Abstract:
A voice packet forwarding apparatus and method is provided in a digital communication system including a switched vocoder module to directly pass a voice packet received from a packet terminal to a digital communication network or a PCM signal decoded from the voice packet to the digital communication network. The switched vocoder module is also provided to directly pass a voice packet received from the digital communication network to the packet terminal or transmit a voice packet coded from a PCM signal received from the digital communication network. In the presence of additional data to be transmitted to the packet terminal, a data inserter is provided to insert the additional data in the voice packet received from the switched vocoder module and transmit the voice packet with the additional data to the packet terminal. A controller is provided to control the switched vocoder module and the data inserter.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. §119(a) of Korean Patent Application No. 10-2005-0006883 entitled “Apparatus and Method for Forwarding Voice Packet in a Digital Communication System” filed in the Korean Intellectual Property Office on Jan. 25, 2005, the entire disclosure of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a digital communication system. In particular, the present invention relates to an apparatus and method for forwarding voice packets over a network. 
     2. Description of the Related Art 
     A voice coder, namely a vocoder is used to reduce the amount of data taken to transmit information in digital voice communications. The vocoder, which is a combination of a voice encoder and a voice decoder, exists between a terminal and a communication system. The voice encoder converts an input Pulse Code Modulation (PCM) voice signal to a very small-size voice packet by using a predetermined coding algorithm. The voice packet is delivered to a destination through the communication system and the voice decoder recovers the voice packet to the original PCM voice signal by using a predetermined algorithm. Under this typical communication environment, voice communications are conducted in the order of input-coding-transmission-decoding-output. Coding-decoding can be repeated many times in a particular communication environment. 
     Many types of vocoders are available to digital mobile communication systems like Voice over Internet Protocol (VoIP), but different vocoders are not compatible with each other. Some vocoders support generation of voice packets at a plurality of data rates. These vocoders are called variable-rate vocoders. Typical examples of the variable-rate vocoders are Qualcomm Code Excited Linear Prediction (QCELP) and Enhanced Variable Rate Codec (EVRC) used for Code Division Multiple Access (CDMA). The variable-rate vocoders encode a voice signal according to the characteristics of the input voice signal or at a data rate requested by the communication system. With respect to a full rate, QCELP and EVRC can encode voice packets at a rate of ½, ¼ or ⅛. 
     There are generally two types of terminals in digital communications. One is a packet terminal that includes a vocoder and which transmits/receives voice packets to/from a communication system. A mobile terminal for digital mobile communications is a typical example. The other is a Public Switched Telephone Network (PSTN) terminal connected directly to a PSTN, for voice communications. The PSTN terminal does not have a vocoder and transmits/receives an analog signal to/from the communication system. 
     In the digital communication system, the transport format of a voice packet is determined according to the types of terminals and the positions and operations of vocoders, and communication performance is correspondingly determined. Best performance can be achieved by appropriately determining the position and operation of at least one vocoder according to the characteristics of two terminals connected for communication and their communication environment. The factors affecting the determination include the types of the terminals, the types of vocoders used in the terminals, packet data rate, transmission or non-transmission of additional data, and transmission or non-transmission of a message. 
     During communications between a packet terminal and a PSTN terminal, upon receipt of a voice packet from the packet terminal, a voice decoder in a communication system converts the voice packet to a PCM signal and provides it to the PSTN terminal. Upon receipt of a PCM signal from the PSTN terminal, a voice encoder in the communication system converts the PCM signal to a voice packet and provides it to the packet terminal. In the case of transmitting voice information together with a message to the packet terminal, the communication system decreases the data rate of the voice packet, relying on the features of the variable-rate voice encoder and inserts as much of the message as the rate decrease in the voice packet allows, prior to transmission. For this purpose, such a module generates control/signaling messages and adjusts the data rates of voice packets that may reside in the communication system. The packet terminal extracts the voice information and the message separately from the received voice packet and recovers the voice information to the PCM voice signal. 
     In a CDMA mobile communication system, for example, upon generation of a message to be transmitted to a Mobile Station (MS) on a radio channel, a Base Station (BS) decreases the rate of a voice packet destined for the MS to, for example, ½ and inserts the message in the voice packet. This message transmission scheme is called “Dim and Burst”. In an extreme case, the rate of the voice packet is decreased to zero and only the message is constructed into a packet. This scheme is called “Blank and Burst”. Due to the entire loss of the voice information at a message transmission time point, the Blank and Burst method is inferior to the Dim and Burst method in terms of voice quality. 
     In communications between two terminals, if each use the same kind of vocoders, the communication system forwards an input voice packet to the peer terminal for normal communications. This is called “Packet Bypass”. However, if the two terminals use different, incompatible kinds of vocoders, the communication system converts an input voice packet appropriately. As described above, if additional data is to be inserted in a voice packet, even when the same kind of vocoders are used, the rate of the voice packet typically must be reduced. Consequently, voice packet conversion is required for transmission. 
     The steps by which voice packets are transmitted and processed depend on a communication environment in the digital communication system. Traditionally, voice packets are forwarded in the following way. 
       FIG. 1  illustrates a configuration of a typical PSTN-based voice packet forwarding system. 
     Referring to  FIG. 1 , when a call is set up between a first packet terminal  110  and a PSTN terminal  102 , a voice encoder of the first packet terminal  110  converts an input voice signal to a voice packet  112 . A first voice decoder  114  generates a PCM voice signal  116  from the voice packet  112  by using the same voice decoding scheme as used in the first packet terminal  110 . A PSTN switch (not shown) in a network  100  converts the PCM voice signal  116  to an analog signal  104  and provides the analog signal  104  to the PSTN terminal  102 . 
     The PSTN switch of the network  100  converts an analog signal  106  received from the PSTN terminal  102  to a PCM signal  118 . A first voice encoder  120  converts the PCM signal  118  to a voice packet  122  by using the same voice coding scheme as used in the first packet terminal  110 . 
     If there is no need for inserting additional data such as a control/signaling message, the voice packet  122  bypasses a first data inserter  124 , thus maintaining its data rate. This voice packet  126  with the same data rate, is provided to the first packet terminal  110 . However, in the presence of additional data to be transmitted to the first packet terminal  110 , the first voice encoder  120  generates the voice packet  122  at a decreased rate. The first data inserter  124  inserts the additional data in the voice packet  122  and transmits the resulting voice packet  126  to the first packet terminal  110 . The first packet terminal  110  converts the voice packet  122  to a PCM signal by using its voice decoding operation and outputs the PCM signal audibly to a user. 
     Regarding packet transmission and processing between the first packet terminal  110  and a second packet terminal  130 , the first voice decoder  114  converts the voice packet  112  from the first packet terminal  110  to the PCM signal  116  and transmits the PCM signal  116  to a second voice encoder  136  through the network  100 . As the PCM signal  116  passes through the network  100 , it becomes a PCM signal  134 . The second voice encoder  136  generates a voice packet  138  from the PCM signal  134  by using the same voice encoding scheme as used in the second packet terminal  130 . A second data inserter  140  generates a voice packet  142  by inserting additional data in the voice packet  138  and transmits the voice packet  142  to the second packet terminal  130 . Communications in the direction from the second packet terminal  130  to the first packet terminal  110  are conducted in substantially the same manner through a second voice decoder  132 , the network  100 , the first voice encoder  120 , and the first data inserter  124 . 
     As described above, coding-decoding is repeated in the process of input-coding-decoding-coding-decoding-output in a total single direction communication path. This is called Tandem Coding. In contrast, one coding-decoding is performed for communications between the first packet terminal  110  and the PSTN terminal  102 . 
     While the typical system configuration illustrated in  FIG. 1  is applicable to all communication environments, it suffers from communication quality degradation caused by information loss from repeated coding-decoding operations. To solve this problem, another system configuration has been developed and is illustrated in  FIG. 2 . 
       FIG. 2  illustrates the configuration of a typical voice packet forwarding system supporting packet bypass. As illustrated in  FIG. 2 , if first and second packet terminals  202  and  210  use the same type of vocoders, each of them can interpret voice packets received from the peer terminal. Hence, no particular processing is needed for packet transmission between the first and second packet terminals  202  and  210 . 
     Referring to  FIG. 2 , a voice packet from the first packet terminal  202  is directly provided to a network  200  by use of a bypass  204 . The network  200  forwards the voice packet to the peer party in substantially the same manner. The voice packet is output from the network  200  by use of a bypass  214 . If the bypassed voice packet from the network  200  has a full rate and a second data inserter  216  is to insert additional data in the voice packet, the second data inserter  216  deletes voice information in the voice packet and forms a new voice packet with the additional data only by using a Blank and Burst method. 
     In the opposite direction, a voice packet from the second packet terminal  210  is directly provided to the network  200  by use of a bypass  212 . The network  200  forwards the voice packet to a first data inserter  208  by use of a bypass  206 . Similarly, the first data inserter  208  forwards the bypassed voice packet to the first packet terminal  202 , or when needed, generates a new voice packet with additional data only, and transmits the new voice packet to the first packet terminal  202 . 
     Since one coding-decoding is sufficient in the system illustrated in  FIG. 2 , voice quality is improved relative to that in the system of  FIG. 1 . Forwarding a voice packet without any further processing is called “Packet Bypass” or “Tandem-Free Operation”. 
     Despite the benefit of improved voice quality between packet terminals, the above system has many constraints in its applicability. If the two packet terminals  202  and  210  use different kinds of vocoders, the Tandem-Free Operation is not viable. In addition, since Dim and Burst is used in the case of decreasing a packet rate to insert additional data during packet transmission, voice quality is decreased. 
       FIG. 3  illustrates the configuration of another typical voice packet forwarding system using a packet converter and a bypass instead of a voice decoder and a voice encoder. 
     Referring to  FIG. 3 , during communications from a first packet terminal  222  to a second packet terminal  230 , a voice packet from the first packet terminal  222  is provided to a second packet converter  236  through a bypass  224  and a network  220 . The second packet converter  236  converts the voice packet in substantially the same manner as used in a vocoder of the second packet terminal  230  or at a system-requested data rate. If data is to be inserted, a second data inserter  234  inserts the data in the converted voice packet. An empty space for the data insertion is prepared by the second packet converter  236 . The voice packet from the second data inserter  234  is then forwarded to the second packet terminal  230 . 
     In the opposite direction, a voice packet from the second packet terminal  230  is provided to the network  220  through a bypass  232 . The network  220  forwards the voice packet to a first data inserter  228  through a first packet converter  226 . The first data inserter  228 , when needed, inserts additional data in the voice packet and provides it to the first packet terminal  222 . An empty space for the data insertion is prepared by the first packet converter  226 . 
     If the two terminals  222  and  230  use the same kind of vocoders and no rate conversion is required, the packet converters  226  and  236  function virtually as bypasses without any practical packet conversion. Voice packet forwarding performance is determined predominantly by the performance of the packet converters  226  and  236 , and the tandem operation as illustrated in  FIG. 1  is avoided. Therefore, the system of  FIG. 3  outperforms that of  FIG. 1 . Furthermore, additional data transmission is possible in the Dim and Burst manner by using rate conversion in the packet converters  226  and  236 . 
     However, voice packet conversion involves re-search or quantization of many voice parameters that determine the characteristics of an input voice signal beyond simple bit reordering or format conversion. Therefore, the voice packet conversion requires very complex mathematical computations. 
     In this regard, the above-described voice packet forwarding methods have their own drawbacks. The communication system illustrated in  FIG. 1  is most typical and applicable to any communication environment, but faces the problem of voice quality degradation caused by tandem coding. Despite increased voice quality through voice packet bypass compared to the communication system of  FIG. 1 , the applicability of the communication system illustrated in  FIG. 2  is limited to an environment where two terminals use the same kind of vocoders. In addition, when additional data such as a control/signaling message is generated, voice quality is decreased because of the Blank and Burst method used. 
     While the system illustrated in  FIG. 3  is effective in solving the problems of the systems illustrated in  FIGS. 1 and 2 , packet conversion is performed on all possible combinations of different voice packets for every kind of vocoder available for communications, and rate conversion is also needed for each variable-rate vocoder. Moreover, in the case of using a new kind of vocoder, the system configuration and operation needs to be modified to support packet conversion between the new kind of vocoder and every existing vocoder. 
     Once one of the schemes illustrated in  FIGS. 1 ,  2  and  3  is chosen at a call connection according to a communication environment, the chosen scheme is kept unchanged during the call. That is, the voice packet forwarding schemes are mutually independent without compatibility between them. However, it may occur that a communication environment for a packet terminal is changed during a call due to handoff in mobile communications, for example, such that the type of vocoder is changed or the data rate of a channel is restricted. In this case, the use of the above described technology brings distinctive interruptions when the environment is changed, thereby decreasing voice quality and creating noise. 
     Accordingly, there exists a need for developing a voice packet forwarding system and method that ensures excellent performance against frequent changes in communication environments. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to substantially solve at least the above and other problems and/or disadvantages, and to provide at least the advantages below. Accordingly, embodiments of the present invention provide an apparatus and method for forwarding voice packets and supporting voice packet conversion required for the voice packet forwarding by introducing an integrated configuration and concept in a communication environment where different types of communication terminals and different types of vocoders are used and various forms of connections are provided. 
     Embodiments of the present invention provide an apparatus and method for forwarding and processing voice packets, which are commonly applicable to substantially every communication environment and ensure optimum performance in substantially every communication environment in a digital communication system. 
     Embodiments of the present invention also provide an apparatus and method for forwarding and processing voice packets, which can support substantially every communication environment by using an integrated system configuration. 
     Embodiments of the present invention also provide an apparatus and method for forwarding and processing voice packets, which ensure excellent performance against frequency environmental changes by setting a communication environment on a frame basis of a vocoder. 
     Embodiments of the present invention also provide an apparatus and method for forwarding and processing voice packets, which enable optimal voice packet forwarding and processing according to a communication environment, utilizing the functions of an existing voice encoder and decoder, without the need for developing a novel packet processing module. 
     According to one aspect of the present invention, in a voice packet forwarding apparatus in a digital communication system, a switched vocoder module is provided and directly passes a voice packet received from a packet terminal to a digital communication network, or a PCM signal decoded from the voice packet to the digital communication network, and directly passes a voice packet received from the digital communication network to the packet terminal, converts the voice packet and transmits the changed voice packet to the packet terminal, or transmits a voice packet coded from a PCM signal received from the digital communication network. Here, the packet terminal is able to communicate with another packet terminal or a PSTN terminal over the digital communication network. A data inserter, in the presence of additional data to be transmitted to the packet terminal, is provided and inserts the additional data in the voice packet received from the switched vocoder module and transmits the voice packet with the additional data to the packet terminal. A controller is provided and controls the switched vocoder module and the data inserter. The switched vocoder module comprises a voice encoder for generating the voice packet by encoding the received PCM signal, a voice decoder for generating the PCM signal by decoding the received voice packet, and a plurality of switches for connecting inputs and outputs of the digital communication network, the packet terminal, the voice decoder, and the voice encoder. 
     According to another aspect of the present invention, in a method of forwarding and processing a voice packet to connect a packet terminal to a digital communication network in a digital communication system, the packet terminal being capable of communicating with another packet terminal or a PSTN terminal over the digital communication network, it is determined whether communications with the digital communication network are PCM interfacing or packet interfacing. If the communications with the digital communication network are PCM interfacing, a voice packet received from the packet terminal is decoded to a PCM signal by a voice decoder and transmitted to the network, and a PCM signal received from the digital communication network is encoded to a voice packet by a voice encoder and transmitted to the packet terminal. If the communications with the digital communication network are packet interfacing, it is determined whether packet bypass is possible. If the packet bypass is possible, the voice packet received from the packet terminal is passed directly to the digital communication network and the voice packet received from the digital communication network is passed directly to the packet terminal. If the packet bypass is not possible, the voice packet received from the packet terminal is transmitted to the digital communication network, decoded to the PCM signal by the voice decoder, encoded to the voice packet by the voice encoder, and transmitted to the packet terminal. In the presence of a message to be transmitted to the packet terminal, the message is inserted into the directly passed voice packet or the voice packet coded from the decoded PCM signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates the configuration of a typical PSTN-based voice packet forwarding system; 
         FIG. 2  illustrates the configuration of a typical voice packet forwarding system supporting packet bypass; 
         FIG. 3  illustrates the configuration of another typical voice packet forwarding system using a packet converter and a bypass instead of a voice decoder and a voice encoder; 
         FIG. 4  illustrates a configuration of a voice packet forwarding system according to an exemplary embodiment of the present invention; 
         FIG. 5  is a detailed block diagram of a switched vocoder module according to an exemplary embodiment of the present invention; 
         FIGS. 6A ,  6 B and  6 C illustrate voice packet forwarding operations in exemplary mode 1, mode 2, and mode 3, respectively, according to an embodiment of the present invention; 
         FIG. 7  is a flowchart illustrating a voice packet forwarding operation according to an exemplary embodiment of the present invention; 
         FIGS. 8A and 8B  are detailed flowcharts illustrating a PCM interfacing operation according to an exemplary embodiment of the present invention; and 
         FIGS. 9A and 9B  are detailed flowcharts illustrating a packet interfacing operation according to an exemplary embodiment of the present invention. 
     
    
    
     Throughout the drawings, like reference numerals will be understood to refer to like parts, components and structures. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Exemplary embodiments of the present invention will now be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. 
       FIG. 4  illustrates a configuration of a voice packet forwarding system according to an exemplary embodiment of the present invention. Referring to  FIG. 4 , first and second packet terminals  310  and  330  and a PSTN terminal  302 , are connected to a digital communication network  300  for communications. Particularly, connections are made between the packet terminals  310  and  330  and the network  300  through first and second switched vocoder modules  314  and  332 , respectively, that are capable of voice coding and decoding. The switched vocoder modules  314  and  332  provide improved performance by realizing optimal communications on a frame basis according to a communication environment. A frame can be defined, for example, as a minimum unit of voice coding/decoding, that is, unit data or a unit time interval corresponding to unit data. 
     In operation, at a call setup between the first packet terminal  310  and the PSTN terminal  302 , the first packet terminal  310  converts an input voice signal to a voice packet  312  by using its internal voice encoder. The first switched vocoder module  314  creates a PCM voice signal  316  with the voice packet  312  by using the same voice decoding as used in the first packet terminal  310 . The network  300  converts the PCM voice signal  316  to an analog signal  304  by using its internal PSTN switch (not shown) and forwards the analog signal  304  to the PSTN  302 . In this way, voice from the first packet terminal  310  is transmitted to the PSTN terminal  302 . 
     The PSTN switch of the network  300  converts an analog signal  306  received from the PSTN terminal  302  to a PCM signal. The first switched vocoder module  314  converts the PCM signal to a voice packet of the same kind as used in the vocoder of the first packet terminal  310 . 
     If there is no need for inserting additional data such as a control/signaling message or a short message, the first switched vocoder module  314  creates the voice packet by encoding the PCM signal from the network  300  at a full rate. The voice packet is forwarded to the first packet terminal  310 , bypassing a first data inserter  324 . However, in the presence of additional data to be inserted, the first switched vocoder module  314  creates a voice packet by encoding the PCM signal at a lower rate than the full rate. The first data inserter  324  inserts the additional data in the voice packet and transmits the resulting voice packet to the first packet terminal  310 . The low rate is determined such that a space large enough to accommodate the additional data is spared from the voice packet according to the size of the additional data. The voice decoder of the first packet terminal  310  converts the received voice packet to a PCM signal and outputs the PCM signal in an audible form to a user. 
     Now a description will be made of exemplary packet forwarding and processing for communications between the first and second packet terminals  310  and  330 . 
     The first switched vocoder module  314  simply passes a voice packet from the first packet terminal  310  to the network  300 . The network  300  forwards the voice packet to the second switched vocoder module  332 . The second switched vocoder module  332  simply passes the voice packet to the second packet terminal  330 . When needed, the second switched vocoder module  332  converts the received voice packet, and a second data inserter  334  inserts additional data in the converted voice packet and transmits the resulting voice packet to the second packet terminal  330 . Communications from the second packet terminal  330  to the first packet terminal  310  are conducted in substantially the same manner. 
     As described above, the first and second switched vocoder modules  314  and  332  substitute for vocoders, bypasses, and packet converters that operate separately according to different communication environments. Advantageously, the switched vocoder modules  314  and  332  each have a simple configuration comprising a voice encoder, a voice decoder, a plurality of switches, and a controller. 
       FIG. 5  is a detailed block diagram of the switched vocoder modules  314  and  332  according to an exemplary embodiment of the present invention. While the first and second vocoder controllers  328  and  336  are provided separately from the first and second switched vocoder modules  314  and  332 , it can be further contemplated as another embodiment of the present invention that the vocoder controllers  328  and  336  are incorporated in the switched vocoder modules  314  and  332 , respectively. Since the switched vocoder modules  314  and  332  are substantially identical in configuration and operation, and vocoder controllers  328  and  336  are substantially identical in configuration and operation, only the first switched vocoder module  314  and the first vocoder controller  328  connected to the first packet terminal  310  will be described by way of example. 
     The first switched vocoder module  314  comprises a voice encoder  314   e  for converting a PCM signal to a voice packet, a voice decoder  314   b  for converting a voice packet to a PCM signal, and a plurality of switches  314   a ,  314   c ,  314   d  and  314   f  for selecting the input and output signals of the voice encoder  314   e  and the voice decoder  314   b . The first vocoder controller  328  collects information about communications between the first and second packet terminals  310  and  330 , and outputs a switch control signal  340  for controlling the switches  314   a ,  314   c ,  314   d  and  314   f , and a data insertion control signal  342  for controlling the voice encoder  314   e  and the voice decoder  314   b , thereby controlling the switched vocoder module  314  and the first data inserter  324 . While not shown, the vocoder controller  328  can be divided into a switch controller and a vocoder controller in another embodiment of the present invention. 
     The voice encoder  314   e  and the voice decoder  314   b  support PCM-based and packet-based connectivity to the network  300 . Embodiments of the present invention can be based on the assumption that a connection standard in correspondence to a communication environment is automatically used, and that there are no limits in connection standards, but is not limited thereto. A connection between the vocoder  314   e  and  314   b  and the packet terminal is established via a packet-based channel. The first packet terminal  310  also comprises a vocoder with a voice encoder and a voice decoder and transmits/receives voice packets. 
     In an exemplary operation, when the two terminals  310  and  302 , or  310  and  330 , access the network  300  and thus are connected to each other, the first vocoder controller  328  determines a control function in relation to communications by comprehensively analyzing the characteristics of the two terminals, the need for transmitting additional data, information about voice communications, and other communication information. The first vocoder controller  328  analyzes a communication situation frame by frame and provides corresponding communication information. The communication information analyzed contains the types of the terminals, the types of vocoders in the terminals, information about whether additional information such as a control/signaling message is to be transmitted, and an allowed rate for an output channel. According to the communication information, communication environments are divided largely into three exemplary modes, but are not limited thereto. 
     For example, the first vocoder controller  328  can receive communication information about the terminals  310 ,  330  and  302  from switching systems (not shown) or access networks (not shown) that serve the terminals. Since the paths and means by which the first vocoder controller  328  receives the communication information are beyond the scope of embodiments of the present invention, their detailed description is not provided herein. 
     In mode 1, voice is transmitted in the form of a PCM signal between the vocoder  314   e  and  314   b  and the network  300 , like communications between the packet terminal  310  and the PSTN terminal  302 . Voice transmission in the form of a packet between the vocoder  314   e  and  314   b  and the network  300  corresponds to mode 2. An example of mode 2 is a mobile-to-mobile call between the packet terminals  310  and  330 . Mode 3 is a special case of mode 2. This is the case where a call can be conducted by simply transmitting voice packets between the two packet terminals  310  and  330  in the communication system. If the two packet terminals  310  and  330  use the same kind of vocoders and there is no need for changing packet rate during transmission, communications are made in mode 3. However, if packet rate needs to be decreased for transmission of additional data even though the same type of vocoders are used, this case corresponds to mode 2. In the case where the terminals  310  and  330  use the same kind of vocoders and packet rate is limited due to transmission of additional data, communications are made in mode 3 as far as the rate of an input voice packet is not higher than the rate limit. 
     At a call connection, the first vocoder controller  328  determines a mode by comprehensively analyzing a given communication situation, determines the type and operation of the vocoder  314   e  and  314   b  according to the mode, and correspondingly controls the operations of the switches  314   a ,  314   c ,  314   d  and  314   f . The control operation is performed on a frame basis. 
     In mode 1, the voice encoder  314   e  and the voice decoder  314   b  are set to operate in the same manner as those of the first packet terminal  310 . In mode 2 and mode 3, the voice encoder  314   e  is set to operate in the same manner as the voice decoder of the first packet terminal  310 , and the voice decoder  314   b  is set to operate in the same manner as the voice encoder of the second packet terminal  330 . 
     Control of the voice encoder  314   e  and the voice decoder  314   b  can involve setting a maximum allowed rate for the voice encoder  314   e , noise cancellation and pre-filtering of the voice encoder  314   e , post-filtering of the voice decoder  314   b , and changing a coding weight parameter for the voice encoder  314   e . The first vocoder controller  328  adjusts different parameters for different modes. For instance, the first vocoder controller  328  excludes noise cancellation in the voice encoder  314   e  and post-filtering in the voice decoder  314   b  in mode 2 and mode 3. In mode 3, the first vocoder controller  328  can reduce computation volume by simplifying the search operation of the voice encoder  314   e  and the voice decoder  314   b . In order to avoid signal discontinuity at a mode change, the first vocoder controller  328  can create a time delay during voice coding and decoding in each mode. The control of the voice encoder  314   e  and the voice decoder  314   b  can be changed according to a voice coding algorithm used and thus, its detailed operation is not provided herein. 
     The first vocoder controller  328  controls switching on a frame basis according to received communication information and outputs commands to the switches  314   a ,  314   c ,  314   d  and  314   f . In the illustrated case of  FIG. 5 , the first and second switches  314   d  and  314   f  determine the input and output to and from the voice encoder  314   e , respectively, and the third and fourth switches  314   a  and  314   c  determine the input and output to and from the voice decoder  314   b , respectively. 
       FIGS. 6A ,  6 B and  6 C illustrate exemplary switching of the switches  314   a ,  314   c ,  314   d  and  314   f  in mode 1, mode 2, and mode 3, respectively. 
     Referring to  FIG. 6A , in mode 1, the first switch  314   d  switches a PCM signal received from the network  300  to the voice encoder  314   e  and the second switch  314   f  switches a voice packet generated from the voice encoder  314   e  to the first data inserter  324 . The third switch  314   a  switches a voice packet received from the first packet terminal  310  to the voice decoder  314   b  and the fourth switch  314   c  switches a PCM signal generated from the voice decoder  314   b  to the network  300 . 
     Referring to  FIG. 6B , in mode 2, the third switch  314   a  switches a voice packet received from the network  300  to the voice decoder  314   b , and the first switch  314   d  switches a PCM signal generated from the voice decoder  314   b  to the voice encoder  314   e . The second switch  314   f  switches a voice packet generated from the voice encoder  314   e  to the first data inserter  324 , and the fourth switch  314   c  switches a voice packet directly received from the packet terminal  310  to the network  300 . 
     Referring to  FIG. 6C , the switches  314   a ,  314   c ,  314   d  and  314   f  operate in mode 3 basically in substantially the same manner as in mode 2, except that a bypass function is performed in mode 3 to avoid voice quality degradation caused by tandem coding. Specifically, the second switch  314   f  simply passes a voice packet received from the network  300  to the first data inserter  324 . At the same time, the third switch  314   a  switches the voice packet from the network  300  to the voice decoder  314   b , and the first switch  314   d  can switch a PCM signal generated from the voice decoder  314   b  to the voice encoder  314   e . While the voice encoder  314   e  generates a voice packet by encoding the PCM signal generated from the voice decoder  314   b , the voice packet is not provided to the first data inserter  324 . These operations of the voice decoder  314   b  and the voice encoder  314   e  are performed to update their internal states for the purpose of ensuring the continuous operation of the voice encoder  314   e  against a mode change. The updating is required due to the characteristics of a voice coding algorithm utilizing a previous voice signal. 
     The operation of the above voice packet forwarding and processing system will now be described in the context of CDMA digital mobile communications. 
     In CDMA mobile communications, the first terminal  310  corresponding to a cellular phone and the third terminal  302  corresponding to a Plain Old Telephone (POT) phone communicate with each other always in mode 1. The first vocoder controller  328  restricts the maximum rate of an output voice packet in a frame delivering additionally a message to ½, and supports transmission of the additional data by Dim and Burst. If the type of vocoder in the first terminal  310  is changed during the call, associated information is provided to the first vocoder controller  328 , and the first vocoder controller  328  controls the voice encoder  314   e  and the voice decoder  314   b  to operate in the substantially same manner as the vocoder of the first terminal  310  by transmitting a command. 
     In one of mode 1, mode 2 and mode 3, communications are made between the first and second terminals  310  and  330  corresponding to cellular phones in CDMA digital mobile communications. While mode 1 and mode 2 are basically available for a call, it is preferable to introduce packet bypass to mode 1 and mode 2 in order to prevent tandem coding-caused voice quality degradation. The basic concept of mode determination in the first vocoder controller  328  is as follows. 
     The first vocoder controller  328  basically uses tandem coding and, in a special case, it applies packet bypass in mode 3. In this case, frequent transition is required between the tandem coding and the packet bypass. The voice encoder  314   e  and the voice decoder  314   b  ensure more stable performance in transitions between mode 2 and mode 3 using the same input signal. That is, although the voice encoder  314   e  and the voice decoder  314   b  receive different signals in mode 1 and mode 3, the same input is used in mode 2 and mode 3. Therefore, no discontinuity is observed in the operations of the voice encoder  314   e  and the voice decoder  314   b , thereby preventing performance degradation. 
     Hence, communications are made between the terminals  310  and  330  basically in mode 2. According to the rates of the terminals  310  and  330 , or according to a maximum rate determined depending on whether additional data is to be transmitted, the voice encoder  314   e  is controlled for message transmission in Dim and Burst. If the condition of the same vocoders in the two terminals  310  and  330  and no packet conversion is fulfilled during a mode 2 operation, mode 2 is transitioned to mode 3. If the condition is not fulfilled during the mode 3 operation, mode 3 is returned to mode 2. During which time, the voice encoder  314   e  and the voice decoder  314   b  operate normally to guarantee operation continuity for the voice encoder  314   e  and the voice decoder  314   b  for the case of returning to mode 2. If the type of the vocoder of either of the terminals is changed due to a handoff during a call, the first vocoder controller  328  selects mode 2 or mode 3 according to the type of the changed vocoder. 
     Therefore, when the two terminals  310  and  330  use the same kind of vocoders in CDMA mobile communications, a frame delivering an additional message is forwarded in mode 2 and any other general frame is forwarded in mode 3. In the former case, the frame is very simply processed by data insertion of the first data inserter  324  in the Dim and Burst manner, whereas in the latter case, voice quality is improved for the general frame through packet bypass. The voice packet forwarding apparatus of embodiments of the present invention significantly improves communication quality by supporting both packet bypass and Dim and Burst without adding a complex module. 
     In the case where the two terminals  310  and  330  use different types of vocoders, they communicate in mode 2. If the type of the vocoder in either of the terminals is changed due to a handoff during a call and thus, the vocoders of the terminals become identical in type, mode 2 is transitioned to mode 3 in the middle of the call, thereby achieving performance improvement through packet bypass. Even though handoff occurs frequently at a cell boundary in the mobile communication system, communications are made without interruptions by transitions between mode 2 and mode 3. 
       FIG. 7  is a flowchart illustrating a voice packet forwarding operation and processing according to an exemplary embodiment of the present invention. A call setup for starting a call and a call release for ending the call in a digital communication system are not described herein because they are beyond the scope of the present invention. Communication signals are transmitted/received on a frame basis in the exemplary embodiment of the present invention described in  FIG. 7 , but are not limited thereto. 
     Referring to  FIG. 7 , the vocoder controller recognizes the start of a frame in step  410  and determines whether communications are conducted between the switched vocoder module and the network on a packet basis in step  420 . In the case of non-packet-based communications, that is, PCM-based communications, the vocoder controller performs PCM interfacing by using a control operation in mode 1 in step  430 . For example, the vocoder controller connects the first to fourth switches  314   d ,  314   f ,  314   a  and  314   c  to inputs A, C, G and E, respectively as shown in  FIG. 6A . 
     In the case of packet-based communications, the vocoder controller performs packet interfacing by using a control operation in mode 2 or mode 3 in step  450 . For example, the vocoder controller connects the first to fourth switches  314   d ,  314   f ,  314   a  and  314   c  to inputs B, C or D, H and F, respectively, as shown in  FIGS. 6B and 6C . The vocoder controller selects the input C or D depending on whether packet bypass is available or not. 
     In step  490 , the vocoder controller determines whether the current frame is the last one and if so, the call is terminated. If the call still proceeds, the vocoder controller returns to step  410  to process the next frame. If the call is terminated, the vocoder controller ends the procedure. 
       FIGS. 8A and 8B  are detailed flowcharts illustrating the PCM interfacing operation of step  430  according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 8A , the voice decoder decodes a voice packet received from the terminal and outputs the resulting PCM signal in step  432 . In step  434 , the PCM signal is forwarded to the network. 
     Referring to  FIG. 8B , the vocoder controller determines a maximum allowed rate for the voice encoder by checking whether additional data is to be transmitted in step  436 . If there is no need for transmitting the additional data, an available maximum rate is selected. If the additional data needs to be transmitted, a lower rate is selected to spare a space that is large enough to accommodate the message. The voice encoder generates a voice packet by encoding the PCM signal received from the network at the determined rate in step  438 , and transmits the voice packet to the data inserter in step  440 . If the additional data is to be transmitted, the message is inserted into the voice packet and then transmitted to the peer terminal in step  442 . 
       FIGS. 9A and 9B  are detailed flowcharts illustrating the packet interfacing operation of step  450  according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 9A , a voice packet from the terminal is directly transmitted to the network in step  452 . 
     Referring to  FIG. 9B , the vocoder controller compares the vocoder of the terminal with that of the peer terminal and determines whether packet bypass is available according to the presence or absence of additional data to be transmitted to the peer terminal in step  454 . If the vocoders are of the same type and no additional data is to be transmitted, or if a space to accommodate the additional data is already spared in the voice packet received from the network, the vocoder controller determines that packet bypass is available. 
     If packet bypass is not available, the voice decoder decodes the voice packet received from the network to a PCM signal in step  456 , and outputs it to the voice encoder in step  458 . In step  460 , the vocoder controller determines a maximum rate for the voice encoder according to the presence or absence of the additional data. The voice encoder encodes the PCM signal received from the voice decoder to a voice packet at the determined rate in step  462 , and transmits the voice packet to the data inserter in step  464 . In the presence of the additional data, the data inserter inserts the additional data into the voice packet in step  470   a  and transmits the resulting voice packet to the peer terminal in step  472   a.    
     However, if packet bypass is available in step  454 , the voice packet from the network is directly provided to the data inserter in step  466 . In step  468 , the vocoder controller determines whether an additional message is to be transmitted by the voice packet. In the absence of the additional message, the voice packet is transmitted to the terminal in step  472   b . In the presence of the additional data, the data inserter inserts the additional data into the voice packet in step  470   b  and transmits the resulting voice packet to the peer terminal in step  472   b.    
     In the case where the packet bypass is available, voice coding and decoding are also performed so as to ensure the operation continuity of the voice encoder in step  474 . To be more specific, the voice decoder generates a PCM signal by decoding a voice packet received from the network in step  476  and outputs the PCM signal to the voice encoder in step  478 . In step  480 , the voice encoder generates a voice packet by encoding the PCM signal. The voice packet is not provided to either the data inserter or the peer terminal. 
     In accordance with embodiments of the present invention as described above, since packet bypass is adaptively performed for digital mobile communications between terminals in a digital communication system such as VoIP, voice quality degradation caused by iterative voice coding and decoding is prevented and additional data or message can be inserted into a voice packet very simply, while minimizing voice quality degradation. Furthermore, excellent communication performance is provided even under a fast changing communication environment. 
     While the present invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.