Abstract:
A codec automatic setting system of an IAD in a DSL network includes a codec table, a codec negotiator, a codec detector, and a codec selector. The codec table stores at least one codec list. The codec negotiator incorporates an applicable codec list from the codec table into a call request message, transmits the same to a reception IAD, and receives a response message when an arbitrary terminal requests a call. The codec detector detects a codec list from the response message. The codec selector compares the codec list of the reception IAD with the codec list stored in the codec table, and selects a settable codec. An automatic codec switching function in an IAD system of a network supporting a DSL automatically switches a codec into an optimum codec according to a DSL connection band and forming and release of a VoIP call, thereby securing an optimum communication quality for an Internet phone.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) AND CLAIM OF PRIORITY 
       [0001]    The present application makes reference to and claims all benefits accruing under 35 U.S.C. §119 from an application for “CODEC AUTOMATIC SETTING SYSTEM OF IAD AND CONTROL METHOD THEREOF IN DSL NETWORK” earlier filed in the Korean Intellectual Property Office on 5 Oct. 2007 and there duly assigned Serial No. 2007-100450. 
       TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method for automatically switching a voice over Internet protocol (VoIP) communication codec during narrowband digital subscriber line (DSL) connection in an integrated access devices (IAD) system and, more particularly, to offer/answer processing technology of an session initiation protocol (SIP) standard protocol for allowing an Internal protocol (IP) terminal to provide services where voice quality is secured. 
       BACKGROUND OF THE INVENTION 
       [0003]    A household and a company use an asymmetric digital subscriber line (ADSL) allowing digital information transmission through a telephone circuit in order to use the Internet and a telephone. 
         [0004]    The ADSL has an asymmetric structure that uses most of channels for downward transmission of sending information toward a user side, and assigns a very small portion of the channels for information from the user (i.e., for upward transmission). 
         [0005]    For example, German line DSL100 in Europe assigns 1 Mbps for downward transmission and 128 Kbps for upward transmission, and German line DSL2000 assigns 2 Mbps for downward transmission and 192 Kbps for upward transmission. 
         [0006]    In a related art, to use an Internet phone under a narrowband upward condition, the number of simultaneous calls is limited, the system is fixedly set using a codec using a small bandwidth, or silence suppression of a VoIP codec or a multi-frame function of a real-time transport protocol (RTP) payload has been used. 
         [0007]    However, in case of limiting the number of simultaneous calls because of a problem in a bandwidth in an Internet phone system using a related art ADSL, there has been a problem of increasing user inconvenience. 
         [0008]    In addition, in the case where the system is fixedly set using a codec using a small bandwidth, sound quality reduction occurs. 
         [0009]    Further, the silence suppression, which is used to reduce a bandwidth on the basis of the fact that about 50% of calls are a bundle, is vulnerable to a packet loss, a delay, and a jitter influence caused by a network load, so that voice quality reduction may be caused. 
         [0010]    Meanwhile, using multi-frame transmission in order to reduce overhead can reduce a bandwidth to some degree and thus used in a limited way, but is against packet division used in a narrowband condition. Accordingly, 30 m or more is not used. 
       SUMMARY OF THE INVENTION 
       [0011]    To address the above-discussed deficiencies of the prior art, it is a primary object to provide a codec automatic setting system of an IAD and a control method thereof in a DSL network, that switches to an optimum codec suitable for a bandwidth of VoIP communication to secure best voice quality during narrowband DSL connection. 
         [0012]    According to an aspect of the invention, the codec automatic setting system of an integrated access device (IAD) in a digital subscriber line (DSL) network may include a codec table storing at least one codec list; a codec negotiator incorporating an applicable codec list from the codec table into a call request message, transmitting the same to a reception IAD, and receiving a response message including a settable codec list when an arbitrary terminal requests a call; a codec detector detecting a codec list from the response message received from the reception IAD; and a codec selector comparing the codec list of the reception IAD detected from the response message through the codec detector with the codec list stored in the codec table, and selecting a settable codec. 
         [0013]    The codec negotiator may transmit the response message including the applicable codec list from the codec table when receiving a call request message including a codec list from a transmission IAD. 
         [0014]    The codec negotiator may transmit a response message including a codec list applicable from the codec table to the reception IAD when receiving a call re-request message from the reception IAD. 
         [0015]    When an arbitrary terminal requests a call additionally, the codec negotiator may incorporate a codec list processable by the reception IAD in communication into a call re-request message, transmit the same to the reception IAD, and receive a response message including a settable codec list. 
         [0016]    The codec selector may compare the codec list of the reception IAD detected from the response message with the codec list stored in the codec table, and then selects a settable codec. 
         [0017]    The codec selector may detect a codec list to be incorporated into a call request message, a response message, and a call re-request message from the codec table depending on an uplink frequency band and a set call. 
         [0018]    The codec table may include at least one of G.711, G.726-32, G.726-16, G.729, G.723.1 and T.38 Fax mode. 
         [0019]    According to another aspect of the invention, the control method of automatically setting a codec of an IAD in a DSL network may include, when an arbitrary terminal requests a call, incorporating an applicable codec list from a codec table into a call request message, transmitting the same to a reception IAD, and receiving a response message including a settable codec list; detecting the codec list from the response message received from the reception IAD; and comparing the codec list of the reception IAD detected from the response message with the codec list stored in the codec table, and selecting a settable codec. 
         [0020]    The control method may further include, when receiving a call request message including a codec list from a transmission IAD, transmitting a response message including an applicable codec list from the codec table; detecting the codec list from the call request message received from the transmission IAD; and comparing the codec list of the reception IAD detected from the call request message with the codec list stored in the codec table, and selecting a settable codec. 
         [0021]    The control method may further include, when receiving a call re-request message from the reception IAD, transmitting a response message including a codec list applicable from the codec table to the reception IAD; detecting a codec list from the call re-request message received from the transmission IAD; and comparing the codec list of the reception IAD detected from the call re-request message with the codec list stored in the codec table, and resetting a settable codec. 
         [0022]    The control method may further include, when an arbitrary terminal requests a call additionally, incorporating a codec list processable by the reception IAD in communication into a call re-request message, transmitting the same to the reception IAD, and receiving the response message including a settable codec list; detecting a codec list from the response message received from the reception IAD; and comparing the codec list of the reception IAD detected from the response message with the codec list stored in the codec table, and resetting a settable codec. 
         [0023]    The control method may further include: comparing the codec list of the reception IAD detected from the response message with the codec list stored in the codec table, and selecting a settable codec. 
         [0024]    The control method may further include: detecting a codec list to be incorporated into a call request message, a response message, and a call re-request message from the codec table depending on an uplink frequency band and a set call. 
         [0025]    The codec list of the codec table may include at least one of G.711, G.726-32, G.726-16, G.729, G.723.1 and T.38 Fax mode. 
         [0026]    The incorporating of the codec list applicable from the codec table into the call request message, the transmitting the same to the reception IAD, and the receiving the response message including the settable codec list when the arbitrary terminal requests the call may include: determining whether a call is a voice call or a fax call when the arbitrary terminal requests the call; when the call is the fax call in the determining of whether the call is the voice call or the fax call, determining whether a T.38Fax codec is available; and when the T.38Fax codec is available in the determining of whether the T.38Fax codec is available, incorporating G.711Fax and T.38Fax into a call request message, a call re-request message, or a response message when setting a codec list, and transmitting the same. 
         [0027]    The control method may further include, when the T.38Fax codec is not available in the determining of whether the T.38Fax codec is available, incorporating only G.711Fax into the call request message, the call re-request message, or the response message when setting the codec list, and transmitting the same. 
         [0028]    According to the present invention as set for above, in the codec automatic setting system of an IAD and the control method thereof in a DSL network, the automatic codec switching function in the IAD system of a DSL supporting network automatically switches a codec into an optimum codec according to a DSL connection band and forming and releasing of a VoIP call, thereby securing an optimum communication quality for an Internet phone. 
         [0029]    Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0030]    For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
           [0031]      FIG. 1  is a view illustrating an ADSL network including an IAD according to an embodiment of the present invention; 
           [0032]      FIG. 2  is a block diagram of a codec automatic setting system of an IAD in a DSL network according to an embodiment of the present invention; 
           [0033]      FIG. 3  is a view illustrating a signal flow as a codec is reset to a reception IAD in communication when an additional call is generated to a transmission IAD of the codec automatic setting system of the IAD in the DSL network of  FIG. 2  according to an embodiment of the present invention; 
           [0034]      FIG. 4  is a view illustrating a signal flow as a codec is reset to a reception IAD in communication when an additional call is generated from a second transmission IAD to a transmission IAD of the codec automatic setting system of the IAD in the DSL network of  FIG. 2  according to an embodiment of the present invention; 
           [0035]      FIG. 5  is a view illustrating a signal flow as a codec is reset to a reception IAD in communication when an additional call is generated from a second transmission IAD to a reception IAD of the codec automatic setting system of the IAD in the DSL network of  FIG. 2  according to an embodiment of the present invention; 
           [0036]      FIG. 6  is a view illustrating a signal flow as a codec is reset to a reception IAD in communication when an additional call is generated from a reception IAD to a second reception IAD of the codec automatic setting system of the IAD in the DSL network of  FIG. 2  according to an embodiment of the present invention; 
           [0037]      FIGS. 7A to 7D  are views of codec lists included in a call request message, a call re-request message, and a response message according to an embodiment of the present invention; 
           [0038]      FIG. 8  is a flowchart illustrating a control method of automatically setting a codec of an IAD in a DSL network according to an embodiment of the present invention; 
           [0039]      FIG. 9  is a flowchart illustrating a method of setting a codec of a reception IAD in the control method of automatically setting the codec of the IAD in the DSL network of  FIG. 8  according to an embodiment of the present invention; 
           [0040]      FIG. 10  is a flowchart illustrating a method of resetting a codec of a reception IAD in the control method of automatically setting the codec of the IAD in the DSL network of  FIG. 8  according to an embodiment of the present invention; 
           [0041]      FIG. 11  is a flowchart illustrating a method of resetting a codec of a transmission IAD in the control method of automatically setting the codec of the IAD in the DSL network of  FIG. 8  according to an embodiment of the present invention; and 
           [0042]      FIG. 12  is a flowchart illustrating detailed operations of an operation of incorporating an applicable codec list into a call request message INVITE and transmitting the same in the control method of automatically setting the codec of the IAD in the DSL network of  FIG. 9  according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0043]      FIGS. 1 through 12 , discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged communication system. 
         [0044]      FIG. 1  is a view illustrating an ADSL system including an IAD. The ADSL system includes an IP network  1 , an ADSL  2 , an IAD  10 , and at least one terminal  20 . 
         [0045]      FIG. 2  is a block diagram of a codec automatic setting system of an IAD in a DSL network according to the present invention. The codec automatic setting system of the IAD in the DSL network includes: a codec table  100 , a codec negotiator  200 , a codec detector  300 , and a codec selector  400 . 
         [0046]    The codec table  100  stores at least one codec list. Here, the codec list includes at least one of G.711, G.726-32, G.726-16, G.729, G.723.1 and T.38 Fax mode. The codec list has an RTP packet illustrated in Table 1. 
         [0000]    
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 G.726- 
                 G.726- 
                   
                   
                   
               
               
                 Header 
                 G.711 
                 16 
                 32 
                 G.729 
                 G.723.1 
                 T38 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Link 
                 8 
                 8 
                 8 
                 8 
                 8 
                 8 
               
               
                 control 
               
               
                 Identifier 
                 2 
                 2 
                 2 
                 2 
                 2 
                 2 
               
               
                 Ethernet 
                 14 
                 14 
                 14 
                 14 
                 14 
                 14 
               
               
                 header 
               
               
                 PPP over 
                 6 
                 6 
                 6 
                 6 
                 6 
                 6 
               
               
                 Ethernet 
               
               
                 Point to 
                 2 
                 2 
                 2 
                 2 
                 2 
                 2 
               
               
                 Point 
               
               
                 IP header 
                 20 
                 20 
                 20 
                 20 
                 20 
                 20 
               
               
                 UDP header 
                 8 
                 8 
                 8 
                 8 
                 8 
                 8 
               
               
                 RTP header 
                 12 
                 12 
                 12 
                 12 
                 12 
                 46 
               
               
                 Payload 
                 160 
                 40 
                 80 
                 20 
                 24 
               
               
                   
               
             
          
         
       
     
         [0047]    An actual use bandwidth is calculated including all of the headers as well as a payload. 
         [0048]    At this point, in case of the payloads of G.711, G.726-16, G.726-32, and G.729, a multi-frame of 20 ms is used as a reference, and a multi-frame of 30 ms is used as a reference for G.723.1. In the present invention, a bandwidth for each codec is determined using calculations below. 
         [0049]    Since a bandwidth is 232 bytes in the case of G.711a/u 20 ms, the bandwidth is 92.8 kbps in terms of kbps. 
         [0050]    Since a bandwidth is 112 bytes in the case of G.726-16a ms, the bandwidth is 44.8 kbps in terms of kbps. 
         [0051]    Since a bandwidth is 152 bytes in the case of G.726-32 20 ms, the bandwidth is 60.8 kbps in terms of kbps. 
         [0052]    Since a bandwidth is 92 bytes in the case of G.729 20 ms, the bandwidth is 36.8 kbps in terms of kbps. 
         [0053]    Since a bandwidth is 96 bytes in the case of G.723.1 30 ms, the bandwidth is 38.4 kbps in terms of kbps. 
         [0054]    Since a bandwidth is 106 bytes in the case of T.38 Fax mode, the bandwidth is 42.4 kbps in terms of kbps. 
         [0055]    Also, a voice quality for each codec is converted into a MOS (mean opinion score) value as illustrated in Table 2. 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 CODEC 
                 G.711 
                 G.726-32 
                 G.726-16 
                 G.729 
                 G.723.1 
               
               
                   
               
             
             
               
                 MOS 
                 4.1 
                 3.85 
                 3.72 
                 3.92 
                 3.65 
               
               
                   
               
             
          
         
       
     
         [0056]    Table 2 shows MOS values measured by a PESQ algorithm using VQT equipment, and sound quality reduction may be generated depending on the characteristic of a codec used in VoIP communication. 
         [0057]    When an arbitrary terminal  20  requests a call, the codec negotiator  200  incorporates a codec list applicable from the codec table  100  into a call request message INVITE, transmits the same to a second IAD  10 , and receives a response message  200  OK including a settable codec list. 
         [0058]    Meanwhile, when receiving a call request message INVITE including a codec list from a first IAD  10 , the codec negotiator  200  transmits a response message  200  OK including a codec list applicable from the codec table  100 . 
         [0059]    When receiving a call re-request message Re-INVITE from the second IAD  10 , the codec negotiator  200  transmits a response message  200  OK including a codec list applicable from the codec table  100  to the second IAD  10 . 
         [0060]    On the other hand, when the arbitrary terminal  20  requests a call additionally, the codec negotiator  200  incorporates a codec list processable by the second IAD  10  in communication into a call re-request message Re-INVITE, transmits the same to the second IAD  10 , and receives a response message  200  OK including a settable codec list. Meanwhile, the call re-request message is transmitted even when a call is released to request resetting of a codec. 
         [0061]    At this point, codec lists incorporated into a call request message, a response message, and a call re-request message by the codec negotiator  200  are illustrated in  FIGS. 7A to 7D . 
         [0062]    Meanwhile, when an arbitrary terminal  20  requests a call, the codec negotiator  200  determines whether the call is a voice call or a fax call. When the call is the fax call and T.38Fax codec is available, the codec negotiator  200  incorporates G.711Fax and T.38Fax into a call request message, a call re-request message, or a response message when setting a codec list. When the T.38Fax is not available, the codec negotiator  200  incorporates only G.711Fax into the call request message, call re-request message, or response message when setting the codec list, and transmits the same. 
         [0063]    In addition, the codec detector  300  detects a codec list from a response message received from the second IAD  10 . 
         [0064]    Also, the codec selector  400  compares the codec list of the second IAD  10  detected from the response message through the codec detector  300  with the codec list stored in the codec table  100 , and selects a settable codec. 
         [0065]    In addition, the codec selector  400  compares the codec list of the second IAD  10  detected from the response message with the codec list stored in the codec table  100 , and selects a settable codec. 
         [0066]    Here, the codec selector  400  detects a codec list to be incorporated into a call request message, a response message, and a call re-request message from the codec table  100  depending on an uplink frequency band and a set call. 
         [0067]    Descriptions of the general functions and detailed operations of the above-described elements are omitted, and instead, the operations relevant to the present invention are described below. 
         [0068]    First, when the arbitrary terminal  20  requests a call, the codec negotiator  200  of the transmission IAD  10 - 1  checks a codec list that can be set from the codec table  100 , incorporates the codec list into a call request message INVITE, and transmits the same to a relevant reception IAD  10 - 2  as illustrated in  FIG. 3 . 
         [0069]    That is, because the transmission IAD  10 - 1  does not use a bandwidth currently, it can use all of the codecs. Therefore, the codec negotiator  200  incorporates a codec list including all information for G.711, G.726-32, G.726-16, G.729, G.723.1, and T.38 Fax mode into a call request message INVITE as illustrated in  FIG. 7A . 
         [0070]    After that, when receiving a response message  200  OK including a codec list of the reception IAD  10 - 2  from the reception IAD  10 - 2 , the transmission IAD  10 - 1  detects the codec list of the reception IAD  10 - 2  included in the response message  200  OK through the codec detector  300 . 
         [0071]    When the codec list detected through the codec detector  300  includes codec information for G.711, G.726-32, G.726-16, G.729, G.723.1, and T.38 Fax mode as illustrated in  FIG. 7A , the codec selector 400 sets the G.711 that can provide a best voice quality as a codec. 
         [0072]    Therefore, the transmission IAD  10 - 1  and the reception IAD  10 - 2  provide VoIP voice communication through a G.711 codec. 
         [0073]    After that, when the arbitrary terminal  20  requests a second call to the transmission IAD  10 - 1  in communication, the codec negotiator  200  of a relevant IAD resets a codec with the reception IAD  10 - 2  in communication as a call to be provided increases by two. 
         [0074]    That is, the codec negotiator  200  searches for a codec that can be provided through a providable bandwidth and the rest bandwidth, except a bandwidth for a codec provided for call setting, from the codec table  100 , incorporates only a settable codec list (as illustrated in  FIG. 7B ) into a call re-request message Re-INVITE, and transmits the same to the reception IAD  10 - 2 . For example, the transmission IAD  10 - 1  having an uplink bandwidth of 128 kbps sets one G.711 codec (92.8 kbps). Then, 29.2 kbps remains from the bandwidth of the uplink. 
         [0075]    Therefore, when the arbitrary terminal  20  requests a second call to the transmission IAD  10 - 1  in communication, the codec negotiator  200  should perform codec renegotiation with the reception IAD  10 - 2  in communication. 
         [0076]    At this point, the transmission IAD  10 - 1  incorporates the codec list illustrated in  FIG. 7B  into a call request message INVITE, and transmits the same to a second reception IAD  10 - 4  to perform operations for call setting and codec setting. 
         [0077]    Meanwhile, the codec negotiator  200  incorporates the rest of the codec list ( FIG. 7B ) obtained by subtracting G.711 from the codec list stored in the codec table  100 , into a call re-request message, and transmits the same to the reception IAD  10 - 2  in communication. 
         [0078]    After that, when receiving a response message including a codec list from the reception IAD  10 - 2 , the transmission IAD  10 - 1  detects the codec list of the reception IAD  10 - 2  from the response message through the codec detector  300 . 
         [0079]    Subsequently, the transmission IAD  10 - 1  compares the codec list of the reception IAD  10 - 2  detected through the codec detector  300  with the codec list transmitted to the reception IAD  10 - 2  to change (reset) codec setting into a settable codec. That is, the codec selector  400  changes G.711 into G.726-32. The codec may be set in the order of a bandwidth size. 
         [0080]    Meanwhile, since only codec information (illustrated in  FIG. 7B ) for G.726-32, G.726-16, G.729, G.723.1, and T.38 Fax mode from the transmission IAD  10 - 1  in communication is incorporated into a call re-request message Re-INVITE even when the uplink bandwidth of the reception IAD  10 - 2  is 192 kbps, the codec selector  400  of the reception IAD  10 - 2  changes codec setting into the G.726-32 (60.8 kbps) having the greatest bandwidth. 
         [0081]    On the other hand, when receiving a call request message or a call re-request message from the transmission IAD  10 - 1 , the reception IAD  10 - 2  detects a codec list from the message, detects a settable codec list from the codec table  100 , incorporates the detected codec list into a response message, and transmits the same to the transmission IAD  10 - 1 . 
         [0082]    Meanwhile, when the second transmission IAD  10 - 3  tries a call request to the transmission IAD  10 - 1 , the operations illustrated in  FIG. 4  are performed. 
         [0083]    Also, when the second transmission IAD  10 - 3  tries a call request to the reception IAD  10 - 2 , the operations illustrated in  FIG. 5  are performed. 
         [0084]    In addition, when the reception IAD  10 - 2  tries a call request to a second reception IAD  10 - 4 , the operations illustrated in  FIG. 6  are performed. 
         [0085]    Hereinafter, a control method of automatically setting a codec of an IAD in a DSL network according to the present invention having the foregoing construction is described with reference to  FIGS. 8 to 12 . 
         [0086]    First, when the arbitrary terminal  20  requests a call, an applicable codec list from the codec table  100  is incorporated into a call request message INVITE, transmitted to a destination reception IAD  10 - 2 , and a response message  200  OK including a settable codec list is received as illustrated in  FIG. 8  (S 11 ). 
         [0087]    Subsequently, a codec list is detected from the response message received from the reception IAD  10 - 2  (S 12 ). 
         [0088]    Then, the codec list of the reception IAD  10 - 2  detected from the response message is compared with the codec list stored in the codec table  100 , and a settable codec list is then selected (S 13 ). 
         [0089]    On the other hand, when a call request message INVITE including a codec list is received from the transmission IAD  10 - 1 , a response message  200  OK into which an applicable codec list from the codec table  100  is incorporated is transmitted as illustrated in  FIG. 9  (S 14 ). 
         [0090]    The codec list is detected from the call request message (S 15 ). 
         [0091]    Subsequently, the codec list of the transmission IAD  10 - 1  detected from the call request message is compared with the codec list stored in the codec table  100 , and a settable codec is then selected (S 16 ). 
         [0092]    On the other hand, when a call re-request message Re-INVITE is received from the reception IAD  10 - 2 , a response message  200  OK into which an applicable codec list from the codec table  100  is incorporated is transmitted to the reception IAD  10 - 2  as illustrated in  FIG. 10  (S 17 ). 
         [0093]    A codec list is detected from a call re-request message (S 18 ). 
         [0094]    Simultaneously, the codec list of the reception IAD  10 - 2  detected from the call re-request message is compared with the codec list stored in the codec table  100 , and a settable codec is then reset (S 19 ). 
         [0095]    Meanwhile, when the arbitrary terminal  20  requests a call additionally, a codec list processable by the reception IAD  10 - 2  in communication is incorporated into a call re-request message Re-INVITE, and transmitted to the reception IAD  10 - 2 , and a response message  200  OK including a settable codec list is then received as illustrated in  FIG. 11  (S 20 ). 
         [0096]    The codec list is detected from the response message (S 21 ). 
         [0097]    Simultaneously, the codec list of the reception IAD  10 - 2  detected from the response message is compared with the codec list stored in the codec table  100 , and a settable codec is then reset (S 22 ). 
         [0098]    Meanwhile, the codec selector  400  selects a codec list to be incorporated into a call request message, a response message, and a call re-request message from the codec table  100  depending on an uplink frequency band and a set call. 
         [0099]    Here, the codec list of the codec table  100  includes at least one of G.711, G.726-32, G.726-16, G.729, G.723.1 and T.38 Fax mode. 
         [0100]    Meanwhile, the operation S 11  of incorporating the codec list into the call request message INVITE, transmitting the same to the destination IAD  10 - 2 , and receiving the response message  200  OK including the settable codec list will be described with reference to  FIG. 12 . 
         [0101]    When the arbitrary terminal  20  requests a call, whether the call is a voice call or a fax call is determined (S 111 ). 
         [0102]    When the call is the fax call in the operation (S 111 ) of determining whether the call is the voice call or the fax call, whether a T.38Fax codec is available is determined (S 112 ). 
         [0103]    When the T.38Fax codec is available (YES) in the operation (S 112 ) of determining whether the T.38Fax codec is available, G.711Fax and T.38Fax are incorporated into a call request message, a call re-request message, or a response message and transmitted while a codec list is set (S 113 ). 
         [0104]    On the other hand, when the T.38Fax codec is not available (NO) in the operation (S 112 ) of determining whether the T.38Fax codec is available, only G.711Fax is incorporated into a call request message, a call re-request message, or a response message and transmitted while the codec list is set (S 114 ). 
         [0105]    At this point, when the call is the voice call in the operation (S 111 ) of determining whether the call is the voice call or the fax call, an applicable codec list from the codec table  100  is incorporated into a call request message INVITE and transmitted (S 115 ). 
         [0106]    Although the present disclosure has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims.