Abstract:
A system and method for managing resources in a distributed Voice-over-IP (VoIP) speech coder pool arrangement including multiple speech coders each comprised of a first number of front-end modules and a second number of back-end modules. Each call session has assigned thereto a front-end module and a back-end module cooperatively functioning as a speech coder. In a preferred embodiment, voice samples are passed through the assigned front-end module to the back-end module where they are encoded and placed as speech frames on the IP network. As soon as absence of speech is detected by a Voice Activity Detector (which may or may not be shared by multiple front-end modules), processing is handed over to the front-end module freeing up the back-end module to sit idle resulting in reduced power consumption. In alternate embodiment, the freed-up back-end module is reassigned to a new VoIP call session for maximum resource utilization.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. patent application Ser. No. 09/164,879 filed Oct. 1, 1998 (Sullivan, et al.) entitled “System and Method for Providing High-Speed Data Access Using a Distributed Modem”; co-pending U.S. Patent Application entitled “Distributed XDSL Modem Pool with Front-End Idle Mode Processing”, filed contemporaneously herewith; and co-pending U.S. Patent Application entitled “Distributed Voiceband Modem Pool with Front-End Idle Mode Processing”, also filed contemporaneously herewith. 
    
    
     TECHNICAL FIELD 
     This invention relates to distributed modem pooling techniques for maximizing resource utilization in a packet-switched communication network with integrated telephony services, and more particularly, for maximizing resource utilization of the network during speech/voice processing. 
     BACKGROUND 
     Voice-over-IP (VoIP) is an emerging technology where packet-switched networks employ the Internet Protocol (IP) to offer telephony services. Examples of such networks are the Internet and private IP-based corporate local area and wide area networks (LANs and WANs). The main advantage of using this technology over Plain Old Telephone Service (POTS) is the cost savings obtained from its reduced bandwidth requirements. Unlike POTS/PSTN where a continuously available circuit-switched DSO (64 kbps) connection is dedicated to the call for its entire life, VoIP shares the network bandwidth with other information types like data and video. This is possible because speech in a VoIP network typically travels (is processed) at the low rate of either 6.3 kbps or 5.3 kbps. Therefore, for example, a corporation can use a portion of the bandwidth of its existing IP network to offer VoIP to its employees and, consequently, do away with their traditional phone services and their associated costs. 
     VoIP achieves its bandwidth efficiency through the use of two techniques: Speech Compression and Discontinuous Transmission (DTX). The former employs source encoding methods to represent the sampled voice signal in compressed form that can be decoded and decompressed later at the receiving end. The compression ratio achieved by this technique can be as high as 24:1, resulting in tremendous bandwidth reduction. In Discontinuous Transmission, bandwidth reductions are achieved by detecting silence in the phone conversation and, in response, either shutting down the transmitter on the non-speaking end or sending smaller frames than the regular speech ones. With DTX, the bandwidth can theoretically be halved since, ideally, in conversations one person is talking and the other is listening. 
     FIG. 1 conceptually illustrates the basic architecture of a typical VoIP WAN network  10  like the internet. An IP network  15  forms the backbone of the WAN network. Telephone internet gateways  20 ,  25 ,  30  connected to the IP network provide the telecommunications interface infrastructure which allows the exchange of information—such as video, text, and/or speech—in the form of one or more recognized protocols between telephony-capable peripheral devices attached thereto. For example, one more personal computers  35  connected to an associated telephone gateway  20 —using a modem  36  or the like equivalent device—are coupled to exchange information (including speech/voice information) therebetween or with other telephony devices ( 41 - 44 ) connected to the network  10 —over for example the Public Switched Telephone Network (PSTN)  40 —via their associated telephone gateways  25 ,  30 . (Telephony devices  41 - 44  can include both wireless as well as wireline devices interfaced thereto in a conventional manner over the PSTN or a dedicated telephony-based network.) The PSTN or the like interface network receives and converts speech data into digital samples which are then communicated to the associated internet telephony gateway. 
     All digital processing of speech samples and associated control tasks are done by special hardware in each telephone gateway in response to appropriate analog (e.g., from modem  36 ) or digital (e.g., DSO signals from the PSTN) speech samples originating in the form of spoken speech from such devices as PC  35  and telephony devices  41 - 44 . The spoken speech samples are received via actual physical layer links  37  connecting the outside world to the telephone gateways internal speech processing hardware. A diagrammatic depiction of such hardware is shown in FIG.  2 . The hardware includes a pulse code modulated (PCM) sample IO handler  50 , an auto gain control (AGC) and echo cancellation devices  55 , a voice coder/decoder (CODEC)  60 , a line coder  65  and an IP network interface device  70  configured to operate in a known fashion. A typical input to voice CODEC  60 , for example, might be an 8 kHz 16-bit linear PCM signal which corresponds to a data rate of 128 kbps (8000×16). In one implementation, CODEC  60  operates on blocks of 240 input samples  56  called frames, each frame having a duration of 30 msec (240/8000). The output of CODEC  60  is at one of two bit-rates: 6.3 kbps or 5.3 kbps. The higher rate has greater quality and the lower rate is, obviously, more bandwidth efficient. At 6.3 kbps, the length of the output codeword is 189 bits (0.03 sec×6300 bps), and at 5.3 kbps its 159 bits. The highest compression ratio is, therefore, achieved by the 5.3 kbps Codec where the input 128 kbps PCM signal is compressed  24  fold. 
     A block diagram of the operational processing logic of CODEC  60  is shown in FIG.  3 . Discontinuous transmission (DTX) and silence compression of input samples  56  are handled by a Voice Activity Detector (VAD)  61  and by Comfort Noise Generator (CNG)  62 . The VAD  61  reliably detects the presence or absence of speech and conveys that information to the CNG  62 . Although this information is passed on a frame by frame basis, the determination of the presence or absence of speech is made over multiple successive frames. 
     The CNG  62  creates a noise signal that matches the actual background noise. It essentially computes and encodes parameters that can be used at the receiving end to synthesize this artificial noise. These parameters constitute the Silence Descriptor (SID) frames  63  which use less bits ( 40 ) than the normal speech ones and are transmitted during inactive periods. This transmission, however, is not periodic. That is, for each inactive (non-speech) frame the CNG  62  makes a decision of sending a SID frame  63  or not based on variations of the power spectrum of the background noise. As long as this spectrum remains relatively unchanged, SID frames  63  stop getting sent and the system&#39;s transmitting modules remain idle. At the receiving end, on the other hand, the decoder always uses the last SID frame  63  received to generate the silence comfort noise. 
     In a typical CODEC, a speech coder  64  processes the speech portion of the PCM samples output from VAD  61  to generate—using appropriate coding algorithms—encoded speech frames  63 ′. This processing dominates the horsepower requirements of a CODEC. Typical power consumption by coding algorithms might be 20 million instructions per second (MIPS). Comparatively, the requirements of the comfort noise generator  62  are negligible and do not exceed 1 MIPS. (The VAD  61  is immaterial to this discussion since it is common to the generation of both SID frames  63  and encoded speech frames  63 ′.) 
     A CNG 1 MIPS estimation represents normal operations where SID frames  63  are not being continuously built and sent. For peak processing estimations, however, one might assume that the background noise&#39;s power spectrum is constantly changing and new SID frames must be computed and sent continuously. Exact numbers for this peak condition are not available but, nonetheless, one can safely estimate that they do not exceed a very generous 5 MIPS. To verify this, one can use the ratio of the number of bits in a SID frame  63  to the number of bits in a regular speech frame as a rough indicator. The minimum such ratio is for a 5.3 kbps coder which is 40/159=0.251. Multiplying this number with 20 MIPS gives us 5.03 MIPS as the horsepower needed to compute a SID frame  63 . The 20 and 5 MIPS estimates produce a 4:1 ratio between full-time and idle-time frame generation processing. (This very rough estimation assumes that SID frame generation is as complex as that of encoded speech frame generation and that the relationship between number of bits and MIPS is linear, which is not remotely the case. Nonetheless, given that the generation of comfort noise parameters is far less complex than coding, 5 MIPS can be taken as an adequate estimate to show the difference in bandwidth resource utilization between full-time speech processing (when both encoded speech frames and SID frames are being generated) and idle-time processing (when only SID frames are being generated.) 
     The inventors have determined that what is needed are ways to better manage processing resources during multiple VoIP call sessions by freeing up (or reassigning) resources during idle-time processing. Of course, paramount to any efficient alternative solution is that the VoIP call session&#39;s physical link remain substantially uninterrupted and that no significant impairment of the call is noticeable by the participants. 
     SUMMARY 
     The invention provides a system and method for managing resources in a distributed Voice-over-IP (VoIP) speech coder pool system arrangement including multiple speech coders each comprised of a first number of front-end modules and a second number of back-end modules. Each call session has assigned thereto a front-end module and a back-end module cooperatively functioning as a speech coder. In the preferred embodiment, voice samples are passed through the assigned front-end module to the back-end module where they are encoded and placed as speech frames on the IP network. As soon as absence of speech is detected by a Voice Activity Detector (which may or may not be shared by multiple front-end modules), processing is handed over to the front-end module freeing up the back-end module to sit idle resulting in reduced power consumption. In alternate embodiment, the freed-up back-end module is reassigned to a new VoIP call session for maximum resource utilization. 
     Each front-end module is configured for keeping alive the physical layer link between the telephony user and the telephony gateway during non-speech portions of the call, while at the same time allowing speech processing hardware strategically located in the back-end modules. In this manner, the call session is allowed to be kept active in a reduced rate mode throughout the duration of the idle call session. The reduced rate idle-mode requires less processing capability than the high rate data-mode and can be maintained by front-end modules of lower capability and lower cost. Because the processing of the idle data are now handled by the front-end module, the back-end module may be reassigned to another call session at that point; or to the extent the front-end to back-end link is maintained, the back-end processing demands are reduced which may allow the back-end module to provide service to another call session. When speech data appear on the subscriber link, a negotiation may be performed to bring any of the available high-capability back-end modules back on line. 
     In another preferred embodiment, the front-end modules are provided with sufficient processing power, memory and programmed functionality to maintain more than one subscriber link active, at least during idle processing of such links. 
     In yet another embodiment, the front-end modules and back-end modules are each fully functional encoder devices capable of operating as either front-end devices or back-end devices, on an as needed basis. 
     In another preferred embodiment, the front-end modules are connected to the public switched telephone system via DSO circuits, which may be delivered physically as DS 1 , DS 3 , or any other commonly provisioned organization of circuits. 
     In another preferred embodiment, the front-end mode are connected via an appropriate voice-band or broadband type modem link to the call participant. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the basic architecture of a typical VoIP WAN network. 
     FIG. 2 is a block diagram of the speech processing hardware in a conventional telephone gateway. 
     FIG. 3 is a very high level block diagram of the voice CODEC of FIG. 2 used for processing digital samples representative of voice and non-voice speech during a VoIP call session. 
     FIG. 4 is a block diagram generally showing a distributed speech coder having a front-end module and a back-end module in accordance with the present invention. 
     FIG. 5 is a block diagram of a distributed speech coder pool system as might be employed in a telephone gateway including a plurality of distributed. speech coders communicably interconnected in accordance with a preferred embodiment of the present invention. 
     FIG. 6 shows the operational flow for performing idle-mode processing in response to a detection of non-speech data in accordance with the preferred embodiment. 
    
    
     DETAILED DESCRIPTION 
     The following description is made in the context of using the invention in a telephone gateway as might be employed in a VoIP network as shown in FIG.  1 . Related aspects of the invention is a voice-band solution described in the co-pending US application, titled “Distributed Voice-Band Modem Pool with Front-End Idle Mode Processing”, and in the xDSL solution described in the co-pending US application, titled “Distributed xDSL Modem Pool with Front-End Idle Mode Processing”, both of which were filed contemporaneously with the present application. 
     The invention provides a system and method for freeing up voice coder resources, preferably located at a central location, to make resources available for new telephony call sessions. This generally involves reassigning certain resources set up to process a call session upon detection of a transition of the call session from active data, when user speech data are being sent, to idle-mode, when no speech data are being sent. 
     The invention differs from prior art resource management approaches in that the physical layer link is maintained throughout the duration of the telephony call session. While in accordance with the preferred embodiments some of the resources are dynamically reassigned, sufficient resources are reserved to the call session to keep the link alive. Accordingly, as active speech data appear on the link indicative of a transition back to active-mode, the limited reserved resources request at that time that additional resources be provided to handle the processing of the incoming speech samples. 
     A key aspect of the invention is that because the physical layer link to the user terminal device is maintained, the user at that device who was previously sitting idle experiences little to no delay in the transition back to active-mode, and at no time has the call session been prematurely interrupted for lack of voice activity. This is generally achieved by having the Voice Activity Detector (VAD) identify a break in speech from the incoming voice samples, which in turn allows the front-end module to coordinate the back-end module relinquishing control of speech processing functions until speech type data are re-detected by the VAD. 
     Given a limited number of speech coders available to handle incoming calls and an equal number of active call sessions, a distributed speech coder pool solution as, will be described herein below, allows new telephony call sessions to be initiated without in any way detrimentally affecting or tearing down active call sessions which may or may not be presently idle. 
     The invention aims to take advantage of the benefits of a so-called distributed speech coder (CODEC) pool system architecture. A distributed speech coder pool generally encompasses the notion of distributed voice CODECs extended to operate in banks or pools; or other equipment that performs the functions of CODECs for a plurality of subscriber lines. Distributed CODEC processing as shall be described herein below is analogous to the concept of distributed modem pools, including voice-band and broadband type modems pool environments as has been described in greater detail in related co-pending U.S. patent application Ser. No. 09/164,879 filed Oct. 1, 1998 titled “System and Method for Providing High-Speed Data Access Using a Distributed Modem”, which application is incorporated herein by reference. 
     The basic operation of a distributed CODEC  100  generally configured in accordance with the invention is shown in block diagrammatic form in FIG.  4 . 
     Distributed CODEC  100  includes a front-end module  120  and a back-end module  140  having an interconnection path/bus  144 . The distributed CODEC  100  is “distributed” because the first-end module  120  is physically separate but electrically coupled to the back-end module  140 . For example, the front-end module  120  and the back-end module  140  may be on separate circuit boards, or on entirely separate chassis. The front-end module  120  may be separated from the back-end module  140  by distances that may be measured in fractions of an inch, or in miles. 
     Referring to the VoIP network configuration shown in FIG. 1, distributed CODEC  100  may reside as part of the CODEC hardware co-located in any one of telephone gateways  20 ,  25 ,  30  and coupled to receive voice/speech samples from, for example, either a PC  35  user or a telephony device  41 - 44  user over an appropriate subscriber link coupled to the voice CODEC as shall be described in greater detail below. 
     FIG. 5 is a block diagram of a distributed speech coder pool system as might be employed in a telephone gateway including a plurality of distributed speech coders communicably interconnected in accordance with a preferred embodiment of the present invention. 
     In accordance with a first preferred embodiment, the front-end module  120  would be coupled to receive speech data samples over a voice-band, broadband or the like subscriber line  12  over which a subscriber link (including a physical layer link) between an appropriate modem-equipped user terminal device ( 36 ) is established during a given call session. (As is well known, modem-equipped user terminal devices essentially encode sourced speech as analog signals at very high data rates.) Once received at the telephone gateway ( 20 ), the analog modem representation of speech signals are converted by appropriate circuitry and associated coding algorithms, into, for example, a 16-bit linear PCM sample stream for processing by voice processing hardware substantially as explained above in connection with the description of FIG.  2 . 
     In an alternate embodiment, the sourced speech originates from a telephony device such as telephony devices  41 - 44  in FIG.  1  and transmitted to the telephony gateway in any one of various known ways, including as a digital voice stream over a DSO link from the PSTN  40  to the telephony device. In the case of a cellular or two-way radio implementation, the sourced speech may arrive at the appropriate telephony gateway  25 ,  30  over the PSTN  40  as shown in FIG. 1, or by way of a direct link (not shown) between the cellular or two-way infrastructure to the appropriate telephony gateway. The sourced speech is received by telephony gateways  25 ,  30  as a digitally represented packet frame and may be converted as necessary into, for example, a 16-bit linear PCM sample stream for processing in the same way as might be processed the modem received signals in the preceding embodiment. The present invention aims to utilize a distributed CODEC architecture within a telephony gateway (versus the nondistributed architecture of FIG. 3) to process the incoming PCM sample stream in a manner which maximizes use of CODEC resources during idle (non-speech) portions of an ongoing (uninterrupted) VoIP call session. 
     In a distributed CODEC  100  as contemplated herein in accordance with a preferred embodiment shown in FIG. 4, the front-end module  120  includes front-end module controller  125  which may comprise Voice Activity Detector (VAD)  126 , idle processing logic  127  and memory  128  for analyzing the incoming PCM samples to determine if there is a break in speech of sufficient length (more than a few seconds) and take appropriate action. The back-end module  140  is connected to the front-end module  120  and receives the PCM samples by way of interconnection path/bus  144  therefrom when speech is detected and receives no samples otherwise. 
     The back-end module  140  comprises a back-end module controller  143  including at least a conventional speech coder  145  and Comfort Noise Generator  147  for isolating speech patterns from the PCM samples and generating therefrom appropriate encoded speech frames  148  and SID frames  149 , respectively. Frames  148  and  149  are then communicated to, for example, a line coder  65  and IP network interface device  70  for transmission as packetized IP protocol-compatible data over an IP network, such as the internet, in a conventional manner. 
     In the event of a break in speech (i.e, no encoded speech frames to be generated by the speech coder  145 ), rather than tearing down the call, the front-end module  120  will instead take action to cause the back-end module controller  143  to relinquish control of the resources assign to the VoIP session. Once control is relinquished, the back-end module  140  may be dynamically assigned to another call or put on stand-by (requiring little or no MIPS resources) until the break in speech is ended and voice is again detected by the VAD  126  in front-end module  120 . 
     In an alternate implementation shown in FIG. 5, the VAD  126  is external to the front-end module and shared by each of plural parallel arranged front-end modules  120 ( 1 )- 120 (m) cooperatively defining a pool  120 ′ of front-end modules. The pool  120 ′ of front end modules  120 ( 1 )- 120 (m) in turn is connected to a pool  140 ′ of back-end modules  140   a - 140   n.  In this regard, it should be appreciated that since the back-end modules may be dynamically assigned on an as needed basis, a one-to-one correspondence with front-end modules  120  is not needed (i.e., m not equal to n). 
     By allowing the VAD  126  functionality to be shared among plural front-end modules  120 , the latter can be provided at very low cost as such would require only reduced processing capability limited to transitioning resources—including back-end resources—to and from idle-mode. 
     Also, while in the illustrative embodiment certain signal processing functions such as auto gain control and echo cancellation are not included as part of the distributed CODEC functionality, such processing and other similar speech processing functions may be optionally included therein and performed preferably by, for example, by appropriate circuitry disposed within the front-end modules  120 ( 1 )- 120 (m). 
     The purpose of the idle processing logic  127  is to maintain the corresponding call session physical layer link alive as the call session transitions into idle (non-speech) mode, and operates to reestablish control of the same or different back-end module  140  as the call session transitions back to active (speech) mode. 
     In accordance with a preferred embodiment, it is envisioned that the front-end modules are provided with sufficient processing power, memory and programmed functionality to maintain more than one subscriber link active, at least during idle processing of such links. 
     In yet another embodiment, the front-end modules and back-end modules are each fully functional encoder devices capable of operating as either front-end devices or back-end devices, on an as needed basis. 
     The speech coder  145  may include a processing element to process the data from the front-end module  120  and to perform high level functions such as, for example, data compression and error detection and correction. The speech coder  145  and CNG  147  may be constituted by a single processing element capable of performing all necessary high level functions. The single processing element may include a digital signal processor (DSP) or a general purpose processor. Alternatively, more than one processor may be used. 
     FIG. 6 shows an operational flow routine  200  for performing idle-mode processing in response to a detection of non-speech data in accordance with the preferred embodiment. 
     At step  220 , a call session is initiated over a subscriber link for establishing a VoIP call session between a telephony device (or PC user) with an associated telephony gateway, as generally described above. The telephony gateway includes a plurality of modules including front-end modules  120 ( 1 )- 120 (m) and back-end modules  140 ( 1 )-(n). Call initiation involves assigning a front-end module  120  and a back-end module  140  to establish the current voice call session. 
     It is contemplated that such resource management may be performed dynamically using predefined statistical multiplexing techniques. A dedicated central processing unit external to the modules may be employed to assign and deassign resources. In an alternate solution, resource management is performed by the modules themselves and preferably by the communicably coupled DSPs in the respective modules. 
     The front-end module  120  assigned to the current call session analyzes the incoming speech PCM data sample stream and then sends them over link  144  to the assigned back-end module  140  (step  230 ). The back-end module  140 , in turn, formats and converts the digital samples into intelligible encoded speech frames packets and simultaneously also generates SID frames based on noise level patterns identified from the digital samples (step  240 ). 
     Before (or as part of) the CODEC module setup, session characteristics associated with the call and the corresponding subscriber link are first saved (step  280 ) to allow for rapid transition from no back-end module functionality to almost instantaneous assignment of thereof. 
     In accordance with a preferred embodiment, the back-end module  140  may them be put back in the “available” list of modems in the modem pools, and thus is de-assigned from the call session (steps  290 ,  300 ). Such deassignment may be on a temporary or permanent basis, based on system designer options. Of course, it is also possible to maintain the assignment of the back-end module  140  to the current call session but configured to also simultaneously encode voice samples from other incoming calls on an as needed basis. 
     Upon relinquishing of non-speech sample processing by the back-end module  140 , control of non-speech processing is exclusively turned over to the front-end module  120 , and more specifically, to the associated idle-processing logic  127 . The VAD  126  monitors the incoming sample stream for speech data all the while keeping the physical layer link alive with the call participant (step  310 ). 
     Upon detection of speech type samples (i.e., switch to active-mode), action is taken by the front-end module  120  to have additional resources provided to handle the full rate of incoming active data (step  330 ). Such action may involve (or be followed by) the assignment of a back-end module  140  (step  340 ), retrieval of saved session characteristics (step  350 ), and establishment of communications between the front-end module  120  and newly assigned back-end module  140  (step  360 ) in a seamlessly quick manner using the saved session characteristics. 
     Once back in active-mode processing, each digitized sample is passed from the front-end module  120  to the new back-end module  140  in the same manner as prior to the idle-mode condition. It should be appreciated that resources may be assigned and de-assigned indefinitely while the link is kept alive at the user terminal device or until such time as a timeout condition is reached. When the call session is terminated for whatever reason, all resources (modules  120 ,  140 ) are de-assigned and placed back in the pool of available modems (step  380 ,  390 ). 
     Thus, in the preferred embodiment just described, when a front-end module identifies that its associated call session has entered idle-mode, the associated back-end module relinquishes control throughout the duration of the call session. Because the processing of the idle data are now handled by the front-end module, the back-end module may be reassigned to another call session at that point; or to the extent the front-end to back-end link is maintained, the back-end processing demands are reduced which may allow the back-end module to provide service to another call session. 
     It should be appreciated that the relinquishing of control by an assigned back-end module in response to a detection of non-speech data amounts to a hand-over between front-end and back-end. Such hand-over should realistically occur within a 30 msec window. As discussed above, a CODEC typically receives frames of voice samples every 30 msec. Accordingly, all compression operations must complete in a 30 msec time period before the next frame arrives. For purposes of the present invention, a limitation for successful implementation demands that the sum of the time it takes to perform a hand-over and the time needed to produce a compressed speech frame must not exceed 30 msec. 
     Implementation 
     The invention may be implemented in hardware or software, or a combination of both (e.g., programmable logic arrays). Unless otherwise specified, the algorithms included as part of the invention are not inherently related to any particular computer or other apparatus. In particular, various general purpose machines may be used with programs written in accordance with the teachings herein, or it may be more convenient to construct more specialized apparatus to perform the required method steps. However, preferably, the invention is implemented in one or more computer programs executing on programmable systems each comprising at least one processor, at least one data storage system (including volatile and non-volatile memory and/or storage elements), at least one input device, and at least one output device. The program code is executed on the processors to perform the functions described herein. 
     Each such program may be implemented in any desired computer language (including machine, assembly, or high level procedural, logical, or object oriented programming languages) to communicate with a computer system. In any case, the language may be a compiled or interpreted language. 
     Each such computer program is preferably stored on a storage media or device (e.g., ROM, CDROM, or magnetic or optical media) readable by a general or special purpose programmable computer, for configuring and operating the computer when the storage media or device is read by the computer to perform the procedures described herein. The inventive system may also be considered to be implemented as a computer-readable storage medium, configured with a computer program, where the storage medium so configured causes a computer to operate in a specific and predefined manner to perform the functions described herein. 
     A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.