Abstract:
A call processing method and system in which the use of the MGW between the RNC and the peer party is limited to only certain situations. This invention uses RFC 3267 (AMR/IETF) on the RNC. This will avoid inserting a MGW context for framing conversion only. This invention provides an optimization that sets up calls from the RNC, thus avoiding the need for a MGW in the path for basic calls. With this optimization, the usage of DSP and packet resources and the number of MGW chassis, as well as bearer path delay, are all reduced.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a method and system for bypassing Media Gateways in wireless networks. 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 mobile 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 (CS) Core Network (CN) based on the GSM. 
     The specification of the UMTS is currently being developed by a standardization group called the Third Generation Partnership Project (3GPP). In the 3GPP specifications, a Mobile Switching Center/Visitor Location Resister (MSC/VLR), which is a core network node belonging to a circuit switched region, is divided into Mobile Switching Center (MSC) servers and Media Gateways (MGWs). 
     In particular, the 3GPP is currently developing new worldwide standards for the creation, delivery, and playback of multimedia over new, high-speed wireless networks. These standards will enable the free sharing of multimedia between a variety of devices, including cell phones, Personal Digital Assistants (PDAs), and notebook and desktop computers. 3GPP devices include, in addition to a voice communication interface, capability for communication of data and display of data, including video. 
     3GPP specifications mandate the use of a Media Gateway (MGW) in the bearer path for voice calls and Circuit Switched Data (CSD) calls in the circuit core network. The MGW receives packetized voice from the Radio Network Controller (RNC) via Iu User Plane Protocol (IuUP) framing. Packetized voice is the process of converting audio signals into digital packet format, transferring these packets through a packet network, reassembling these packets into their original data form, and then recreating the audio signals. The voice or CSD packets then traverse through the MGW on route toward the destination. It is existing art (and in the process of standardization in 3GPP) to use IETF standard protocols to carry voice and data between MGWs in the core network. In many cases, the MGW does nothing more than convert the type of packetization used for the voice and data, i.e., converting between IuUP framing and IETF framing formats, where no transcoding of voice to another format is necessary. The newly reframed packets are then sent on toward the destination, which can be another MGW, a RNC, an IP Multimedia Subsystem (IMS), and so on. Using a MGW solely for performing a reframing function is not efficient from a cost and voice quality perspective. 
     Accordingly, the present invention contemplates a new and improved method and system that resolves the above-referenced difficulties and others. More particularly, this invention devises a method and system for eliminating MGWs from the bearer path when they are not needed. This invention is only applicable to an MSC when originating or terminating a call for a wireless subscriber. The prior art describes means of bypassing MGWs at transit exchanges and handover anchors, but does not describe the case of MGW bypass when the MSC is controlling access to a subscriber device via a RAN. This invention achieves at least two purposes for network providers: reducing cost by using less equipment in the network and improving voice quality by reducing bearer delay. 
     SUMMARY OF THE INVENTION 
     3GPP standards specify the use of IP in transporting packetized voice signals between the RNC and the MGW in the MSC. These packets are encapsulated within IuUP framing. When these same packets are transported into the core network (e.g., inter-MSC traffic) through a media gateway, they are encapsulated within NbUP framing when controlled by the Bearer Independent Call Control (BICC) protocol. When the use of the Session Initiation Protocol (SIP) is standardized within 3GPP, these packets will be transported into the core network via standardized Internet Engineering Task Force (IETF) formats. Example IETF methods include AMR (IETF RFC 3267) and G.711 (IETF RFC 3551). 
     The ability to carry native 3GPP AMR voice samples and CSD within the core network for the bearer path between the subscriber and the peer party is existing art. This allows transcoding to be eliminated for mobile-to-mobile calls. It also allows transcoding to be pushed to the edges of the service provider&#39;s network for mobile-to-land calls. Note, however, that it is still necessary for a MGW to perform framing conversion between the IuUP format used between the MGW and the RNC, and the IETF format used within the core network. Besides additional Digital Signal Processor (DSP) and packet network resources used to convert IuCS to IETF frames, additional delays are introduced resulting in higher costs when a CN MGW is in the bearer path. 
     The next step, and the subject of this invention, is the ability to limit the need for the MGW between the RNC and the core network. Instead of using the IuUP framing protocol at the bearer interface to the RNC according to the current 3GPP standard, this invention uses standardized IETF framing formats, such as IETF RFC 3267 (AMR—Adaptive Multi-Rate) and IETF RFC 4040 (Clearmode for CSD). This avoids the need to insert a MGW context only for the purpose of framing conversion. This invention also provides an optimization in the bearer path from the RNC to the peer party, thus avoiding the need for any MGW in the path for basic calls. With this optimization, the usage of DSP and packet resources and the number of MGW chassis, as well as bearer path delay, are all reduced. 
     Thus, methods and systems are provided for bypassing media gateways at MSCs that control radio access to user equipment in wireless networks. 
     In one aspect of the invention, a method of processing calls in a wireless telecommunications network is provided. The method comprises: the originating or terminating MSC receiving a call attempt from a calling party to a called party; determining whether an MGW is necessary at the MSC to begin to process the call; setting up a call bearer path between the calling party and the called party via a Radio Network Controller (RNC) controlled by the MSC and via an MGW at the MSC only if necessary to process the call; receiving a call event at the MSC; determining whether an MGW at the MSC is necessary to process the call event; and inserting or removing an MGW at the MSC depending on the requirements of the call event. 
     In another aspect of the invention, a method of processing calls in a wireless telecommunications network whereby an originating or terminating Mobile Switching Center (MSC) does not allocate a Media Gateway (MGW) resource at the beginning of a call. The method comprises: performing initial bearer negotiation with media capability information but without media connection information until media capability negotiation is complete; and determining that no condition exists requiring the inclusion of a MGW resource. Optionally, the method may further comprise including media connection information in subsequent bearer negotiation messages that does not reference any MGW resource. 
     In yet another aspect of the invention, a system for processing calls in a wireless telecommunications network is provided. The system comprises: means for receiving a call attempt from a calling party to a called party at an originating or terminating MSC; means for determining whether an MGW is necessary at the MSC to begin to process the call; means for setting up a call bearer path between the calling party and the called party via a Radio Network Controller (RNC) controlled by the MSC and via an MGW at the MSC only if necessary to process the call; receiving a call event at the MSC; determining whether a MGW at the MSC is necessary to process the call event; and inserting or removing an MGW at the MSC depending on the requirements of the call event. 
     In yet another aspect of the invention, a system for processing calls in a wireless telecommunications network whereby an originating or terminating Mobile Switching Center (MSC) does not allocate a Media Gateway (MGW) resource at the beginning of a call is provided. The system comprises: means for performing initial bearer negotiation with media capability information but without media connection information until media capability negotiation is complete; and means for determining that no condition exists requiring the inclusion of a MGW resource. Optionally, the system may further comprise means for including media connection information in subsequent bearer negotiation messages that does not reference any MGW resource. 
     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 invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. 
         FIG. 1  is a schematic view illustrating an exemplary construction of a typical Universal Mobile Telecommunication System described in the 3GPP specifications. 
         FIG. 2  is a flow chart of a call processing method for improving network efficiency according to aspects of the present invention. 
         FIG. 3  is a message signaling diagram showing call establishment between two instances of CS user equipment (UE) when call progress information is inserted into the call bearer path to the calling party at a media gateway being controlled by the originating MSC in accordance with aspects of the present invention. 
         FIG. 4  is a message signaling diagram showing call establishment between two instances of CS user equipment (UE) when call progress information is inserted into the call bearer path to the calling party at a media gateway being controlled by the terminating MSC in accordance with aspects of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, which are for purposes of illustrating the exemplary embodiments only and not for purposes of limiting the claimed subject matter,  FIG. 1  provides a view of a system into which the presently described embodiments may be incorporated. As shown generally,  FIG. 1  is a view illustrating an exemplary structure of a UMTS  2  as described in the 3GPP specifications. In the drawings, like reference numerals have been used throughout to designate identical elements. 
     The MSC server  10  on the originating side of the UMTS  2  is connected to a MGW  12  via an Mc interface. The originating MGW  12  connects to other media gateways via an Nb interface. The originating MSC server  10  and the originating MGW  12  are connected to a Universal Mobile Telecommunications System Radio Access Network (UTRAN) via an Iu interface. The originating MSC  10  and the originating MGW  12  may be connected to a Public Switched Telephone Network (PSTN) as well (not shown in  FIG. 1 ). Here, the UTRAN includes a Radio Network Controller (RNC)  14  and cell sites (Node Bs) (not shown in  FIG. 1 ). The user terminal (or User Equipment: UE) is shown as  16 . The originating MSC server  10  uses MEGACO/ITU H.248 as the protocol for controlling the originating MGW  12 . The Mc, Nb, and Iu interfaces are defined in current 3GPP specifications. 
     Similarly, the MSC server  20  of the terminating side is connected to a MGW  22  via an Mc interface. The terminating MSC server  20  and the terminating MGW  22  are connected to a UTRAN via an Iu interface. The terminating MSC  20  and the terminating MGW  22  may be connected to a PSTN as well. Here, the UTRAN includes a RNC  24  and cell sites (Node Bs) (not shown in  FIG. 1 ). The user terminal (or UE) is shown as  26 . The terminating MSC server  20  uses MEGACO/ITU H.248 as the protocol for controlling the terminating MGW  22 . It is to be understood, however, that additional network components may be present, including intermediate nodes (e.g., tandem exchanges, Gateway MSCs). 
     The MSC servers  10  and  20  are connected via an Nc interface, while the MGWs  12  and  22  are connected via an Nb interface. The MSC servers  10  and  20  are generally connected to each other using the Session Initiation Protocol (SIP) as defined by IETF RFC 3261 and associated extensions, to initiate interactive user sessions that involve the exchange of voice or data. Further, the MSC servers  10  and  20  control the MGWs  12  and  22  and include a Visitor Location Register (VLR) function to manage mobility of the terminals. 
     The MSC servers  10  and  20  may be connected to each other using Bearer Independent Call Control (BICC) rather than SIP. While this patent describes procedures using SIP, it also applies to BICC protocol or any other signaling protocol controlling packet bearers. 
     Further, the MSC servers  10  and  20  are connected to the RNCs  14  and  24 , respectively, via a Radio Access Network Application Part (RANAP). RANAP represents a radio access network (RAN) signaling protocol that enables each MSC to control its UTRAN. 
     The MGWs  12  and  22  perform substantial media functions and bearer InterWorking (I/W) functions for the network. The MGWs  12  and  22  may additionally perform functions such as transcoding, echo cancellation, modem, bridging, Lawful Intercept delivery, and media packet framing interworking. MGWs  12  and  22  interwork between their CN bearer and their Iu bearer. The Iu bearers use IETF framing as an aspect of this invention. The RNCs  14  and  24  are connected to their respective UEs  16  and  26  via radio access bearers (and Node Bs not shown). 
     Two control planes ( 30 A and  30 B) exist in the standards for call establishment. The first control plane  30 A conveys call information between the MSC server, the UTRAN and the UE. The second control plane  30 B goes between the RNC and the MGW and/or MSC to establish the IP Iu bearer. A user plane  32  includes nodes  12 ,  14 ,  16 ,  22 ,  24 , and  26  through which user traffic travels. Connections between the UEs  16  and  26  and the. RNCs  14  and  24 , respectively, are established by means of radio access bearers (note that NodeB connections have been omitted for clarity), while connections between the RNCs  14  and  24  and the MGWs  12  and  22 , respectively, are established through Iu bearers. Moreover, a connection between the MGWs  12  and  22  through a transit network is established by a core network (CN) bearer. An end-to-end connection between the UEs  16  and  26  may be established through the above-described bearers. 
     Referring to  FIG. 1 , when a call is established between the two UEs  16  and  26 , each MGW ( 12  and  22 ) generates a logical entity referred to as a “context” for processing the corresponding call, and provides “terminations” for each context, which define each external connection to a context. The terminations on each context define the endpoints associated with Iu or CN bearers comprising an end-to-end connection between the two UEs  16  and  26 . At this time, the MSC servers  10  and  20  must manage information regarding the bearer encoding (or codec) and network addresses and ports on each termination of the MGWs  12  and  22 , respectively, to establish and maintain the end-to-end bearer path. When both the ingress and egress paths of a MGW use the same codec, transcoding is not necessary at that MGW. If no media gateway in the bearer path performs transcoding so that the UEs transparently exchange codec frames along the bearer path, this condition is known as Transcoder Free Operation (TrFO). If one of the endpoints is required to transcode, for example, when transcoding to PCM for a UE in the PSTN, but no other media gateway in the bearer path performs transcoding, the condition is known as Remote Transcoder Operation (RTO). TrFO or RTO is desired to minimize the number of transcoding stages performed in the network, thus minimizing the bearer delay and voice quality degradation introduced by each transcoding stage. 
     The establishment of TrFO or RTO in the network requires the end-to-end negotiation of codec capabilities throughout the network to maximize the number of bearer segments using the same codec. SIP uses the Session Description Protocol (SDP) defined in IETF RFC 4566 and the corresponding SDP offer/answer negotiation procedures defined in IETF RFC 3264 to perform the end-to-end codec negotiation. 
     The voice packets then traverse through the originating MGW  12  on route toward the destination, for example, the UE  26  on the terminating side. In many cases, the originating MGW  12  does nothing more than convert the type of packetization used for the voice, i.e., no transcoding of voice to another format is necessary. For example, the originating MGW  12  performs framing conversion between AMR/IuUP/RTP on the RNC side and AMR/RTP/IP on the CN side. The newly reframed packets are then sent on toward the destination (which can be another MGW, an RNC, an IMS system, and so on). Thus, the originating MGW  12  is in bearer paths even when it does not do transcoding, which is one of its main functions. Using the originating MGW  12  solely for a reframing function is not efficient from a cost and voice quality viewpoint. 
     The solution to this problem involves eliminating the MGWs  12  and/or  22  from the voice path when transcoding is not involved. To accomplish this goal, the RNC framing and vocoding is changed to be the same (or a subset of) that used in the CN. The RNC supports AMR/RTP and CSD bearers as used in the CN; thus, the MGWs  12  and  22  are not needed for framing conversion when carrying these bearers through the network. Of course, the MGW  12  or  22  may be added and subtracted from the bearer path as necessary. In this regard, the MGW is only utilized for services such as hard handover, multiparty, call forwarding, tones and announcements, and Customized Applications for Mobile network Enhanced Logic (CAMEL). The MGW is subsequently removed after that service completes (per  FIG. 2 ). This solution applies equally as well to intra-MSC and inter-MSC calls. 
     IuCS is the circuit switched (CS) interface between the RNC and the MSC. This consists of several protocols and underlying ATM or IP hardware with an integrated management entity. With respect to the exemplary embodiment of the invention, the IuCS control plane protocol stack (3G TS 25.413) is not modified, i.e., it remains as RANAP/SCCP/M3UA/SCTP/IP/ETHERNET, while the IuCS user plane protocol stack changes to become RTP/UDP/IP/ETHERNET. 
     The exemplary embodiment of a call processing method  100  for improving network efficiency is set forth in  FIG. 2 . The method  100  includes the following steps. Initially, a call from a calling party originates at the MSC  10  on the originating side or is delivered to the MSC  20  on the terminating side ( 102 ). Next, the MSC  10  or  20  determines if an MGW  12  or  22  is needed immediately ( 104 ). If so, the MGW is allocated and inserted as part of the initial bearer path establishment ( 106 ). Else, the call bearer path is set up directly via the RNC  14  or  24  ( 108 ). In this regard, the MGW  12  or  22  is bypassed. 
     Once the call bearer path has been set up, any number of call events may occur ( 110 ) at a node that allow the removal of the MGW  12  or  22  that is already in the call bearer path or that require the insertion of the MGW  12  or  22  into the call bearer path. Such call events may include call answer, tones and announcements, call conferencing (e.g. 3-way or 6-way calling), resuming to a 2-party call, wire tapping (CALEA), handover to another RNC or to a GSM system, services such as call forwarding and multi-party conferencing, etc. This applies to intra-MSC and inter-MSC calls. 
     Thus, when a call event occurs ( 110 ) at a node in the UMTS  2 , a determination is made as to whether the originating MGW  12  (or the terminating MGW  22 ) is needed ( 112 ). If it is needed, then the appropriate MGW ( 12  or  22 ) is added to the call bearer path if it is not already present ( 114 ). When the MGW ( 12  or  22 ) is no longer needed, another call event occurs that allows it to be subtracted from the session. Of course, if the MGW ( 12  or  22 ) is not needed, then the triggering call event is processed without a MGW or the MGW is removed if currently being used ( 116 ). 
     To further explain the operation of the present invention, two message signaling diagrams are presented in  FIGS. 3 and 4 . The following two scenarios in  FIGS. 3 and 4  illustrate two common end-to-end call scenarios and possible methods for removing and inserting CN MGWs on an as-needed basis within the scenarios.  FIGS. 3 and 4  show call scenarios in which either the originating MSC or the terminating MSC, respectively, generates in-band call progress information towards the calling party. These figures show the establishment of the originating radio access bearer before the establishment of the core network (CN) bearer and establishment of the terminating radio access bearer either before or after establishment of the CN bearer. Other variations exist for radio access bearer establishment and can be used to achieve the same result. For example, some implementations could choose to establish the CN bearer (via INVITE) before establishing the bearer with the originating RNC. These FIGS also show the sending of the  180  Ringing response as the first SIP provisional response message. Other variations exist in the SIP message sequence (e.g., sending the  183  Call Progress response as the first SIP provisional response message) that can achieve the same result in supporting SDP offer/answer negotiation procedures and progressing the call. Additionally, the current flows in this description convey a sequential flow of messages. Reasonable implementations could use parallel message exchange to optimize and reduce the time needed to complete the signaling. One example of the latter case is performing MGW “Add” commands in parallel with the response to the  180  Ringing response. SIP and 3GPP messaging provide the building blocks with which an MSC server brings MGWs in and out of the bearer path as needed. These scenarios and procedures also apply to other network types such as CDMA. 
       FIG. 3  shows a message signaling diagram when a network subscriber desires to establish a session with another network participant and the call establishment signaling in the network determines that the originating MSC is to provide call progress information to the calling party. The procedure begins with a mobile origination triggered by a message from the UE  16  that is not shown in  FIG. 3 . After a Setup Message is received from an originating UE, the originating MSC  10  requests media resources to be allocated at the originating RNC  14 . Since the peer party IP address is not known at this point, the originating MSC  10  sets the IP address to 0.0.0.0 in the RAB assignment message ( 201 ). The originating RNC  14  responds to the RAB Assignment by establishing the radio access bearer and returning an RAB Assignment Complete message ( 202 ) to the originating MSC  10  with the IP address/port (R 1 ) assigned at the RNC  14  for the Iu bearer connection. While a radio access bearer assignment message exchange has occurred, the Iu bearer is not yet established at this point because the originating RNC  14  does not have the IP address/port of the Iu or CN bearer endpoint. 
     The call setup is then continued from the originating MSC  10  to the terminating MSC  20 . An INVITE request ( 203 ) is sent from the originating MSC  10  to the terminating MSC  20  with a list of supported codecs and CN bearer connection information to the remote end. It is desirable to prevent the flow of early media before all necessary resources have been allocated in the bearer path. The method shown in the flow to accomplish this is the use of the unspecified address of 0.0.0.0 for the originating IP address. Alternative methods, which are not illustrated in the flow, such as the use of SIP preconditions procedures (per IETF RFC 3312) or a separate media line for early media, are also possible. The terminating MSC  20  responds by sending a  100  Trying response ( 204 ). The terminating MSC  20  indicates that the called party&#39;s terminal device or phone is ringing by sending a  180  Ringing response ( 205 ) to the originating MSC  10 . The coding of the P-Early-Media header with the “inactive” parameter in this message indicates that the terminating MSC  20  is not providing call progress tones to the calling party and that it expects the originating MSC  10  to provide a ringback tone to the calling party. The ringback is a signaling tone indicating that a called party is being alerted. In parallel, the terminating MSC  20  initiates paging and authentication of the called mobile (which is not shown in the call flows). 
     The originating MSC determines that it needs to insert a MGW in the bearer path to provide the ringback call progress tone to the originator as indicated by the  180  Ringing response ( 205 ) to the initial INVITE request. The originating MSC does the following in parallel: responds to the  180  Ringing response with a PRACK request ( 206 ) (which supports a reliable exchange of the  180  Ringing provisional response per IETF RFC 3262), and requests an IP address/port assignment from MGW  12  by sending a H.248 Add command ( 208 ) to MGW  12 . The Add command includes the codec selected for the UE  16  during the RAB assignment procedure ( 201 ,  202 ) and the ringback call progress tone indicator (cg/rt) to be played to the originating UE  16 . The MGW  12  responds with the IP address/port (O 1 ) in the AddReply message ( 209 ). 
     Next, the originating MSC  10  completes the establishment of the Iu bearer connection between the allocated originating MGW  12  and the originating RNC  14 . It does this by sending an RAB assignment message ( 210 ) containing the MGW IP address/port (O 1 ) to the originating RNC  14 . The RNC  14  confirms the RAB Assignment message by returning an RAB assignment complete message ( 211 ) containing the same RNC  14  IP address/port (R 1 ). 
     Upon receiving the PRACK request ( 206 ) from the originating MSC  10 , the terminating MSC  20  responds with a  200  OK (PRACK) response ( 207 ) to complete the SIP PRACK sequence. 
     After the terminating MSC  20  receives a paging response from UE  26 , it allocates media resources on the terminating RNC  24 . It forwards a RAB Assignment message ( 212 ) to the terminating RNC  24  with a remote IP address set to 0.0.0.0 to establish the terminating radio access bearer. In response, the terminating RNC  24  sends an RAB Assignment Complete message ( 213 ) to the terminating MSC  20  with the terminating RNC IP address/port (R 2 ). The terminating MSC  20  sets up the terminating Iu connection without the remote IP address/port since it does not yet have a valid IP address/port of the originating side. The terminating MSC  20  then communicates the terminating RNC  24  media information to the originating MSC  10  by sending a  183  Session Progress response ( 214 ) to the originating MSC  10 . The  183  Session Progress response includes the selected codec (e.g., AMR) and the IP address/port of the terminating RNC  14  (R 2 ). The originating MSC  10  responds with another PRACK request ( 215 ). The terminating MSC  20  responds with a  200  OK (PRACK) response ( 216 ) to complete the SIP PRACK sequence. The originating MSC saves the remote IP address/port (R 2 ) and codec received in the  183  Session Progress response for future use. 
     When the mobile answers, the terminating MSC  20  sends a  200  OK (INVITE) response ( 217 ) to the originating MSC  10 , which responds with an ACK request ( 218 ). 
     The initial codec negotiation is completed, but an end-to-end media path has not yet been established. Furthermore, the call answer call event requires the cessation of ringback to the calling party, making the tone generation function of MGW  12  unnecessary. To fully establish an end-to-end media path, the originating MSC ( 10 ) sends a re-INVITE request ( 219 ) with the preferred Codec (e.g., AMR) received in step  214 . It also includes the RNC  12  IP address/port (R 1 ) in the re-INVITE request. 
     In this example, since a common codec is negotiated between the radio access bearer connection and the core network bearer path and the framing protocol is also the same (RFC 3267) between these two interfaces, neither codec transcoding nor framing conversion is necessary. Therefore, the originating MGW  12  is no longer needed and the originating MSC  10  can remove it from the call bearer path. To remove the originating MGW  12  from the bearer path, the originating MSC  10  sends a Subtract message ( 220 ) to the originating MGW  12  ( 220 ), which responds with a SubtractReply message ( 224 ). In parallel, the originating MSC  10  assumes that the remote CN bearer address/port (R 2 ) will not change and sends an RAB assignment message ( 223 ) to the originating RNC  14  containing the terminating RNC IP address/port (R 2 ). The originating RNC  14  responds with an RAB Assignment Complete message ( 225 ). If the remote CN bearer address/port returned in the subsequent  200  OK response ( 226 ) is not the assumed value (R 2 ) then the originating MSC  10  can update the originating RNC  14  with another RAB assignment message (not shown). 
     When the re-INVITE request is received at the terminating MSC  20 , the terminating MSC  20  sends an RAB assignment message to the terminating RNC  24  ( 221 ) with the IP address/port of the originating RNC (R 1 ) received in the re-INVITE request, thus connecting the bearer path from RNC  12  to RNC  14 . RNC  14  confirms receipt by returning an RAB Assignment Complete message ( 222 ). 
     Finally, the terminating MSC  20  sends a  200  OK (re-INVITE) response ( 226 ) to the originating MSC  10  with the IP address/port of the terminating RNC (R 2 ), and the originating MSC  10  responds with an ACK request ( 227 ). The media now flows between the RNC  14  and RNC  24  without any intervening MGWs in the path after call answer. 
     It was shown in  FIG. 3  that a media gateway may be necessary during call establishment in order to provide the ringing call progress tone. However, one option to optimize this method even more is that the RNC could be instructed to provide the ringback call progress tone. In this way, the initial call setup does not use a MGW. Additionally, the RNC could support and be instructed to play common announcements such as when the terminating party does not answer or is busy. The additional signaling required for the RNC to provide call progress tones is not shown. 
     If at any time a call event occurs at either MSC  10  or MSC  20  for which it would be desirable to insert or remove a local MGW, the MSC can initiate a standard re-INVITE transaction by sending the re-INVITE request to its peer MSC. The re-INVITE request will include SDP with codec and address information for the desired local bearer configuration (with or without the local MGW). This is a standard mid-call SDP offer/answer scenario so no figure is included to describe the message flow. 
       FIG. 4  represents a message signaling diagram when the call establishment signaling in the network determines that the terminating MSC is to provide the call progress information to the calling party. The procedure begins with a mobile origination triggered by a message from the UE  16  that is not shown in  FIG. 4 . The initial message flows for  FIG. 4  are identical to  FIG. 3  through the initial INVITE request. After a Setup message is received from an originating UE, an RAB assignment is sent from the originating MSC  10  to the originating RNC  14  ( 301 ) with the remote IP address set to 0.0.0.0 to establish the radio access bearer. The originating RNC  14  responds by sending an RAB Assignment Complete message ( 302 ) with the IP address/port R 1  assigned at the RNC to be used for this session. An INVITE request ( 303 ) is then sent from the originating MSC  10  to the terminating MSC  20  with the codec list and IP address set to 0.0.0.0. 
     When the terminating MSC  20  receives the INVITE request, it responds with a  100  Trying response ( 304 ). Also, the terminating MSC  20 , because it determines that it will provide the call progress tone, needs to insert a MGW in the media path to do so. Terminating MSC  20  sends an Add request ( 305 ) with the remote IP address/port received in the INVITE request, the selected codec and the ringback call progress tone indicator (cg/rt) to the terminating MGW  22 , though the call progress media can not flow until the bearer path is fully established with an exchange of valid address information. The selected codec is usually the first codec in the offered list, if supported by the terminating MGW  22 , to avoid any transcoding at the originating MGW  12 . The terminating MGW  22  responds with an AddReply message ( 306 ) including the IP address/port (T 1 ) assigned by the MGW  22 . The terminating MSC  20  then sends a  180  Ringing response ( 307 ) to the originating MSC  10  with the terminating MGW IP address/port (T 1 ) and selected codec (e.g., AMR). It also includes the P-Early-Media header coded with “sendonly” indicating that the terminating MSC  20  is providing the call progress tone. In parallel, the terminating MSC  20  initiates paging and authentication of the called mobile (which is not shown in the call flows). 
     The originating MSC  10  needs to establish the bearer path between RNC  14  and the remote network endpoint (MGW  22 ) and therefore sends an RAB assignment ( 308 ) to the originating RNC  14  with remote IP address/port set to T 1 . RNC  14  returns an RAB Assignment Complete message ( 309 ) to the originating MSC  10 . The originating MSC  10  sends a PRACK request ( 310 ) to the terminating MSC  20  with IP address/port of originating RNC  14  (R 1 ) and the selected codec. 
     The terminating MSC  20  then sends a H.248 Modify message ( 311 ) to the terminating MGW  22  with remote IP address/port of R 1 . MGW  22  responds with a ModifyReply message ( 312 ). In parallel with  311  the terminating MSC  20  sends a  200  OK (PRACK) response ( 313 ) to the originating MSC  10 . At this point, the bearer path is connected between the originating RNC  14  and the terminating MGW  22  to provide call progress tone to the originating UE  16  using the selected codec (e.g., AMR). Therefore, in this scenario no transcoding is needed so far. 
     Upon receiving a page response message (not shown) from the terminating UE  26 , the terminating MSC  20  establishes the radio access bearer connection between the terminating RNC  24  and the terminating UE  26 . (The page response provides an indication to the terminating MSC which RNC to use for the call.) MSC  20  sends an RAB Assignment message ( 314 ) with an unspecified address (0.0.0.0) to the terminating RNC  24  to prevent any backward media flow from the terminating RNC  24 . The terminating RNC  24  responds with an RAB Assignment Complete message ( 315 ) including the RNC IP address/port R 2  to be used for this call. 
     When the terminating UE  26  answers, the terminating MSC  20  establishes the bearer path directly between the originating and terminating RNCs by sending an UPDATE request ( 316 ) to the originating MSC  10  with the RNC  24  IP address/port R 2  and the selected codec (e.g., AMR). 
     Because no transcoding is required (as AMR was negotiated as a common codec) and the framing protocol (RFC 3267) is the same on the originating RNC  14  and on the terminating RNC  24  it is possible to remove all MGWs from the bearer path between the originating RNC  14  and the terminating RNC  24 . Since the call progress tone is no longer needed at this point, the terminating MGW  22  may now be removed. The terminating MSC  20  sends a Subtract message ( 317 ) to the terminating MGW  22 , which responds with a SubtractReply message ( 318 ) and removes itself from the bearer path. 
     Upon receiving the UPDATE request, the originating MSC  10  sends an RAB Assignment message ( 319 ) with the IP address/port R 2  to the originating RNC  14 , which responds with an RAB Assignment Complete message ( 320 ) with R 1  as the IP address/port to use on RNC  14 . The originating MSC  10  then sends a  200  OK (UPDATE) response ( 321 ) to the terminating MSC  20  with the IP address/port of RNC  14  (R 1 ). 
     The terminating MSC  20  then sends an RAB Assignment message ( 322 ) to the terminating RNC  24  with the far end IP address/port R 1 . The terminating RNC  24  responds with an RAB Assignment Complete message ( 323 ). This completes the establishment of the end-to-end bearer path. The terminating MSC  20  sends a  200  OK (INVITE) response ( 324 ) to the originating MSC  10 , which responds with an ACK request ( 325 ) to complete call setup. At this point media flows directly between RNC 1  and RNC  2 . Since no transcoding is required, a MGW is not needed in the path. 
     As in the scenario of  FIG. 3 , if at any time a call event occurs at either MSC  10  or MSC  20  for which it would be desirable to insert or remove a local MGW, the MSC can initiate a standard re-INVITE transaction by sending the re-INVITE request to its peer MSC. The re-INVITE request will include SDP with codec and address information for the desired local bearer configuration (with or without the local MGW). This is a standard mid-call SDP offer/answer scenario so no figure is included to describe the message flow. 
     This invention does not apply to intermediate MSCs and MSCs that interface to other networks (e.g. tandems, Gateway MSCs) since they do not involve a radio access network. Insertion and removal of MGWs at call setup and as the result of call events at these nodes (not shown in any figures) can be realized using SIP third party call control procedures as described in prior art. 
     The detailed description presented above is represented largely in terms of processes and symbolic representations of operations performed by conventional computer components, including a central processing unit (CPU), memory storage devices for the CPU, and connected display devices. These operations include the manipulation of data bits by the CPU, and the maintenance of these bits within data structures that reside in one or more of the memory storage devices. Such data structures impose a physical organization upon the collection of data bits stored within computer memory and represent specific electrical or magnetic elements. These symbolic representations are the means used by those skilled in the art of computer programming and computer construction to most effectively convey teachings and discoveries to others skilled in the art. 
     For the purposes of this discussion, a process is generally conceived to be a sequence of computer-executed steps leading to a desired result. These steps generally require physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical, magnetic, or optical signals capable of being stored, transferred, combined, compared, or otherwise manipulated. It is conventional for those skilled in the art to refer to these signals as bits, values, elements, symbols, characters, terms, objects, numbers, records, files or the like. It should be kept in mind, however, that these and similar terms should be associated with appropriate physical quantities for computer operations, and that these terms are merely conventional labels applied to physical quantities that exist within and during operation of the computer. 
     In addition, it should be understood that the programs, processes, methods, etc. described herein are not related or limited to any particular computer or apparatus. Rather, various types of general purpose machines may be used with programs constructed in accordance with the teachings described herein. Similarly, it may prove advantageous to construct specialized apparatus to perform the method steps described herein by way of dedicated computer systems with hard-wired logic or programs stored in nonvolatile memory, such as read only memory. 
     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.