LAN-WAN-LAN communication method, and apparatus for LAN-WAN connection

Claims

What is claimed is:

1. A LAN-WAN-LAN communication method in a LAN-WAN-LAN connecting apparatus, for performing communication between a local area network (LAN) on a transmitting side and a local area network (LAN) on a receiving side via a wide area network (WAN) within limits of a band allocated by said apparatus, the method comprising the steps of:

demultiplexing a frame, which has been received from the LAN on the transmitting side, into data constituting smallest units of information, and transmitting this data via a plurality of lines;

multiplexing data, which is sent to the receiving side via each line, on the receiving side so as to generate a frame and sending the generated frame to the LAN on the receiving side;

obtaining traffic between the LAN on the transmitting side and the LAN on the receiving side when a request for transfer of a new frame has been issued from a terminal which is connected to the LAN on the transmitting side;

obtaining a band required for the transfer of the new frame on the basis of the traffic;

performing communication upon increasing the number of lines used in LAN-WAN-LAN communication in such a manner that the band obtained in said band obtaining step is satisfied;

providing a table for storing a list of idle lines;

investigating whether a sufficient number of idle lines that satisfy the obtained band are available;

reducing a band to obtain a new band in the absence of the sufficient number of idle lines that satisfy the obtained band; and

increasing the number of lines to be used in said communication so as to satisfy the new band.

2. A LAN-WAN-LAN communication method in a LAN-WAN-LAN connecting apparatus, for performing communication between a local area network (LAN) on a transmitting side and a local area network (LAN) on a receiving side via a wide area network (WAN) within limits of a band allocated by said apparatus, the method comprising the steps of:

demultiplexing a frame, which has been received from the LAN on the transmitting side, into data constituting smallest units of information, and transmitting this data via a plurality of lines;

multiplexing data, which is sent to the receiving side via each line, on the receiving side so as to generate a frame and sending the generated frame to the LAN on the receiving side;

obtaining traffic between the LAN on the transmitting side and the LAN on the receiving side when a request for transfer of a new frame has been issued from a terminal which is connected to the LAN on the transmitting side;

obtaining a band required for the transfer of the new frame on the basis of the traffic;

performing communication upon increasing the number of lines used in LAN-WAN-LAN communication in such a manner that the band obtained in said band obtaining step is satisfied;

transmitting a synchronizing pattern to all lines used on the transmitting side at an initial connection between LANs;

receiving the synchronizing pattern and performing synchronization of each line on the receiving side and notifying the transmitting side of establishment of synchronization;

after synchronization is established, demultiplexing data, which has been accepted from the LAN on the transmitting side, into data constituting smallest units of information, and transmitting this data to the receiving side via each of the lines used; and

on the receiving side, multiplexing data, which is sent to the receiving side via each line and sending the multiplexed data to the LAN on the receiving side.

3. A LAN-WAN-LAN communication method in a LAN-WAN-LAN connecting apparatus, for performing communication between a local area network (LAN) on a transmitting side and a local area network (LAN) on a receiving side via a wide area network (WAN) within limits of a band allocated by the LAN-WAN-LAN connecting apparatus, the method comprising the steps of:

demultiplexing a frame, which has been received from the LAN on the transmitting side, into data constituting smallest units of information, and transmitting this data via a plurality of lines;

multiplexing data, which is sent to the receiving side via each line, on the receiving side so as to generate a frame and sending the generated frame to the LAN on the receiving side;

obtaining traffic between the LAN on the transmitting side and the LAN on the receiving side when a request for transfer of a new frame has been issued from a terminal which is connected to the LAN on the transmitting side;

obtaining a band required for the transfer of the new frame on the basis of the traffic;

increasing the number of lines used in LAN-WAN-WAN communication in such a manner that the band obtained is satisfied;

in a case where the number of lines is increased, transmitting data on at least one line thus far in communication between LANs upon setting a synchronizing flag in the data, and transmitting data for which said synchronizing flag has been set on an additional line used in communication between LANs;

before synchronization is established, on the receiving side, multiplexing only the data on lines used thus far in communication between LANs, sending the multiplexed data to the LAN on the receiving side, synchronizing lines for which synchronizing flags have been detected and notifying the transmitting side of establishment of synchronization;

after synchronization is established, and on the transmitting side, demultiplexing data accepted from the LAN on the transmitting side into data constituting smallest units of information, and transmitting this data to the receiving side via the lines used thus far as well as the additional line; and

on the receiving side, multiplexing data sent to the receiving side via each line and sending this multiplexed data to the LAN on the receiving side.

4. A LAN-WAN-LAN communication method in a LAN-WAN-LAN connecting apparatus, for performing communication between a local area network (LAN) on a transmitting side and a local area network (LAN) on a receiving side via a wide area network (WAN) within limits of a band allocated by said apparatus, the method comprising the steps of:

demultiplexing a frame, which has been received from the LAN on the transmitting side, into data constituting smallest units of information, and transmitting this data via a plurality of lines;

multiplexing data, which is sent to the receiving side via each line, on the receiving side so as to generate a frame and sending the generated frame to the LAN on the receiving side;

monitoring traffic between LANs;

obtaining a band required for communication between LANs, based upon monitored traffic;

performing communication upon increasing, decreasing or maintaining the number of lines, which are used in communication between LANs, based upon the obtained band;

in a case where the number of lines is increased, transmitting data on at least one line used thus far in communication between LANs upon setting a synchronizing flag in the data, and transmitting data for which said synchronizing flag has been set on an additional line used in communication between LANs;

before synchronization is established, on the receiving side, multiplexing only the data on lines used thus far in communication between LANs, sending the multiplexed data to the LAN on the receiving side, synchronizing lines for which synchronizing flags have been detected and notifying the transmitting side of establishment of synchronization;

after synchronization is established, on the transmitting side, demultiplexing data accepted from the LAN on the transmitting side into data constituting smallest units of information, and transmitting this data to the receiving side via the lines used thus far as well as the additional line; and

on the receiving side, multiplexing data sent to the receiving side via each line and sending this multiplexed data to the LAN on the receiving side.

5. A LAN-WAN-LAN communication method in a LAN-WAN-LAN connecting apparatus, for performing communication between a local area network (LAN) on a transmitting side and a local area network (LAN) on a receiving side via a wide area network (WAN) within limits of a band allocated by said apparatus, the method comprising the steps of:

demultiplexing a frame, which has been received from the LAN on the transmitting side, into data constituting smallest units of information, and transmitting this data via a plurality of lines;

multiplexing data, which is sent to the receiving side via each line, on the receiving side so as to generate a frame and sending the generated frame to the LAN on the receiving side;

monitoring traffic between LANs;

obtaining a band required for communication between LANs based upon monitored traffic;

performing communication upon increasing, decreasing or maintaining the number of lines, which are used in communication between LANs, based upon the obtained band;

in a case where the number of lines is decreased, and on the transmitting side, transmitting data, on a line to be disconnected, to the receiving side upon setting a line-disconnection flag in the data, and transmitting data on other lines without setting a line-disconnection flag in the data; and

on the receiving side, recognizing said line to be disconnected by detecting the line-disconnection flag, and thenceforth multiplexing data sent to the receiving side via lines other than the line to be disconnected and sending this multiplexed data to the LAN on the receiving side.

6. A LAN-WAN connecting apparatus in a communication system for allocating a band within which communication between a local area network (LAN) on a transmitting side and a LAN on a receiving side via a wide area network (WAN) is performed and performing said communication between LANs within limits of the allocated band, the apparatus comprising:

a demultiplexer and multiplexer means for demultiplexing a frame, which has been accepted from the LAN on the transmitting side, into data constituting smallest units of information, transmitting this data to a plurality of lines, multiplexing data, which is sent to the receiving side via each line so as to generate a frame, and sending the generated frame to the LAN on the receiving side;

means for obtaining traffic between the LAN on the transmitting side and the LAN on the receiving side when a request for transfer of a new frame has been issued from a terminal which is connected to the LAN on the transmitting side;

band calculating means for obtaining a band required for the transfer of the new frame on the basis of the traffic; and

line increasing means for increasing the number of lines used in LAN-WAN-LAN communication in such a manner that the band obtained is satisfied.

7. A LAN-WAN connecting apparatus in a communication system for allocating a band within which communication between a local area network (LAN) on a transmitting side and a LAN on a receiving side via a wide area network (WAN) is performed and performing the communication between LANS within limits of the allocated band, the apparatus comprising:

a demultiplexer and multiplexer means for demultiplexing a frame, which has been accepted from the LAN on the transmitting side, into data constituting smallest units of information, transmitting this data to a plurality of lines, multiplexing data, which is sent to the receiving side via each line so as to generate a frame, and sending the generated frame to the LAN on the receiving side;

monitoring means for monitoring traffic between LANs;

band calculating means for obtaining a band required for communication between LANs based upon the traffic; and

line increasing and decreasing means for increasing, decreasing or maintaining the number of lines, which are used in communication between LANs, based upon the band obtained, wherein said demultiplexer and multiplexer comprises:

means which, in a case where the number of lines is increased, is for transmitting data on at least one line used thus far in communication between LANs upon setting a synchronizing flag in the data, and transmitting data for which said synchronizing flag has been set on an additional line used in communication between LANs; and

demultiplexing means which, after synchronization is established, is for demultiplexing data accepted from the LAN on the transmitting side and transmitting this data to the receiving side via the lines used thus far as well as the additional line.

8. The apparatus according to claim 7, wherein said demultiplexer and multiplexer means comprises:

means which, before synchronization is established, is for multiplexing only the data on lines used thus far in communication between LANs, sending the multiplexed data to the LAN on the receiving side, synchronizing lines for which said synchronizing flags have been detected and notifying the transmitting side of establishment of synchronization; and

multiplexing means which, after synchronization is established, is for demultiplexing data sent to the receiving side via the lines used thus far as well as the additional line and sending the data to the LAN on the receiving side.

9. A LAN-WAN connecting apparatus in a communication system for allocating a band within which communication between a local area network (LAN) on a transmitting side and a LAN on a receiving side via a wide area network (WAN) is performed and performing the communication between LANS within limits of the allocated band, the apparatus comprising:

a demultiplexer and multiplexer means for demultiplexing a frame, which has been accepted from the LAN on the transmitting side, into data constituting smallest units of information, transmitting this data to a plurality of lines, multiplexing data, which is sent to the receiving side via each line so as to generate a frame, and sending the generated frame to the LAN on the receiving side;

monitoring means for monitoring traffic between LANs;

band calculating means for obtaining a band required for communication between LANs based upon monitored traffic; and

line increasing and decreasing means for increasing, decreasing or maintaining the number of lines, which are used in communication between LANs, based upon the band obtained, wherein said demultiplexer and multiplexer means comprises:

means which, in a case where the number of lines is decreased, is for transmitting data, on a line to be disconnected, to the receiving side upon setting a line-disconnection flag in the data, and transmitting data on other lines without setting a line-disconnection flag in the data; and

multiplexing means for recognizing said line to be disconnected based upon the line-disconnection flag, and thenceforth multiplexing data sent to the receiving side via lines other than the line to be disconnected and sending this multiplexed data to the LAN on the receiving side.

10. A communication method in a connecting apparatus for performing communication between first type networks via a second type network within an allocated band allocated by said apparatus, the method comprising the steps of:

obtaining traffic between a first type network on a transmitting side and a first type network on a receiving side when a request for transfer of a new frame has been issued from a terminal which is connected to the first type network on the transmitting side;

obtaining a band required for the transfer of the new frame on the basis of the traffic obtained in said traffic obtaining step; and

performing communication upon increasing the allocated band for communication by an amount equivalent to the band obtained in said band obtaining step.

11. A communication method in a connecting apparatus, for performing communication between a first type network on a transmitting side and a network first type on a receiving side via a second type network within a band allocated by said apparatus, the method comprising the steps of:

demultiplexing a frame, which has been received from the first type network on the transmitting side, into data constituting smallest units of information, and transmitting this data via a plurality of lines;

multiplexing data, which is sent to the receiving side via each line, on the receiving side so as to generate a frame and sending said generated frame to the first type network on the receiving side;

obtaining traffic between the first type network on the transmitting side and the first type network on the receiving side when a request for transfer of a new frame has been issued from a terminal which is connected to the first type network on the transmitting side;

obtaining a band required for the transfer of the new frame on the basis of the traffic; and

performing communication upon increasing the number of lines used in communication in such a manner that the band obtained is satisfied.

12. The method according to claim 11, wherein the traffic between LANs is obtained using a number of frames residing in a buffer provided between LANs or time of residence in the buffer.

13. The method according to claim 11, wherein the traffic between LANs is obtained using line utilization rate calculated based upon a band allocated to LAN-WAN-LAN communication and the band in actual use.

14. The method according to claim 11, wherein there is provided a table for storing required bands in correspondence with traffic or types of application, the required band being obtained by referring to said table.

15. The method according to claim 11, wherein there is provided a table for storing optimum numbers of lines commensurate with bands, the number of lines being decided by referring to said table.

16. A communication method in a connecting apparatus, for performing communication between a first type network on a transmitting side and a first type network on a receiving side via a second type network within a band allocated by said apparatus, the method comprising the steps of:

demultiplexing a frame, which has been received from the first type network on the transmitting side, into data constituting small units of information, and transmitting this data via a plurality of lines;

multiplexing data, which is sent to the receiving side via each line, on the receiving side so as to generate a frame and sending the generated frame to the first type network on the receiving side;

monitoring traffic between the first type network on the transmitting side and the first type network on the receiving side;

obtaining a band required for communication between the first type network on the transmitting side and the first type network on the receiving side based upon monitored traffic; and

performing communication upon increasing, decreasing or maintaining the number of lines, which are used in communication between first type networks, based upon the obtained band.

17. The method according to claim 16, wherein a line-tariff monitoring timer is provided for each line, the traffic being obtained before the line-tariff monitoring timer runs out of time.

18. The method according to claim 16, further comprising reserving a prescribed number of lines in a case where a line is disconnected, and refraining from using these reserved lines in communication with other LANs.

19. The method according to claim 16, wherein the reserved lines are used as new lines in a case where the number of lines is to be increased.

20. A connecting apparatus in a communication system for allocating a band within which communication between a first type network on a transmitting side and a first type network on a receiving side via a second type network is performed and performing the communication between first type networks within an allocated band, the apparatus comprising:

demultiplexer and multiplexer means for demultiplexing a frame, which has been accepted from the first type network on the transmitting side, into data constituting small units of information, transmitting this data to a plurality of lines, multiplexing data, which is sent to the receiving side via each line so as to generate a frame, and sending the generated frame to the first type network on the receiving side;

monitoring means for monitoring traffic between LANs;

band calculating means for obtaining a band required for communication between first type networks, based upon monitored traffic; and

line increasing and decreasing means for increasing, decreasing or maintaining the number of lines, which are used in communication between first type network, based upon the band obtained.

21. A communication method in a connecting apparatus, for performing communication between first type networks via a second type network within an allocated band, which is allocated by the connecting apparatus, the method comprising the steps of:

identifying a type of application of a terminal that has issued a request for transfer of a new frame, by referring to an application type information in the new frame, when said request for transfer of a new frame has been issued;

obtaining a band required for the transfer of the new frame on the basis of the type of application; and

performing communication by increasing the allocated band for communication by an amount equivalent to the band obtained.

22. A communication method in a connecting apparatus, for performing communication between a first type network on a transmitting side and a first type network on a receiving side via a second type network within a band which is allocated by the connecting apparatus, the method comprising the steps of:

demultiplexing a frame, which has been received from the first type network on the transmitting side, into data constituting small units of information, and transmitting this data via a plurality of lines;

multiplexing data, which is sent to the receiving side via each line, on the receiving side so as to generate a frame and sending the generated frame to the first type network on the receiving side;

identifying a type of application of a terminal that has issued a request for transfer of a new frame, when said request for transfer of a new frame has been issued;

obtaining a band required for the transfer of the new frame on the basis of the type of application; and

performing communication upon increasing the number of lines used in communication in such a manner that the band obtained is satisfied.

23. A connecting apparatus in a communication system for allocating a band within which communication between a first type network on a transmitting side and a first type network on a receiving side via a second type network is performed and for performing the communication between first type networks within an allocated band, the apparatus comprising:

demultiplexer and multiplexer means for demultiplexing a frame, which has been accepted from the first type network on the transmitting side, into data constituting small units of information, transmitting this data to a plurality of lines, multiplexing data, which is sent to the receiving side via each line so as to generate a frame, and sending the generated frame to the first type network on the receiving side;

means for identifying a type of application of a LAN terminal which has issued a request for transfer of a new frame, when said request for transfer of a new frame has been issued;

band calculating means for obtaining a band required for the transfer of the new frame on the basis of the type of application; and

line increasing means for increasing the number of lines used in communication in such a manner that the band obtained is satisfied.

BACKGROUND OF THE INVENTION

This invention relates to a LAN-WAN-LAN communication method whereby communication between LANs is achieved via a wide area network, as well as to a LAN-WAN connecting apparatus. More particularly, the invention relates to a LANN-WAN-LAN communication method, and a LAN-WAN connecting apparatus, in which LAN-WAN-LAN communication is performed by band allocation and line selection in conformity with traffic and type of application.

In order to interconnect LANs (local area networks) that are remote from each other, a so-called wide area network (WAN) such as a public switched telephone network, public data switched network, ISDN or leased line is used as the intermediary. Routers are provided between each LAN and the WAN. The router is connected to the LAN to control physical layers, a data link layer and a network layer, which are first, second and third layers, and is connected to the WAN to control the interface to the WAN as well as the network thereof. When a terminal sends out a frame with an attached network address in a communication system thus interconnecting LANs via a WAN, the router accepts the frame, refers to the network address and sends this frame to the LAN of the other party via the WAN. The router connected to the LAN of the other party accepts the frame and sends it to the LAN, where the destination terminal accepts the frame.

In presently existing WANs, lines have a narrow band. When there is a large increase in traffic in a connection between LANs, the data transfer rate in a WAN suddenly drops to 2400 bps, and this declines to 64 Kbps even when a comparatively fast ISDN Bch is employed. Consequently, a file transfer that ends in several seconds within one and the same LAN is slowed down at the moment the data is passed through the WAN.

Accordingly, consideration has been given to interconnecting LANs by borrowing a fast leased line. However, employing a leased line results in higher communication cost. In particular, since the characteristic of communication using a LAN is such that data does not flow at all times but only when the necessary data is generated (so-called bursty transmission), the fact that there are long periods of idleness between short periods of transmission becomes a problem when a leased line is used.

SUMMARY OF THE INVENTION

Accordingly, a first object of the present invention is to provide a LAN-WAN-LAN communication method, as well as a LAN-WAN connecting apparatus, whereby a user can be given the impression of communication being performed over a single LAN without the intermediary of a WAN.

A second object of the present invention is to provide a LAN-WAN-LAN communication method, as well as a LAN-WAN connecting apparatus, in which band is allocated and line selection is performed in dependence upon traffic and type of application so that high-speed transfer and economical communication can be achieved even via a WAN.

A third object of the present invention is to provide a LANN-WAN-LAN communication method, as well as a LAN-WAN connecting apparatus, whereby the allocated band can be increased and decreased in conformity with traffic to enable economical, high-speed communication.

A fourth object of the present invention is to provide a LAN-WAN-LAN communication method, as well as a LAN-WAN connecting apparatus, in which the allocated band can be controlled by increasing or decreasing the number of lines multiplexed on the basis of traffic.

A fifth object of the present invention is to provide a LAN-WAN-LAN communication method, as well as a LAN-WAN connecting apparatus, in which synchronization of data among a number of lines can be performed without momentary interruption of data transfer when the allocated band is controlled by increasing or decreasing the number of lines multiplexed.

A sixth object of the present invention is to provide a LAN-WAN-LAN communication method in which a virtual circuit can be established and maintained (a released line reserved) automatically when the quantity of data transferred becomes small, whereby it is possible to avoid failure to acquire a line because the line is busy when it becomes necessary to widen the allocated band, and start-up of data transmission can be performed smoothly.

In accordance with the present invention, the first and second objects set forth above are attained by determining traffic between LANs when a new frame transfer request has been issued or identifying the type of application of a LAN terminal that has issued a frame transfer request, obtaining a band required for the new frame transfer based upon the traffic or type of application, and performing communication upon increasing the number of lines used in communication between LANs in such a manner that the band obtained is satisfied.

In accordance with the present invention, the first through fourth objects set forth above are attained by demultiplexing data, which has been accepted from a LAN on a transmitting side, into data constituting smallest units of information, transmitting this data via a plurality of lines, multiplexing data, which arrives via each line, on a receiving side and sending the multiplexed data to LAN on the receiving side, monitoring traffic between LANs, obtaining a band required for communication between LANs based upon the traffic, and performing communication upon increasing, decreasing or maintaining the number of lines, which are used in communication between LANs, based upon the band.

Further, in accordance with the present invention, the fifth object set forth above is attained by the following in a case where the number lines is increased: transmitting data on at least one line used thus far in communication between LANs upon setting a synchronizing flag in the data, and transmitting data identical with the data for which the synchronizing flag has been set on an additional line used anew in communication between LANs; before synchronization is established, and on a receiving side, multiplexing only the data on lines used thus far in communication between LANs, sending the multiplexed data to a LAN on the receiving side, synchronizing lines for which the synchronizing flags have been detected and notifying a transmitting side of establishment of synchronization; and, after synchronization is established, and on the transmitting side, demultiplexing data accepted from the LAN on the transmitting side into data constituting smallest units of information, transmitting this data to the receiving side via the lines used thus far and additional lines, and, on the receiving side, multiplexing the data that arrives from each line and sending this multiplexed data to the LAN on the receiving side.

Further, in accordance with the present invention, the fifth object set forth above is attained by the following in a case where the number lines is decreased: on the transmitting side, transmitting data, on a line to be disconnected, to the receiving side upon setting a line-disconnection flag in the data, and transmitting data on the other lines without setting a line-disconnection flag in the data; and, on the receiving side, recognizing a line to be disconnected by detecting the line-disconnection flag, and thenceforth multiplexing data sent to the receiving side via lines other than the line to be disconnected and sending this multiplexed data to the LAN on the receiving side.

Further, in accordance with the present invention, the sixth object set forth above is attained by the following in a case where a line is disconnected: reserving a prescribed number of lines in advance and refraining from using these lines in communication with other LANs.

Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for describing the general features of the present invention;

FIG. 2 is a diagram illustrating the configuration of a communication system according to the invention;

FIG. 3 is a diagram showing the construction of a routing functional unit;

FIGS. 4(A-C) are diagrams for describing a required-band table, in which FIG. 4A is a correspondence table showing the correspondence between traffic and required bands, FIG. 4B is a correspondence table showing the correspondence between applications and required bands, and FIG. 4C a memory for storing allocated bands, namely bands allocated between LANs;

FIGS. 5(A-B) are diagrams for describing an LCR table, in which FIG. 5A is a correspondence table showing the correspondence between connection destinations and contract lines and FIG. 5B is a correspondence table showing the correspondence among required bands, types of lines and numbers of lines;

FIG. 6 is a diagram for describing a line-resource management table;

FIG. 7 is a is a diagram showing the construction of a demultiplexing/multiplexing functional units and the peripheral elements;

FIG. 8 is a diagram for describing the constitution of a frame;

FIG. 9 is a diagram showing the construction of a demultiplexer;

FIG. 10 is a diagram showing the construction of a multiplexer;

FIG. 11 is a flowchart showing processing for increasing a band;

FIG. 12 is a flowchart showing processing for decreasing a band;

FIG. 13 is a flowchart showing processing for synchronization at the time of initial connection;

FIG. 14 is a flowchart showing processing for data transfer when lines are increased;

FIG. 15 is a diagram for describing synchronizing and data transfer processing executed when lines are increased, in which FIGS. 15A, 15B and 15C illustrate the contents of flag data transferred to a receiving side from a transmitting side up to establishment of synchronization;

FIG. 16 is a flowchart illustrating data transfer processing executed when lines are reduced; and

FIGS. 17(A-C) are diagrams for describing synchronizing and data transfer processing executed when lines are reduced, in which FIGS. 17A, 17B and 17C illustrate the contents of flag data transmitted from a transmitting side to a receiving side up to the moment of line disconnection.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(a) Overview of the Invention

FIG. 1 is a diagram for describing the general features of the present invention. Shown in FIG. 1 are LANs 11, 12, terminals 11a, 12a such as personal computers connected to the LANs 11, 12, respectively, a wide area network (WAN) 13 (public switched network, a data packet switched network, a leased line, etc.), and LAN-WAN connection apparatus 14, 15, which are identical in construction.

The LAN-WAN connection apparatus 14 includes: a LAN interface 14a; a traffic/application monitoring functional unit 14b which, when a new frame transfer request is produced, obtains the required band (transfer rate) by monitoring the traffic between the LANs or the type of application of the terminal, or obtains the traffic prevailing during communication and performs monitoring to determine whether a band greater than that necessary has been allocated. The apparatus 14 further includes a line monitoring functional unit 14c which increases the number of lines, on the basis of the required band designated by the traffic/application monitoring functional unit, when there is a new frame transfer request, and decreases the number of lines if a band greater than necessary has been allocated. The apparatus 14 further includes a demultiplexing/multiplexing functional unit 14d for demultiplexing data, which enters from the LAN interface, into data on a plurality of lines and then outputting the data, and for multiplexing data, which has entered from a plurality of lines, and inputting the multiplexed data to the LAN interface 14a. Also included are WAN interfaces 14e-1.about.14e-n connected to respective ones of the lines.

The traffic/application monitoring functional unit 14b monitors the traffic between LANs or the type of application of a LAN terminal. The unit 14b obtains the band necessary for LAN-WAN-LAN communication based upon the traffic prevailing when a request for transfer of a new frame between LANs is issued, or on the basis of the type of application. The line monitoring functional unit 14 increases the number of lines used in the communication between LANs so as to satisfy the band obtained. Since it is thus arranged to increase the band in conformity with the traffic or the type of application, data can be transferred at high speed and the user can be given the impression of communication being performed within a single LAN without the intermediary of a WAN. Further, a table is provided for storing the types of optimum lines or the number of lines in conformity with the required band. The line monitoring functional unit 14c refers to the table to decide the types of lines or the number of lines. This makes possible economical, high-speed communication.

Furthermore, the number of lines used in communication between LANs is increased in conformity with the traffic or type of application. The demultiplexing/multiplexing functional unit 14d demultiplexes the data, which has been accepted from the LAN 11 on the transmitting side, into data on a plurality of lines and transmits the data on these lines. The demultiplexing/multiplexing functional unit on the receiving side multiplexes data that has arrived via each line and sends the multiplexed data to the LAN 12 on the receiving side. As a result of this arrangement, even though the lines are narrow-band lines, the number of lines bundled is controlled to make the bundle slender or thick, whereby the band can be controlled from the narrow-band to the wide-band region.

In a case where the number of lines is increased dynamically during communication, synchronization between lines is necessary. However, rather than pass a synchronizing pattern, the demultiplexing/multiplexing functional unit 14d attaches a synchronizing flag to the transmission data, transmits the data and performs synchronization using the flag. In accordance with this arrangement, even if the number of lines increases during communication, synchronization of the data among a number of lines can be carried without a momentary interruption in the data transfer. This makes it possible to perform a data transfer continuously without a decline in communication efficiency or a decline in reliability.

Furthermore, a table is provided for storing data indicating, for each line (each of the WAN interfaces 14e-1.about.14en), whether the line is idle or not. In a case where enough idle lines for satisfying the required band do not exist, the required band is reduced and data communication is performed using the idle lines. This makes possible optimum LAN-to-LAN communication conforming to the existing conditions.

The traffic/application monitoring functional unit 14b monitors traffic between LANs during communication and, on the basis of the traffic monitored, decides whether to decrease, maintain or increase the number of lines presently in use. In a case where it is required to decrease the number of lines, the line monitoring functional unit 14c disconnects the lines that are unnecessary. As a result, economical LAN-to-LAN communication commensurate with traffic can be performed while maintaining high speed. A line-tariff monitoring timer is provided for each line. The traffic is determined and the above-mentioned decision to disconnect unnecessary lines is made before the line-tariff monitoring timer runs out of time, i.e., immediately before the line user fee increases. This makes it possible to reduce lines in economical fashion.

Furthermore, in a case where the number of lines is reduced, the demultiplexing/multiplexing functional unit 14d sets a line-disconnection flag for data transmitted via a line to be disconnected and then transmits this data. Data transmitted via other lines that will not be disconnected is transmitted without the unit 14d setting the line-disconnection flag. On the receiving side a disconnected line is recognized by the line-disconnection flag, after which the data that arrives via li