Patent Publication Number: US-2005135376-A1

Title: Media gateway and method of managing local channel thereof

Description:
This application claims the priority of Korean Patent Application No. 2003-95385, filed on Dec. 23, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a media gateway and a method of managing a local channel thereof, and more particularly, to a media gateway and a method of managing a local channel thereof which interfaces a public switched telephone network (PSTN) and voice over Internet protocol (VoIP).  
      2. Description of the Related Art  
      As network infrastructure technologies that support the Internet become more advanced and Internet networks become more widespread, data transmission/reception between remote locations can be achieved at increasingly lower cost. VoIP makes it possible to provide user-to-user voice call service over the Internet, because a VoIP network provides an infrastructure and an infrastructure protocol that enable transmission of voice data over an IP network.  
      IP network service charges are relatively low. Thus, by using VoIP, expensive international telephone charges can be saved. In order to interface a PSTN and VoIP, devices that convert different call control protocols and voice control protocols suitably for respective networks are needed. Among such devices, a media gateway converts voice control protocol.  
      As an interface unit for a PSTN and a VoIP network, a gateway that converts the call control protocol and the voice control protocol at the same time has been used. A media gateway manages media channels under the control of a media gateway controller through a media gateway control protocol, i.e., media gateway control (MEGACO) or media gateway control protocol (MGCP).  
      As one example of a call processing method using a conventional gateway, a VoIP gateway management method is disclosed in Korean Patent Publication No. 2003-0035404. The disclosed VoIP gateway management method enables rapid telephone calling without any waiting period after opening a voice channel. A destination gateway checks whether or not an H.245 address is contained in a call setup message that is provided from a source gateway. If it is found that an H.245 address is contained, the destination gateway makes an H.245 TCP connection and sends an H.245 message to the source gateway. Then, the destination gateway sends a call connection message to the source gateway.  
      Another example of a call processing method using a conventional gateway, more specifically, a VoIP call processing apparatus and method, is disclosed in Korean Patent Publication No. 2003-0052759. This reference discloses a gateway and a call setup method that are applicable to various PBXs relatively without being influenced by an operating system of PBX system, a call setup, a communication protocol of voice signal, and a variation of an embedded voice data processing unit, in a case where the gateway is embedded into PBX in which a call control signal and a voice data transmission/reception channel are separated from each other. The call processing apparatus includes a call control unit, a media data processing unit, and a system utility. The call control unit converts call control signals, which are generated in a PSTN-IP heterogeneous network, into signals suitable for opposite networks, and then transmits the converted signals. Also, the call control unit controls call processing resource management and media transmission/reception channels according to the call setup. The media data processing unit performs processes such as conversion and compression of voice data into data suitable for opposite networks. The system utility manages system resources, such as memory and a timer.  
     SUMMARY OF THE INVENTION  
      The present invention provides a media gateway and a method of managing a channel thereof, in which the media gateway provides VoIP service through an interface of a PSTN network and an IP network and local channels of DSPs are operated independently through centralized channel management.  
      According to an aspect of the present invention, there is provided a media gateway, which includes: a main control processor which selects one of at least one DSP (digital signal processor) in response to a call setup request, selects a TDM (time division multiplexing) channel of the selected DSP, assigns a session number to the selected DSP in order to distinguish the call inside the selected DSP, and assigns a channel that is in an idle status among predetermined RTP (realtime transport protocol) available channels as an RTP channel for the call; a DSP to which the session number with respect to the TDM channel is assigned by the main control processor if the main control processor selects the TDM channel of the DSP among a plurality of DSPs each including a plurality of TDM channels, the DSP storing the session number; and a T-switch which dynamically connects the plurality of TDM channels with a plurality of E1 links.  
      According to another aspect of the present invention, there is provided a method of managing a channel in a media gateway, which includes: selecting one of at least one DSP in response to a call setup request, selecting a TDM channel of the selected DSP, and assigning a session number to the selected DSP in order to distinguish the call inside the selected DSP; assigning a channel that is in an idle status among predetermined RTP available channels as an RTP channel for the call; and connecting the assigned TDM channel and the assigned RTP channel.  
      The local DSPs of the media gateway can operate with only channel information necessary for the DSPs themselves without storing system-level channel information. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
       FIG. 1  illustrates a configuration of a media gateway that performs channel management according to the present invention;  
       FIG. 2  illustrates a channel management data structure of the media gateway according to the present invention;  
       FIG. 3  is a flowchart illustrating a method of managing a local channel during call processing in the media gateway according to the present invention;  
       FIG. 4  is a flowchart illustrating another method of managing a local channel during call processing in the media gateway according to the present invention; and  
       FIG. 5  is a flowchart illustrating a method of managing a channel in the media gateway according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art.  
       FIG. 1  illustrates a configuration of a media gateway that performs channel management according to the present invention.  
      Referring to  FIG. 1 , media gateway  100  according to the present invention includes a main control processor  110 , a plurality of digital signal processors (DSPs)  150 ,  160 ,  170  and  180 , a T-switch  190 , and E1 framers  191  to  198 . The main control processor  110  includes a channel management unit  120 , a MEGACO slave  130 , and a RTP transmission/reception unit  140 . Also, the channel management unit  120  includes a channel management data unit  122  and a host interface (HI) master  124 . At least one or more DSPs  150 ,  160 ,  170  and  180  are provided. The DSPs  150 ,  160 ,  170  and  180  include HI slaves  151 ,  161 ,  171  and  181 , channel data units  152 ,  162 ,  172  and  182 , codec units  153 ,  163 ,  173  and  183 , and a plurality of TDMs  154  and  155 ,  164  and  165 ,  174  and  175 ,  184  and  185 , respectively. Here, TDM indicates time division multiplexing.  
      In the media gateway  100  of the present invention, the main control processor  110  and the DSPs  150 ,  160 ,  170  and  180  operate in master/slave status. The main control processor  110  and the DSPs  150 ,  160 ,  170  and  180  perform communication as a host interface using the HI master  124  and the HI slaves  151 ,  161 ,  171  and  181 . The host interface is an interface for data transmission/reception between main control processor (host processor) and a sub processor.  
      The channel management data unit  122  of the channel management unit  120  stores data used for managing internal resources (channels (TDM, E1, RTP, etc.), codec, switch) of the media gateway  100 . The channel management data structure stored in the channel management data unit  122  will be described in detail later with reference to  FIG. 2 . The RTP transmission/reception unit  140  transmits/receives VoIP voice data from users during a call. The RTP transmission/reception unit  140  is installed in the main control processor  110  as software and manages transmission/reception of voice data.  
      The DSPs  150 ,  160 ,  170  and  180  include the codec units  153 ,  163 ,  173  and  183  which perform encoding/decoding of VoIP voice data. The TDMs  154  and  155 ,  164  and  165 ,  174  and  175 ,  184  and  185  which are PSTN interface, the HI slaves  151 ,  161 ,  171  and  181  which manage communication with the channel management unit  120  of the main control processor, and the channel data units  152 ,  162 ,  172  and  182  which store data (session number for distinguishing calls within the DSP, etc) related to the local channels of the DSPs.  
      The T-switch  190  dynamically connects the TDMs  154  and  155 ,  164  and  165 ,  174  and  175 ,  184  and  185  of the DSPs  150 ,  160 ,  170  and  180  with the E1 framers  191  to  198 . The E1 framers  191  to  198  are directly connected with a PSTN exchange through E1 trunk.  
      The MEGACO slave  130  receives MEGACO protocol messages with predetermined commands from an external media gateway controller (MGC) through a media gateway control protocol (MGCP). The MEGACO slave  130  interprets the commands contained in the received MEGACO protocol messages and sends the interpreted commands to the channel management unit  120 .  
      The commands contained in the MEGACO protocol messages include a channel assignment, a channel data modification, a channel connection, a channel disconnection, and the like. If the channel management unit  120  receives a command from the MEGACO slave  130 , the channel management unit  120  performs an operation corresponding to the command.  
      Specifically, if the received command is the channel assignment, the channel management unit  120  assigns a local channel and maps the assigned local channel into an external channel. Then, the channel management unit  120  stores assigned local channel information and mapping information in the channel management data unit  122 .  
      The local channel is a channel for mapping internal resources of the media gateway  100  independently of the external channel. For example, if only one DSP is present inside the media gateway, the channel number requested by the MGC can be used as is. Meanwhile, if two or more DSPs are present, it must be determined which DSP&#39;s channel numbers will be used among channel numbers 1 to 60 of the DSP1, channel numbers 1 to 60 of the DSP2 and channel numbers 1 to 60 of a DSP#N. Accordingly, the channel management unit  120  determines which channel number of which DSP among the plurality of DSPs will be assigned as the local channel. The external channel is a channel over which the media gateway  100  communicates with an external device.  
      If the received command is the channel data modification, the channel management unit  120  modifies the channel management data for the corresponding local channel. Also, if the received command is the channel connection, the channel management unit  120  selects one of the DSPs  150 ,  160 ,  170  and  180  using a DSP selection algorithm and selects the TDMs  154  and  155 ,  164  and  165 ,  174  and  175 ,  184  and  185  of the selected DSP using a TDM selection algorithm. In other words, the channel management unit  120  selects an available channel among the local channels of the selected DSP as the local channel, which will be mapped with the external channel, and then registers the selected local channel in the channel management data unit  122  of the channel management unit  120 .  
      The channel management unit  120  uses the DSP selection algorithm and the TDM selection algorithm. The DSP selection algorithm selects a DSP by comparing performance weight data and DSP load. The performance weight data is data that is applied when programs for codec operate inside the DSP, and the DSP load is calculated by the number of codec processing according to DSP. The TDM selection algorithm selects a TDM having a small number of active TDM channels. The channel management unit  120  can also use other DSP selection algorithms and TDM selection algorithms.  
      The channel management unit  120  sends data related to TDM channels of the selected DSP to the channel data unit of the selected DSP. The channel management unit  120  sends data to the DSP through the HI master  124 .  
      In call processing, the E1 framers  191  to  198  for external E1 link connection are dynamically connected with the DSPs  150 ,  160 ,  170  and  180  through the T-switch  190 . If a media connection for a call that is requested through the MEGACO slave  130  is completed, voice data is transmitted/received between PSTN telephones connected to the E1 framers  191  to  198  and IP terminals connected to the RTP transmission/reception unit  140 .  
      In other words, PSTN voice data is input through the E1 framers  191  to  198 , the T-switch  190 , and the TDMs  154  and  155 ,  164  and  165 ,  174  and  175 ,  184  and  185  of the selected DSP, to the codec units  153 ,  163 ,  173  and  183 , which are internal voice data modulators of the DSPs. The PSTN data is converted into a realtime transport protocol (RTP) Payload suitable for the IP network in the codec units  153 ,  163 ,  173  and  183 . Then, the RTP Payload is transmitted to the RTP transmission/reception unit  140  through HI slaves  151 ,  161 ,  171  and  181  and the HI master  124 . Reversely, data from the IP terminal is transmitted through the HI master  124  of the channel management unit  120  and the HI slaves  151 ,  161 ,  171  and  181  of the DSP, and is converted into voice data suitable for the PSTN network by DSP internal voice data modulation. Then, the voice data is transmitted to the PSTN network.  
       FIG. 2  illustrates the channel management data structure of the media gateway according to the present invention.  
      Referring to  FIG. 2 , the channel management data unit  122  of the channel management unit in  FIG. 1  includes a TDM channel management table  200 , a RTP channel management table  240 , and a connection management table  260 .  
      The TDM channel management table  200  stores data necessary for connection of a PSTN terminal in the media gateway  100 . If the channel. management unit  120  receives a TDM channel assignment request from the MEGACO slave  130 , the channel management unit  120  selects a DSP and assigns an available TDM channel of the selected DSP in order for mapping to the external channel. Then, the channel management unit  120  stores data of the assigned channel in the TDM channel management table  200 .  
      Specifically, ChannellD  202  is an identifier that the channel management unit  120  calculates based on TDM channel assignment request data (E1 link number, channel number) and that distinguishes the assigned TDM channels. Status  204  indicates a status of the assigned local channel and maintains the channel status during a call.  
      E1TrunkID  206  is an identifier that distinguishes the E1 trunk according to the TDM channel assignment request of the MEGACO slave  130 . E1ChID  208  is an identifier that distinguishes the local channel of the E1 trunk. DspID  210 , TdmID  212  and TdmChID  214  respectively store identification data about the DSPs  150 ,  160 ,  170  and  180 , the TDMs  154  and  155 ,  164  and  165 ,  174  and  175 ,  184  and  185 , and the TDM local channels, which are decided by the channel management unit  120  according to the TDM channel assignment request of the MEGACO salve  130 . Referring to data of the TDM channel management table  200 , the channel management unit  120  manages the DSP local channels and the E1 framer channels, which are necessary for VoIP processing.  
      The RTP channel management table  240  stores data necessary for connection with the IP terminal in the media gateway  100 . If the channel management unit  120  receives the RTP channel assignment request from the MEGACO slave  130 , the channel management unit  120  assigns an RTP channel and stores information on the assigned RTP channel in the RTP channel management table  240 .  
      In detail, ChannelID  241  stores an identifier of an available RTP channel, which is found from Status  242  of the RTP channel after the channel management unit  120  receives the RTP channel assignment request from the MEGACO slave  130 . The Status  242  indicates a current status of the RTP channel and status information is changed if the channel management unit  120  assigns the ChannelID  241  to the available RTP channel.  
      If the channel management unit  120  receives the channel data modification request from the MEGACO slave  130 , the channel management unit  120  modifies the remaining fields  243  to  255  of the RTP channel management table  240 .  
      LocalIPAddr  243  stores an IP address of the media gateway  100 . LocalRtpPort  240  stores a port number of the media gateway  100 , of which a counterunit terminal must be notified in order to receive an RTP Payload from the counterunit terminal. LocalRtcpPort  245  stores a port number of the media gateway  100 , through which a realtime transport control protocol (RTCP) message is received.  
      RemoteIPAddr  246  stores an IP address of an IP terminal to which the media gateway  100  must send the RTP Payload. RemoteRtpPort  247  stores an RTP port number of a counterunit terminal. RemoteRtcpPort  248  stores an RTCP port number and NoTxPayloadType  249  stores the number of payload types that are available in the counterunit terminal. TxPayloadType  250  stores a payload type. The RTP channel management table  240  includes fields  251  to  255  that store jitter-related information. Referring to data stored in the RTP channel management table  240 , the channel management unit  120  transmits/receives RTP data to/from a terminal of an IP-side VoIP user.  
      The connection management table  260  is a table that associates the TDM channel management table  200  with the RTP channel management table  240 . In other words, the connection management table  260  combines the TDM channel management table  200  and the RTP channel management table  240  into a set by associating the channelID  202  of the TDM channel management table  200  and the ChannelID  241  of the RTP channel management table  260  with respect to one VoIP call.  
      In detail, ConnectionID  262  stores values that distinguish a VoIP call. Status  264  stores a connection status and DspID  266  stores values that identify DSPs in which connection is processed.  
      DspSessionID  268  is used to distinguish DSP local calls in the channel management unit  120 . The DspSessionID  268  is used to store information on a TDM channel and an RTP channel with respect to one VoIP call in channel data units  152 ,  162 ,  172  and  182  of the DSPs  150 ,  160 ,  170  and  180 .  
      In other words, the channel management unit  120  uses the DspSessionID  268  to distinguish VoIP channel information that is necessary in the DSP. The ConnectionID  262  is call connection management data at level of the entire media gateway  100 . The DspSessionID  268  is local data that is used inside the DSPs  150 ,  160 ,  170  and  180 .  
      Channel1  270  and Channel2  274  are channel identifiers associated with connection. Channel1Type  272  and Channel2Type  276  indicate channel types of the Channel1  270  and the Channel2  274 , respectively.  
       FIG. 3  is a flowchart illustrating a method of managing a local channel during call processing in the media gateway according to the present invention.  
       FIG. 3  shows a media gateway controller (MGC)  300 , a MEGACO slave  302 , a channel management unit  304 , a DSP  306  and a T-switch  308 .  
      The MEGACO slave  302  receives an add command of outgoing and incoming channel assurance from the MGC  300  (S 310 ). The add command that instructs the outgoing channel assurance includes an E1 link number and a channel number. The MEGACO slave  302  sends E1 channel assignment request data to the channel management unit  120  (S 312 ). Here, the E1 channel assignment request data includes the E1 link number and the channel number, which are contained in the received add command.  
      The E1 link is a trunk line that is physically connects the gateway and the PSTN exchange. In a case where one E1 link is divided into 32 time slots and transmits/receives electrical signals, the channel number indicates logical channels for the respective 32 time slots.  
      The channel management unit  304  assigns local channel numbers based on the E1 link number and the channel number contained in the channel assignment request data (for example, local channel number=(E1 line number −1)×number of data channels per link+channel number). Then, the channel management unit  304  assigns session numbers for the selected DSP (numbers for identifying calls that are used inside one DSP), and requests channel data assignment to the selected DSP (S 314 ).  
      The DSP  306  that receives the channel data assignment request assigns memory with respect to the channel data, stores the session number in the assigned memory, and sends the assignment result to the channel management unit  304  (S 316 ). The channel management unit  304  sends the assigned local channel number 1 to the MEGACO slave  302  (S 318 ).  
      If the MEGACO slave  302  requests the assignment for the RTP channel (S 320 ), the channel management unit  304  searches for a channel of idle status among channels regulated as available during initialization of the media gateway  100  and assigns the channel as the RTP channel. The channel management unit  304  sends the assignment result to the MEGACO slave (S 322 ).  
      If the channel management unit  304  receives from the MEGACO block a change request of channel mode (Send_Only mode, Receive_Only mode, Send_Receive mode, etc.) for the local channel 1 that is the TDM channel (S 324 ), the channel management unit  304  sends the TDM channel data change information (TDM number, TDM channel number, previously assigned session number) to the selected DSP  306  (S 326 ). The DSP  304  changes the mode of the corresponding TDM channel and then sends the result to the channel management unit  304  (S 328 ). Then, the channel management unit  304  sends the received result and the local channel number 1 to the MEGACO slave (S 330 ).  
      If the channel management unit  304  receives from the MEGACO slave  302  the channel mode change request for the local channel 2 that is the RTP channel (S 332 ), the channel management unit  304  changes the channel management data according to a call processing scenario and sends the result to the MEGACO slave  302  (S 334 ). In  FIG. 3 , since the RTP channel is the incoming channel, the channel mode change request is a request for a change from Receive_Only mode to Send_Receive mode.  
      If the channel management unit  304  receives a request for connection of the assigned local channels 1 and 2 from the MEGACO slave  302 , the channel management unit  304  performs an association of the two channels (local channels 1 and 2) through the connection management table  260  (S 338 ). In other words, since the channel IDs and channel type information of the two channels are stored in the connection management table, information on the connection can be obtained by referring to only the associative management table. If necessary, the information on the respective channels can be obtained by searching the TDM (or RTP) channel management table alone. The channel management unit  304  connects the TDM and the E1 framer through the T-switch based on a status (e.g., one-way direction) of the TDM channel and then sends the result to the MEGACO slave (S 340 ).  
       FIG. 4  is a flowchart illustrating a method of managing the local channel during call processing in the media gateway according to the present invention.  
      If the MEGACO slave  402  receives information on a remote channel from the MGC  400  through a modify message (S 412 ), the MEGACO slave  402  requests a channel data modification to the channel management unit  404  (S 414 ). If the local channel 1 that is the channel data modification target is a TDM channel (S 414 ), the channel management unit  404  connects the TDM and the E1 framer through the T-switch based on a status (e.g., bi-direction) of the TDM channel (S 416 ), and sends the result to the MEGACO slave  402  (S 418 ). If data for the incoming channels is all received, the bidirectional connection is achieved.  
      If the local channel 2 that is the channel data modification target is an RTP channel (S 420 ), the channel management unit  404  modifies the local channel management data (RemoteIPAddress, RemoteRtpPort, RemoteRtcpPort, TxPayloadType, Jitter information), and sends the result to the MEGACO slave  402  (S 422 ).  
      If the channel management unit  404  receives an information update request (channel data modification request) for a remote terminal from the MEGACO slave  402 , the channel management unit  404  sends data (PayloadType, Jitter information), necessary for DSP local voice information modification to the selected DSP  406  together with the session number (S 426 ). The DSP  406  modifies the corresponding data and then sends the result to the channel management unit  404  (S 428 ).  
      The channel management unit  404  sends the received result to the MEGACO slave  402  (S 430 ). In a case where the status of the RTP channel data is bidirectional, if voice data transmission/reception for an RTP Payload is started (S 432 ), voice data between the DSP  406  of the media gateway  100  and the RTP Pump  410  is transmitted to both terminals (S 434 ).  
       FIG. 5  is a flowchart illustrating a method of managing the channel in the media gateway according to the present invention.  
      Since a concept of channel disconnection is equal to that of channel connection, only channel connection will be described below. For simplicity, abnormal processing portions are omitted in the drawing.  
      If the channel management unit  120  starts, the channel management unit  120  initializes resources which need to be managed inside the channel and media gateway  100  (S 502 ), and waits for messages from the MEGACO slave  130 , the HI master  124  communicating with the DSPs  150 ,  160 ,  170  and  180 , and the respective devices (S 504 ).  
      If the TDM channel assignment request message is transmitted from the MEGACO slave  130  to the channel management unit  120  (S 510 ), the channel management unit  120  assigns the local channel which is intended to be mapped to the external channel, among idle channels of the TDM channel, based on the received E1 link number and channel number (S 511 ).  
      The channel management unit  120  assigns a DSP local SessionID for communication with the DSPs  150 ,  160 ,  170  and  180  (S 512 ) and sends the channel data assignment request message to the DSP (S 513 ). If the channel management unit  120  receives a response to the channel data assignment request from the DSP (S 514 ), the channel management unit  120  sends its results, including the assigned local channel number, to the MEGACO slave  130  (S 515 ).  
      If the RTP channel assignment request message is transmitted from the MEGACO slave  130  to the channel management unit  120  (S 520 ), the channel management unit  120  assigns an idle channel (S 521 ) and sends the assigned local channel number to the MEGACO slave  130  (S 522 ).  
      If the channel mode change request with respect to the already assigned channel is transmitted from the MEGACO slave  130  to the channel management unit  120  (S 530 ), the channel management unit  120  checks the type of the requested channel (S 531 ).  
      If the channel type is an RTP channel (S 531 ), the channel management unit  120  changes the channel status into the requested mode and sends the result to the MEGACO slave  130  (S 538 ). The local channel number appointed between the MEGACO slave  130  and the channel management unit  120  is transmitted as the result.  
      If the channel type is a TDM channel (S 531 ), the channel status is checked (S 532 ). If the channel is in an already connected status, the channel management unit  120  connects the T-switch suitably for the channel status (S 536 ) and sends the result to the MEGACO slave  130  (S 537 ). The local channel number appointed between the MEGACO slave and the channel management unit is transmitted as the result.  
      If the channel status is in a channel standby status (S 532 ), the channel management unit  120  sends TDM data to the DSP and receives the result (S 534 ). The channel management unit  120  sends the received result to the MEGACO slave  130  using the local channel number (S 535 ).  
      If the channel connection request message is transmitted from the MEGACO slave  130  to the channel management unit  120  (S 540 ), the channel management unit  120  associates the two channels (local channels 1 and 2) through the associative management table  160  (S 541 ). The channel management unit  120  connects the TDM and the E1 framer through the T-switch (S 541 ) and sends the result to the MEGACO slave  130  (S 542 ).  
      If the RTP data update request message is transmitted from the MEGACO slave  130  to the channel management unit  120  (S 550 ), the channel management unit  120  sends the data (PayloadType, Jitter information) necessary for DSP internal voice information conversion to the selected DSP  406  together with the session number (S 551 ).  
      The DSP modifies the corresponding data and sends the result to the channel management unit  120  (S 552 ). The channel management unit  120  sends the received result to the MEGACO slave  130  (S 553 ). If voice data transmission/reception for an RTP Payload is started, voice data between the internal DSP and the RTP pump is transmitted to both terminals (S 554 ).  
      Since the media gateway of the present invention uses a centralized channel management method in call processing, the local DSPs of the media gateway can operate with only channel information necessary for the DSPs themselves without storing system-level channel information, and can provide channels for the requested E1 link in view of the outside of the system.  
      The present invention can also be embodied as computer readable codes on a computer readable recording medium. The computer readable recording medium is any data storage device that can store data which can thereafter be read by a computer system. Examples of the computer readable recording medium include read-only memory (ROM), random-access memory (RAM), CD-ROMs, magnetic tapes, floppy disks, optical data storage devices, and carrier waves (such as data transmission over the Internet). The computer readable recording medium can also be distributed over network of coupled computer systems so that the computer readable code is stored and executed in a decentralized fashion.  
      While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.