Patent Publication Number: US-7586868-B2

Title: Method and apparatus for controlling distributed transcoders

Description:
REFERENCE(S) TO RELATED APPLICATION(S) 
     The present application claims priority from provisional application Ser. No. 60/487,182, entitled “METHOD AND APPARATUS FOR CONTROLLING DISTRIBUTED TRANSCODERS,” filed Jul. 14, 2003, which is commonly owned and incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to cellular communication systems, and, in particular, to transcoding functionality in a cellular communication system 
     BACKGROUND OF THE INVENTION 
     In a typical Code Division Multiple Access (CDMA) cellular network, such as a second generation (2G) CDMA communication network, transcoders are located in a radio access network (RAN), and in particular in a base station controller (BSC) located in the RAN. The transcoders receive compressed voice packets from a mobile station and convert the voice packets to pulse code modulated (PCM) signals for transmission through a circuit switched core network included in the cellular network. The BSCs then transmit the PCM signals upstream through the circuit switched core network and, via the core network, to a Public Switched Telephone Network (PSTN) coupled to the operator&#39;s cellular network. Similarly, PCM signals received by a 2G CDMA cellular network from a PSTN that are intended for a mobile station serviced by the RAN are transmitted as PCM signals through the circuit switched core network to the RAN, where the transcoder in the RAN converts the PCM signals to compressed voice packets. The RAN then transmits the compressed voice packets to the mobile station. 
     The development of next generation CDMA networks, such as a cdma2000 cellular network, have allowed system operators to install packet switched core networks in parallel with the circuit switched core networks, thereby permitting data packets to be transmitted through the cellular networks in parallel with circuit switched signals. The installation of such packet switched core networks permits a system operator to transmit voice data as a compressed voice packet through the packet switched core network, rather than transmit voice data as PCM signals through the circuit switched core network. To facilitate transmission of voice through a cellular network in a data packet format, operators of cdma2000 cellular networks have expressed an interest in relocating the transcoders closer to the PSTN. In addition, a relocation of transcoders to a more centralized location deeper in the cellular network can reduce system costs by providing a more centralized transcoder function, as opposed to a widely distributed, RAN-based, transcoder function, and by permitting voice services to be transported over more of a backhaul network in a compressed format as opposed to an uncompressed format. 
     However, when a legacy CDMA communication system is upgraded with a relocated transcoder function, a result may be a provision of a transcoding function in each of a core network and a RAN. Furthermore, some systems may service mobile stations that are not capable of transmitting voice in a format compatible for transmission over a packet switched core network, thus necessitating a transcoding function in a RAN. A result is that multiple transcoding functions may reside along a voice signal&#39;s path. The problem then arises as to which of the multiple transcoding functions shall encode/decode received voice. 
     Therefore, there exists a need for controlling where, in the cellular system, the transcoding function is performed, and what transcoding type(s) should be used in the serving network elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a wireless communication system in accordance with an embodiment of the present invention. 
         FIG. 2A  is a logic flow diagram of a method executed by the communication system of  FIG. 1  in controlling which transcoder of a first transcoder residing in a first network element and a second transcoder residing in a second network element that is downstream from the first network element may transcode voice traffic in accordance with an embodiment of the present invention. 
         FIG. 2B  is a continuation of the logic flow diagram of  FIG. 2A  depicting a method executed by the communication system of  FIG. 1  in controlling which transcoder of a first transcoder residing in a first network element and a second transcoder residing in a second network element that is downstream from the first network element may transcode voice traffic in accordance with an embodiment of the present invention. 
         FIG. 2C  is a continuation of the logic flow diagrams of  FIGS. 2A and 2B  depicting a method executed by the communication system of  FIG. 1  in controlling which transcoder of a first transcoder residing in a first network element and a second transcoder residing in a second network element that is downstream from the first network element may transcode voice traffic in accordance with an embodiment of the present invention. 
         FIG. 3  is a logic flow diagram of a method executed by the communication system of  FIG. 1  in transferring a transcoding function from a first transcoder residing in a first network element and a second transcoder residing in a second network element that is downstream from the first network element in accordance with another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     To address the need for a method and an apparatus that controls where, in the cellular system, the transcoding function is performed and what transcoding type(s) should be used in the serving network elements, a wireless communication system is provided that comprises an infrastructure having a first network element that is upstream of a second network element, wherein the first network element comprises a first transcoder and the second network element comprises a second transcoder. The communication system controls a transcoding of voice by determining a first bearer type supported by the first transcoder, determining a second bearer format type mutually supported by the infrastructure and a mobile station serviced by the infrastructure, and selecting one of the first transcoder and the second transcoder to transcode the voice based on the first bearer format type and the second bearer format type. 
     Generally, an embodiment of the present invention encompasses a method for controlling a transcoding of voice in a wireless communication system having an infrastructure comprising a first network element that is upstream of a second network element, wherein the first network element comprises a first transcoder and the second network element comprises a second transcoder. The method includes determining a first bearer format type supported by the first transcoder, determining a second bearer format type mutually supported by a mobile station and the infrastructure, and selecting one of the first transcoder and the second transcoder to transcode the voice based on the first bearer format type and the second bearer format type. 
     Another embodiment of the present invention encompasses a transcoder controller comprising at least one memory device that maintains a first bearer format type supported by a first transcoder associated with a first network element. The transcoder controller further comprises a processor coupled to the at least one memory device that receives a second bearer format type mutually supported by a mobile station and a second transcoder associated with a second network element and selects one of the first transcoder and the second transcoder to transcode voice based on the first bearer format type and the second bearer format type. 
     Still another embodiment of the present invention encompasses a distributed transcoding system comprising a first network element comprising a first transcoder, a second network element comprising a second transcoder, wherein the second network element is downstream from the first network element, and a transcoder controller coupled to each of the first network element and the second network element. The transcoder controller maintains a first bearer format type supported by the first transcoder, receives a second bearer format type that is mutually supported by a mobile station and the second transcoder, and selects one of the first transcoder and the second transcoder to transcode voice based on the first bearer format type and the second bearer format type. 
     The present invention may be more fully described with reference to  FIGS. 1-3 .  FIG. 1  is a block diagram of a wireless communication system  100  in accordance with an embodiment of the present invention. Communication system  100  includes a Radio Access Network (RAN)  106  that comprises multiple base stations (BSs)  110 ,  120 . Each BS of the multiple BSs  110 ,  120  includes a respective at least one base transceiver station (BTS)  112 ,  122  operably coupled to a respective base station controller (BSC)  114 ,  124 . Each BSC  114 ,  124  optionally includes a respective transcoder  116 ,  126  that is capable of is capable of decoding voice data packets received from MS  102  into at least one of multiple bearer formats, such as SMV (IS-893), EVRC (IS-127), 13 k-QCELP (IS-733), 8 k-QCELP (IS-96C), and G.711, for conveyance to a public network  190  or a remote packet voice network  192  and is further capable of encoding voice data received from public network  190  or remote packet voice network  192  in at least one of multiple bearer formats into voice data packets for conveyance to MS  102 . 
     Communication system  100  further comprises a mobile station (MS)  102  in wireless communication with a BS, such as BS  110 , of RAN  106  via an air interface  104 . Air interface  104  comprises a forward link (not shown) having multiple communication channels, such as one or more forward link control channels, one or more forward link traffic channels, and a forward link paging channel, and a reverse link (not shown) having multiple communication channels, such as one or more reverse link control channels, one or more reverse link traffic channels, and a reverse link access channel. 
     Each BS  110 ,  120 , preferably a respective BSC  114 ,  124  of the BS  110 ,  120 , is coupled to an Inter-BS Packet Transport network  140  via a respective signaling interface  130 ,  134  and a respective bearer interface  132 ,  136 . Inter-BS Packet Transport network  140  is further coupled to a packet switched controller  144  via a signaling interface  142 , thereby providing a signaling link between each BS of the multiple BSs  110 ,  120  and the packet switched controller. Inter-BS Packet Transport network  140  is still further coupled to a wide area packet transport network  154  via a bearer interface  150 . In turn, wide area packet transport network  154  is further coupled to a local Media Gateway (MGW)  160  via a bearer traffic interface  156 , thereby providing a bearer traffic link between each BS of the multiple BSs  110 ,  120  and Media Gateway  160 . Wide area packet transport network  154  is also coupled to packet switched controller  144  via a signaling interface  148  and to remote packet voice network  192  via a signaling interface  194  and a bearer interface  196 . Preferably, each of signaling interfaces  130 ,  134 , and  142  comprises an A1 interface that has been modified to support an exchange of signaling messages in a packet voice format (which interfaces are depicted in  FIG. 1  as A1 p  interfaces). In addition, preferably each of bearer interfaces  132 ,  136 ,  150 , and  156  comprises an A2 interface that has been modified to support the exchange of bearer traffic in a packet voice format (which interfaces are depicted in  FIG. 1  as A2 p  interfaces). 
     Media Gateway  160  optionally includes a transcoder  161  that, similar to transcoders  116  and  126 , is capable of is capable of decoding voice data packets received from MS  102  into at least one of multiple bearer formats, such as SMV, EVRC, 13 k-QCELP, 8 k-QCELP, and G.711, for conveyance to public network  190  or remote packet voice network  192  and is further capable of encoding voice data received from public network  190  or remote packet voice network  192  in at least one of the multiple bearer formats into voice data packets for conveyance to MS  102 . 
     Packet switched controller  144  includes a transcoder controller  145  that determines a transcoder of the multiple transcoders  116 ,  161  possible in communication system  100  that will encode and decode voice traffic during a communication session involving MS  102 . Transcoder controller  145  includes a processor  146 , such as one or more microprocessors, microcontrollers, digital signal processors (DSPs), combinations thereof or such other devices known to those having ordinary skill in the art, and one or more associated memory devices  147 , such as random access memory (RAM), dynamic random access memory (DRAM), and/or read only memory (ROM) or equivalents thereof, that maintain data and programs that may be executed by the corresponding processor. The one or more memory devices  147  may further maintain bearer format types supported by one or more of transcoder  161  of Media Gateway  160  and transcoder  116  of BSC  114 . Preferably, packet switched controller  144  comprises a packet voice Soft Switch that is available from Motorola, Inc, of Schaumburg, Ill., and that has been modified to perform the functions described herein. 
     Media Gateway  160  is further coupled to public network  190 , preferably a Public Switched Telephone Network (PSTN), via a bearer interface  162 , preferably a pulse code modulation (PCM) interface, and to packet switched controller  144  via a signaling interface  164 . Public network  190  is further coupled to packet switched controller  144  via a signaling interface  152 , preferably an ISDN User Part (ISUP) interface. Inter-BS Packet Transport network  140 , packet switched controller  144 , wide area packet transport network  154 , Media Gateway  160 , and interconnecting interfaces  130 ,  132 ,  142 ,  148 ,  150 ,  152 ,  156 ,  162 ,  164 ,  194 , and  196  may be collectively referred to as a packet switched core network and provide a packet voice communication link between each BS  110 ,  120  and each of public network  190  and remote packet voice network  192 . 
     BS  110 , preferably BSC  114  of BS  110 , is further coupled to a circuit switched controller  174 , preferably a circuit switched MSC, via a signaling interface  170 , preferably an A1 interface, and a bearer interface  172 , preferably an A2 interface. In turn, circuit switched controller  174  is coupled to public network  190  via a bearer interface  176 , preferably a PCM interface, and a signaling interface  178 , preferably an ISUP interface. Circuit switched controller  174  and the associated interfaces  170 ,  172 ,  176 , and  178  coupling the circuit switched controller  174  to BS  110  and public network  190  may be collectively referred to as a circuit switched core network and provide a circuit switched communication link between BS  110  and public network  190 . RAN  106 , BS  110 , packet switched controller  144 , Media Gateway  160 , transport networks  140  and  154 , and circuit switched controller  174  are collectively referred to herein as an infrastructure  180  of communication system  100 . 
     Communication system  100  comprises a wireless packet voice communication system. In order for MS  102  to engage in a voice communication with an external network  190 ,  192  connected to infrastructure  180 , each of BS  110 , packet switched controller  144 , Media Gateway  160 , transport networks  140  and  154 , and circuit switched controller  174  operates in accordance with well-known wireless telecommunications protocols. By operating in accordance with well-known protocols, a user of MS  102  can be assured that MS  102  will be able to communicate with infrastructure  180  and establish a communication link with an external network  190 ,  192  via the infrastructure. Preferably, communication system  100  operates in accordance with the 3GPP2 and TIA/EIA (Telecommunications Industry Association/Electronic Industries Association) IS-2001, or IOS (Inter Operability Specification), standards, which provides a compatibility standard cdma2000 or 1xEV-DO systems and wherein each communication channel of the multiple communication channels of each of the forward link and the reverse link of air interface  104  comprises one or more orthogonal codes, such as Walsh codes. The standard specifies wireless telecommunications system operating protocols, including radio system parameters and call processing procedures. However, those who are of ordinary skill in the art realize that communication system  100  may operate in accordance with any one of a variety of wireless packet-oriented voice communication systems, such as a Global System for Mobile communication (GSM) communication system, a Time Division Multiple Access (TDMA) communication system, a Frequency Division Multiple Access (FDMA) communication system, or an Orthogonal Frequency Division Multiple Access (OFDM) communication system. 
     When MS  102  establishes a communication session with infrastructure  180 , it may be possible for any one of multiple serially-distributed transcoders  116 ,  161  located along a voice bearer path associated with the MS to encode and decode voice traffic originating from or destined for the MS. In addition, communication system  100  may determine to establish the communication session as a transcoder free operation (TrFO), wherein voice traffic propagates through the communication system without being processed by any transcoder  116 ,  161  of infrastructure  180 . Accordingly, communication system  100  provides for a selection of a transcoder of the multiple serially-distributed transcoders  116 ,  161  to transcode the voice traffic when the voice traffic is to be transcoded by the infrastructure. 
     Referring now to  FIGS. 2A ,  2 B, and  2 C, a logic flow diagram  200  is provided that illustrates a method by which communication system  100  controls which transcoder of a first transcoder residing in a first network element, such as transcoder  161  in Media Gateway  160 , and a second transcoder residing in a second network element, such as transcoder  116  in BS  110 , that is downstream from the first network element, shall transcode voice traffic in accordance with an embodiment of the present invention. Logic flow diagram  200  begins when MS  102  conveys to infrastructure  180 , and in particular to BS  110  via BTS  112 , and BS  110  receives ( 202 ) from MS  102 , a request for service, that is, a request to establish a voice, or a voice and data, communication session. Preferably, the request for service comprises an Origination Message as is known in the art, which Origination Message requests service and requires a Layer 2 acknowledgment. 
     In response to receiving the request for service, BS  110  acknowledges ( 204 ) the request, preferably by conveying a Base Station Acknowledgment Order, as is known in the art, to MS  102 . In addition, BS  110  determines ( 206 ) whether communication resources, such as a forward link traffic channel and a reverse link traffic channel in air interface  104 , are available to support the requested communication session. Upon determining that such resources are available, BS  110  assembles a service request message that requests service from packet switched controller  144  and that further requests an allocation of a communication link between the BS and Media Gateway  160 . Preferably the service request message comprises a CM Service Request Message as is known in the art, which CM Service Request Message is conveyed as part of a Complete Layer 3 Information message. BS  110  then conveys ( 208 ) to packet switched controller  144 , and the packet switched controller, and in particular transcoder controller  145 , receives ( 212 ) from BS  110 , the service request message. Unless otherwise specified herein, all functions described herein as being performed by transcoder controller  145  are performed by processor  146  of the transcoder controller. Upon conveying the service request message, BS  110  starts ( 210 ) a first timer, T 303 , while awaiting a response to the service request message from packet switched controller  144 . Since MS  102  is awaiting an allocation of a traffic channel, such as a forward link traffic channel and a reverse link traffic channel of air interface  104 , and in the meanwhile is not being power controlled, it is desirable to provide an allocation of the requested communication link with a minimum amount of delay. Accordingly, the first timer counts down a predetermined first time period corresponding to an acceptable period of time that the BS may await the response to the service request message. 
     When a network element upstream from BS  110  includes a transcoder, such as media gateway  160  and transcoder  161 , packet switched controller  144 , preferably transcoder controller  145 , may determine ( 214 ) the bearer format types supported by the upstream network element, that is, by transcoder  161  of the Media Gateway, by reference to the one or more memory devices  147  of the transcoder controller. Further, in response to receiving the service request message, packet switched controller  144 , preferably transcoder controller  145 , conveys ( 216 ) to BS  110  an assignment message informing of an allocation of a communication link between BS  110  and Media Gateway  160  for the communication session involving MS  102 , that is, an allocation of communication links in interfaces  130 ,  132 ,  142 ,  150 , and  156 , and a bearer address, preferably an A2 p  bearer address, of Media Gateway  160  or remote packet voice network  192 . When the upstream network element, that is, Media Gateway  160 , includes a transcoder, that is, transcoder  161 , the assignment message may further include one or more requested bearer format types for conveyance of the voice data over the allocated A2 p  communication links, which requested bearer format types corresponds to bearer format types supported by the transcoder of the upstream network element. When support for multiple bearer format types exist, a priority of preference may be included. In such an instance, packet switched controller  144  determines the requested bearer format types by reference to the one or more memory devices  147 , which one or more memory devices may store the bearer format types supported by the upstream network element. The BS may use the requested format type information to determine and convey a service option assignment to the MS. The BS may preferentially assign a service option to the MS, corresponding to one of the requested bearer format types included in the assignment message. Preferably, the assignment message comprises a modified version of an Assignment Request message as is known in the art, which Assignment Request message that has been modified to optionally convey the information described herein. Upon conveying the assignment message, packet switched controller  144 , preferably transcoder controller  145 , further starts ( 218 ) a second timer, T 10 . The second timer counts down a predetermined second time period corresponding to an acceptable period of time that packet switched controller  144  may await a response from BS  110  to the conveyed assignment message. 
     When the first time period, as measured by the first timer, expires ( 220 ) prior to BS  110  receiving an assignment message from packet switched controller  144 , logic flow diagram  200  returns to step  208 , where BS  110  reconveys the service request message to the packet switched controller. When BS  110  receives the assignment message prior to the expiration of the first time period, then BS  110  stops ( 222 ) the first timer, T 303 . In addition, based on the determination that forward and reverse link traffic channels are available for allocation to MS  102  for the requested call, BS  110  and MS  102  set up ( 224 ) the call over air interface  104  in accordance with well known call set up procedures. 
     For example, in one embodiment of the present invention, the step of setting up ( 224 ) the call may include the following steps, which steps are provided merely to illustrate one method for setting up a call and are not intended to limit the invention in any way. One of ordinary skill in the art realizes that many schemes exist for setting up a call, which schemes may be used herein without departing form the spirit and scope of the present invention. Upon determining that forward and reverse link traffic channels are available for allocation to MS  102 , BS  110  conveys a channel assignment message to MS  102  over a forward link paging channel of air interface  104 . The channel assignment message informs of the forward and reverse link traffic channels allocated to the communication session. In response to receiving the channel assignment message, MS  102  conveys a traffic channel preamble to BS  110  over the allocated reverse link traffic channel. When BS  110  acquires the allocated reverse link traffic channel, the BS so informs MS  102  via the allocated forward link traffic channel. Preferably BS  110  informs MS  102  that the BS has acquired the allocated reverse link traffic channel by conveying a Base Station Acknowledgment Order to the MS, which Base Station Acknowledgment Order requires a Layer 2 acknowledgment. 
     Upon being informed that BS  110  has acquired the allocated reverse link traffic channel, MS  102  acknowledges receipt of the acquisition information to BS  110 , preferably by conveying a Mobile Station Acknowledgment Order to the BS, and further conveys null traffic channel data to the BS over the allocated reverse link traffic channel. 
     Upon receiving the acknowledgement of receipt of the acquisition information and the null traffic channel data from MS  102 , BS  110  conveys to MS  102  information requesting a list of service configurations supported by MS  102 . Preferably, the information requesting a list of service configurations is included in a Status Request/Status Response Order. Upon receiving the selected bearer format type information from the transcoder controller  145 , BS  110  further conveys to MS  102  information specifying a service configuration of the call, including a requested bearer format type. Preferably, the information specifying a service configuration of the call is included in a Service Connect Message/Service Response Order. Upon receiving the information specifying a service configuration of the call, MS  102  informs BS  110  that the MS is able to support the specified service configuration, including the requested bearer format type, preferably by conveying a Service Connect Completion Message to the BS, and begins processing bearer traffic in accordance with the specified service configuration. 
     After a radio traffic channel is set up, that is, after a forward link traffic channel and a reverse link traffic channel in air interface  104  are established and fully interconnected, BS  110  determines ( 226 ), based in part on the service option assigned to the MS, the bearer format types mutually supported by each of MS  102  and transcoder  116  of BSC  114 . BS  110  conveys ( 228 ) to packet switched controller  144 , and in particular transcoder controller  145 , information concerning the mutually supported bearer format types, which mutually supported bearer format may be formatted in an order of preference by the BS. Preferably, BS  110  informs packet switched controller  144  of the supported bearer format types in a modified version of an Assignment Complete message as is known in the art, which Assignment Complete message is modified to include an ordered preference of bearer format types data field. For example, BS  110  may inform of the bearer format types mutually supported by MS  102  and BS  110  by embedding in the bearer format type data field of the modified Assignment Complete message one or more of the following values, which values each corresponds to a following bearer format type, 
     ‘0001’=SMV, 
     ‘0010’=EVRC, 
     ‘0011’=13 k-QCELP, 
     ‘0100’=8 k-QCELP, and 
     ‘0101’=G.711. 
     BS  110  may qualify the bearer format types mutually supported by MS  102  and BS  110  according to available format types that may have been received in the Assignment Request message. 
     When packet switched controller  144 , and in particular transcoder controller  145 , fails to receive ( 230 ) the information concerning the mutually supported bearer format types prior to an expiration of the second time period, as measured by the second timer, logic flow diagram  200  proceeds to step  216 , where the packet switched controller, and in particular the transcoder controller, reconveys the assignment message to BS  110 . When transcoder controller receives ( 230 ) the information concerning the bearer format types mutually supported by MS  102  and infrastructure  180 , and more specifically BS  110 , prior to an expiration of the second time period, transcoder controller  145  stops ( 232 ) the second timer, T 10 , and stores ( 234 ), in the one or more memory devices  147  of transcoder controller  145  and further in association with BSC  109 , or BS  110 , the mutually supported bearer format types received from BS  110 . 
     Based at least in part on the information received from BS  110  concerning the mutually supported bearer format types, transcoder controller  145  determines ( 236 ), that is, selects, a bearer format type for transport of voice packets over the packet switched core network, that is, between BS  110  and Media Gateway  160 , during the communication session. The selection of the bearer format type for transport of voice packets over the packet switched core network may be further based on the bearer format types supported by Media Gateway  160 , that is, by transcoder  161  of the Media Gateway, which bearer format types are known to the transcoder controller and may be included in the assignment message conveyed to BS  110  and subsequently reflected in the mutually supported bearer format types conveyed by the BS to packet switched controller  144 . Based at least in part on the mutually supported bearer format types received from BS  110  and bearer format types supported by one or more of transcoder  116  of BS  110  and transcoder  161  of Media Gateway  160 , transcoder controller  145  further determines ( 238 ) where to transcode in infrastructure  180 , that is, selects a transcoder of transcoder  116  of BSC  114 /BS  110  and transcoder  161  of Media Gateway  160  to transcode voice during the communication session involving MS  102 , thereby producing a selected transcoder. 
     In one embodiment of the present invention, transcoder controller  145  may select a bearer format type based on minimizing number of bearer interworking functions, that is, based on minimizing a quantity of separate transcoding functions that are applied to the bearer traffic. Transcoder controller  145  may then preferentially, or in default, select a transcoder location of Media Gateway  160 , that is, transcoder  161 , and may select a transcoder location of BS  110 , that is, transcoder  116 , if the selected bearer format type is not supported or available in Media Gateway  160 . In other embodiments of the present invention, transcoder controller  145  may determine the bearer format type and where to transcode the voice data in infrastructure  180  based on bearer format types, such as one or more compressed bearer format types, such as SMV, EVRC, 13 k, or 8 k, or one or more uncompressed bearer format type, such as G.711, supported by a transcoder of remote packet voice network  192  or by a transcoder of a destination MS (not shown) serviced by remote packet voice network  192 . In still other embodiments of the present invention, transcoder controller  145  may determine the bearer format type and where to transcode voice in infrastructure  180  based on bearer format type preferences for packet switched core network  192 , which preferences are maintained in the one or more memory devices  147  of the transcoder controller. However, those who are of ordinary skill in the art realize that many algorithms may be used herein for determining the bearer format type and where to transcode in infrastructure  180  without departing from the spirit and scope of the present invention. 
     For example, as noted above in one embodiment of the present invention, with respect to performing step  238 , transcoder controller  145  may be configured to select the transcoder of the upstream network element, that is, transcoder  161  of Media Gateway  160 , whenever the transcoder of the upstream network element supports the bearer format type determined by the transcoder controller to be used for transport of voice packets over the packet switched core network. In the event that the transcoder of the upstream network element does not support the determined bearer format type and the transcoder of the downstream network element, that is, transcoder  116  of BS  110 , supports the determined bearer format type, then transcoder controller  145  selects the transcoder of the downstream network element. 
     In another embodiment of the present invention, with respect to performing step  238 , transcoder controller  145  may be configured to select the transcoder of the downstream network element, that is, transcoder  116  of BS  110 , whenever the transcoder of the downstream network element supports the bearer format type determined by the transcoder controller to be used for transport of voice packets over the packet switched core network. In the event that the transcoder of the downstream network element does not support the determined bearer format type and the transcoder of the upstream network element, that is, transcoder  161  of Media Gateway  160 , supports the determined bearer format type, transcoder controller  145  may select the transcoder of the upstream network element. 
     In still another embodiment of the present invention, with respect to performing step  238 , transcoder controller  145  may select one of the transcoder of the upstream network element and the transcoder of the downstream network element based on a network-element processing load. For example, when the processing load of the upstream network element, that is, Media Gateway  160 , exceeds a processing load threshold, transcoder controller  145  may select the transcoder of the downstream network element, that is, transcoder  116  of BS  110 . Otherwise, transcoder controller  145  selects the transcoder of the upstream network element. In still another embodiment of the present invention, with respect to performing step  238 , transcoder controller  145  may select one of the transcoder of the upstream network element and the transcoder of the downstream network element based on a load of the packet switched core network in order to assure that voice data is transported in a compressed format over the packet switched core network when the network load is high, that is, exceeds a network load threshold. 
     In response to determining a bearer format type for transport of voice packets over the packet switched core network and further determining where to perform transcoding in infrastructure  180 , transcoder controller  145  instructs ( 240 ) a network element associated with the selected transcoder, such as BS  108  or BSC  110  with respect to transcoder  116  or Media Gateway  160  with respect to transcoder  161 , via a corresponding signaling interface, to transcode the bearer path of the communication session. In response to receiving the transcoding instruction, the network element inserts ( 242 ) the selected transcoder into the bearer path and the selected transcoder transcodes the voice. Logic flow  200  then ends. In another embodiment of the present invention, instead of instructing a network element associated with the selected transcoder to transcode, transcoder controller  145  may instruct ( 244 ) network elements not associated with the selected transcoder not to transcode the bearer path of the communication session. In response to receiving the instruction not to transcode, the network element removes ( 246 ) the selected transcoder into the bearer path so that only the selected transcoder transcodes the voice. 
     By determining a bearer format type for a transcoding of voice during a communication session and selecting a transcoder of multiple transcoders  116 ,  161 , that are associated with respective network elements  110 ,  161 , to transcode the voice based on the determined bearer format type and bearer format types supported by one or more of the multiple transcoders, communication system  100  is able to control where, in the system, the transcoding function is performed and what transcoding type(s) should be used in the serving network elements. A downstream network element, such as BS  110 , determines one or more bearer format types mutually supported by the downstream network element and an MS  102  engaging in the communication session. The downstream network element conveys the mutually supported bearer format types to a transcoder controller  145  that determines a bearer format type for transport of voice packets over the packet switched core network based at least in part on the mutually supported bearer format types, which mutually supported bearer format types may reflect one or more bearer format types supported by an upstream network element, such as Media Gateway  160 . Based on the determined bearer format type, transcoder controller  145  then determines where to perform the transcoding, that is, whether to transcode in the downstream network element or the upstream network element, or more particularly in a transcoder of the downstream network element or a transcoder of the upstream network element. The transcoder of the selected element of then performs the transcoding function. 
     For example, upon selecting the transcoder of the upstream network element, that is, transcoder  161  of Media Gateway  160 , to transcode the communication session, transcoder controller  145  may instruct Media Gateway  160  to insert transcoder  161  in the path of the bearer traffic, that is, to transcode the voice packets exchanged during the communication session and/or may instruct BS  110  not to insert transcoder  116  in the path of the bearer traffic, that is, the voice data. By way of another example, upon selecting the transcoder of the downstream network element, that is, transcoder  116  of BS  110 , to transcode the communication session, transcoder controller  145  may instruct BS  110  to insert transcoder  116  in the path of the bearer traffic and/or may instruct Media Gateway  160  not to insert transcoder  161  in the path of the bearer traffic. And if transcoder controller  145  determines not to transcode in infrastructure  180 , then the transcoder controller may not instruct each of Media Gateway  160  and BS  110  to insert their respective transcoders  116 ,  161 , in the path of the bearer traffic or may instruct each of Media Gateway  160  and BS  110  not to insert their respective transcoders  116 ,  161 , in the path of the bearer traffic. 
     In yet another embodiment of the present invention, packet switched controller  144 , and in particular transcoder controller  145 , may determine during the course of the communication session to transfer a transcoding function from a selected transcoder, such as, transcoder  161  of Media Gateway  160 , to a non-selected transcoder, such as transcoder  116  of BS  110 . Referring now to  FIG. 3 , a logic flow diagram  300  is provided that illustrates a transfer of a transcoding function within infrastructure  180  by communication system  100  in accordance with another embodiment of the present invention. Logic flow diagram  300  begins when, during the course of an active communication session, transcoder controller  145  determines ( 302 ) to transfer a transcoding function from a first transcoder that has been selected to transcode voice, such as a transcoder  161  in Media Gateway  160 , to a second transcoder, that is, a non-selected transcoder, that was not selected to transcode voice, such as transcoder  116  in BS  110 , wherein the selected transcoder is either upstream of, or downstream of, the non-selected transcoder. 
     Upon determining to transfer the transcoding function from the selected transcoder to the non-selected transcoder, transcoder controller  145  conveys ( 304 ) a first transcoder transfer message comprising information concerning a requested bearer format type to the network element associated with the non-selected transcoder, that is, BS  110 . The requested bearer format type may be a new bearer format type, that is, may or may not be the same bearer format type as the bearer format type currently being applied to the voice communications. By conveying a request bearer format type to the network element associated with the non-selected transcoder, transcoder controller  145  is able to change both bearer format type and transcoding location during an on-going communication session. Preferably, transcoder controller  145  determines the requested bearer format type based on the bearer format types supported by the non-selected transcoder, that is, transcoder  116 , and stored in the one or more memory devices  147 . However, in another embodiment of the present invention, transcoder controller  145  may not know that the network element associated with the non-selected transcoder supports the requested bearer format type until the transcoder controller receives an acceptance of the requested bearer format type from the network element. The first transcoder transfer message may further comprise the bearer address, that is, the A2 p  address, of the network element associated with the selected transcoder, that is, Media Gateway  160 , and an indication of when the transfer is to be effective, such as a frame count, a time stamp, or a reception of data packets with a changed bearer format type. Preferably, the transcoder transfer message comprises a Change Bearer Request Message. 
     In response to receiving the first transcoder transfer message, the network element associated with the non-selected transcoder, that is BS  110 , conveys ( 306 ) a transcoder transfer response message, preferably a Change Bearer Response Message, to transcoder controller  145  informing of acceptance of the requested bearer format type. The transcoder transfer response message may further inform of a bearer address, preferably an A2 p  address, of the network element associated with the non-selected transcoder, that is, BS  110 . In response to receiving the first transcoder transfer message, and at the indicated time when a transfer time is indicated, the network element associated with the non-selected transcoder that is BS  110 , inserts ( 308 ) the non-selected transcoder, that is, transcoder  116 , into the bearer path of the bearer traffic, that is, the voice data, and the non-selected transcoder begins transcoding the voice data. 
     Transcoder controller  145  further conveys ( 310 ) a second transcoder transfer message to the network element associated with the selected transcoder, that is, Media Gateway  160 , instructing the network element associated with the selected transcoder to remove the selected transcoder, that is, transcoder  161 , from the path of the bearer traffic. In an embodiment of the present invention wherein transcoder controller  145  does not know that the network element associated with the non-selected transcoder supports the requested bearer format type, the transcoder controller may not convey the second transcoder transfer message until the transcoder controller receives an acceptance of the requested bearer format type from the network element associated with the non-selected transcoder. Like the first transcoder transfer message, the second transcoder transfer message may include an indication of when the transfer is to be effective. Upon receiving the second transcoder transfer message, or at the indicated time when a transfer time is indicated, the network element associated with the selected transcoder, that is, Media Gateway  160 , removes ( 312 ) the selected transcoder, that is, transcoder  161 , from the path of the bearer traffic. Logic flow  300  then ends ( 314 ). 
     By permitting transcoder controller  145  to change a bearer format type and a transcoding location during the course of an on-going communication session, communication system  100  is capable of adapting to changes in bearer interactions with other core network elements, such as conference bridges, announcement servers, and so on. Accordingly, communication system  100  provides a distributed transcoding system comprising multiple network elements that are each capable of transcoding voice during a communication session, wherein the communication system is capable of both determining a bearer format type and a transcoding location for the communication session and of changing the bearer format type and the transcoding location for the communication session when desirable. 
     While the present invention has been particularly shown and described with reference to particular embodiments thereof, it will be understood by those skilled in the art that various changes may be made and equivalents substituted for elements thereof without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather then a restrictive sense, and all such changes and substitutions are intended to be included within the scope of the present invention. 
     Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. As used herein, the terms “comprises,” “comprising,” or any variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. It is further understood that the use of relational terms, if any, such as first and second, top and bottom, and the like are used solely to distinguish one entity or action from another entity or action without necessarily requiring pr implying any actual such relationship or order between such entities or actions.