and accomplishes the same purpose. Use of such an identification token allows a subscriber to travel without his or her personal mobile phone, which may be configured to operated on frequencies that are not used in the visited environment, and to use a locally available mobile phone without incurring costs in establishing a new account.  
           [0007]    Although convenient, the use of such identification tokens to access account information of a subscriber can be insecure. Currently, such identification tokens are programmed to transmit private information, such as a cryptographic key used for message encryption or an authentication key for identifying the subscriber, to the mobile phone. A person contemplating the theft of account information can accomplish his or her goal by programming a mobile phone to retain private information after the identification token has been removed, or to transmit the private information to another storage unit during the legitimate use of the mobile phone. Mobile phones that have been tampered in this manner will hereafter be referred to as “rogue shells.” Hence, there is a current need to preserve the security of the private information stored on an identification token while still facilitating the use of said private information to access communication services.  
         SUMMARY  
         [0008]    A novel method and apparatus for providing secure authentication to a subscriber roaming outside his or her home system is presented. In one aspect, a subscriber identification token is configured to provide authentication support to a mobile unit, wherein the mobile unit conveys information to the subscriber identification token for transformation via a secret key.  
           [0009]    In another aspect, a subscriber identification module for providing local authentication of a subscriber in a communication system is presented, comprising: a memory; and a processor configured to implement a set of instructions stored in the memory, the set of instructions for: generating a plurality of keys in response to a received challenge; generating an authentication signal based on a received signal and a first key from the plurality of keys, wherein the received signal is transmitted from a communications unit communicatively coupled to the subscriber identification module, and the received signal is generated by the communications unit using a second key from the plurality of keys, the second key having been communicated from the subscriber identification module to the communications unit; and transmitting the authentication signal to the communications system via the communications unit.  
           [0010]    In another aspect, a subscriber identification module is presented, comprising: a key generation element; and a signature generator configured to receive a secret key from the key generation element and information from a mobile unit, and further configured to output a signature to the mobile unit.  
           [0011]    In another aspect, an apparatus for providing secure local authorization of a subscriber in a communication system is presented, comprising a subscriber identification module configured to interact with a communications unit, wherein the subscriber identification module comprises: a key generator for generating a plurality of keys from a received value and a secret value, wherein at least one communication key from the plurality of keys is delivered to the communications unit and at least one secret key from the plurality of keys is not delivered to the communications unit; and a signature generator for generating an authorization signal from both the at least one secret key and from an authorization message, wherein the authorization message is generated by the communications unit using the at least one communication key.  
           [0012]    In another aspect, a method for providing authentication of a subscriber using a subscriber identification device is presented, comprising: generating a plurality of keys; transmitting at least one key from the plurality of keys to a communications device communicatively coupled to the subscriber identification device and holding private at least one key from the plurality of keys; generating a signature at the communications device using both the at least one key transmitted to the communications device and a transmission message; transmitting the signature to the subscriber identification device; receiving the signature at the subscriber identification device; generating a primary signature from the received signature; and conveying the primary signature to a communications system.  
           [0013]    In another aspect, a method for providing authentication of a subscriber using a subscriber identification device, comprising: generating a plurality of keys; transmitting at least one key from the plurality of keys to a communications device communicatively coupled to the subscriber identification device and holding private at least one key from the plurality of keys; assigning a weight to the transmission message at the communications device in accordance with a relative importance of the transmission message; generating a signature at the communications device using both the at least one key transmitted to the communications device and the transmission message; transmitting the signature to a communications system if the assigned weight to the transmission message indicates that the transmission message is unimportant; and transmitting the signature to the subscriber identification device if the assigned weight to the transmission message indicates that the transmission message is important, whereupon the subscriber identification device generates a primary signature from the received signature signal, and then conveys the primary signature to a communications system. 
       
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a diagram of an exemplary data communication system.  
         [0015]    [0015]FIG. 2 is a diagram of a communication exchange between components in a wireless communication system.  
         [0016]    [0016]FIG. 3 is a diagram of an embodiment wherein a subscriber identification token provides encryption support to a mobile unit. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0017]    As illustrated in FIG. 1, a wireless communication network  10  generally includes a plurality of mobile stations (also called subscriber units or user equipment)  12   a - 12   d , a plurality of base stations (also called base station transceivers (BTSs) or Node B)  14   a - 14   c , a base station controller (BSC) (also called radio network controller or packet control function  16 ), a mobile switching center (MSC) or switch  24 , a packet data serving node (PDSN) or internetworking function (IWF)  20 , a public switched telephone network (PSTN)  22  (typically a telephone company), and an Internet Protocol (IP) network  18  (typically the Internet). For purposes of simplicity, four mobile stations  12   a - 12   d , three base stations  14   a - 14   c , one BSC  16 , one MSC  18 , and one PDSN  20  are shown. It would be understood by those skilled in the art that there could be any number of mobile stations  12 , base stations  14 , BSCs  16 , MSCs  18 , and PDSNs  20 .  
         [0018]    In one embodiment the wireless communication network  10  is a packet data services network. The mobile stations  12   a - 12   d  may be any of a number of different types of wireless communication device such as a portable phone, a cellular telephone that is connected to a laptop computer running IP-based, Web-browser applications, a cellular telephone with associated hands-free car kits, a personal data assistant (PDA) running IP-based, Web-browser applications, a wireless communication module incorporated into a portable computer, or a fixed location communication module such as might be found in a wireless local loop or meter reading system. In the most general embodiment, mobile stations may be any type of communication unit.  
         [0019]    The mobile stations  12   a - 12   d  may be configured to perform one or more wireless packet data protocols such as described in, for example, the EIA/TIA/IS-707 standard. In a particular embodiment, the mobile stations  12   a - 12   d  generate IP packets destined for the IP network  24  and encapsulate the IP packets into frames using a point-to-point protocol (PPP).  
         [0020]    In one embodiment the IP network  24  is coupled to the PDSN  20 , the PDSN  20  is coupled to the MSC  18 , the MSC  18  is coupled to the BSC  16  and the PSTN  22 , and the BSC  16  is coupled to the base stations  14   a - 14   c  via wirelines configured for transmission of voice and/or data packets in accordance with any of several known protocols including, e.g., E 1 , T 1 , Asynchronous Transfer Mode (ATM), IP, Frame Relay, HDSL, ADSL, or xDSL. In an alternate embodiment, the BSC  16  is coupled directly to the PDSN  20 , and the MSC  18  is not coupled to the PDSN  20 . In another embodiment of the invention, the mobile stations  12   a - 12   d  communicate with the base stations  14   a - 14   c  over an RF interface defined in the 3 rd    Generation Partnership Project  2 “3 GPP 2”, “Physical Layer Standard for cdma2000 Spread Spectrum Systems,” 3GPP2 Document No. C.P0002-A, TIA PN-4694, to be published as TIA/EIA/IS-2000-2-A, (Draft, edit version 30) (Nov. 19, 1999), which is fully incorporated herein by reference.  
         [0021]    During typical operation of the wireless communication network  10 , the base stations  14   a - 14   c  receive and demodulate sets of reverse-link signals from various mobile stations  12   a - 12   d  engaged in telephone calls, Web browsing, or other data communications. Each reverse-link signal received by a given base station  14   a - 14   c  is processed within that base station  14   a - 14   c . Each base station  14   a - 14   c  may communicate with a plurality of mobile stations  12   a - 12   d  by modulating and transmitting sets of forward-link signals to the mobile stations  12   a - 12   d . For example, as shown in FIG. 1, the base station  14   a  communicates with first and second mobile stations  12   a ,  12   b  simultaneously, and the base station  14   c  communicates with third and fourth mobile stations  12   c ,  12   d  simultaneously. The resulting packets are forwarded to the BSC  16 , which provides call resource allocation and mobility management functionality including the orchestration of soft handoffs of a call for a particular mobile station  12   a - 12   d  from one base station  14   a - 14   c  to another base station  14   a - 14   c . For example, a mobile station  12   c  is communicating with two base stations  14   b ,  14   c  simultaneously. Eventually, when the mobile station  12   c  moves far enough away from one of the base stations  14   c , the call will be handed off to the other base station  14   b.    
         [0022]    If the transmission is a conventional telephone call, the BSC  16  will route the received data to the MSC  18 , which provides additional routing services for interface with the PSTN  22 . If the transmission is a packet-based transmission such as a data call destined for the IP network  24 , the MSC  18  will route the data packets to the PDSN  20 , which will send the packets to the IP network  24 . Alternatively, the BSC  16  will route the packets directly to the PDSN  20 , which sends the packets to the IP network  24 .  
         [0023]    [0023]FIG. 2 illustrates a method for authenticating a subscriber using a mobile phone in a wireless communication system. A subscriber travelling outside of the range of his or her Home System (HS)  200  uses a mobile unit  220  in a Visited System (VS)  210 . The subscriber uses the mobile unit  220  by inserting a subscriber identification token. Such a subscriber identification token is configured to generate cryptographic and authentication information that allows a subscriber to access account services without the need for establishing a new account with the visited system. A request is sent from the mobile unit  220  to the VS  210  for service (not shown in figure). VS  210  contacts HS  200  to determine service to the subscriber (not shown in figure).  
         [0024]    HS  200  generates a random number  240  and an expected response (XRES)  270  based on knowledge of the private information held on the subscriber identification token. The random number  240  is to be used as a challenge, wherein the targeted recipient uses the random number  240  and private knowledge to generate a confirmation response that matches the expected response  270 . The random number  240  and the XRES  270  are transmitted from the HS  200  to the VS  210 . Other information is also transmitted, but is not relevant herein (not shown in figure). Communication between the HS  200  and the VS  210  is facilitated in the manner described in FIG. 1. The VS  210  transmits the random number  240  to the mobile unit  220  and awaits the transmission of a confirmation message  260  from the mobile unit  220 . The confirmation message  260  and the XRES  270  are compared at a compare element  280  at the VS  210 . If the confirmation message  260  and XRES  270  match, the VS  210  proceeds to provide service to the mobile unit  220 .  
         [0025]    Mobile unit  220  sends the random number  240  to the subscriber identification token  230  that has been inserted inside the mobile unit  220  by the subscriber. A Secure Key  300  is stored on the subscriber identification token  230 . Both the Secure Key  300  and the random number  240  are used by a key generator  250  to generate the confirmation message  260 , a cryptographic Cipher Key (CK)  290 , and an Integrity Key (IK)  310 . The CK  290  and IK  310  are conveyed to the mobile unit  220 .  
         [0026]    At the mobile unit  220 , the CK  290  can be used to encrypt communications between the mobile unit  220  and the VS  210 , so that communications can be decrypted only by the intended recipient of the message. Techniques for using a cryptographic key to encrypt communications are described in co-pending U.S. patent application 09/143,441, filed on Aug. 28, 1998, entitled, “Method and Apparatus for Generating Encryption Stream Ciphers,” assigned to the assignee of the present invention, and incorporated by reference herein. It should be noted that other encryption techniques can be used without affecting the scope of the embodiments described herein.  
         [0027]    The IK  310  can be used to generate a message authentication code (MAC), wherein the MAC is appended to a transmission message frame in order to verify that the transmission message frame originated from a particular party and to verify that the message was not altered during transmission. Techniques for generating MACs are described in co-pending U.S. patent application No. 09/371,147, filed on Aug. 9, 1999, entitled, “Method and Apparatus for Generating a Message Authentication Code,” assigned to the assignee of the present invention and incorporated by reference herein. It should be noted that other techniques for generating authentication codes may be used without affecting the scope of the embodiments described herein.  
         [0028]    Alternatively, the IK  310  can be used to generate an authentication signature  340  based on particular information that is transmitted separately or with the transmission message. Techniques for generating an authentication signature are described in U.S. Pat. No. 5,943,615, entitled, “Method and Apparatus for Providing Authentication Security in a Wireless Communication System,” assigned to the assignee of the present invention and incorporated by reference herein. The authentication signature  340  is the output of a hashing element  330  that combines the IK  310  with a message  350  from the mobile unit  220 . The authentication signature  340  and the message  350  are transmitted over the air to the VS  210 .  
         [0029]    As seen in FIG. 2, the cryptographic key  290  and the integrity key  310  are transmitted from the subscriber identification token  230  to the mobile unit  220 , which proceeds to generate data frames for public dissemination over the air. While this technique may prevent an eavesdropper from determining the values of such keys over the air, this technique does not provide protection from attack by a rogue shell. A rogue shell can be programmed to accept the CK  290  and the IK  310 , and to then store the keys rather than purging the presence of such keys from local memory. Another method to steal keys is to program the mobile unit  220  to transmit received keys to another location. The CK  290  and the IK  310  can then be used to fraudulently bill unauthorized communications to the subscriber. This rogue shell attack is particularly effective in systems wherein the random number generated at the Home System  200  is used in a manner that is insecure, such as the case when the same generated keys are used for an extended period of time.  
         [0030]    An embodiment that protects against a rogue shell attack uses the processors and memory in the subscriber identification token to generate an electronic signature that cannot be reproduced by a mobile unit without the insertion of the subscriber identification token.  
         [0031]    [0031]FIG. 3 illustrates an embodiment for performing local authentication of a subscriber in a wireless communication system. In this embodiment, the subscriber identification token  230  is programmed to generate an authentication response based on a key that is not passed to the mobile unit  220 . Hence, if the mobile unit used by a subscriber is a rogue shell, the rogue shell cannot recreate the appropriate authentication responses.  
         [0032]    Similar to the method described in FIG. 2, the mobile unit  220  generates a signature signal based upon an IK  310  that is received from the subscriber identification token  230  and a message that is to be sent to the VS  210 . However, in the exemplary embodiment, the signature signal is not passed to the VS. The signature signal is passed to the subscriber identification token  230 , and is used along with an additional key to generate a primary signature signal. The primary signature signal is sent out to the mobile unit  220 , which in turn transmits the primary signature signal to the VS  210  for authentication purposes.  
         [0033]    HS  200  generates a random number  240  and an expected response (XRES)  270  based on knowledge of the private information held on the subscriber identification token  230 . The random number  240  and the XRES  270  are transmitted to the VS  210 . Communication between the HS  200  and the VS  210  is facilitated in the manner described in FIG. 1. The VS  210  transmits the random number  240  to the mobile unit  220  and awaits the transmission of a confirmation message  260  from the mobile unit  220 . The confirmation message  260  and the XRES  270  are compared at a compare element  280  at the VS  210 . If the confirmation message  260  and the XRES  270  match, the VS  210  proceeds to provide service to the mobile unit  220 .  
         [0034]    Mobile unit  220  conveys the random number  240  to the subscriber identification token  230  that has been electronically coupled with the mobile unit  220  by the subscriber. A Secure Key  300  is stored on the subscriber identification token  230 . Both the Secure Key  300  and the random number  240  are used by a key generator  250  to generate the confirmation message  260 , a Cryptographic Key (CK)  290 , an Integrity Key (IK)  310 , and a UIM Authentication Key (UAK)  320 . The CK  290  and IK  310  are conveyed to the mobile unit  220 .  
         [0035]    At the mobile unit  220 , the CK  290  is used for encrypting transmission data frames (not shown in FIG. 3). The IK  310  is used to generate a signature signal  340 . The signature signal  340  is the output of a signature generator  330  that uses an encryption operation or a one-way operation, such as a hashing function, upon the IK  310  and a message  350  from the mobile unit  220 . The signature signal  340  is transmitted to the subscriber identification token  230 . At the subscriber identification token  230 , the signature signal  340  and the UAK  320  are manipulated by a signature generator  360  to generate a primary signature signal  370 . The primary signature signal  370  is transmitted to the mobile unit  220  and to the VS  210 , where a verification element  380  authenticates the identity of the subscriber. The verification element  380  can accomplish the verification by regenerating the signature signal  340  and the primary signature signal  370 . Alternatively, the verification element  380  can receive the signature signal  340  from the mobile unit  220  and only regenerate the primary signature signal  370 .  
         [0036]    The regeneration of the signature signal  340  and the primary signature signal  370  at the VS  210  can be accomplished by a variety of techniques. In one embodiment, the verification element  380  can receive a UAK  390  and an integrity key from the Home System  200 . When the verification element  380  also receives the message  350  from the mobile unit  220 , the signature signal can be generated and then used to generate the primary signature element.  
         [0037]    The signature generator  360  within the subscriber identification token  230  can comprise a memory and a processor, wherein the processor can be configured to manipulate inputs using a variety of techniques. These techniques can take the form of encryption techniques, hashing functions, or any nonreversible operation. As an example, one technique that can be implemented by the subscriber identification token is the Secure Hash Algorithm (SHA), promulgated in Federal Information Processing Standard (FIPS) PUB  186 , “Digital Signature Standard,” May 1994. Another technique that can be performed by the subscriber identification token is the Data Encryption Standard (DES), promulgated in FIPS PUB 46, January 1977. The use of the term “encryption” as used herein does not necessarily imply that operations must be reversible. The operations may be non-reversible in the embodiments described herein.  
         [0038]    The key generator  250  can also comprise a memory and a processor. Indeed, in one embodiment, a single processor can be configured to accomplish the functions of the signature generator  360  and the key generator  250 . Verification can be performed by calculating the same result from the same inputs at the verification element  380 , and comparing the calculated and transmitted values.  
         [0039]    A subscriber identification token used in a CDMA system or a GSM system, also known as an R-UIM or a USIM, respectively, can be configured to generate the primary signature signal  370  in the manner described above, i.e., all messages generated by the mobile unit are encrypted and authenticated. However, since the central processing unit in such tokens can be limited, it may be desirable to implement an alternative embodiment, wherein a weight of importance is assigned to a message frame so that only important messages are securely encrypted and authenticated. For example, a message frame containing billing information has more need for increased security than a message frame containing a voice payload. Hence, the mobile unit can assign a greater weight of importance to the billing information message frame and a lesser weight of importance to the voice message frame. When the subscriber identification token receives the signature signals generated from these weighted messages, the CPU can assess the different weights of importance attached to each signature signal and determine a primary signature signal for only the heavily weighted signature signals. Alternatively, the mobile unit can be programmed to convey only the “important” signature signals to the subscriber identification token. This method of selective primary signature signal generation increases the efficiency of the subscriber identification token by lightening the processing load of the subscriber identification token.  
         [0040]    The embodiments described above prevent unauthorized use of a subscriber&#39;s account by requiring a more secure transaction between the subscriber identification token and the mobile unit. Since the mobile unit cannot generate a primary signature signal without knowledge of the secret UAK, the mobile unit that is programmed to act as a rogue shell cannot misappropriate subscriber information for wrongful purposes.  
         [0041]    The embodiments described above also maximize the processing capability of the subscriber identification token by operating on a signature signal, rather than a message. Typically, a signature signal will have a shorter bit length than a message. Hence, less time is required for the signature generator in the subscriber identification to operate on a signature signal rather than a transmission message frame. As mentioned above, the processing capability of the subscriber identification token is usually much less than the processing capability of the mobile unit. Hence the implementation of this embodiment would provide secure authentication of messages without sacrificing speed.  
         [0042]    However, it should be noted that improvements in processor architectures occur at an almost exponential pace. Such improvements consist of faster processing times and smaller processor sizes. Hence, another embodiment for providing local authentication can be implemented wherein the primary signature signal can be generated directly from a message, rather than indirectly through a short signature signal. A mobile unit can be configured to pass a message directly to the subscriber identification token, one with the capability to generate a primary signature signal quickly, rather than passing the message to a signature generating element within the mobile unit. In another embodiment, only a limited number of messages need be passed directly to the subscriber identification token, in accordance with the degree of security needed for said messages.  
         [0043]    It should be noted that while the various embodiments have been described in the context of a wireless communication system, the various embodiments can be further used to provide secure local authentication of any party using an unfamiliar terminal connected in a communications network.  
         [0044]    Thus, novel and improved methods and apparatus for performing local authentication of a subscriber in a communication system have been described. Those of skill in the art would understand that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, software, firmware, or combinations thereof. The various illustrative components, blocks, modules, circuits, and steps have been described generally in terms of their functionality. Whether the functionality is implemented as hardware, software, or firmware depends upon the particular application and design constraints imposed on the overall system. Skilled artisans recognize the interchangeability of hardware, software, and firmware under these circumstances, and how best to implement the described functionality for each particular application.  
         [0045]    Implementation of various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented or performed with a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components. A processor executing a set of firmware instructions, any conventional programmable software module and a processor, or any combination thereof can be designed to perform the functions described herein. The processor may advantageously be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. The software module could reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary processor is coupled to the storage medium so as to read information from, and write information to, the storage medium. In the alternative, the storage medium may reside in an ASIC. The ASIC may reside in a telephone or other user terminal. In the alternative, the processor and the storage medium may reside in a telephone or other user terminal. The processor may be implemented as a combination of a DSP and a microprocessor, or as two microprocessors in conjunction with a DSP core, etc. Those of skill would further appreciate that the data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description are represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof.  
         [0046]    Various embodiments of the present invention have thus been shown and described. It would be apparent to one of ordinary skill in the art, however, that numerous alterations may be made to the embodiments herein disclosed without departing from the spirit or scope of the invention.

Technology Classification (CPC): 7