CA2020212C

CA2020212C - System for controlling data transmission in atm switching network

Info

Publication number: CA2020212C
Application number: CA002020212A
Authority: CA
Inventors: Ryuji Hyodo; Eisuke Iwabuchi; Tetsuo Nishino; Osamu Isono; Tetsuo Tachibana; Toshimasa Fukui
Original assignee: Fujitsu Ltd
Current assignee: Fujitsu Ltd
Priority date: 1989-07-03
Filing date: 1990-06-29
Publication date: 1993-11-09
Anticipated expiration: 2010-06-29
Also published as: US5067123A; AU5860990A; EP0406759B1; EP0406759A3; CA2020212A1; JPH0335637A; AU612938B2; DE69031580D1; EP0406759A2; JP2964151B2; DE69031580T2

Abstract

ABSTRACT OF THE DISCLOSURE
A transmission control system includes an ATM
switching network, terminals and terminal adapters respectively connected to the plurality of terminals.
The transmission control system also includes a table circuit registering virtual channels presently formed in the ATM switching network in a management table, and a retrieval part for receiving a communication request from one of the terminals and for determining whether or not the management table registers a virtual channel corresponding to the communication request at present.
Data output from the one of the terminals is transferred to a different one of the terminals addressed by the communication request through the virtual channel corresponding to the communication request and corresponding two of the terminal adapters when the retrieval part determines that the management table has the virtual channel corresponding to the communication request. A virtual channel setting part registers the virtual channel corresponding to the communication request in the management table when the retrieval processor determines that the management table does not have the virtual channel corresponding to the communication request. Data output from the one of the terminals is transferred to the different one of the terminals addressed by the communication request through the virtual channel registered in the management table by the virtual channel setting part.

Description

TITLE OF THE INVENTION

SYSTEM FOR CONTROLLING DATA TRANSMISSION IN ATM

SWITCHING NETWORK

FIELD OF THE INVENTION
. _ The present invention generally relates to a system for controlling data transmission in an ATM (asynchronous transfer mode) switching network.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features and advantages of the present inventlon will become apparent from the following detailed description when read in conjunction with the accompanying drawings, in which:
FIG. 1 is a block diagram of a conventional system which employs a handler;
FIG. 2 is a block diagram of another conventional ~y~tem~
FIG. 3 is a block diagram illustrating the princlple of the pre~ent invention;
FIG. 4 is a block diagram of a system according to a fir~t preferred embodiment of the present invention;
FIGS. 5, 6, 7 and 8 are flowcharts illustrating the operation of the system shown in FIG. 4;
FIGS. 9, 10 and ll are block diagrams, each illustrating the operatlon of the sy~tem shown ln FIG. 4;
FIG. 12 ls a block dlagram of a 6ystem according to a ~econd preferred embodlment of the present lnvention;
FIGS. 13, 14 and 15 are flowcharts illustratlng the operatlon of the ~y~tem shown in FIG. 12; and ~

-~V

A

.

FIGS. 16, 17 and 18 are block diagrams, each illustrating the operation of the system shown in FIG. 12.
BACKGROUND OF THE INVENTION
There is known an ATM switching network in which data is transferred in an asynchronous transfer mode. In the ATM
switching network, data is divlded into predetermined-length data pieces. A header is added to each predetermined-length data piece so that a cell is formed. In the ATM æystemr cells are transferred. ATM switching networks realize connection oriented ~ervices. Thus, ATM switching networks have advantages presented by the line switching and packet switching. Howeverr ATM
swltchlng networks are not oriented to connectionless services as provided by the packet swltchlng.
In order to realize connectionless services by using an ATM swltchlng networkr it i8 necessary to define a fixed virtual channel in the ATM switching system since the ATM switching network ls a connection-oriented services networkr as described above.
Referring to FIG. lr an ATM switching network 63 is provlded wlth a handler 62. A plurality of terminal equipment devlces (herelnafter slmply referred to as terminals) 61-1 - 61-n are coupled to the ATM swltchlng network 63. It wlll be noted that for the sake of slmpllcity, terminal adapters are omitted whlch derlved predetermlned-length cells from data and execute the rever~e process. The handler 62 sets addresses of the terminals 61-1 - 61-n on the ATM swltchlng network 63 and correspondlng flxed vlrtual channels ~lllustrated by the broken lines) by uslng a network management functlon. It wlll be noted that virtual A

.. ..
, channels are called virtual circuits for packet data. Further, the handler 62 assigns virtual channel identification numbers to the terminals 61-1 - 61-n.
Each of the terminals 61-1 - 61-n divides data into cells, each including the called party address, by using the virtual channel identification number assigned to the own, and ~ends the cells to the handler 62 through the ATM switching network 63. The handler 62 assembles received cells and identifies the called party address from the assembled original data. Then, the handler 62 seeks a correæponding virtual channel based on the identified party address. After that, the handler 62 dlvides the assembled data into cells again by using the virtual channel ldentification number of the related virtual channel, and sends the cells to the destination terminal. With the above-mentioned arrangement, it is possible to transfer data between the termlnals through the ATM switching network 63 by the connection-orlented services as provided by the packet switching system.
Referring to FIG. 2, a plurality of terminals 64-1 -64-n are connected to an ATM switchlng network 65. It wlll be noted that no handler is coupled to the ATM switching network.
Flxed vlrtual channels illustrated by broken lines are provided between terminals which have a posslbility that they communicate with each other. It can be considered that the system shown in FIG. 2 shares the function of the handler 62 shown ln FIG. 1 wlth the termlnals 64-1 - 64-n. The terminals can communlcate with each other through the fixed virtual channels via whlch cells are tran~ferred.
However, the conventional system shown in FIG. 1 has the A

disadvantages described below. It is necessary to provide the number of virtual channels equal to the number of terminals accommodated in the ATM switching network 63. When terminals are added to the system, the same number of virtual channels must be provided ln the ATM switching network 63. There iæ a limit of the number of virtual channels provided in the ATM switching network 63. For this reason, the number of terminals which can be coupled to the ATM switching network 63 is also limited. It should be noted that virtual channels are provided even for some terminals which are not often used. When terminals are added to or removed from the ATM switching network 63, the handler 62 must be modified 80 that the procedure for setting or releasing virtual channels are changed. Further, the handler 62 is required to have a large data processing ability to assemble and disassemble cells. Thus, the system employing the handler 62 is expenslve.
The conventional system shown in FIG. 2 has the dlsadvantages descrlbed below. As descrlbed previously, vlrtual channels are provided between termlnals which have a possibility that they communlcate wlth each other. When a large number of termlnals have ~uch a posslblllty, it i~ necessary to provide a large number of virtual channels in the ATM switching network 65.
It wlll be noted that a limited number of virtual channels is provided in the ATM switchlng network 65. Thu~, the number of terminals whlch can be accommodated in the ATM switching network 65 18 also llmlted. When termlnals are added to or removed from the ATM swltchlng network 65, all vlrtual channels related to these termlnals must be provided or removed.

: . . . ~ . , , ''. ~ ' ' :,. . - ,, .:, SUMMARY OF THE INVENTION
It is a general object of the present invention to provide an improved system for controlling data transmission in the ATM switching system in which the aforementioned disadvantages are eliminated.
A more specific object of the present invention is to provide a system for controlling data transmission in the ATM
switching system in which virtual channels are dynamically set so that data transmission between termlnals is improved.
In accordance with the present invention, there is provided a transmlssion control system comprising: an asynchronous transfer mode (ATM) switching network provlding vlrtual channels; a plurallty of terminals; a plurality of termlnal adapters connected to said ATM switching network and respectively connected to said plurality of termlnals, each of sald termlnal adapters coupllng a correspondlng one of sald termlnals to sald ATH switching network, data belng transferred between any palr of said terminals through two corresponding termlnal adapter~ from among sald termlnal adapters and said ATM
swltchlng network; management table means, lncludlng a management table, for reglsterlng the vlrtual channels presently formed ln sald ATM ~witchlng network in said management table, each of said virtual channels mutually connecting two corresponding terminal adapters through said ATM switching network; retrleval means for recelving a communication reque~t from a flrst one of sald termlnals and for determlnlng whether sald management table reglsters a vlrtual channel correspondlng to the communlcatlon request, data output from the flr~t one of said termlnals belng ~'1 ~, transferred to a second one of said termlnals addressed by the communication request through the virtual channel corresponding to the communication request and the two corresponding terminal adapters corresponding to the first and second ones of said terminals when said retrieval means determines that said management table has the virtual channel corresponding to the communication request; virtual channel setting means for registering the vlrtual channel corresponding to the communication request in said management table when said retrieval means determlnes that said management table does not have the virtual channel corresponding to the communication request, data output from the first one of said terminals being transferred to the second one of said terminals addressed by the communication request through the virtual channel registered in sald management table by said virtual channel æetting means; and virtual channel releaslng means for deletlng any of said vlrtual channels from sald management table of sald management table means satlsfying a predetermined condltion of virtual channel use wlthln sald ATM
switching network to release sald virtual channels unllkely to be reused. The management table means, retrleval means and vlrtual channel setting means can be provlded ln each of the terminal adapters or the ATM switching network.
In accordance with the present lnvention, there ls further provlded a transmission control system, comprlsing, an a~ynchronou~ transfer mode ~ATM) switching network providing virtual channels; a plurality of termlnals; and a plurality of termlnal adapter~ connected to sald ATM switchlng network and re~pectively connected to said plurality of terminals, each of ~.
' ' ' .: .' ' ', '' ' . :' . .
,: :
:..... . . . .. . . .
.. .. ..
~- . . . .. .
.

6a 27879-29 said terminal adapters coupling a corresponding one of said terminals to said ATM swltching network, data being transferred between any pair of terminals through corresponding terminal adapters from among said terminal adapters and one of said virtual channels in said ATM switching network, each of said terminal adapters including management table means, including a management table, for registering said virtual channels presently formed in sald ATM switching network in sald management table; retrieval means for receiving a communication request from a first one of said terminals and for determining whether said management table registers a vlrtual channel corresponding to the communication request, data output from the first one of said terminals being transferred to a second one of said terminals addressed by the communication request through the virtual channel corresponding to the communication request when said retrieval means determines that sald management table has the virtual channel corresponding to the communication request registered therein; virtual channel ~etting means for reglstering the virtual channel corresponding to the communlcation request ln sald management table when said retrleval means determines that said management table does not have the virtual channel correspondlng to the communication request, data output from the first one of said terminals being transferred to the second one of sald terminals addressed by the communlcatlon reque~t through the vlrtual channel reglstered ln ~ald management table by sald vlrtual channel settlng means; and vlrtual channel releaslng means for deletlng any of sald vlrtual channel~ ~rom cald management table of ~ald management table means ~atlcfylng a predetermlned condltlon of vlrtual channel use wlthin ~.

6b 27879-29 said ATM switching network to release said virtual channels unlikely to be reused.
In accordance wlth the present invention, there is further provided a transmission control system comprising: a plurality of terminals; a plurality of terminal adapters respectively connected to said plurality of terminals; and an asynchronous transfer mode (ATM) switching network connected to said terminal adapters, providing virtual channels, each of said terminal adapters coupling a corresponding one of said terminals to said ATM switching network, data being transferred between a pair of said terminals through two corresponding terminal adapters from among said terminal adapters and one of said virtual channels in said ATM switching network, said ATM switching network lncluding management table means, including a management table, for registering said virtual channels presently formed in said ATM
~witching network in said management table; retrieval means for recelving a communlcation request from a first one of said termlnals and for determining whether said management table pre~ently registers a virtual channel corresponding to the communlcatlon request, data output from the first one of said termlnals belng transferred to a second one of said terminals addressed by the communicatlon request through the virtual channel correspondlng to the communlcatlon request and the two correspondlng termlnal adapter~ correspondlng to the flrst and second ones of sald termlnals when ~ald retrieval means determines that ~ald management table has the vlrtual channel correspondlng to the communicatlon request reglstered therein; vlrtual channel settlng means for registerlng the vlrtual channel correspondlng to ' ' ' ' " ~
': ' . , , ' ' . .
..
.

6c 27879-29 said communication request in the management table when said retrieval means determines that said management table does not have the virtual channel corresponding to the communication request, data output from the first one of said terminals being transferred to the second one of said terminals addressed by the communication request through the virtual channel registered in said management table by said virtual channel setting means; and virtual channel releasing means for deleting any of said virtual channels from said management table of said management table means satisfying a predetermined condition of virtual channel use within sald ATM switching network to release said virtual channels unlikely to be reused.
In accordance with the present invention, there is further provided a transmission control system for an asynchronous transfer mode switching network providing virtual channels to connect terminals together, comprising. a management table for storing channel ldentifiers of the virtual channels formed by the asynchronous transfer mode ~witching network; retrieval means for recelving a communication request for a communication channel from a flrst terminal to a ~econd terminal and for determining whether sald management table includes an identifier for the communication channel among the channel identifiers stored therein, data from the fir~t termlnal belng transmitted to the second termlnal addressed by the communicatlon request through the communlcation channel when sald retrleval means determine~ that said management table lncludes the identlfler for the communication channel;
vlrtual channel settlng means for sending the identifler for the communlcatlon ahannel to said management table when said retrieval ~, 6d 27879-29 means determines that said management table does not include the identifier for the communication channel, data from the first terminal being transmltted to the second terminal addressed by the communication request through the communication channel after the identifier therefore is stored in said management table; and virtual channel releasing means for deleting at least one of the channel identifiers from said management table satisfying a predetermined condition of virtual channel use to release the channel identifiers of said virtual channels unlikely to be reused.
In accordance with the present invention, there is further provided a method ~or controlling the transmission of data between terminals through an asynchronous transfer mode swi.tching network providing virtual channels, comprising the steps of: ~a) recelvlng a communicatlon request for a communication channel from a first terminal to communicate with a second terminal; (b) determinlng whether the communication channel is identifled in a management table containing channel ldentifiers for all presently provldlng vlrtual channels; (c) storing an identifier for the communlcatlon channel ln the management table when the ldentlfier for the communication channel ls not ln the management table; (d) transferrlng data from the flrst terminal to the second terminal through the communication channel when the identlfier for the communlcation channel 1B stored ln the management table after elther of steps ib) and ~c); and ~e) releasing the virtual channels when a predetermined condition of virtual channel use withln the a~ynchronou~ transfer mode swltchlng network ls satl~led.

' ~ - , , 6e 27879-29 DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 3, there is illustrated the prlnciple of a system for controlling data transmission in an ATM swltching network accordlng to the present invention. A plurality of terminals 1-1 - l-n are coupled to an ATM switching network 3 through terminal adapters 2-1 - 2-n, respectively. Data ls transferred between the terminals 1-1 - l-n through the ATM
switching network 3 in an asynchronous transfer mode (ATM). A
managemen~ table 4 which manages virtual channels formed in the ATM swltchlng network 3 ls coupled to the ATM switching network 3.
A table controller 5 controls the management table 4. The table controller 5 determines whether or not vlrtual channels corresponding to communlcation requests from the terminals 1-1 -l-n have been provlded ln the ATM switching network 3. When the table controller 5 determines that the ATM switching network 3 has a vlrtual channel corre~ponding to the communlcatlon request from a termlnal, cells are transferred through this virtual channel.
On the other hand, when the table controller 5 determines that the ATM swltching network 3 does not have a virtual channel corresponding to the communication request, the table controller 5 deflnes the corresponding vlrtual channel in the management table 4. The termlnal adapter 2 dlvldes data output from the terminal whlch generates the communicatlon request lnto cells, each havlng the header and send~ the same ln the a~ynchronou~ transfer mode.
Data output by each of the termlnals 1~ n is ln conformlty to, for example, the CCITT X.25 recommendation. The cells pass through the ATM ~witching network 3 and is sent to the destination termlnal. When a predetermined conditlon 1~ satisfied, the virtual channel 1~ deleted from the management A

- 7 - 2~2~2~2 1 table 4. For example, when a predetermined time elapses from the time when the last cell is transferred, the corresponding virtual channel is deleted from the management table 4. It will be noted that if the terminal 1-n has the function of the adapter 2-n, the terminal has the configuration surrounded by the one-dot chain line shown in FIG.3.
The management table 4 manages the relationship between the virtual channels (VCI/VPIl, VCI/VPI2, ...) and addresses (ADl, AD2, ...) of the terminals 1-1, 1-2, ... on the ATM switching network 13. The table controller 5 retrieves the management table 4, and sets or releases virtual channels. For example, the table controller 5 operates as follows.
The table controller 5 determines whether or not the management table 4 has the called party address in response to the communication request. When the table controller 5 finds out the called party address in the management table 4, it recognizes that the related virtual channel directed to the other party terminal has been formed in the ATM switching network 3. When the table controller 5 does not find the called party address, it registers a virtual channel coupling two terminals being considered in the management table 4.
Data output from the terminal which generates the communication request is divided into cells by the corresponding terminal adapter. Then, the cells are sent to the ATM switching network 3. As a result, it is possible to transfer data between terminals through the ATM switching network 3 by defining the corresponding virtual channel even if the ATM switching network 3 does not have the corresponding virtual channel at present.
When the predetermined condition is satis~ied, virtual channels de~ined in the management table 4 are deleted there~rom. For example, virtual channels de~ined in the management table 4 are deleted - 8 - 2~212 1 for every predetermined time. Alternatively, it is possible to delete each virtual channel from the management table 4 when a predetermined time elapses from the time when the corresponding virtual channel was last used. With the above-mentioned arrangement, it becomes possible to provide virtual channels dynamically and thus utilize the same effectively and efficiently.
A description will now be given of a first preferred embodiment of the present invention.
Referring to FIG.4, there is illustrated a system according to the first preferred embodiment of the present invention. A terminal 11 is coupled to an ATM
switching network 13 through a terminal coupler 12.
The terminal coupler 12 is composed of an adaptation layer processor (hereinafter simply referred to as an ADP processor) 20, a signal processor 21, an ADP
processor 22, an ATM processor 23, a physical layer processor 24 and a table processor 25. Each of the ADP
processors 20 and 22 executes a conventional adaptation , layer process. The ATM processor 23 assembles and disassembles cells. The physical layer processor 24 has the function of converting a receive signal level into a transmit signal level and vise versa. In a case where the ATM switching network 13 shown in FIG.4 is coupled to other ATM switching networks (not shown) through optical transmission channels, the physical layer processor 24 carries out a conversion between optical signals and electrical signals.
The table processor 25 is made up of a table controller 26 and a management table 27. The table controller 26 is composed o~ a retrieval processor 28 and a setting/relea~ing processor 29. The retrieval processor 28 accesses the management table 27 and determines whether or not the virtual channel corresponding to the communication request is defined in the management table 27. The setting/releasing ~ 9 ~ 2~2~2~2 1 processor 29 writes virtual channels into the management table 27 and eliminates the same therefrom.
The management table 27 defines storage areas, each of which defines the party address AD, the virtual channel identification number VCI/VPI, and time T. The time T
indicates when the virtual channel being considered was last used. Information about the time T is used for eliminating virtual channels from the management table 27. For example, when a predetermined time elapses from the time T, the corresponding virtual channel is deleted from the management table 27. Alternatively, it is possible to eliminate the virtual channel being considered when it is determined that the number of times that the present virtual channel is used is less than a predetermined number of times. As will be described later, it is possible to employ an additional condition, such as a release inhibit condition.
The terminal 11 sends the terminal adapter 12 data to which the called party address, that is, the called (destination) terminal is added. The ADP
processor 20 receives the data and then transfers the called party address to the table processor 25. The table processor 25 controls the retrieval processor 28 so that it determines whether or not the virtual channel corresponding to the called party address is presently registered in the management table 27. When the retrieval processor 28 finds the called party address in the management table 27, the retrieval processor 28 sends the same to the ATM processor 23.
Also, the ADP processor 20 divides the received data into cells, each having the header. The header includes in~ormation which indicates the position of the cell having this header in a series of cells. Each cell with the header added is trans~erred to the ATM
proce~sor 23, which writes the virtual channel identi~ication number addressed to the destination terminal into the header. Then, each cell is sent to , ' ~ ' ' '' , ' :.
' "''. : ~ , 2a2~2~2 1 the ATM switching network 13 through the physical layer processor 24 ~
If the retrieval processor 28 determines that the virtual channel corresponding to the called party address is not registered in the management table 27 the retrieval processor 28 controls the setting/releasing processor 29 so that it sends a virtual channel setting request to the signal processor 21s The signal processor 21 determines a virtual channel identification number corresponding to the called party address, and informs the setting/releasing processor 29 of the determined virtual channel identification number. Alternatively, it is possible to generate the corresponding virtual channel by communicating with the ATM switching network 13 through the ATM processor 23 and the physical layer processor 24 in accordance with a call set-up procedure. This procedure will be described later.
The ATM switching network 13 sends 20 information about the virtual channel setting request to the destination terminal. The setting/releasing processor 29 writes the virtual channel identification number VCI/VPI of the destination terminal into the management table 27~ Then, the setting/releasing 25 processor 29 informs the ATM processor 23 of the virtual channel identification number VCI/VPI. The ATM
processor 23 writes the virtual channel identification number VCI/VPI into the header of each cell, which is then sent to the ATM switching network 13 through the ATM processor 23 and the physical layer processor 24~
In the case where the virtual channel setting request information is received from the ATM switching network 13, the signal processor 21 of the terminal adapter 12 which is located on the destination side in thi8 case analyzes the same, and sends the party address AD and the virtual channel identification number VCI/VPI to the setting/releasing processor 29 of , - ' "' ' - 11 - 2~2~2~ 2 1 the table processor 27. By using the received information, the setting/releasing processor 29 registers the corresponding virtual channel in the management table 27.
Each virtual channel defined in the management table 27 is released, that i5, deleted from the management table 27 as follows. A timer is provided in the signal processor 21 or the table processor 25. In the configuration shown in FIG.4, a timer is provided in the signal processor 21. Each time the timer indicates a predetermined time, the setting/releasing processor 29 releases virtual channels in the management table 27 unconditionally.
Alternatively, when the time passage between the time T
and the present time is equal to or greater than a predetermined passage time, the setting/releasing processor 29 releases the corresponding virtual channel unconditionally. Alternatively, when the number of times that the virtual channel being considered has been used is equal to or greater than a predetermined ; number of times, the setting/releasing processor 29 releases it. Alternatively, when the setting/releasing processor 29 determines that an available storage capacity of the management table 27 becomes less than a predetermined capacity which corresponds to, for example, one virtual channel, at least one virtual channel is released unconditionally or at least one virtual channel which is not frequently used is released. Alternatively, when a large number of virtual channels have already been realized in the ATM
switching network 13 so that it becomes impossible to provide a new virtual channel, at least one virtual channel i8 released unconditionally or at least one virtual channel which is not frequently used is released.
It is possible to use a flag which inhibits the virtual channel from being released so that a ,; ; .

- 12 - 2~20212 1 virtual channel having a high degree of importance can be prevented from being deleted from the management table 27. Thereby, it is possible to dynamically register virtual channels in the management 27 and thus utilize them effectively and efficiently. The flag can be provided in the management table 27, for example.
FIG.5 is a flowchart of a process realized by the table processor 25 shown in FIG.4. In FIG.5, "IDLE" denotes a process waiting state of the table processor 25. The called party address is output to the retrieval processor 28 from the ADP processor 20 (step 1), and the retrieval processor 28 starts to retrieve the management table 27 (step 2). The retrieval processor 28 determines whether or not the virtual channel corresponding to the called party address is presently registered in the management table 27 (step 3). When the result obtained at step 3 is YES, the retrieval processor 28 informs the ATN
processor 23 of the virtual channel identification number VCI/VPI of the other party terminal (step 4).
On the other hand, when the management table 27 does not define the corresponding virtual channel at present, the retrieval processor 28 instructs the setting/releasing processor 29 to send the virtual channel setting request to the signal processor 21 (step 5). The signal processor 21 generates the virtual channel corresponding to the called party address by its internal process or transferring related in~ormation between the own and the ATM network 13.
Then, the signal processor 21 informs the setting/releasing processor 29 of the table processor 25 o~ the fact that the virtual channel has been set.
This procedure is carried out by sending the called party address and the virtual channel identification number VCI/VPI to the setting/releasing processor 29 (step 6). Then, the setting/releasing processor 29 in~orms the ATM processor 23 of the informed virtual - 13 - 20~2~2 1 channel identification number VCI/VPI (step 7). After that, the setting/releasing processor 29 writes the virtual channel defined by the signal processor 21 into the management table 27.
In the case where the system shown in FIG.4 receives the virtual channel setting request from the source terminal, it is sent to the signal processor 21 through the physical layer processor 24, the ATM
processor 23 and the ADP processor 22. The signal processor 21 analyzes the received virtual channel setting request and generates, from the analyzed results, the calling party address and the virtual channel identification number VCI/VPI of the virtual channel connected to the source terminal. Then, the signal processor 21 informs the setting/releasing processor 29 of the calling party address and the virtual channel identification number VCI/VPI (step 9). Then, the signal processor 21 registers the corresponding virtual channel in the management table 27 (step 10).
FIG.6 is a flowchart of the procedure for unconditionally releasing virtual channels. The procedure shown in FIG.6 is executed by the table processor 25. As has been described previously, the timer for measuring the predetermined time is provided in the table processor 25 or the signal processor 21.
When the timer indicates the predetermined time, it lets the setting/releasing processor 29 of this fact ~step 11). The setting/releasing processor 29 generates a request to release the virtual channels de~ined in the management table 27 (step 12). Then, the setting/releasing processor 29 deletes the virtual channel being considered from the management table 27 (5tep 13). A~ter that, the setting/releasing processor 29 determines whether or not all the virtual channels defined in the management table 27 have been deleted ~step 14). When the result at step 14 is NO, the . . .
, . -' ., .:

- 14 - 2~2~2~2 1 procedure returns to step 12. On the other hand, when the result at step 14 is YES, the setting/releasing processor 29 sets the timer so that it starts to measure the predetermined time (step 15).
On the other hand, the signal processor 21 analyzes the virtual channel releasing request and generates the calling/called party address and the virtual channel identification number VCI/VPI (step 16). Then, the signal processor 21 generates the request to release the corresponding virtual channel (step 16), and sends the same to the setting/releasing processor 29. In response to this request, the setting/releasing processor 29 deletes the specified virtual channel from the management table 27 (step 17).
FIGS.7 and 8 are flowcharts of the procedures for monitoring the passage time on the side of the ATM
switching network 13 and releasing virtual channels on the basis of the monitored passage time. The procedure shown in FIG.7 is related to the terminal adapter 12, and the procedure shown in FIG.8 is related to the ATM
switching network 13. The ATM switching network 13 is provided with a table which defines the virtual channel identification number VCI/VPI and time T, and a timer for measuring a predetermined time. When the timer built in the ATM switching network 13 indicates the predetermined time, the ATM switching network 13 determines whether or not the passage time between the time when the virtual channel being considered was last used and the present time exceeds a predetermined threshold time. This determination procedure is carried out ~or each o~ the virtual channels defined in the management table provided in the ATM switching network 13. When the determination result is YES, the request to delete the corresponding virtual channel ~rom the management table provided in the ATM switching network 13 is generated (step 21). Then, the virtual - 15 - 2~2~212 1 channel defined by the virtual identification number VCI/VPI and time T are deleted from the management table in the ATM switching network 13 (step 22). Then, it is determined that all the virtual channels defined in the management table in the ATM switching network 13 have been processed (step 23). When the result at step 23 is NO, the procedure returns to step 21. On the other hand, when the result at step 23 is YES, the timer built in the ATM switching network 13 i5 started to measure the predetermined time (step 24).
On the other hand, the setting/releasing processor 29 is informed of thé request which is generated at step 21 (step 18). Then, the setting/releasing processor 29 deletes the corresponding virtual channel from the management table 27 (step 19). As described above, the procedures shown in FIGS.7 and FIG.8 are related to the arrangement in which the ATM switching network 13 monitors the predetermined time by the timer provided therein and it is determined whether or not the passage time from the time when the virtual channel of interest was last used exceeds the predetermined threshold time.
Alternatively, it is possible to utilize the number of times that each virtual channel defined in the management table in the ATM switching network 13 was used in the past. In this case, it is determined whether or not the number o~ times that the virtual channel being considered exceeds to a predetermined number o~ times.
FIGS.9 through 11 are diagrams illustrating the operation of the system shown in FIG.4. Referring to FIG.9, terminals TE1 and TE2 are coupled to the ATM
switching network 13 through terminal adapters TAl and TA2, respectively. In the illustrated case, address AD2 o~ another party terminal, virtual channel identi~icatlon number VCI/VPI12 and last used time tl are registered in a management table 27-1 of the 2~2~2~2 1 terminal adapter TAl. Also, address ADl of another party terminal, virtual channel identification number VCI/VPI21 and last used time t2 are registered in a management table 27-2 of the terminal adapter TA2.
It is now assumed that data is sent from the terminal TEl to the terminal TE2. The terminal adapter TAl retrieves the management table 27-1 and determines whether or not the virtual channel addressed to the terminal TE2 is presently defined in the management table 27-1. In the case illustrated in FIG.9, the address of the terminal TE2 and the virtual channel identification number AD2 are defined in the management table 27-1. Thus, the terminal adapter TAl divides data to be transmitted into cells, each having the header containing the virtual channel identification number VCI/VPI12. Then, the terminal adapter TAl sends the cells to the ATM switching network. Time t3 when all the cells have just been set to the ATM switching network 13 is written into the management table 27-1 of the terminal adapter TA1.
The terminal adapter TA2 receives the cells ~rom the ATM switching network 13 and assembles the received cells into the original data. The assembled data is sent to the terminal TE2. Time t4 when the data has just been sent to the terminal TE2 is written into the management table 27-2 of the terminal adapter TA2.
FIG.10 illustrates a case where data is sent ~rom the terminal TE1 to a terminal TE3. The terminal adapter TA1 retrieves the management table 27-1 in the way as described previously. In this case, the ATM
switching network 13 does not have any virtual channel which mutually connects the terminal adapters TAl and ; TA3. Thus, each o~ the management tables 27-1 and 27-3 does not have any in~ormation about the virtual channel which connects the terminal adapters TA1 and TA3, that is, the address o~ the other party terminal and the 2!~2~212 1 virtual channel identification number VCI/VPI. Thus, the terminal adapter TA1 requests the ATM network 13 to set the corresponding virtual channel (VP).
The ATM switching network 13 selects an idle virtual channel and informs each of the terminal adapters TA1 and TA3 of the selected virtual channel.
The selected virtual channel identified by VCI/VPI13 or VCI/VPI31 is defined in the management tables 27-1 and 27-3, respectively. The terminal adapter TA1 divides data to be transmitted into cells, each having the header including the virtual channel identification number VCI/VPI13. Then, the cells are sent to the ATM
switching network 13 from the terminal adapter TA1.
Time t5 when all the cells have just been output to the ATM switching network 13 is written into the management table 27-1 of the terminal adapter TA1. The terminal adapter TA3 receives the cells from the ATM switching network 13 and assembles the same into the original data. Then, the terminal adapter TA3 sends the original data to the terminal TE3. Time t6 when the original data has just been output to the terminal TE3 i8 written into the management table 27-3 of the terminal adapter TA3. By the above-mentioned procedure, the address AD3 of the terminal TE3, the virtual channel identification number VCI/VPI13 and the last used time t5 are defined in the management table 27-1. Similarly, the address ADl of the terminal TEl, the virtual channel identification number VCI/VPI31 and the last used time t6 are defined in the management table 27-3.
FIG.11 illustrates a case where the management table 27-2 of the terminal adapter TA2 is referred to and the virtual channel releasing request is generated due to the fact that the passage time from the last used time when the virtual channel VP
addressed to the terminal TE1 was last used exceeds the predetermined threshold time. The terminal adapter TA2 ", 2~2~712 1 ssnds the virtual channel releasing request to the ATM
switching network 13. In response to this request, the ATM switching network 13 informs each of the terminal adapters TA1 and TA2 of the occurrence of the virtual channel releasing request. Thereby, the address AD2 of the terminal TE2, the virtual channel identification number VCI/VPI12 and time t3 are deleted from the management table 27-1. Similarly, the address AD1 of the terminal TE1, the virtual channel identification number VCI/VPI21 and time t4 are deleted from the management table 27-2.
A description will now be given of a second preferred embodiment of the present invention with reference to FIG.12. It will be noted that a management table as provided in each of the aforementioned terminal adapters is provided in the ATM
switching network and that each terminal adapter does not have any management table. Referring to FIG.12, the system is composed of a terminal 31, a terminal adapter 32 and an ATM switching network 33. It will be noted that the configuration of the ATM switching network 33 corresponds to a network controller provided at an arbitrary position thereof. The terminal adapter 32 is of the same structure as the terminal adapter 12 (FIG.12~ except that the terminal adapter 32 does not have any management table. That is, the terminal adapter 32 is composed of an ADP processor 40, a signal processor 41, an ADP processor 42, an ATM processor 43 and a physical layer processor 44.
The ATM switching network 33 includes a physical layer processor 50, an ATM processor 51, an ADP processor 52, a signal processor 53 and a table ! processor 54. The table processor 54 is made up of a table controller 55 and a management table 56. The table controller 55 is comprised of a retrieval processor 57 and a setting/releasing processor 58. The above-mentioned elements of the ATM switching network - 19 - 2~2~2 1 33 operate in the same as corresponding elements shown in FIG.4. The management table 56 is provided for each of the terminals 31 (only one management table 56 is shown in FIG.12).
FIG.13 is a flowchart of the operation of the table processor 54 shown in FIG.12. The retrieval processor 57 receives the called party address from the signal processor 53 of the AT~ switching network 33 (step 31). Then, the retrieval processor 57 retrieves the management table 56 (step 33), and determines whether or not a virtual channel corresponding to the received called party address is presently defined in the management table 56 (step 33). When the result obtained at step 33 is YES, the retrieval processor 57 informs the signal processor 53 of the virtual channel identification number VCI/VPI of the defined virtual channel (step 34). On the other hand, when it is determined that no virtual channel related to the received called party address is presently defined in the management table 56 (step 33), the retrieval processor 57 instructs the setting/releasing processor 58 to request the signal processor 53 to generate a request to set the virtual channel corresponding to the called party address (step 35). Then, the signal processor 53 selects an idle virtual channel and informs the setting/releasing processor 58 of the calling party address and the virtual channel identification number VCI/VPI (step 36). The virtual channel identification number VCI/VPI thus obtained is sent to the ATM processor 51 (step 37). Then, the setting/releasing processor 58 registers the selected virtual channel in the management table 56 (step 38).
As described above, a virtual channel is set in response to the communication request addressed to a terminal which is not de~ined in the management table 56 so that cells can be transmitted through this virtual channel.

, 2~2~12 1 FIG.14 is a flowchart of the procedure for unconditionally releasing virtual channels. When the timer indicates the predetermined time (step 41) or when the virtual channel releasing request is generated (step 46), the setting/releasing processor 58 is notified of the virtual channel to be released (step 42), and deletes the same from the management table 56 (step 43). When all the virtual channels defined in the management table 56 have been checked (step 44), the setting/releasing processor 58 starts the timer to measure the predetermined time (step 45).
FIG.15 is a flowchart of the procedure for conditionally releasing virtual channels. When the timer indicates the predetermined time (step 51) or when the virtual channel releasing request is generated (step 52), the setting/releasing processor 58 determines whether or not the passage time from the time when the virtual channel being considered was last used exceeds the predetermined threshold time (step 53). When the result obtained at step 53 is YES, the setting/releasing processor 58 deletes the virtual channel being considered from the management table 56 (steps 54 and 55). Then it is determined, at step 56, whether or not all the virtual channels defined in the management table 56 have been checked. The procedure at step 56 is also executed when the result obtained at step 53 is NO. When the result at step 56 is NO, the procedure returns to step 53. On the other hand, when the result at step 56 is YES, the setting/releasing processor 58 sets the timer so that it starts to measure the predetermined time (step 57).
A ~escription will now be given of the operation of the system according to the second preferred embodiment of the present invention.
Referring to FIG.16, the address AD2 of the terminal TE2, the virtual channel identification number VCI/VPI12 and time tll when the virtual channel 2~2~1212 l identification number VCI/VPI12 was last used are defined in a management table 56-l provided for the terminal TEl. The address AD1 of the terminal TE1, the virtual channel identification number VCI/VPI21 and time tl2 when the virtual channel identification number VCI/VPI21 was last used are defined in a management table 56-2 provided for the terminal TE~.
Data is output to the terminal adapter TAl from the terminal TE1. The terminal adapter TAl sends a virtual channel identification number notice request to the ATM switching network 33 by using a call set-up message defined by the CCITT recommendations or the like. The retrieval processor 57 of the ATM switching network 33 determines whether or not the requested virtual channel identification number is registered in the management table 56 at present. In the illustrated case, the virtual channel identification number VCI/VPI12 is registered in the management table related to the terminal TE1. Thus, the setting/releasing ¦ 20 processor 58 informs the terminal TE1 of the virtual channel identification number VCI/VPI12 through the signal processor 53, the ADP processor 52, the ATM
processor 51 and the physical layer processor 50. In response to the notice of the virtual channel identification number VCI/VPI12, the terminal adapter TA1 divides data into cells and writes the virtual channel identification number VCI/VPI12 into the header of each of the cells. Then, the terminal adapter TA1 sends the cells to the ATM switching network 33.
The ATM switching network 33 sends the , received cells to the terminal adapter TA2, which assembles the received cells into the original data.
j Then, the terminal adapter TA2 sends the original data to the terminal TE2. The timers provided in the ATM
switching network 33 for the terminals TE1 and TE2 start to measure the predetermined time when the last cell is processed. Time tl3 when the last cell was ... . ..

~ , - 22 - 2~2~212 1 received is written into the management table 56-1, and time tl4 when the last cell was output is written into the management cell 56-2.
FIG.17 is a diagram illustrating a case where there is no virtual channel between the terminal adapters TA1 and TA3. No information is presently defined in a management table 56-3 which is provided in the ATM switching network 33 and related to the terminal TE3. It is now assumed that data is transferred from the terminal TEl to the terminal TE3.
The termin~l adapter TAl sends the virtual call identification number notice request to the ATM
switching network 3 3 in the same manner as described before. The retrieval processor 57 of the ATM
switching network 33 retrieves the management table 56-1 related to the terminal TE1 and determines whether or not the virtual channel addressed to the terminal TE3 is defined in the management table 56-1. In the illustrated case, no virtual channel is defined in the management table 56-1. The setting/releasing processor 58 registers address AD3 of the terminal TE3 and the virtual channel identification number VCI/VPI13 in the management table 56-1. Similarly, the setting/releasing processor 58 registers address AD1 and the virtual channel identification number VCI/VPI31 in the management table 56-3. Then, the ATM switching network 33 informs the terminal adapter TAl of the virtual channel identification number VCI/VPI13.
The terminal adapter TA1 divides data into cells, each having the header containing the virtual channel identification number VCI/VPI13. Then, the terminal adapter TAl sends the cells to the ATM
switching network 33 and then the terminal adapter TA3. The terminal adapter TA3 assembles the cells into the original data, which is then sent to the terminal TE3. Time tl5 when the ATM switching network receives the last cell is written into the management table 56-1 .. .. . . . .

, - 23 - 2a202~2 1 under the control of the setting/releasing processor 58. Similarly, time tl6 when the ATM switching network outputs the last cell is written into the management table 56-3. Then, the timers provided for the terminals TE1 and TE2 are started to measure the predetermined time.
FIG.18 is a diagram illustrating the procedure for releasing virtual channels. It is now assumed that the management tables 56-1 and 56-2 have the contents shown in FIG.18. The setting/releasing processor 58 accesses each of the management tables 56-1 and 56-2 and extracts virtual channels, each of which has a passage time which starts from the last used time and which exceeds the predetermined threshold time. Then, the setting/releasing processor 58 deletes these virtual channels from the corresponding management tables 56-1 and 56-2. For example, if both of the passage time between the time tl3 and the present time and the passage time between time tl4 and the present time exceed the predetermined threshold time, the corresponding virtual channels are deleted so that the contents of the management tables 56-1 and 56-2 are changed, as shown in FIG.18. That is, the virtual channel between the terminals TEl and TE2 is deleted.
The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention. For example, when the number o~ virtual channels in each of the management tables becomes equal to or greater than a predetermined number of virtual channels, a virtual channel which has not yet been used for a period of time longer than a predetermined period of time from the last used time is set to be a candidate to be deleted. Alternatively, a virtual channel which is not o~ten used is set to be a candidate to be deleted.

. . .

2~2~212 1 According to the aforementioned first embodiment of the present invention, virtual channels realized in the ATM switching network are managed on the terminal side. Thus, it is possible to reduce a load on the ATM switching system. According to the aforementioned second embodiment of the present invention, virtual channels are managed on the ATM
switching network side so that they can easily be released from the management tables.
It will be noted that The ATM switching network 13 shown in FIG.4 has the configuration of the ATM switching network 33 except the table processor 54. The aforementioned timer or counter used in the first embodiment of the present invention is provided in the signal processor 21 or the setting/releasing processor 29. Similarly, the timer or counter used in the second embodiment of the present invention is provided in the signal processor 53 or the setting/releasing processor 58. The table processor 25 or 54 can be formed of a central processing unit, a mlcroprocessor or the like.

I

Claims

1. A transmission control system comprising:
an asynchronous transfer mode (ATM) switching network providing virtual channels;
a plurality of terminals;
a plurality of terminal adapters connected to said ATM
switching network and respectively connected to said plurality of terminals, each of said terminal adapters coupling a corresponding one of said terminals to said ATM switching network, data being transferred between any pair of said terminals through two corresponding terminal adapters from among said terminal adapters and said ATM switching network;
management table means, including a management table, for registering the virtual channels presently formed in said ATM
switching network in said management table, each of said virtual channels mutually connecting two corresponding terminal adapters through said ATM switching network;
retrieval means for receiving a communication request from a first one of said terminals and for determining whether said management table registers a virtual channel corresponding to the communication request, data output from the first one of said terminals being transferred to a second one of said terminals addressed by the communication request through the virtual channel corresponding to the communication request and the two corresponding terminal adapters corresponding to the first and second ones of said terminals when said retrieval means determines that said management table has the virtual channel corresponding to the communication request;
virtual channel setting means for registering the virtual channel corresponding to the communication request in said management table when said retrieval means determines that said management table does not have the virtual channel corresponding to the communication request, data output from the first one of said terminals being transferred to the second one of said terminals addressed by the communication request through the virtual channel registered in said management table by said virtual channel setting means; and virtual channel releasing means for deleting any of said virtual channels from said management table of said management table means satisfying a predetermined condition of virtual channel use within said ATM switching network to release said virtual channels unlikely to be reused.

2. A transmission control system as claimed in claim 1, further comprising timer means for repeatedly measuring a predetermined time, wherein said virtual channel releasing means comprises means for deleting all of said virtual channels from said management table when said timer means indicates the predetermined time, so that all of said virtual channels are released.

3. A transmission control system as claimed in claim 1, further comprising count means for counting a number of times that each of said virtual channels registered in said management table has been used and for comparing the number of times obtained for each of said virtual channels with a predetermined number of times, and wherein said virtual channel releasing means comprises means for deleting, from said management table, any virtual channel used less than the predetermined number of times.

4. A transmission control system as claimed in claim 1, further comprising timer means for measuring a passage of time for each of said virtual channels registered in said management table, the passage of time starting at a last used time when a corresponding virtual channel was last used and ending at a present time, and for comparing the passage of time obtained for each of said virtual channels with a predetermined threshold time, and wherein said virtual channel releasing means comprises means for deleting, from said management table, the corresponding virtual channel for each passage of time longer than the predetermined threshold time.

5. A transmission control system as claimed in claim 1, further comprising determining means for determining whether said management table has an idle area sufficient to register a new virtual channel, and wherein said virtual channel releasing means comprises means for releasing at least one of said virtual channels from said management table when said determining means determines that said management table does not have an idle area sufficient to register the new virtual channel.

6. A transmission control system as claimed in claim 5, further comprising count means for counting a number of times that each of said virtual channels registered in said management table has been used and for comparing the number of times obtained for each of said virtual channels with a predetermined number of times, and wherein said means for releasing in said virtual channel releasing means deletes, from said management table, any virtual channel used less than the predetermined number of times.

7. A transmission control system as claimed in claim 5, further comprising timer means for measuring a passage of time for each of said virtual channels registered in said management table, the passage of time starting at a last used time when a corresponding virtual channel was last used and ending at a present time, and for comparing the passage of time obtained for each of said virtual channels with a predetermined threshold time, and wherein said means for releasing in said virtual channel releasing means deletes, from said management table, the corresponding virtual channel for each passage of time longer than the predetermined threshold time.

8. A transmission control system as claimed in claim 1, wherein said management table includes a flag indicating that a corresponding one of said virtual channels is inhibited from being deleted from said management table.

9. A transmission control system as claimed in claim 1, wherein said management table includes a first area for storing an address of each of said terminals related to said virtual channels registered in said management table, and a second area for storing an identification number of each of said virtual channels presently registered in said management table.

10. A transmission control system as claimed in claim 4, wherein said management table includes a first area for storing an address of each of said terminals related to said virtual channels registered in said management table, a second area for storing an identification number of each of said virtual channels presently registered in said management table, and a third area which registers the last used time for each of said virtual channels presently registered.

11. A transmission control system as claimed in claim 1, wherein said data comprises cells, each having a predetermined length of data.

12. A transmission control system as claimed in claim 1, wherein each of said terminal adapters comprises request means for generating a virtual channel setting request when said retrieval means determines that said management table does not have the virtual channel corresponding to the communication request and for sending the virtual channel setting request to said ATM switching network, so that the virtual channel corresponding thereto is formed in said ATM switching network in response to the virtual channel setting request.

13. A transmission control system as claimed in claim 12, wherein:
said ATM switching network comprises formation indication means for informing said virtual channel setting means of formation of the virtual channel corresponding to the virtual channel setting request; and said virtual channel setting means registers formation of the virtual channel in said management table when said virtual channel setting means is informed by said formation indication means.

14. A transmission control system, comprising:
an asynchronous transfer mode (ATM) switching network providing virtual channels;
a plurality of terminals; and a plurality of terminal adapters connected to said ATM
switching network and respectively connected to said plurality of terminals, each of said terminal adapters coupling a corresponding one of said terminals to said ATM switching network, data being transferred between any pelf of terminals through corresponding terminal adapters from among said terminal adapters and one of said virtual channels in said ATM switching network, each of said terminal adapters including management table means, including a management table, for registering said virtual channels presently formed in said ATM
switching network in said management table;
retrieval means for receiving a communication request from a first one of said terminals and for determining whether said management table registers a virtual channel corresponding to the communication request, data output from the first one of said terminals being transferred to a second one of said terminals addressed by the communication request through the virtual channel corresponding to the communication request when said retrieval means determines that said management table has the virtual channel corresponding to the communication request registered therein;
virtual channel setting means for registering the virtual channel corresponding to the communication request in said management table when said retrieval means determines that said management table does not have the virtual channel corresponding to the communication request, data output from the first one of said terminals being transferred to the second one of said terminals addressed by the communication request through the virtual channel registered in said management table by said virtual channel setting means; and virtual channel releasing means for deleting any of said virtual channels from said management table of said management table means satisfying a predetermined condition of virtual channel use within said ATM switching network to release said virtual channels unlikely to be reused.

15. A transmission control system as claimed in claim 14, wherein each of said terminal adapters comprises timer means for repeatedly measuring a predetermined time, and wherein said virtual channel releasing means comprises means for deleting all of said virtual channels from said management table when said timer means indicates the predetermined time, so that all of said virtual channels are released.

16. A transmission control system as claimed in claim 14, wherein each of said terminal adapter means comprises count means for counting a number of times that each of said virtual channels registered in said management table has been used and for comparing the number of times obtained for each of said virtual channels with a predetermined number of times, and wherein said virtual channel releasing means comprises means for deleting from said management table, any virtual channel used less than the predetermined number of times.

17. A transmission control system as claimed in claim 14, wherein each of said terminal adapters comprises timer means for measuring a passage of time for each of said virtual channels registered in said management table, the passage of time starting at a last used time when a corresponding virtual channel was last used and ending at a present time, and for comparing the passage of time obtained for each of said virtual channels with a predetermined threshold time, and wherein said virtual channel releasing means comprises means for deleting, from said management table, the corresponding virtual channel for each passage of time greater than the predetermined threshold time.

18. A transmission control system as claimed in claim 14, wherein each of said terminal adapters comprises determining means for determining whether said management table has an idle area sufficient to register a new virtual channel, and wherein said virtual channel releasing means comprises means for releasing at least one of said virtual channels from said management table when said determining means determines that said management table does not have an idle area sufficient to register the new virtual channel.

19. A transmission control system as claimed in claim 18, wherein each of said terminal adapters comprises count means for counting a number of times that each of said virtual channels registered in said management table has been used and for comparing the number of times obtained for each of said virtual channels with a predetermined number of times, and wherein said means for releasing in said virtual channel releasing means deletes, from said management table, any virtual channel used less than the predetermined number of times.

20. A transmission control system as claimed in claim 18, wherein each of said terminal controllers comprises timer means for measuring a passage of time from a time when each of said virtual channels registered in said management table was last used and for comparing the passage of time obtained for each of said virtual channels with a predetermined threshold time, and wherein said means for releasing in said virtual channel releasing means deletes, from said management table, a virtual channel for each passage of time longer than the predetermined threshold time.

21. A transmission control system as claimed in claim 14, wherein said management table includes a flag indicating that a corresponding one of said virtual channels is inhibited from being deleted from said management table.

22. A transmission control system as claimed in claim 14, wherein said management table includes a first area for storing an address of each of said terminals related to said virtual channels registered in said management table, and a second area for storing an idnetification number of each of said virtual channels registered in said management table at present.

23. A transmission control system as claimed in claim 19, wherein said management table includes a first area for stoning an address of each of said terminals related to said virtual channels registered in said management table, a second area for stoning an identification number of each of said virtual channels registered in said management table at present, and a third area for stoning a last used time for each of said virtual channels presently registered.

24. A transmission control system as claimed in claim 14, wherein said data comprises cells, each having a predetermined length of data.

25. A transmission control system as claimed in claim 14, wherein each of said terminal adapters comprises request means for generating a virtual channel setting request when said retrieval means determines that said management table does not have the virtual channel corresponding to the communication request and for sending said virtual channel setting request to said ATH switching network, so that the virtual channel corresponding thereto is formed in said ATM switching network in response to the virtual channel setting request.

26. A transmission control system as claimed in claim 25, wherein, said ATM switching network comprises formation indication means for informing the virtual channel setting means of formation of the virtual channel corresponding to said virtual channel setting request; and said virtual channel setting means registers formation of the virtual channel in said management table when said virtual channel setting means is formed by said formation indication means.

27. A transmission control system comprising:
a plurality of terminals;
a plurality of terminal adapters respectively connected to said plurality of terminals; and an asynchronous transfer mode (ATM) switching network connected to said terminal adapters, providing virtual channels, each of said terminal adapters coupling a corresponding one of said terminals to said ATM switching network, data being transferred between a pair of said terminals through two corresponding terminal adapters from among said terminal adapters and one of said virtual channels in said ATM switching network, said ATM switching network including management table means, including a management table, for registering said virtual channels presently formed in said ATM
switching network in said management table;
retrieval means for receiving a communication request from a first one of said terminals and for determining whether said management table presently registers a virtual channel corresponding to the communication request, data output from the first one of said terminals being transferred to a second one of said terminals addressed by the communication request through the virtual channel corresponding to the communication request and the two corresponding terminal adapters corresponding to the first and second ones of said terminals when said retrieval means determines that said management table has the virtual channel corresponding to the communication request registered therein;
virtual channel setting means for registering the virtual channel corresponding to said communication request in the management table when said retrieval means determines that said management table does not have the virtual channel corresponding to the communication request, data output from the first one of said terminals being transferred to the second one of said terminals addressed by the communication request through the virtual channel registered in said management table by said virtual channel setting means; and virtual channel releasing means for deleting any of said virtual channels from said management table of said management table means satisfying a predetermined condition of virtual channel use within said ATM switching network to release said virtual channels unlikely to be reused.

28. A transmission control system as claimed in claim 27, wherein said ATM switching network comprises timer means for repeatedly measuring a predetermined time, and wherein said virtual channel releasing means comprises means for deleting all of said virtual channels from said management table when said timer means indicates the predetermined time, so that all of said virtual channels are released.

29. A transmission control system as claimed in claim 27, wherein said ATM switching network comprises count means for counting a number of times that each of said virtual channels registered in said management table has been used and for comparing the number of times obtained for each of said virtual channels with a predetermined number of times, and wherein said virtual channel releasing means comprises means for deleting, from said management table, any virtual channel used less than the predetermined number of times.

30. A transmission control system as claimed in claim 27, wherein said ATM switching network comprises timer means for measuring a passage of time for each of said virtual channels registered in said management table, the passage time starting when a corresponding virtual channel was last used and for comparing the passage time obtained for each of said virtual channels with a predetermined threshold time, and wherein said virtual channel releasing means comprises means for deleting, from said management table, the corresponding virtual channel for each passage time longer than the predetermined threshold time.

31. A transmission control system as claimed in claim 27, wherein said ATM switching network comprises determining means for determining whether said management table has an idle area sufficient to register a new virtual channel, and wherein said virtual channel releasing means comprises means for releasing at least one of said virtual channels from said management table when said determining means determines that said management table does not have an idle area sufficient to register a new virtual channel.

32. A transmission control system as claimed in claim 31, wherein said ATH switching network comprises count means for counting a number of times that each of said virtual channels registered in said management table has been used and for comparing the number of times obtained for each of said virtual channels with a predetermined number of times, and wherein said means for releasing in said virtual channel releasing means deletes, from said management table, any virtual channel used less than the predetermined number of times.

33. A transmission control system as claimed in claim 31, wherein said ATM switching network comprises timer means, for measuring a passage of time for each of said virtual channels registered in said management table, the passage of time starting at a last used time when a corresponding virtual channel was last used and ending at a present time, and for comparing the passage of time obtained for each of said virtual channels with a predetermined threshold time, and wherein said means for releasing in said virtual channel releasing means deletes, from said management table, the corresponding virtual channel for each passage of time greater than the predetermined threshold time.

34. A transmission control system as claimed in claim 27, wherein said management table includes a flag indicating that a corresponding one of said virtual channels is inhibited from being deleted from said management table.

35. A transmission control system as claimed in claim 27, wherein said management table includes a first area for storing an address of each of said terminals related to said virtual channels registered in said management table, and a second area for storing an identification number of each of said virtual channels presently registered in said management table.

36. A transmission control system as claimed in claim 31, wherein said management table includes a first area for storing an address of each of said terminals related to said virtual channels registered in said management table, a second area for storing an identification number of each of said virtual channels presently registered in said management table, and a third area which registers the last used time for each of said virtual channels presently registered.

37. A transmission control system as claimed in claim 27, wherein said data comprises cells, each having a predetermined length of data.

38. A transmission control system as claimed in claim 27, wherein each of said terminals comprises request means for generating a virtual channel notice request subsequent to the communication request and for sending the virtual channel notice request to said ATM switching network, and wherein said ATH switching network comprises means for sending back each said terminal information about the virtual channel obtained by said control means in response to the virtual channel notice request.

39. A transmission control system as claimed in claim 38, wherein when said retrieval means determines that said management table does not have the virtual channel corresponding to said communication request, said virtual channel setting means registers the virtual channel corresponding to the communication request in said management table.

40. A transmission control system for an asynchronous transfer mode switching network providing virtual channels to connect terminals together, comprising:
a management table for storing channel identifiers of the virtual channels formed by the asynchronous transfer mode switching network;
retrieval means for receiving a communication request for a communication channel from a first terminal to a second terminal and for determining whether said management table includes an identifier for the communication channel among the channel identifiers stored therein, data from the first terminal being transmitted to the second terminal addressed by the communication request through the communication channel when said retrieval means determines that said management table includes the identifier for the communication channel;
virtual channel setting means for sending the identifier for the communication channel to said management table when said retrieval means determines that said management table does not include the identifier for the communication channel, data from the first terminal being transmitted to the second terminal addressed by the communication request through the communication channel after the identifier therefore is stored in said management table; and virtual channel releasing means for deleting at least one of the channel identifiers from said management table satisfying a predetermined condition of virtual channel use to release the channel identifiers of said virtual channels unlikely to be reused.

41. A transmission control system as claimed in claim 40, further comprising determining means for determining whether said management table has an idle area sufficient to register a new virtual channel, and wherein said virtual channel releasing means releases at least one of said virtual channels from said management table when said management table does not have an idle area sufficient to register the new virtual channel.

42. A method for controlling the transmission of data between terminals through an asynchronous transfer mode switching network providing virtual channels, comprising the steps of, (a) receiving a communication request for a communication channel from a first terminal to communicate with a second terminal;
(b) determining whether the communication channel is identified in a management table containing channel identifiers for all presently providing virtual channels;
(c) storing an identifier for the communication channel in the management table when the identifier for the communication channel is not in the management table;
(d) transferring data from the first terminal to the second terminal through the communication channel when the identifier for the communication channel is stored in the management table after either of steps (b) and (c); and (e) releasing the virtual channels when a predetermined condition of virtual channel use within the asynchronous transfer mode switching network is satisfied.

43 43. A method of controlling the transmission of data as claimed in claim 42, wherein step (e) comprises releasing the virtual channels when the management table does not have an idle area sufficient to register a new virtual channel.