KR100793433B1 - Method and system for controlling call mode of vocoder - Google Patents

Abstract

A method and a system for controlling vocoder communication mode are provided to quickly transfer to a transcoding mode in case that a hard handoff occurs between MSCs(Mobile Switching Centers) while a calling terminal is executing a call with a called terminal in vocoder communication mode. An originating BSC(Base Station Controller), receiving an outgoing call request from a calling terminal, transmits it to an originating MSC(310). The originating MSC transmits a location information request message to an HLR in order to obtain the location information of a called mobile terminal corresponding to the outgoing call(315). The HLR transmits a router request message to a serving MSC, and receives a router response message from the serving MSC. Using the router response message, the HLR transmits a location information response message to the originating MSC(320). The originating MSC analyzes the received response message, and judges whether the communication call between the calling and called terminals is linked with an intelligent network service or executing a data call(325). In case the communication call is neither linked with an intelligent network service or nor executing a data call, the originating MSC transmits an ISUP IAM(Initial Address Message), which contains information to request transfer from vocoder communication to bypass mode, to a serving MSC(330).

Description

Vocoder communication mode control method and system therefor {Method and system for controlling call mode of vocoder}

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram schematically showing a configuration of a general imaging device.

2 is a block diagram illustrating a schematic functional configuration of a system for controlling a vocoder bypass or transcoding mode when a hard handoff occurs according to an exemplary embodiment of the present invention.

3 is a flowchart illustrating a process of bypassing a voice call when an originating exchange exchange performs a mobile foreign call according to an embodiment of the present invention.

4 is a diagram illustrating parameters of an ISUP message according to a preferred embodiment of the present invention.

5 is a flowchart illustrating a method of controlling a vocoder communication mode when a hard handoff occurs in an exchange period according to a movement of a terminal according to an exemplary embodiment of the present invention.

<Explanation of symbols for the main parts of the drawings>

210a, 210b: terminal

220a, 220b: base station

230a, 230b: base station controller

240: exchanger

250: home location register

The present invention relates to a mobile communication system, and more particularly, to a vocoder communication mode control method and system capable of efficiently controlling a communication mode of a vocoder in an exchange.

In general, in a mobile communication system, a base station controller includes a vocoder that modulates or demodulates a voice during a call between a calling terminal and a called terminal. The vocoder included in the base station controller is generally used in bypass mode and transcoding mode.

In general, when the call between the mobile communication terminal and the wired terminal is operated in the transcoding mode, and the call between the mobile communication terminal is operated in the bypass mode. That is, in the case of using the same vocoder, it is not necessary to vocode voice signals separately, and thus generally operates in a bypass mode due to the nature of a call between the same mobile communication terminals. As such, there is an advantage in that the sound quality can be improved by removing the modulation error and the demodulation error generated during the modulation or demodulation of the voice due to the operation in the bypass mode.

1 is a flowchart illustrating a bypass mode switching process of a vocoder according to the related art.

Referring to FIG. 1, a vocoder bypass mode switching process according to the prior art will be described. First, when a caller attempts to access a call through the calling terminal 10a, the calling terminal 10a is connected to the base station 10a. A call connection request is attempted to the originating exchange 40a via 20a and base station controller 30a. The calling side exchange 40a transmits the ringing tone signal "alerting with info" to the calling terminal 10a, and transmits the paging signal "alerting with info" to the calling terminal 10a.

That is, when the calling exchange 40a transmits an alerting signal to the calling party communication control processor 33a, the CCP 33a receives the alerting signal Msg_Alert to transmit it to the selector 32a. do. The selector 32a transmits the "Alerting with info" signal to the calling terminal 10a.

On the other hand, when the called party responds to the "alerting with info" signal transmitted from the selector 32b through the called terminal 10b, the called terminal 10b transmits a connection command (connect_order) requesting a connection to the selector 32b. Done. After receiving the connection command, the selector 32b transmits a bypass command to the vocoder 31b to switch the operation mode of the vocoder 31b to the bypass mode to switch the operation mode of the vocoder to the bypass mode, and then the CCP. The connection message Msg_connect is transmitted to 32b.

Thereafter, the destination CCP 33b transmits a connection signal to the switch 40, and the switch 40 transmits a connection signal to the caller side CCP 33a, and the caller side CCP 33a receiving the corresponding connection signal selects the connection message. To 32a.

The selector 32a receiving the access message transmits a bypass command for the caller side vocoder 31a to switch to the bypass mode. Upon receiving the bypass command, the vocoder 31a switches the operation mode to the bypass mode and transmits a voice call to the called terminal 10b.

This vocoder bypass function is performed through the base station and the base station controller, and the service option information of two or more exchanges in which an exchange period handoff occurs or handles a communication call between an incoming and outgoing terminal (for example, vocoding type) In this different case, there is a problem in that switching to the vocoder bypass mode or the transcoding mode cannot be efficiently performed.

Currently, communication in a mobile communication system has a problem in that a loss of a voice signal due to voice modulation or demodulation occurs due to processing in a vocoder transcoding mode.

Accordingly, an object of the present invention to solve the above-mentioned problem is to quickly switch to transcoding mode when a hard handoff occurs in an arbitrary exchange period while a call between a called and originating terminal is communicating in vocoder bypass mode. To provide a vocoder communication mode control method and a system thereof.

Another object of the present invention is to provide a method and a system for controlling a vocoder communication mode that can set a vocoder communication mode for a communication call via two or more exchanges at an exchange.

Still another object of the present invention is to provide a method and a system for controlling a vocoder communication mode that can be processed in a bypass mode when service option information is the same for a communication call processed in a vocoder transcoding mode due to the characteristics of a mobile communication system.

Other objects of the present invention will be easily understood through the description of the following examples.

In order to achieve the above object, according to an aspect of the present invention, a method for controlling the vocoder communication mode by the calling exchange in the mobile communication system for processing a communication call between the calling terminal and the called terminal.

According to an embodiment of the present invention, in a mobile communication system for processing a communication call between a calling terminal and a called terminal, in a method of controlling a vocoder communication mode by a calling side exchange, the calling terminal and the called terminal according to a communication call Receiving respective service option information from the terminals, wherein the service option information is a voice communication call compression scheme; Comparing the received respective service option information; If the service option information is the same, sending a switch request message for switching the vocoder communication mode to the bypass mode to the called side exchange; And transmitting a setting instruction message for setting a vocoder communication mode to a bypass mode to a base station controller corresponding to the originating terminal when receiving a switch completion message from the called exchange in response to the switch request message. Vocoder control method may be provided.

According to another aspect of the present invention, there is provided a mobile communication system including an originating exchange that controls a vocoder communication mode in a mobile communication system that processes a communication call between an originating terminal and a terminating terminal.

According to an embodiment of the present invention, in a mobile communication system for processing a communication call between a calling terminal and a called terminal, receiving respective service option information from the terminals according to the communication call between the calling terminal and the called terminal. After comparing the received service option information with each other, if the service option information is the same, a switch request message for switching a vocoder communication mode to a bypass mode is transmitted to a called side exchange, and the corresponding switch request message corresponds to the switch request message. Upon receiving a switch completion message from the called side exchange, a mobile communication system including a calling side exchange transmitting a setup instruction message for setting the vocoder communication mode to the bypass mode to a base station controller corresponding to the calling terminal may be provided. have.

As the invention allows for various changes and numerous embodiments, particular embodiments will be illustrated in the drawings and described in detail in the written description. However, this is not intended to limit the present invention to specific embodiments, it should be understood to include all modifications, equivalents, and substitutes included in the spirit and scope of the present invention. In describing the drawings, similar reference numerals are used for similar elements.

Hereinafter, with reference to the accompanying drawings, it will be described in detail a preferred embodiment of the present invention. Hereinafter, the same reference numerals are used for the same components in the drawings, and duplicate descriptions of the same components are omitted. In addition, in describing the present invention, when it is determined that the detailed description of the related known technology may unnecessarily obscure the gist of the present invention, the detailed description thereof will be omitted.

2 is a block diagram illustrating a schematic functional configuration of a system for controlling a vocoder bypass or transcoding mode when a hard handoff occurs according to an exemplary embodiment of the present invention.

Referring to FIG. 2, the system 200 according to the present invention includes terminals 210a and 210b, base stations 220a and 220b, base station controllers 230a and 230b, an exchange 240, and a home location register 250. It is configured by.

The terminals 210a and 210b are terminals capable of receiving a mobile communication service (for example, an incoming call or an outgoing call) through a communication network (for example, a CDMA network or a W-CDMA network). , PDA, smart phone, MBS (Mobile Broadcast System) can include all.

The base stations 220a and 220b are network termination devices directly connected to the subscriber station, and perform baseband signal processing, wired / wireless conversion, and transmission and / or reception of radio signals.

The base station controllers 230a and 230b are located between the base stations 220a and 220b and the switch 240 to perform management and control functions of the base stations 220a and 220b. That is, the base station controllers 230a and 230b may allocate and release radio channels for each terminal, control transmission power of the terminal and the base station, perform soft handoff between cells and determine hard handoff, transcoding and vocoding functions, handoff and GPS clock distribution for signal processing, operation and maintenance of the base station can be performed.

In addition, the base station controllers 230a and 230b may set the communication mode of the vocoder to either one of the bypass mode and the transcoding mode according to the request of the switch 240, and process the voice call in the corresponding communication mode. .

In the present specification, the bypass mode is defined as transmitting voice signals transmitted and received between terminals without additional modulation or demodulation in a mobile communication network.

In addition, the transcoding mode is transmitted and received by the terminals when the voice signal compression schemes between the devices included in the mobile communication network (for example, the called base station controller 220b and the calling base station controller 220a) are different. It is defined as additionally modulating and / or demodulating the speech signal.

A mobile switching center (MSC) 240 is a wired / wireless hybrid digital switching center that provides wired and mobile communication functions, and provides basic and additional service processing, incoming or / and outgoing call processing, and location of a terminal for communication of the terminal. It is responsible for registration and handoff procedure processing and interworking with other communication networks.

In addition, when the terminating terminal 210b or the originating terminal 210a performs a hard handoff, the switch 240 determines the communication mode of the vocoder and issues a mode switching command to the corresponding mode, the base station controllers 230a and 230b. And / or to a target exchange targeted for hard handoff.

For example, suppose that a communication call is established between terminals and a call is made in vocoder bypass mode.

For convenience, an exchange interworking with the originating terminal 210a to process a communication call will be referred to as an originating side exchange, and an exchange interworking with the terminating terminal 210b to process a communication call will be referred to as a called side exchange.

At this time, when the called exchange determines that the service option information received from the calling exchange is the same as the service option information received from the calling terminal 210a, the called exchange switches the communication mode of the vocoder into the bypass mode by the calling exchange. Information instructing to operate may be inserted into an ISUP message and transmitted to the calling exchange. Here, the service option information may be coding scheme information of the communication call.

In addition, the called side exchange is a mobile other station passing through two or more exchanges and the communication call between the calling terminal 210a and the called terminal 210b is bypassed by the vocoder's communication mode when it is determined that the service option information of each exchange is the same. Information indicating to operate in mode may be inserted into an ISUP message and transmitted to the calling exchange. In this case, if the service exchanger information received from the calling exchange is different, the called exchange may insert information indicating that the communication mode of the vocoder operates in the transcoding mode into the ISUP message and transmit the information to the calling exchange.

As another example, when the called exchange exchanges the service option information received from the calling terminal 210a and the called terminal 210b, the information indicating to the calling exchange to operate the vocoder communication mode in the bypass mode is inserted into the ISUP message. Can be sent to the calling exchange. If it is determined that the service option information is different, the called exchange may transmit an ISUP message to which the information regarding the vocoder communication mode setting is not inserted, to the calling exchange. Therefore, since the calling exchange does not include information on the vocoder communication mode setting in the ISUP message received from the called exchange, the calling exchange may instruct the calling base station controller corresponding to the calling exchange to operate the vocoder communication mode in the transcoding mode. Can be.

In addition, when a hard handoff occurs at any exchange while the vocoder communication mode of the calling exchange and the called exchange is operating in the bypass mode, the exchange performing the hard handoff is performed by the hard handoff. Prior to performing the off, the information indicating the transition to the transcoding mode may be inserted into the ISUP message and transmitted to the target exchange. This will be described in detail with reference to the accompanying drawings below.

The home location register (HLR) 250 is a database that stores information on subscribers of the mobile communication service, and all data on the subscriber such as call origination, location registration, and call reception of the mobile communication service subscription terminal. It stores the necessary routing information for the call to the terminal (210a, 210b). For example, the home location register 250 is N0. 7 Common-line signaling may be used to connect data stored for a subscriber to another device (eg, a visitor location register (VLR)). In addition, the home location register 250 is another device in the communication network (for example, exchange 240, operation maintenance device (not shown), authentication sensor (not shown), billing device and short message center (not shown), etc.) ) Is performed.

3 is a flowchart illustrating a process of bypassing a voice call when a calling exchange is performing a mobile station call according to an exemplary embodiment of the present invention, and FIG. 4 illustrates parameters of an ISUP message according to an exemplary embodiment of the present invention. The illustrated figure. Hereinafter, a method in which the calling side exchange 302 switches the vocoder communication mode in response to a request for switching the communication mode of the vocoder to either the bypass mode or the transcoding mode by the called side exchange 303 will be described. Shall be.

In step 310, the originating base station controller 301 receives an outgoing call request from the originating terminal and transmits it to the originating exchange 302.

In step 315, the originating exchange 302 transmits a location information request message (locreq) home location register 250 of the called terminal according to the originating signal received through the originating base station controller 301.

For example, the calling side exchange 302 may transmit the location information request message of the called terminal corresponding to the telephone number information of the called terminal and the telephone number information to the home location register 250 in response to an outgoing call from the calling terminal. have.

Accordingly, the home location register 250 transmits a router request message (Routreq) requesting the subscriber location of the called terminal to the called side exchange 303 interworking with the called terminal corresponding to the originating terminal. 250 may receive from the destination exchange 303 a response message (Routreq [TLDN], hereinafter referred to as " router response message " for convenience) corresponding to the router request message from the destination exchange 303. .

In step 320, the home location registerer 250 uses a router response message received through the destination exchange 303 to respond to a response message (locreq [TLDN, serving MSCID) requested by the originating exchange 302. ], Hereinafter referred to as " location response message " for convenience, is transmitted to the calling exchange 302.

Here, the location response message may include identification information of the exchange performing a communication call corresponding to the called terminal, and service option information of the called terminal.

In step 325, the originating exchange 302 analyzes the location response message received from the home location register 250 to determine whether the communication call between the originating and destination terminals is linked with the intelligent network service or performs a data call.

If it is determined that the data call is not performed without interworking with the intelligent network service, in step 330, the source exchange 302 sends an ISUP IAM (Initial Address Message) message with information requesting to switch the vocoder communication mode to the bypass mode. Insert it into the destination side and send it to the destination exchange (303).

For example, the source exchange 302 may set the vocoder communication mode in the user to user information (UUI) parameter of the ISUP IAM message to a value corresponding to the bypass mode, and set service option information received at the calling terminal. It may transmit to the called side exchange 303. Here, the UUI parameters in the ISUP IAM message and the ISUP ANM (Answer Message) message are illustrated in FIG. 4. In this specification, it is assumed that the exchange 302 transmits the switching information of the vocoder communication mode to another exchange or the base station controller using an ISUP IAM message or an ISUP ANM message. Of course, when transmitting the setting information of the vocoder communication mode using another message, it is natural that parameters inserted to set the vocoder communication mode in the message may be different. In the following description, it is assumed that the vocoder communication mode is inserted as a UUI parameter in an ISUP message.

However, if it is determined that the communication call between the originating and terminating terminal is interworking with the intelligent network service or performs a data call, in step 335, the originating exchange 302 sends the ISUP IAM message not including the vocoder communication mode to the called side exchange 303. send. In addition, even if the vocoder communication mode for the mobile other station is forced on-demand to the first information (e.g., Off) or is not a mobile foreign call, the originating exchange 302 May transmit to the destination exchange 303 an ISUP IAM message in which the UUI parameter into which the vocoder communication mode switching information is inserted is not set.

In step 340, the destination exchange 303 analyzes the ISUP IAM message received from the source exchange 302, and the vocoder communication mode is the bypass mode in the ISUP IAM message, and the service option information is received from the destination terminal. Determine whether or not

If the set value of the vocoder communication mode is a set value corresponding to the bypass mode, and it is determined that the set service option information is the same as the service option information received from the called terminal, the called side exchange 303 in step 345 sends an ISUP ANM message. Set the vocoder communication mode in the UUI parameter to a value corresponding to the bypass mode and send it to the calling exchange 302.

In addition, the called side exchange 303 may transmit a message including a setting value for setting the vocoder communication mode to the called base station controller 304 which is interlocked for communication call processing of the called terminal to switch the vocoder communication mode. have.

However, if the vocoder communication mode is not the bypass mode, and it is determined that the set service option information and the service option information received from the destination terminal are different from each other, in step 350, the called side exchange 303 does not include the vocoder communication mode. Or an ISUP ANM message to the originating exchange 302 and the called base station controller 304.

In step 355, the calling side exchange 302 analyzes the ISUP ANM message received from the called side exchange 303, extracts the vocoder communication mode, and uses the extracted vocoder communication mode, the calling base station controller linked with the calling terminal ( 301 request to switch the vocoder communication mode.

For example, if the vocoder communication mode in the UUI parameter is the bypass mode, the originating exchange 302 may send a message to the originating base station controller 301 containing information indicating switching to the bypass mode. On the other hand, if the setting of the vocoder communication mode in the UUI parameter is a transcoding mode, the originating exchange 302 may transmit a message to the originating base station controller 301 containing information indicating the transition to the transcoding mode. Of course.

As such, when a hard handoff occurs during the exchange period while the call between the calling terminal and the called terminal is communicating in the vocoder bypass mode, the vocoder communication mode may be switched to the transcoding mode. This will be described in detail with reference to the accompanying drawings below.

5 is a flowchart illustrating a method of controlling a vocoder communication mode when a hard handoff occurs in an exchange period according to a movement of a terminal according to an exemplary embodiment of the present invention. Hereinafter, when a hard handoff occurs between the source exchanger 501 and the target exchange 503 in response to the movement of the source terminal 210a during communication between the source terminal 210a and the destination terminal 210b, the vocoder correspondingly corresponds to the case where a hard handoff occurs. A process of switching the communication mode to the transcoding mode will be described. Since the hard handoff method of the exchange period is obvious to those skilled in the art, a separate description thereof will be omitted.

If the originating exchange 501 generates a hard handoff with the target exchange 503 in step 510, the originating exchange 501 instructs the target exchange 503 to switch the vocoder communication mode to the transcoding mode. Is inserted into the FACDIR2 message (facilities directive) and sent to the target exchange 503.

For example, the source exchange 501 may transmit the corresponding FACDIR2 message to the target exchange 503 by setting the vocoder communication mode to a value corresponding to the transcoding mode in the FACDIR2 message.

In the following description, a hard handoff is generated in the originating exchange 501, but the same applies to other exchanges (for example, the destination exchange 502 or another exchange of the first hard handoff). The duplicate description will be omitted.

In step 515 the destination exchange 503 sends a response message to the originating exchange 501 corresponding to the FACDIR2 message received from the originating exchange 501. The target exchange 503 then sends a command to the target base station controller 504 connected in correspondence with the target exchange 503 to instruct the switch to the transcoding mode of the vocoder communication mode. The target exchange 503 interoperates with the target base station controller 504 in response to the request for switching the vocoder communication mode from the source exchange 501 to the transcoding mode in the transcoding mode and the terminating terminal 210b. Can handle communication between them.

For example, target exchange 503 requests handoff to target base station controller 504 and receives a corresponding response message from target base station controller 504.

In operation 520, the target exchange 503 receives a message corresponding to the handoff completion from the target base station controller 504.

Accordingly, in step 525, the target exchange 503 sends a response message corresponding to the handoff completion to the originating exchange 501.

In step 530, the originating exchange 501 receives a response message corresponding to the completion of the handoff from the target exchange 503, and inserts a request to switch the vocoder communication mode to the transcoding mode into the UUI parameter in the ISUP UUI message. The call is sent to the destination exchange 502.

For example, the source exchange 501 sets the UUI parameter to the identification information of the target exchange 503, the vocoder transcoding mode, and the service option information received from the source terminal 210a to set the ISUP UUI message to the destination side exchange ( 502). The destination exchange 502 receives the ISUP UUI message from the originating exchange 501, which indicates the changeover of the vocoder communication mode to the transcoding mode, while processing the communication call in the bypass mode. May transmit a message indicating to switch to the transcoding mode to the called base station controller.

As described above, by providing a method and a system for controlling the vocoder communication mode according to the present invention, when a hard handoff occurs in an arbitrary exchange period while a call between a called and originating terminal is communicating in vocoder bypass mode, This has the effect of switching to transcoding mode.

In addition, the present invention also has the effect that it is possible to set the vocoder communication mode for a communication call via two or more exchanges in the exchange.

In addition, the present invention has an effect of reducing the signal loss due to modulation and demodulation by processing in the bypass mode when the service option information is the same for the communication call processed in the vocoder transcoding mode due to the characteristics of the mobile communication system.

Although the above has been described with reference to a preferred embodiment of the present invention, those skilled in the art to which the present invention pertains without departing from the spirit and scope of the present invention as set forth in the claims below It will be appreciated that modifications and variations can be made.

Claims (11)

Claims [1] A method of controlling a vocoder communication mode in an originating exchange in a mobile communication system for processing communication calls between an originating terminal and a terminating terminal, Receiving respective service option information from the terminals according to a communication call between the calling terminal and the called terminal, wherein the service option information is a voice communication call compression scheme; Comparing the received respective service option information; If the service option information is the same, sending a switch request message for switching the vocoder communication mode to the bypass mode to the called side exchange; And And receiving a switch completion message from the called side exchange in response to the switch request message, transmitting a setting instruction message for setting a vocoder communication mode to a bypass mode to a base station controller corresponding to the calling terminal. Control method. The method of claim 1, The called party exchange receiving the switching request message extracts the service option information of the calling terminal included in the switching request message, and determines whether the same is the same as the service option information received from the called terminal. Is equal to, transmits the switching completion message corresponding to the switching of the vocoder communication mode to the bypass mode to the calling exchange, and sets the vocoder communication mode to the bypass mode to the base station controller corresponding to the called terminal. Vocoder control method characterized in that the. The method of claim 2, If each of the service option information is different, the called side exchange transmits a switching completion message to the calling side exchange, which does not include information corresponding to the vocoder communication mode, And the originating exchange receiving the switch completion message that does not include the information corresponding to the communication mode, instructing the setting of the vocoder communication mode to the transcoding mode. The method of claim 1, Sending a switch message to a target exchange in a transcoding mode, when the exchange duration handoff to the target exchange occurs; Receiving a completion response message corresponding to the transition message to the transcoding mode from the target exchange; And And if it is determined that the handoff is completed by analyzing the received completion response message, transmitting a request message for switching a vocoder communication mode to a transcoding mode to the called exchange. The method of claim 4, wherein And the receiving side exchange receiving the request for switching to the transcoding mode instructs the base station controller corresponding to the terminal performing the communication call to set the vocoder communication mode to the transcoding mode. The method of claim 4, wherein The target exchange receiving the switch message from the vocoder communication mode to the transcoding mode requests to perform a handoff to a target base station controller corresponding to the target exchange, receives a handoff complete message from the target base station controller, and handoffs And transmitting the switch response message including completion information to the originating exchange. In a mobile communication system for processing communication calls between a calling terminal and a called terminal, By receiving respective service option information from the terminals according to the communication call between the calling terminal and the called terminal, and comparing the received respective service option information, if the service option information is the same, bypass the vocoder communication mode. When a switch request message for switching to the mode is transmitted to the called exchange, and a switch completion message is received from the called exchange according to the switch request message, the vocoder communication mode is bypassed to the base station controller corresponding to the calling terminal. A mobile communication system comprising an originating exchange that transmits a configuration indication message for setting to a mode. The method of claim 7, wherein The service option information includes the communication call compression method information. The method of claim 7, wherein The called party exchange receiving the switching request message extracts the service option information of the calling terminal included in the switching request message, and determines whether the same is the same as the service option information received from the called terminal. Is equal to, transmits the switching completion message corresponding to the switching of the vocoder communication mode to the bypass mode to the calling exchange, and sets the vocoder communication mode to the bypass mode to the base station controller corresponding to the called terminal. Mobile communication system, characterized in that for indicating. The method of claim 9, If each of the service option information is different, the called side exchange transmits a switching completion message not including the vocoder communication mode to the calling side exchange, and sends the switching completion message not including information corresponding to the communication mode. And the originating exchange receives the indication of setting the vocoder communication mode to the transcoding mode. The method of claim 7, wherein If an exchange period handoff to a target exchange occurs, the originating exchange sends a switch message for switching the vocoder communication mode to a transcoding mode to the target exchange, and completes corresponding to the switch message from the target exchange. And if it is determined that the handoff is completed after receiving and analyzing the response message, transmitting a switching request message for switching a vocoder communication mode to a transcoding mode to the called exchange.

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