Abstract:
A method and system for providing circuit-switched to IP Multimedia Subsystem voice call continuity with a single radio are provided. The exemplary embodiment takes advantage of a standard RANAP signaling procedure called “directed retry” to perform a much better coordination between the change in radio mode and the transfer of the media path. With the use of “directed retry”, the transfer of the media path does not begin until the handover procedure is complete, thus significantly reducing the likely amount of media disruption.

Description:
This application claims priority from U.S. Provisional Application Ser. No. 60/976,658 filed on Oct. 1, 2007. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method and system for providing circuit-switched to IP Multimedia Subsystem voice call continuity with a single radio. While the invention is particularly directed to the art of telecommunications, and will be thus described with specific reference thereto, it will be appreciated that the invention may have usefulness in other fields and applications. 
     By way of background, wireless telecommunication networks, which are well known, allow mobile devices to communicate with each other and other networks, such as the Internet and the Public Switched Telephone Network (PSTN). In general, the Universal Mobile Telecommunication System (UMTS) is a third generation wireless communications system that has evolved from the Global System for Mobile communication (GSM). The UMTS is intended to provide various mobile communication services by combining a UMTS Terrestrial Radio Access Network (UTRAN) with a Circuit-Switched Core Network (CS CN) based on the GSM and a Packet-Switched Core Network (PS CN) providing General Packet Radio Service (GPRS). The specification of the UMTS is currently being developed by a standardization group called the Third Generation Partnership Project (3GPP). 
     The IP Multimedia Subsystem (IMS) is an architectural framework for delivering internet protocol (IP) multimedia to mobile users via any IP access network providing Packet-Switched (PS) services. It was originally designed by 3GPP, and it is part of the vision for evolving mobile networks beyond GSM. To ease the integration with the Internet, IMS generally uses IETF (i.e., Internet) protocols such as Session Initiation Protocol (SIP). According to 3GPP, IMS is intended to aid the access of multimedia and voice applications from both wireless and wireline terminals, that is, to aid a form of fixed mobile convergence. This is done by having a horizontal control layer that isolates the access network from the service layer. Services need not have their own control functions, as the control layer is a common horizontal layer. 
     LTE/SAE (Long Term Evolution/System Architecture Evolution) is the name given to a project within 3GPP to improve the UMTS mobile phone standard to cope with future requirements. Goals include improving efficiency, lowering costs, improving services, making use of new spectrum opportunities, and better integration with other open standards. LTE/SAE&#39;s radio access is called Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) and is intended to support only packet-switched services. The LTE/SAE PS CN offers PS services via a variety of access networks, such as GSM&#39;s/UMTS&#39;s own radio network (GERAN, UTRAN, and E-UTRAN), WiFi and even “competing” systems such as CDMA2000 and WiMax. Users of non-UMTS radio networks would be provided with an entry-point into the IP network, with different levels of security depending on the trustworthiness of the network being used to make the connection. Users of GSM/UMTS networks would use an integrated system where all authentication at every level of the system is covered by a single system, while users accessing the UMTS network via WiMAX and other similar technologies would handle the WiMAX connection one way (for example, authenticating themselves via a MAC or ESN address) and the UMTS link-up another way. 
     3GPP TS 23.206 defines a feature called Voice Call Continuity (VCC) that provides the capability to transfer the path of an existing voice call between a 3GPP circuit-switched (CS) system (GSM/UMTS) and IMS via packet-switched (PS) access network, and vice versa, to enable the continuation of a voice call when only one of two types of access networks are available. 
     Transfer of the path from IMS to CS is realized by placing a CS call to IMS that takes over for an existing IMS call. Transfer of the path from CS to IMS is realized by placing an IMS call that takes over for an existing CS call. 
     Current procedures in 3GPP TS 23.206 assume that it is possible for the user equipment (UE) to transmit and receive simultaneously in both IMS via PS access and CS access while transferring the media path to minimize disruption of speech and/or other media. Without simultaneous transmit and receive capability in both IMS and CS, the media disruption may last several seconds and be unacceptably long. 
     Simultaneous transmit and receive capability is available with a single radio for IMS over the UMTS packet-switched (PS) domain and for CS over the UMTS circuit-switched (CS) domain. Simultaneous transmit and receive capability requires two radios for IMS over WiFi and CS over GSM or UMTS. This is an acceptable burden for dual-mode WiFi and GSM/UMTS UEs, since these radios do not share chip sets. 
     Simultaneous transmit and receive capability for IMS over LTE and CS over GSM or UMTS also requires two radios, but this is an unacceptable burden on these devices for reasons of cost, form factor, etc., since these radios will be available in common chip sets. 
     Thus, the issue is how to support voice call continuity between IMS over LTE and CS over GSM/UMTS with an acceptably short media disruption during transfer of the media path using only a single radio in the UE. 
     Section 7.19.1 of 3GPP TS 23.882 includes a discussion of the major options currently available. However, the underlying problem with all solutions proposed so far is that there is likely to be an unacceptable disruption in media during the transfer of the media path. Note that this disclosure only focuses on the transfer of the path of a voice call from IMS over LTE to UMTS CS (GSM/UMTS). Other solutions may apply to the transfer from CS to IMS, but they are outside the scope of this disclosure. 
     The ideal solution requires that sufficient information be exchanged between the UE and the network prior to transfer of the media path so that the radio switch from LTE to GSM/UMTS occurs at the same time that the media path is transferred between the networks. If either occurs significantly before the other, there will be a corresponding disruption in the media. This coordination is very difficult. Arguably, the best proposed solution to date initiates the transfer of the media path just before performing an inter-system handover procedure that results in the change of radio mode. Unfortunately, the inter-system handover procedure may take as much as a few seconds to perform in a typical network, leading to a potentially significant disruption in speech or other media. 
     Thus, the present invention contemplates a new and improved method (and system) that resolves the above-referenced difficulties and others. 
     SUMMARY OF THE INVENTION 
     A method and system for providing circuit-switched to IP Multimedia Subsystem voice call continuity with a single radio are provided. The exemplary embodiment takes advantage of a standard RANAP (radio access network application part, defined in 3GPP TS 25.413) signaling procedure called “directed retry” to perform a much better coordination between the change in radio mode and the transfer of the media path between the networks. With the use of “directed retry,” the transfer of the media path does not begin until the handover procedure is complete, thus significantly reducing the likely amount of media disruption. 
     In accordance with an aspect of the present invention, a method of providing circuit-switched to IP Multimedia Subsystem (IMS) voice call continuity with a single radio (UE) is provided. The method comprises: detecting that domain transfer is required and sending this information to the UE as well as sending a relocation request to an inter-working function (IWF); sending a tunneled circuit origination request to the CS CN; receiving a CS radio access bearer (RAB) assignment request from the CS CN network; the IWF responding to the CS RAB assignment request with “directed retry” to indicate that it will honor the request by initiating a CS RAB handover to a target radio access network (RAN); and completing the CS RAB handover by directing the UE to the target RAN. 
     In accordance with another aspect of the present invention, a method of providing circuit-switched to IP Multimedia Subsystem (IMS) voice call continuity with a single radio is provided. The method comprises: receiving a circuit-switched radio access bearer (CS RAB) assignment request and an inter-working function (IWF) responding to the CS RAB assignment request with “directed retry” to indicate that it will honor the request by initiating a CS RAB handover to a target RAN. 
     In accordance with yet another aspect of the present invention, a system for providing circuit-switched (CS) to IP Multimedia Subsystem (IMS) voice call continuity with a single radio (UE) is provided. The system comprises: tunneling means for tunneling CS call control and mobility management messages via a packet-switched core network (PS CN) to a circuit-switched core network (CS CN); detecting means for detecting that domain transfer is required and sending this information to the UE as well as sending a relocation request to an inter-working function (IWF); sending means for sending a tunneled circuit origination request to the CS CN; receiving means for receiving a CS (radio access bearer) RAB assignment request from the CS CN network; an inter-working function (IWF) for responding to the CS RAB assignment request with “directed retry” to indicate that it will honor the request by initiating a CS RAB handover to a target radio access network (RAN); and directing means for completing the CS RAB handover by directing the UE to the target RAN. 
     Further scope of the applicability of the present invention will become apparent from the detailed description provided below. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The present invention exists in the construction, arrangement, and combination of the various parts of the device, and steps of the method, whereby the objects contemplated are attained as hereinafter more fully set forth, specifically pointed out in the claims, and illustrated in the accompanying drawings in which: 
         FIG. 1  is a block diagram of the VCC reference architecture; 
         FIG. 2  is a block diagram of an exemplary communications system suitable for implementing aspects of the present invention; 
         FIG. 3  is a call flow for an embodiment of a method of providing LTE to CS domain transfer; and 
         FIG. 4  is a call flow for another embodiment of a method of providing LTE to CS domain transfer. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings wherein the showings are for purposes of illustrating the exemplary embodiments only and not for purposes of limiting the claimed subject matter,  FIG. 1  provides a simplified view of the VCC architecture as generally described in the 3GPP specifications and others into which the presently described embodiments may be incorporated.  FIG. 1  only shows network elements and interfaces relevant to the procedures in the disclosure. In the drawings, like reference numerals have been used throughout to designate identical elements. 
     Before describing the basic elements shown in  FIG. 1 , it may be helpful to define the logical structure of the system  2 . That is, the system  2  is logically divided into Core Network (CN) and Access Network (AN) infrastructures, as defined in 3GPP TS 23.101 and 3GPP TS 23.110. The CN is logically divided into a CS domain  10 , a PS domain  12  and an IMS  14 . The AN  16  is called BSS for GSM and RNS for UMTS, as defined in the specifications. 
     As noted, the CN consists of a Circuit Switched (CS) domain  10  and a Packet Switched (PS) domain  12 . These two domains differ by the way they support user traffic, as explained below. These two domains are overlapping, i.e., they contain some common entities. 
     The CS domain  10  refers to the set of all the CN entities offering “CS type of connection” for user traffic as well as all the entities supporting the related signalling on the PLMN/PSTN  18 . A “CS type of connection” is a connection for which dedicated network resources are allocated at the connection establishment and released at the connection release. 
     The PS domain  12  refers to the set of all the CN entities offering “PS type of connection” for user traffic as well as all the entities supporting the related signalling on IP networks  20 . A “PS type of connection” transports the user information using autonomous concatenation of bits called packets: each packet can be routed independently from the previous one. 
     The IMS  14  comprises all CN elements (e.g., VCC AS  22 , CSCF  24  and MGCF/MGW  26 ) for provision of IP multimedia services comprising audio, video, text, chat, and a combination of them delivered over the PS domain. 3GPP TS 23.228 defines the core IMS network elements. The Call State Control Function (CSCF)  24  provides for registration of SIP user equipment (UE) and routing of all SIP requests in IMS  14 . The Media Gateway Control Function (MGCF) and corresponding Media Gateway (MGW)  26  provide interworking between the SIP-based session management procedures used within IMS  14  and the protocols used within PLMN/PSTN networks  18 , such as the Integrated Services Digital Network User Part (ISUP). 3GPP TS 23.206 defines the Voice Call Continuity Application Server (VCC AS)  22 , which manages media sessions with dual-mode UEs to select the proper access domain over which to deliver calls, and to perform SIP Third Party Call Control (3PCC) procedures as needed to switch the media path of an ongoing call between the access domains. The VCC AS performs the 3PCC procedures to realize the domain transfer upon receipt of a call from the UE  28  via the target access domain. 
     The system  2  generally includes dual-mode VCC-capable User Equipment (VCC UE)  28 , which supports voice and possibly other media over both IMS and CS domains. The UE  28  may be located anywhere within the coverage area of the RAN  16 , and it may be stationary or mobile. The UE  28  may also be referred to as a mobile station, a terminal, an access terminal, a subscriber unit, a station, etc. In this regard, the UE  28  may be a cellular phone, a personal digital assistant (PDA), a wireless device, a handheld device, a wireless modem, a laptop computer, etc. The UE  28  may communicate with one or more NodeBs in either RAN  16  on the downlink and/or uplink at any given moment. 
       FIG. 2  shows a block diagram of a communication system  100  suitable for providing circuit-switched to IMS voice call continuity with a single radio. Only key network elements and interfaces are shown. 
     The system  100  generally includes a VCC UE  102  and an E-UTRAN  104 . The system requirements for the E-UTRAN  104  are described, for example, in 3GPP TR 25.913, which is incorporated herein by reference. 
     The LTE/SAE PS CN  106  includes an MME (Mobility Management Entity)  108 , an IWF (Inter-Working Function)  110 , a serving GW (Gateway)  112 , and a PDN GW (Packet Data Network Gateway)  114 . 
     The MME  108  manages and stores UE context. It generates temporary identities and allocates them to UEs and checks the authorization as to whether the UE may camp on the LTE radio or on the PLMN. It also authenticates the user. 
     The serving GW  112  routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies. It manages and stores UE contexts, for example, parameters of the IP bearer service and network internal routing information. 
     The PDN GW  114  provides PS connectivity between the UE  102  and external packet data networks by being the point of exit and entry of traffic for the UE  102 . The UE  102  may have simultaneous connectivity with more than one PDN GW  114  for accessing multiple PDNs. Another key role of the PDN GW  114  is to act as the anchor for mobility between 3GPP and non-3GPP technologies such as WiMAX and 3GPP2 (CDMA 1X and EvDO). 
     The IWF  110  is a new network element that helps to enable the invention; it is described later. 
     The UMTS PS CN  116  includes an SGSN (Serving GPRS Support Node)  118  and other entities such as the GGSN (Gateway GPRS Support Node), not shown. The SGSN  118  in the UMTS PS CN  116  is analogous to the Serving GW  112  in the LTE/SAE PS CN  106 . The SGSN  118  routes and forwards user data packets, while acting as the mobility anchor for mobility between RNCs  130 . 
     The UMTS CS CN  120  includes an MSC (Mobile Switching Center)  122  and an MGW  124 . The MSC  122  constitutes the interface between the radio system and the fixed networks. The MSC  122  performs all necessary functions in order to handle the circuit switched services to and from the mobile stations. The MGW  124  adapts between CS radio access bearers (RAB) in the UTRAN/GERAN  126  and bearer channels in the PLMN/PSTN  18 . 
     The UTRAN  126  includes a number of NodeBs  128 , each serving the communication needs of the UE  102  in a respective coverage area. The NodeBs  128  are connected to a radio network controller (RNC)  130 . A NodeB is generally a fixed station that communicates with the UE  102  and may also be referred to as an enhanced NodeB, a base station, an access point, etc. 
     The RNC  130  is connected to an SGSN  118  in the UMTS PS CN  116  and to an MSC  122  in the UMTS CS CN  120 , and provides coordination and control for the NodeBs  128  along with management of the PS RABs and CS RABs allocated for each UE  102 . 
     The main functions of several known reference points (i.e., a conceptual point at the conjunction of two non overlapping functional entities or groups) as shown in  FIG. 2  are set forth below: 
     S 1 : It provides access to E-UTRAN resources for the transport of user plane and control plane traffic. 
     S 4 : It provides the user plane with related control and mobility support between the UMTS PS CN  116  and the Serving GW  112  and is based on Gn reference point as defined between SGSN and GGSN. 
     S 5 : It provides the user plane with related control and mobility support between the PDN GW  114  and the Serving GW  112 . 
     S 10 : It provides the user plane with related control and mobility support between the MMEs  108 . Note that the IWF  110  interacts with MME  108  by emulating another MME  108   
     S 11 : It provides the user plane with related control and mobility support between the MME  108  and the Serving GW  112 . 
     Iu-ps: It provides the user plane with related control and mobility support between the RNC  130  and the SGSN  118  for management of PS RABs. 
     Iu-cs: It provides the user plane with related control and mobility support between the RNC  130  and the MSC  122  for tunneling of CS call control messages and management of CS RABs. 
       FIG. 2  also shows several known interfaces (i.e., a set of technical characteristics describing the point of connection between two telecommunication entities), including Iu-b Interface, Uu Interface, and LTE-Uu Interface. The darker lines represent the user plane (e.g., voice, packet), while the thinner lines represent the control plane (e.g., SIP, RANAP). 
     In addition to the standard elements already described in  FIG. 2 , this invention introduces a new element IWF  110  with unique functions needed to coordinate the domain transfer signaling between the LTE/SAE PS CN  106  and the UMTS CS CN  120 . On the side toward the MME  108 , the IWF  110  emulates a peer MME entity in the LTE/SAE network via the standard S 10  interface. On the side toward the UMTS network, the IWF  110  emulates a UMTS RAN  130  via the Iu-cs and Iu-ps interfaces to the CS CN  120  and PS CN  116 , respectively, providing the functions associated with RANAP (Radio Access Network Application Part). An IWF  110  may also facilitate interworking between LTE/SAE and GSM networks via the A and Gb interfaces using BSSMAP to the GSM CS CN and GSM PS CN, respectively, (not shown), between any PS CN and any CS CN, or between any PS CN and any combination CS CN and PS CN. The IWF  110  may also be collocated with the MME  108  or other network entities while providing the same functions in the architecture. 
     The key RANAP functions provided by the IWF  110  are: tunneling of CS call control messages between the UE  102  and the CS CN  120 ; radio access bearer (RAB) assignment; and RAB relocation. In addition, the IWF  110  coordinates these procedures to realize the invention, as described in the procedures below. In support of the tunneling and relocation functions in the IWF  110 , the UE  102 , E-UTRAN  104  and MME  108  also provide supporting functions as described in the procedures below. In particular, E-UTRAN  104  can identify the need for a domain transfer procedure due to radio conditions and signal this towards both the UE  102  and IWF  110  with new messaging. In addition, the UE  102  has the capability to exchange CS call control messages (for example, as described in 3GPP TS 24.008) with the CS CN  120  via the CS call control tunneling capabilities of the E-UTRAN  104 , MME  108  and IWF  110 . Finally, the IWF  110 , MME  108  and E-UTRAN  104  can forward a relocation/handover command directive to the UE  102  from the UMTS CS CN  120  and optionally from the PS CN  116 , to direct the UE  102  to retune to the UMTS radio as needed to complete relocation/handover procedures. 
     We turn now to  FIGS. 3 and 4 , which show call flows for alternative embodiments of a method of providing CS-to-IMS voice call continuity with a single radio. 
     Step  1 : Prior to the need for “domain transfer” from IMS over LTE to CS over GSM/UMTS, the LTE/SAE network  106  tunnels CS registration messages or location area update messages as needed between the UE  102  and the CS CN  120  so that the UE  102  is prepared to tunnel additional signaling when the time comes to perform the domain transfer. 
     Step  2 : Meanwhile, the IMS session is in progress via the E-UTRAN  104  and LTE/SAE network  106 . 
     Step  3 : When the LTE RAN (E-UTRAN)  104  determines that domain transfer is needed (partly based on radio measurement reports from the UE  102 ), the E-UTRAN  104  signals to the UE  102  that it should initiate the domain transfer. The E-UTRAN  104  also signals to the MME  108  and IWF  110  that relocation is required in support of the domain transfer. The IWF  110  queues the relocation request until it receives a RAB assignment request from the CS CN to synchronize the CS and PS RAB relocation procedures. 
     Step  4 : The UE  102  sends a tunneled CS origination request to the CS CN  10 / 120  via the LTE/SAE network  106 . The purpose of the origination request is to establish a bearer path from the UE  102  to the IMS  14  via the CS RAN  126  and CS CN  10 / 120  to replace the still active bearer path from the UE  102  to the IMS  14  via the LTE/SAE network  12 / 106 . 
     Step  5 : Following the standard procedure for CS origination, the MSC  122  (in the CS CN  10 / 120 ) sends a “CS RAB assignment request” message to the IWF  110  via RANAP over Iu-CS, treating the IWF  110  like an RNC in a CS RAN. The CS RAB, when finally realized in step  7   a  or  7   b  below, will provide the portion of the media path for the call between the UE  102  and the CS CN  10 / 120  via the CS RAN  16 / 126 . 
     Step  6   a : While it is possible for the LTE/SAE network  106  to emulate a CS RAB (possibly even temporarily) before transferring it to the CS RAN, this would force at least two disruptions in the media path during the transition, which is undesirable. As shown in  FIG. 3 , one alternative here (i.e., Step  6   a ) is for the IWF  110  to signal successful completion of the CS RAB assignment without actually doing so (i.e., it lies). The CS CN  10 / 120  (MSC  122 ), having been told that the CS RAB assignment is complete, propagates the CS origination into the network, initiating the transfer of the media path from IMS over LTE to CS over GSM/UMTS. But since the real CS RAB is not yet in place, the media path is blocked. 
     Step  7   a : The procedure continues in  FIG. 3 . In parallel with Step  6   a , the IWF  110  initiates a handover of the (phantom) CS RAB from the E-UTRAN  104  to the CS RAN  126  via existing procedures. When the handover is complete, the UE  102  and the network re-establish the media path via the CS RAN  126  and CS CN  10 / 120 . Unfortunately, there is a disruption of the media path that lasts approximately as long as the handover procedure, which may take an unacceptably long time. 
     Step  6   b : As shown in  FIG. 4 , a better alternative to Step  6   a  is for the IWF  110  to signal “directed retry” in response to the CS RAB assignment request in Step  5 . The “directed retry” indication informs the CS CN  10 / 120  that the IWF  110  (emulating a CS RAN) is incapable of assigning a CS RAB at this time (usually due to resource constraints in a CS RAN) but there is a CS RAN candidate (e.g., UTRAN  126 ) to fulfill the CS RAB assignment request and the IWF  110  will initiate a CS RAB handover procedure to assign this CS RAB in the CS RAN  126 . The “directed retry” procedure was initially created to support “hand-down” from UMTS/UTRAN to GSM/GERAN when a UE  102  originates a call on UMTS but there are insufficient resources in UMTS/UTRAN to support the call. The CS origination is blocked until the MSC  122  receives an indication of successful CS RAB assignment, so the media path is not yet transferred but continues via E-UTRAN  104  and LTE/SAE network  106   
     Step  7   b : The procedure continues in  FIG. 4 . Immediately after the IWF  110  indicates “directed retry” to the CS CN  10 / 120 , it initiates a CS RAB handover to the CS RAN  126  via the CS CN  10 / 120 . To complete the handover signaling, the CS CN  10 / 120  (MSC  122 ) forwards the handover request to the CS RAN  126 , the CS RAN  126  assigns resources for the CS RAB and acknowledges the assignment to the CS CN  10 / 120 , the CS CN  10 / 120  informs the IWF  110  of CS RAB resource assignment in the CS RAN  126 , the LTE/SAE network  106  directs the UE  102  to re-tune its radio to the CS RAN  126 , the CS RAN  126  detects connectivity to the UE  102 , and the CS CN  10 / 120  propagates the CS origination into the network. The signaling of the CS origination into the network initiates the transfer of the media path from IMS over LTE to CS over GSM/UMTS and the transfer of the media path is complete. The disruption in the media path begins when the radio retunes and ends when the transfer of the media path is complete, but these steps are expected to usually take less time than the entire handover procedure. 
     Step  8 : Note that, in parallel with Steps  6  and  7  in  FIG. 3  and  FIG. 4 , the LTE/SAE network  106  and the IWF  110  may also relocate/handover any other PS RABs from E-UTRAN  104  to UTRAN/GERAN  126  to the extent that the target network supports these PS RABs. The PS RAB relocation/handover would occur in parallel with and synchronous with the CS RAB relocation/handover according to existing procedures. 
     Some portions of the above description were presented in terms of algorithms and symbolic representations of operations on data bits performed by conventional computer components, including a central processing unit (CPU), memory storage devices for the CPU, and connected display devices. These algorithmic descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. An algorithm is generally perceived as a self-consistent sequence of steps leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like. 
     It should be understood, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the preceding discussion, it is appreciated that throughout the description, discussions utilizing terms such as “processing” or “computing” or “calculating” or “determining” or “displaying” or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system&#39;s registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices. 
     The exemplary embodiment also relates to systems for performing the operations herein. These systems may be specially constructed for the required purposes, or they may comprise one or more general-purpose computers selectively activated or reconfigured by one or more computer programs stored in the computer(s). Such computer program(s) may be stored in a computer readable storage medium, such as, but is not limited to, any type of disk including floppy disks, optical disks, CD-ROMs, and magnetic-optical disks, read-only memories (ROMs), random access memories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, or any type of media suitable for storing electronic instructions, and each coupled to a computer system bus. 
     The algorithms and displays presented herein are not inherently related to any particular computer or other apparatus. Various general-purpose systems may be used with programs in accordance with the teachings herein, or it may prove convenient to construct more specialized apparatus to perform the methods described herein. The structure for a variety of these systems will be apparent from the description. In addition, the present exemplary embodiment is not described with reference to any particular programming language. It will be appreciated that a variety of programming languages may be used to implement the teachings of the exemplary embodiment as described herein. 
     A machine-readable medium includes any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer). For instance, a machine-readable medium includes read only memory (“ROM”); random access memory (“RAM”); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.); etc. 
     It will be appreciated that variants of the above disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims. 
     The above description merely provides a disclosure of particular embodiments of the invention and is not intended for the purposes of limiting the same thereto. As such, the invention is not limited to only the above-described embodiments. Rather, it is recognized that one skilled in the art could conceive alternative embodiments that fall within the scope of the invention.