JP2002124986A - Communication system and private branch exchange - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a communication system and a private branch exchange capable of rationally solving conflicting matters, that is, the assurance of voice quality and the improvement of transmitting efficiency, while considering an optimal balance as the whole network by controlling communication quality and the line using efficiency of a private line. SOLUTION: This communication system is provided with an exchange node to which a telephone set is connected for exchanging voice data, a line card connected to the exchange node and equipped with a plurality of coder/decoders for encoding data at a prescribed compression rate, and a router for connecting the line card through a private line to a networked communication network. This communication system is also provided with at least two coders/decoders having different encoding systems; a central controller for generating a control signal by recognizing an inter-node or whole communication network transmission band occupancy rate; and a selector for receiving the control signal from the central controller, and for selectively switching the coder/decoder so that the selection of the compression rate at the time of voice composition/encoding can be performed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a communication system in which voice signals between nodes connected to a communication network networked by a dedicated line are converted into IP packets for transmission and reception, and a private branch exchange employing the same. is there.

[0002]

2. Description of the Related Art Generally, when an audio signal is converted into IP packets, encoding at a high compression rate reduces the data amount and the packet amount of the packets, while the decoded audio has a greater degree of deterioration with respect to the original sound. . For example,
G.1 standardized by ITU-T (International Telecommunication Union Telecommunication Standardization Sector) recommendation 729 (CS-ACELP system) and the like. Conversely, if encoding is performed at a low compression rate, decoding can be performed with relatively high fidelity to the original sound, and high-quality voice communication can be performed. G. FIG. 711 (PCM system) and the like, but the compression rate is low, so that the data amount and the packet amount increase.

[0003] When transmitting and receiving a voice signal using an IP packet as described above, since the data amount and the packet amount differ depending on the coding method to be used, the quality of the voice quality is determined. Since it also affects the transmission efficiency of the dedicated line connecting the dedicated line, it is desirable to select an encoding method in consideration of the relationship with the line speed of the dedicated line used.

In recent years, portable telephones that transmit audio signals at a high compression rate in order to prioritize the use efficiency of resources in a wireless section have become widespread, and some degradation of audio is permitted. However, some users who are accustomed to the intra-corporate communication environment, which has used high-quality coding such as 64kPCM as standard, still need high-quality voice communication even in communication using IP packets. Seem.

[0005] In this situation, there is a problem in the prior art that the network is not operated in consideration of both the voice quality desired by the user and the transmission efficiency of the dedicated line.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems in the prior art, and makes it possible to control the communication quality and the line use efficiency of a dedicated line, and to consider an optimal balance as a whole network. It is another object of the present invention to provide a communication system and a private branch exchange that can rationally solve the contradictory issues of ensuring voice quality and improving transmission efficiency.

[0007]

According to a first aspect of the present invention for solving the above-mentioned problems, an audio signal is encoded and transmitted / received at a predetermined compression ratio between nodes of a communication network networked by a dedicated line. In the communication system, if the transmission bandwidth occupancy between nodes or the entire communication network is low, select a coding method with a relatively low compression rate, and if the transmission bandwidth occupancy between nodes or the entire communication network is high, select a compression method. This is a communication method characterized by selecting an encoding method having a relatively high rate.

Therefore, according to the communication system of the first invention of the present application, when the transmission bandwidth occupancy between the nodes or the entire communication network is low, it is possible to perform a speech with high voice quality, Although the voice quality is degraded when the transmission rate becomes higher, the transmission efficiency can be increased, and there is an advantage that the most suitable communication method can be selected for the bandwidth occupation state.

Further, the second invention of the present application is a communication system in which voice signals are encoded and transmitted / received at a predetermined compression rate between nodes of a communication network networked by a dedicated line connection. If the transmission bandwidth occupancy of the communication is too high to secure a new communication bandwidth, the coding scheme of the communication using the coding scheme with a relatively low compression rate that has already been established is changed to the coding scheme with a relatively high compression rate. This is a communication method characterized by changing to an encoding method.

[0010] Therefore, according to the communication system of the second invention of the present application, if the transmission bandwidth occupation ratio between the nodes or the entire communication network is so high that a new communication cannot be established, the communication is already performed. By changing the coding method of the established call to a method with a high compression rate, it becomes possible to lower the transmission bandwidth occupancy and establish a new call,
Further, there is an advantage that the transmission efficiency can be increased, and an optimal communication method can be selected for the band occupation state.

[0011] The third invention of the present application is a communication system in which voice signals are encoded and transmitted at a predetermined compression ratio between nodes of a communication network networked by a dedicated line connection. If the transmission bandwidth occupancy of the communication path is too high to secure a new communication bandwidth, the connection of the communication path is changed using a coding method with a relatively low compression rate that has already been established. This is a communication method characterized in that connection is performed using an encoding method having a relatively high compression ratio.

Therefore, according to the communication system of the third invention of the present application, when the transmission bandwidth occupancy between the nodes or the entire communication network is very high and a new communication cannot be established, the communication is already performed. By changing the connection of the call path of the established call and changing the encoding method to a method with a high compression rate, it becomes possible to lower the transmission bandwidth occupation rate and establish a new call. The transmission efficiency can be increased, and there is an advantage that a communication method most suitable for the bandwidth occupation state of the entire system can be selected.

Further, the fourth invention of the present application is directed to a telephone system, comprising: two or more telephones; an exchange node connected to the telephones for exchanging voice data; and a codec connected to the exchange node and encoding at a predetermined compression rate. In a private branch exchange including a plurality of line cards and a router for connecting the line cards to a communication network networked by a dedicated line connection, a control circuit for recognizing a transmission bandwidth occupancy between nodes and generating a control signal; A private branch exchange, comprising: a central control device that issues a call instruction to the control circuit; and a selector that receives the control signal from the control circuit and selectively switches the codec.

Therefore, according to the private branch exchange of the fourth invention of the present application, the codec is switched according to the transmission bandwidth occupancy between the nodes by the control circuit recognizing the transmission bandwidth occupancy between the nodes. It is possible to realize a call system that performs communication.

According to a fifth aspect of the present invention, there is provided a telephone having two or more telephones, an exchange node connected to the telephones for exchanging voice data, and a codec connected to the exchange node and encoding at a predetermined compression rate. In a private branch exchange including a plurality of line cards and a router for connecting the line cards to a network formed by a dedicated line, a control circuit for generating a control signal and a transmission band occupancy of the entire communication network are recognized. A private branch exchange comprising: a central control unit that issues an instruction to the control circuit; and a selector that receives the control signal from the control circuit and selectively switches the codec.

Therefore, according to the private branch exchange according to the fifth aspect of the present invention, the control circuit recognizes the transmission bandwidth occupancy of the entire communication network, thereby switching the codec in accordance with the transmission bandwidth occupancy of the entire communication network. It is possible to realize a characteristic call system.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A speech system and a private branch exchange according to one embodiment of the present invention will be described below with reference to the drawings.
The following is one embodiment of the present invention and does not limit the present invention.

First Embodiment First, a communication system and a private branch exchange according to a first embodiment of the present invention will be described with reference to FIG. FIG. 1 is a configuration diagram showing a call system and a private branch exchange according to Embodiment 1 of the present invention.

As shown in FIG. 1, Embodiment 1 of the present invention
The networked communication network includes an IP network 43 using a dedicated line, routers 41 and 71 connected thereto, a line card 31 for IP connection connected to the router 41, and an IP network connected to the router 71. A connection line card 61, a switching node 30 connected to the IP network 43 via the router 41 and the line card 31, a router 71 and the line card 6
A switching node 60 connected to the IP network 43 via
It comprises extension telephones 39 and 40 connected to the switching node 30, extension telephones 69 and 70 connected to the switching node 60, and central control units 36 and 66. Each of the line cards 31 has four trunk circuits (hereinafter referred to as IP circuits).
Trunks) 311, 312, 313, and 314. Each IP trunk has codec circuits 32 and 33 and selectors 34 and 35 as shown in IP trunk 311.
, And a control circuit 42. The line card 61 also has four trunk circuits (hereinafter referred to as IP trunks) 611, 612, 613, and 614 therein.
The trunk is composed of codec circuits 62 and 63, selectors 64 and 65, and a control circuit 72, as shown in IP trunk 611. And the central controller 36
Is equipped with a main storage device 37, and the central control device 66 is equipped with a main storage device 67.

The method adopted by the codec circuits 32 and 62 is based on the G.264 standardized by the ITU-T recommendation. 711, and the method adopted by the codec circuits 33 and 63 is G.711. 729. G. FIG. The 711 method has a low compression ratio, and can reproduce relatively faithfully the original sound, which is advantageous in terms of sound quality. On the other hand, the data amount and the packet amount are large, so that it is disadvantageous in terms of transmission efficiency. Conversely, G. The 729 method has a high compression rate, and thus has good transmission efficiency, but has a low voice quality. 71
Inferior to one method. The control circuits 42 and 72 are capable of recognizing the line speed of the IP leased line accommodated therein, manage the use state of each IP trunk circuit, control the selectors 34 and 35, and autonomously control the line of the connection partner. A signal transmission / reception process for call control with a card or an IP trunk is performed.

The extension telephones 39, 40, 69 and 70 have extension numbers 1100, 1200, 2100 and 220, respectively.
0 is assigned. In addition, each of them is an extension subscriber A,
Used by B, C and D. The central control devices 36 and 66 perform I-based operations based on data registered in the main storage devices 37 and 67.
Call control of voice communication is performed while controlling the P trunk and the like. Hereinafter, Embodiment 1 of the present invention will be described with reference to FIG.
Will be described.

Referring to FIG. 1, first, a case will be described in which a call is made from extension subscriber A (extension number: 1100) of exchange node 30 to extension subscriber C (extension number: 2100) of exchange node 60. When the extension subscriber A makes a call, an idle IP trunk 311 required for calling a dedicated line is captured, and the central control unit 36 sends the control circuit 4 of the line card 31.
2 is instructed to make a call to the switching node 60. The control circuit 42 that has received the call instruction has determined that the four subordinate IP trunks have not communicated at all so far,
It recognizes that there is still enough free bandwidth in the IP line that accommodates the communication, and for the current outgoing call from extension subscriber A, G. 71
It is determined that encoding is performed by one method, and selectors 34 and 35 are selected.
To select the audio processing by the codec 32.
Line card 61 receiving an incoming call from IP trunk 311
The control circuit 72 confirms the voice coding method with the control circuit 42 on the transmitting side, and determines that the codec type on the transmitting side is “G. Recognizing that this is the 711 system, the selectors 64, 6
5 to control the codec 62 even in the IP trunk on the called side.
In such a manner that the audio decoding is performed. Later,
In the same manner as in the prior art, the incoming call processing to the extension subscriber C is performed, and if C responds, G.G. A high-quality voice call using the 711 system becomes possible.

Next, while the extension subscribers A and C are communicating, the extension subscriber B of the exchange node 30 (extension number: 12)
00) to the extension subscriber D (extension number:
2200) will be described. When the extension subscriber B makes a call, an idle IP trunk 312 necessary for originating a dedicated line is captured in the same manner as in the above-described example, and the central control unit 36 sends the control circuit 42 of the line card 31 to the switching node 60. Is issued. The control circuit 42 that has received the call instruction recognizes that one of the four subordinate IP trunks (311) is already communicating at this point in time, but there is still an available band on the IP line, and Even for the outgoing call from the subscriber B, G.100 is still excellent in sound quality. Judgment that coding in the 711 system is applicable,
By controlling the selector, The audio processing by the 711-compatible codec is selected. The subsequent incoming operation to the extension subscriber D is the same as in the case of the extension subscriber C described above.

Next, a description will be given of a case where a call is made from another extension subscriber E of the switching node 30 to the extension subscriber F of the switching node 60 while the extension subscribers A and C and B and D are communicating. I do. When the extension subscriber E originates a call, the vacant IP trunk 31 required for originating the leased line is transmitted in the same manner as in the above-described example.
Then, the central control unit 36 issues a call instruction to the switching node 60 to the control circuit 42 of the line card 31. The control circuit 42 having received the call instruction has already transmitted G.G. in the band left on the IP line because two of the four subordinate IP trunks are already communicating at this time. 711
This time, it is recognized that encoding by the G. scheme is impossible, and this time, although the sound quality is inferior, G. 729 is determined to be performed, and the selector is controlled to perform G.729 coding. 7
The audio processing by the 29 compatible codec is selected.

The central controller 36 issues a call instruction to the IP trunk 313, and gives priority to transmission efficiency over speech quality as a codec type to be used. 729
The control circuit 42 is instructed to perform the voice processing by the system, that is, the codec 33. The control circuit 42 receiving this instruction controls the selectors 34 and 35 to control the codec 33
Select audio processing by. The control circuit 72 of the IP trunk 60 that has received the incoming call from the IP trunk 313 checks the voice coding method with the control circuit 42 of the transmitting side, and the codec type of the transmitting side is G.264. 729 system, and controls the selectors 64 and 65 to control the I
The P trunk operates so that speech decoding by the codec 63 is performed. Thereafter, extension subscriber F
When the extension subscriber F responds to the incoming call, the extension subscriber F responds to the G.P. 729 system is used for voice communication. In this case, network operation that prioritizes transmission efficiency over voice quality can be performed.

Next, while the three calls remain in a communication state, a further extension subscriber of the switching node 30 sends the switching node 6
Consider a case where a call is made to a 0 extension subscriber. When an extension subscriber makes a call, an idle IP trunk 314 required for calling a dedicated line is captured, and the central controller 36 instructs the control circuit 42 of the line card 31 to make a call.
Since the other three IP trunks are already in communication, the control circuit 42 determines that the G.100 is in the band left on the IP line.
It recognizes that even encoding in the 729 system is impossible. Therefore, the voice processing of the call of the IP trunk 311 during communication is performed according to G.264. G.711 to G. By switching to the 729 system, the selectors 34 and 35 are controlled to change to the codec 33 side so as to reduce the band occupied by the IP trunk 311 and allocate the band to the call of the IP trunk 314. By doing so, the bandwidth of the IP line is secured even for a new call, and 729 communication is enabled.

According to the communication system and the private branch exchange of the first embodiment, by dynamically changing the coding system of the voice processing including the call during communication, the sound quality is emphasized at the time of low traffic, and the high traffic is used. In some cases, it is possible to operate the system with emphasis on line efficiency, and to always provide services with the best call quality and line use efficiency.

Embodiment 2 Next, a communication system and a private branch exchange according to Embodiment 2 of the present invention will be described with reference to FIG. FIG. 2 is a configuration diagram showing a call system and a private branch exchange according to a second embodiment of the present invention.

As shown in FIG. 2, Embodiment 1 of the present invention
The networked communication network includes an IP network 43 using a dedicated line, routers 41 and 71 connected thereto, a line card 31 for IP connection connected to the router 41, and an IP network connected to the router 71. A connection line card 61, a switching node 30 connected to the IP network 43 via the router 41 and the line card 31, a router 71 and the line card 6
A switching node 60 connected to the IP network 43 via
It comprises extension telephones 39 and 40 connected to the switching node 30, extension telephones 69 and 70 connected to the switching node 60, and central control units 36 and 66. Each of the line cards 31 has four trunk circuits (hereinafter referred to as IP circuits).
Trunks) 311, 312, 313, and 314. Each IP trunk has codec circuits 32 and 33 and selectors 34 and 35 as shown in IP trunk 311.
, And a control circuit 42. The line card 61 also has four trunk circuits (hereinafter referred to as IP trunks) 611, 612, 613, and 614 therein.
The trunk is composed of codec circuits 62 and 63, selectors 64 and 65, and a control circuit 72, as shown in IP trunk 611. And the central controller 36
Is equipped with a main storage device 37, and the central control device 66 is equipped with a main storage device 67. Furthermore, the router 10 is connected to the IP network 43.
It is assumed that an extension telephone network having a similar configuration is connected to the extension telephone network 1 via a line card 91.

The system adopted by the codec circuits 32 and 62 is based on the G.264 standardized by the ITU-T recommendation. 711, and the method adopted by the codec circuits 33 and 63 is G.711. 729. G. FIG. The 711 method has a low compression ratio, and can reproduce relatively faithfully the original sound, which is advantageous in terms of sound quality. On the other hand, the data amount and the packet amount are large, so that it is disadvantageous in terms of transmission efficiency. Conversely, G. The 729 method has a high compression rate, and thus has good transmission efficiency, but has a low voice quality. 71
Inferior to one method.

Extension telephones 39, 40, 69, and 70 have extension numbers 1100, 1200, 2100, and 220, respectively.
0 is assigned. In addition, each of them is an extension subscriber A,
Used by B, C and D. Central control units 36 and 66 are IP
It can recognize the line speed of the entire network, manages the usage status of each IP trunk circuit, and controls the call control of voice communication while controlling the IP trunk based on the data registered in the main storage devices 37 and 67. The control circuit 4
In addition to letting 2, 72 control the selectors 34, 35,
Signal transmission / reception processing for call control with the line card or IP trunk of the connection partner is performed. Hereinafter, the operation of the second embodiment of the present invention will be described with reference to FIG.

Referring to FIG. 2, a case will be described in which a call is made from extension subscriber A (extension number: 1100) of exchange node 30 to extension subscriber C (extension number: 2100) of exchange node 60. When the extension subscriber A makes a call, the central controller 36 instructs the control circuit 42 of the line card 31 to make a call to the switching node 60. The central control unit 36 recognizes that the IP network still has a sufficient free band so far, and for the current outgoing call from the extension subscriber A, even if it occupies a lot of band, the sound quality is high. Excellent G. Judgment to perform encoding by the 711 system is made, and the control circuit 42 controls the selectors 34 and 35 to select the audio processing by the codec 32. The control circuit 72 of the line card 61 that has received the incoming call from the IP trunk 311 checks the voice coding method with the control circuit 42 on the calling side, and the codec type on the calling side is G.264. 711 system, and controls the selectors 64 and 65 to control the I
The P trunk 611 also operates so that speech decoding by the codec 62 is performed. Thereafter, the incoming call processing to the extension subscriber C is performed in the same manner as in the prior art, and if C responds, G.G. A high-quality voice call using the 711 system becomes possible.

Next, while the extension subscribers A and C are communicating, the extension subscriber B of the exchange node 30 (extension number: 12)
00) to the extension subscriber D (extension number:
2200) will be described. When the extension subscriber B makes a call, an idle IP trunk 312 necessary for originating a dedicated line is captured in the same manner as in the above-described example, and the central control unit 36 sends the control circuit 42 of the line card 31 to the switching node 60. Is issued. Central controller 36 has so far configured one of the IP trunks.
One (311) recognizes that the IP network still has a sufficient free band while communication is in progress, and that the outgoing call from extension subscriber A has a high sound quality even if it occupies a large amount of band. Excellent G. Judgment to encode by 711 system,
The control circuit 42 controls the selectors 34 and 35 to select the audio processing by the codec 32. The subsequent incoming operation to the extension subscriber D is the same as in the case of the extension subscriber C described above.

Next, a case where a call is made from another extension subscriber E of the switching node 60 to the extension subscriber F of the switching node 30 while the extension subscribers A and C and B and D are communicating with each other will be described. I do. When the extension subscriber E originates a call, the vacant IP trunk 61 necessary for originating the leased line is used in the same manner as in the above-described example.
3 is captured, and the central controller 66 instructs the control circuit 72 of the line card 61 to call the switching node 30. The central control unit 66 transmits the G.100 signal to the band that has already been left on the IP line at this point. It recognizes that encoding in the 711 system is impossible, and for the current outgoing call from the extension subscriber, the G.711 which is inferior in sound quality but has a small occupied band.
729 encoding, and the control circuit 7
2 by controlling the selectors 64 and 65 so that the G. 729 compatible codec is selected.

The central control unit 66 instructs the IP trunk 613 to make a call, and as a codec type to be used, prioritizes transmission efficiency over speech sound quality. 729
The control circuit 72 is instructed to perform the audio processing by the system, that is, the codec 63. The control circuit 72 having received the instruction controls the selectors 64 and 65 to control the codecs 33.
Select audio processing by. The control circuit 42 of the IP trunk 30 that has received the incoming call from the IP trunk 613 checks the voice coding method with the control circuit 72 of the transmitting side, and the codec type of the transmitting side is G.264. 729 system, and controls the selectors 34 and 35 to control the I
The P trunk operates so that voice decoding by the codec 33 is performed. Thereafter, extension subscriber F
Is processed, and if F responds, the extension subscriber E
To the G. through an IP trunk. 729 system is used for voice communication. In this case, network operation that prioritizes transmission efficiency over voice quality can be performed.

Next, let us consider a case where a call is made from a further extension subscriber G of the switching node 60 to an extension subscriber H on the line card 91 side while the three calls are being communicated. When the extension subscriber G makes a call, an idle IP trunk 614 required for calling a dedicated line is captured, and the central control unit 66 issues a call instruction to the control circuit 72 of the line card 61. The central control unit 66 transmits the G.264 signal in the band left on the IP line at this time. It recognizes that even encoding in the 729 system is impossible. Therefore, an instruction is issued to the control circuit 72 to temporarily suspend the call by the codec 32 between the IP trunk 311 and the IP trunk 611, and
By instructing the control circuit 72 to make a new call to the IP trunk 611 via the codec 63 with the IP trunk 311, the connection of the call path is changed to reduce the bandwidth occupied by the IP trunk 311, Are assigned to the call of the IP trunk 314 by controlling the selectors 64 and 65 to change to the codec 63 side. By doing so, the bandwidth of the IP line is secured for a new call,
G. FIG. 729 communication is enabled.

According to the communication system and the private branch exchange according to the second embodiment, the central control unit recognizes the traffic of the entire IP network, so that the encoding system of the voice processing including the call in communication in the entire IP network system. By dynamically changing the traffic quality, it is possible to operate the system with emphasis on sound quality during low traffic and emphasis on line efficiency during high traffic, and always provide services with the best call quality and line use efficiency. Become.

[0038]

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a first embodiment of the present invention.

FIG. 2 is a configuration diagram of Embodiment 2 of the present invention.

[Explanation of symbols]

30, 60 ... Switching node 31, 61, 91 ... Line card 311, 312, 313, 314, 611, 612, 6
13, 614... IP trunk 32, 62. G.711 codec circuits 33, 63. 729 codec circuit 34, 35, 64, 65 selector 36, 66 central control device 37, 67 main storage device 39, 40, 69, 70 extension subscriber 41, 71 router 42, 72 control circuit 43 IP network

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04Q 11/04 Q F term (Reference) 5K030 GA19 HA08 HB01 HC13 JA13 KA13 KA19 LA07 LB08 LC09 LE17 MA04 MB09 5K051 AA01 BB01 CC01 CC02 CC04 DD01 DD07 DD12 DD13 HH26 HH27 JJ01 JJ13 5K069 AA06 CA01 CA02 CB01 DA02 FA03 FC07 FC11 GA10 5K101 LL01 LL05 MM02 NN21 RR11 SS06 SS08 UU19

Claims (5)

[Claims] 1. A communication system in which voice signals are encoded and transmitted at a predetermined compression ratio between nodes of a communication network networked by a dedicated line connection, and the transmission bandwidth occupancy of the nodes or the entire communication network is low. In this case, a coding method having a relatively low compression rate is selected, and a coding method having a relatively high compression rate is selected when the transmission bandwidth occupancy between nodes or the entire communication network is high. method. 2. A communication system in which a voice signal is encoded and transmitted / received at a predetermined compression rate between nodes of a communication network networked by a dedicated line connection, wherein a transmission bandwidth occupation ratio between the nodes or the entire communication network is: If it is too high to secure a new call bandwidth, change the coding method of the call with the relatively low compression rate that has already been established to a coding method with a relatively high compression rate. A call system characterized by the following. 3. A communication system in which a voice signal is encoded at a predetermined compression ratio between nodes of a communication network networked by a dedicated line connection and transmitted / received, the transmission bandwidth occupancy ratio between the nodes or the entire communication network is: If it is too high to secure a new call bandwidth, the connection of the call path is changed using a coding method with a relatively low compression rate that has already been established, and a code with a relatively high compression rate is used when the connection is changed. A communication method characterized in that the connection is made in a simplified manner. 4. A line card including two or more telephones, an exchange node connected to the telephones for exchanging voice data, a plurality of codecs connected to the exchange nodes and encoding at a predetermined compression rate, In a private branch exchange having a router for connecting the line card to a communication network networked by a dedicated line, a control circuit for recognizing a transmission band occupancy between nodes and generating a control signal, and calling the control circuit for the first period. A private branch exchange, comprising: a central control device that issues an instruction; and a selector that receives the control signal from the control circuit and selectively switches the codec. 5. A line card comprising two or more telephones, an exchange node connected to the telephones for exchanging voice data, and a plurality of codecs connected to the exchange nodes and encoding at a predetermined compression rate. In a private branch exchange having a router for connecting the line card to a communication network networked by a dedicated line, a control circuit for generating a control signal and a transmission bandwidth occupancy of the entire communication network are recognized, and the control circuit is instructed. A private branch exchange, comprising: a central control device for performing the control; and a selector that receives the control signal from the control circuit and selectively switches the codec.