Lan emulation system and atm switch

A LAN emulation system is capable of avoiding a fault without making a line dual by a local dual structure on the premise that a LAN emulation server is made dual. An ATM switch, to which a LAN emulation terminal is connected, is provided with a line fault detecting unit, a routing table in which to define a correspondence a specified address to a line, and a line status table rewritten based on a result of detection by the fault detecting unit. A LAN emulation configuration server (LECS) connected to the ATM switch is constructed to have a dual structure. If a fault occurs in a connection to the LAN emulation configuration server, it is feasible to easily implement a connection to the dual LAN emulation configuration server and, because of no necessity for making dual the line itself, to easily construct a fault avoiding system.

BACKGROUND OF THE INVENTION
 The present invention relates to a technology effective in an application
 to a-LAN emulation technology in an ATM communication network.
 A system for actualizing a LAN emulation in an ATM network involves a data
 transmission between LAN emulation terminals connected to an ATM switch
 via the ATM network. Connected to the ATM switch are a group of servers
 for actualizing a LAN emulation function such as, e.g., a LAN emulation
 server (LES), a Broadcast and Unknown Server (BUS) and a LAN emulation
 configuration server (LECS).
 If a fault occurs in the server group for the LAN emulation, it follows
 that a trouble is caused in communications between LAN emulation clients.
 In the great majority of cases, there exists only one system of the LAN
 emulation configuration server (LECS) in the network, and, if the fault
 occurs in this LECS, there must be a high possibility in which the whole
 network might fall into a system-down.
 There can be considered the following contrivances for avoiding the above
 system-down.
 (1) An implementation system, i.e., the server group for providing the LAN
 emulation service function is constructed as a dual structure.
 (2) The implementation system for providing the LAN emulation service
 function incorporates a line dual function.
 (3) The ATM switch itself incorporates the line dual function.
 In the above contrivances (2) and (3), it is required that the dual
 function based on the same procedures be incorporated into the
 implementation system (the server group) connected to an opposite
 equipment, i.e., the ATM switch existing on the opposite side and
 therefore be incorporated at a plurality of places. This conduces to a
 problem in which the incorporation of the function itself becomes heavy.
 Accordingly, it is considered optimal to adopt the contrivance (1),
 however, there has been made no examination about a concrete method of
 actualizing it.
 It is a primary object of the present invention, which was contrived in
 view of the point described above, to provide a LAN emulation system
 capable of avoiding a fault without making the line dual by a local dual
 structure on the premise that a LAN emulation server is made dual.
 SUMMARY OF THE INVENTION
 To accomplish the above object, according to a first aspect of the present
 invention, an ATM switch, to which a LAN emulation terminal is connected,
 includes a line fault detecting unit, a routing table in which to define a
 correspondence between a specified address and a line, and a line status
 table rewritten based on a result of detection by the fault detecting
 unit. Further, a LAN emulation configuration server (LECS) connected to
 the ATM switch is constructed to have a dual structure.
 A line status is monitored, and, if a fault occurs in a connection to the
 LAN emulation configuration server, it is feasible to easily make a
 connection to the dual LAN emulation configuration server. Besides, the
 line is not required to be made dual, and hence a fault avoiding system
 can be easily constructed.
 According to a second aspect of the present invention, in the LAN emulation
 system according to the first aspect of the invention, one or two or more
 LAN emulation execution servers and address set servers for the LAN
 emulation are connected to the LAN emulation configuration server, and the
 LAN emulation configuration server includes a server registration table in
 which to set a combination of a LAN emulation executing server connected
 to the LAN emulation configuration server and an address set server for
 the LAN emulation.
 According to a third aspect of the present invention, in the LAN emulation
 system according to the second aspect of the invention, the LAN emulation
 configuration server is provided with a plurality of server registration
 tables, and the server registration table alternatively different is
 registered in each of the dual LAN emulation configuration servers.
 According to the second aspect of the invention, the LAN emulation
 execution server is defined as a LES (LAN emulation Server) prescribed in
 an ATM Forum UNI Version 1.0, and the address set server for the LAN
 emulation is defined as a BUS (Broadcast and Unknown Server).
 According to the first and second aspects of the invention, a content of
 the server registration table is changed per dual LAN emulation
 configuration server, whereby a load can be decentralized when a fault
 occurs due to an overload of the line.
 According to a fourth aspect of the present invention, a well-known address
 for the LAN emulation configuration server is set as a specified address
 of the routing table.
 The well-known address is a well-known LECS address prescribed in the ATM
 Forum UNI Version 1.0.
 Thus, the well-known LECS address is used as the specified address of the
 routing table, thereby making it possible to set routing while steering
 clear of a line with an occurrence of fault by use of a SETUP signal
 transmitted from the LAN emulation terminal (LEC).
 According to a fifth aspect of the present invention, an ATM switch
 connected to a LAN emulation configuration server comprises a control unit
 provided corresponding to two or more lines, a switch unit for switching
 the line, a cell inserting/extracting unit for inserting and extracting
 cells transferred via the line, a signaling processing unit for detecting
 a fault in the line, and a line correspondence data unit including a
 routing table in which to define a correspondence of the well-known
 address to the line and a line status table rewritten based on a result of
 detection by the fault detecting unit. Then, the routing table is set so
 as to lead a connection to the well-known address from an external
 terminal to the LAN emulation configuration server selectively from the
 two or more lines on the basis of a change in the line status table.
 With such a construction of the ATM switch, a fault avoiding system can be
 constructed simply by making the LAN emulation configuration server dual
 without making the line dual.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Embodiments of the present invention will be discussed with reference to
 the accompanying drawings.
 FIG. 1 shows a construction of a network system according to the present
 invention.
 An ATM network is constructed of ATM switches 101, 102. LAN emulation
 terminals (LEC#A-LEC#H) are connected to these ATM switches 101, 102.
 Dual LAN emulation configuration servers 103a, 103b are connected to the
 ATM switch 101, and manage communications between the LAN emulation
 terminals.
 FIG. 2 shows structures of routing tables 303, 305 of the ATM switches 101,
 102 in the prior art for a comparison. FIG. 3 shows a structure of a line
 status table as well as showing the structures of the routing tables 303,
 305 in this embodiment.
 According to the prior art, as shown in FIG. 2, well-known addresses are
 registered in addresses in the routing tables 303, 305. The ATM switch
 102, when receiving a SETUP signal addressed to the well-known address
 from the LAN emulation terminal LEC#A, determines a line #2 with reference
 to the routing table 305, and transfers the same SETUP signal to the ATM
 switch 101.
 The ATM switch 101 determines the line #2 corresponding to the relevant
 well-known address with reference to its own routing table 303, and
 transfers the SETUP signal to the LAN configuration server (LECS) 301
 connected to this ATM switch 101.
 By contrast, this embodiment has such differences from the prior art (FIG.
 2) that, as shown in FIG. 3, the dual LAN configuration servers (LECS)
 301a, 301b are connected to the ATM switch 101 which is further provided
 with a line status table 304, and items of line classifications
 (active/standby) are set in the routing table 303.
 Referring to FIG. 3, it is assumed that a fault occurs in a line #3 between
 the ATM switch 101 and the LAN configuration server (LECS) 301a.
 Referring again to FIG. 3, it is presumed that the line #3 be defined as an
 active line, while the line #2 be defined as a standby line when the fault
 happens. As for a fault detecting method, although it will be described in
 greater detail later on, the ATM switch 101 monitors the LAN configuration
 servers (LECS) 301a, 301b and, if a change appears in the status of one of
 these lines (i.e., if the fault occurs), rewrites the line status table
 304. Then, the item of classification of the routing table is rewritten
 based on the line status table. A rewriting condition at this time is that
 the active-system line and the standby-system line are rewritten in such a
 case that "the fault occurs in the active-system line" while "the
 standby-system line is normal" and "a call-in is given to the well-known
 address".
 This process being thus executed, the call-in is given to the well-known
 address not via the line #3 in which the fault has occurred but via the
 normal line #2.
 FIG. 6 is a block diagram illustrating an internal construction of the ATM
 switch 101.
 A line control unit 608 accommodates a plurality of line controllers 1-N
 (609) ad incorporates a function of terminating an UNI/NNI interface.
 A switch unit 607 has a function of exchanging ATM cells to be transferred
 on the cell unit.
 A cell extracting/inserting unit 606 has a function of executing an
 extraction and an insertion of cells for signaling.
 A signaling processing unit 604 incorporates a function of terminating
 L2/L3 of an ATM signaling protocol, and has line fault detecting unit 605.
 An ATM call control unit 601 implements main control of the relevant ATM
 switch 101, and has a function of controlling a transmission and a receipt
 of an SVC call, a connection and a disconnection as well as controlling a
 routing process.
 A data unit 602 includes a routing table 303, a line status table 304 and a
 system correspondence data unit 603. The data unit 602 manages data per
 system and data per line, and provided the ATM call control unit 601 and
 the signaling processing unit 604 with functions to register, delete,
 change and refer to the data.
 FIG. 7 shows functional blocks showing processes between the signaling
 processing unit 604, the data unit 602 and the switch unit 607, which are
 executed in the ATM call control unit 601.
 Referring to FIG. 7, a call processing task designated by 701 has function
 of executing SVC call processes (of connecting and disconnecting the call,
 and managing a status of the call). Herein, a plurality of call processing
 tasks 701 operate in parallel.
 A message analyzing/distributing/editing unit 706 incorporates a function
 of analyzing messages received from the signaling processing unit 604 and
 distributing these messages to the call processing tasks. Further, the
 message analyzing/distributing/editing unit 706 has also a function of
 editing the message requested for a transmission by the call processing
 task 701, and transferring the same message to the signaling processing
 unit 604.
 The call processing task 701 is constructed of a call management unit 702,
 an initialization processing unit 703, a routing processing unit 704 and a
 connection management unit 705.
 The call management unit 702 has a function of managing the call status.
 The initialization processing unit 703 has a function of initializing a
 various items of data of the data unit 602, e.g., the routing table 303.
 The routing processing unit 704 incorporates a function of analyzing a
 call-in address and a call-in sub-address and executing a routing process.
 The connection management unit 705 has a function of managing a status of
 an SVC connection and controlling the switch unit 607.
 Next, a procedure of setting the routing table in this embodiment will be
 explained.
 To begin with, an active-system line number and a standby-system line
 number of the LAN emulation configuration server (LECS) are registered
 beforehand in the system correspondence data unit 603 via an external
 input device such as an unillustrated system console etc.
 Then, similarly, the routing table 303 is registered with two or more line
 numbers corresponding to the well-known addresses.
 Note that the fault detecting unit 605 of the signaling processing unit 604
 detects a change in the status of the line, and the line status table 304
 is rewritten based on a result of this detection.
 To start with, the initializing process is executed by switching ON a power
 supply. At this time, however, the initialization processing unit 703 in
 the call processing task 701 of the ATM call control unit 601 refers to
 the LECS active-system line number of the system correspondence data unit
 603 and, based on this line number, sets "active" in the item of
 classification corresponding to the well-known address of the routing
 table 303. Then, referring to the LECS standby-system line number of the
 system correspondence data unit 603, the initialization processing unit
 703 sets "standby" in the item of classification of the routing table 303
 that corresponds to this line number.
 Further, "fault" is set in all the line status table.
 Next, the signaling processing unit 604 monitors a status of each line in
 accordance with a procedure of a relevant layer, and rewrites the line
 status table 304 when the status changes. Herein, in a normal case,
 "normal" is set in the items of line statuses corresponding to "active"
 and "standby" when normal.
 Next, a procedure of the rewriting process of the routing table when the
 fault happens will be explained with reference to FIG. 8.
 At the first onset, the LAN emulation terminal transfers and receive
 information to and from the LAN emulation configuration server and, for
 this purpose, transmits the SETUP signal designating the well-known
 address to the ATM switch 101.
 The ATM switch 101, upon receiving the SETUP signal (step 801), judges
 whether or not this is the well-known address (step 802). Herein, if
 judged to be the well-known address, with reference to the routing table
 303 and the line status table 304, the ATM switch 101 judges whether the
 line in which the "active" is set in the item of classification of the
 well-known address, is normal or not (step 803).
 Herein, if judged to be normal, the routing process is executed based on
 the routing table 303, and the LAN emulation terminal (LEC) is connected
 to the LAN emulation configuration server (LECS).
 Next, the LAN emulation terminal (LEC) is connected to the LAN emulation
 server (LES) on the basis of the information obtained from the LAN
 emulation configuration server (LECS), and further connected to an address
 set server (BUS)for the LAN emulation owing to an address solving function
 possessed by the LAN emulation server (LES). The LAN emulation terminal
 comes into an operating status owing to the connections to the group of
 those servers.
 Next, as explained in FIG. 3, if the fault occurs in the active-system LAN
 emulation configuration server (LECS) 301a or the line #3 corresponding to
 this server, the fault detecting unit 605 of the signaling processing unit
 604 of the ATM switch 101 detects the fault in the line, and rewrites
 "normal" of the line status corresponding to "active" to "fault" in the
 line status table 304.
 Next, the ATM call control unit 601 of the ATM switch releases the SVC
 connection from all the LAN emulation terminals (LEC) on the basis of the
 line fault notification given from the above fault detecting unit 605. All
 the LAN emulation terminals are thereby brought into the initial status.
 Subsequently, the LAN emulation terminal (LEC) having become the initial
 status, when falling into a connection phase to the LAN emulation
 configuration server (LECS), retransmits the SETUP signal addressed to the
 well-known address to the ATM switch 101.
 Herein, the ATM switch 101 re-executes the processing flow shown in FIG. 8.
 Namely, the ATM call control unit 601 of the ATM switch 101 that receives
 the SETUP signal, when judging that this signal is addressed to the
 well-known address (steps 801, 802), refers to the routing table 303 and
 the line status table 304 in order to make a judgement in step 803. At
 this time, as discussed above, the line in which "active" is set in the
 item of classification of the routing table 101, is conceived "fault", and
 hence the judgement in step 803 becomes a negative logic.
 Next, the ATM call control unit 601 judges whether the line in which
 "standby" is set in the item of classification of the routing table 101,
 is "normal" or not (step 804). Herein, if the standby line is judged to be
 "normal", "active" (#3) is replaced with "standby" in the routing table
 101, and "standby" (#2# is replaced with "active" (step 805). FIG. 3 shows
 the thus rewritten state.
 Subsequently, the routing process is executed in accordance with the
 rewritten routing table 303, and the LAN emulation terminal (LEC) is
 connected to the LAN emulation configuration server (LECS).
 Next, the LAN emulation terminal (LEC) is connected to the LAN emulation
 server (LES) on the basis of the information obtained from the LAN
 emulation configuration server (LECS), and further connected to the
 address set server (BUS) for the LAN emulation owing to the address
 solving function possessed by the LAN emulation server (LES). The LAN
 emulation terminal comes into the operating status owing to the
 connections to the group of those servers.
 FIGS. 4 and 5 show an applied version of this embodiment, wherein a
 plurality of server registration tables 402a, 402b and 403a-403c are set
 respectively in the dual LAN emulation configuration servers 103a, 103b.
 401, and the same management information is set in these tables. The
 management information in those server registration tables is provided to
 set a corresponding relationship between the LAN emulation server (LEC)
 and the address set server (BUS) for the LAN emulation.
 Referring to FIG. 4, LES1/BUS1 is set in the LAN emulation configuration
 server 103a, LES2/BUS2 is set in the server 103b, and LES2/BUS2 and
 LES3/BUS3 are set in the server 401.
 Then, the line between the LAN emulation configuration server 103a and the
 ATM switch 101 is set so that a line fault occurs when coming into an
 overload state. This setting can be easily done by setting a fault
 detection program of the fault detecting unit 605 of the signaling
 processing unit 604.
 Next, as illustrated in FIG. 5, if the connection to the LAN emulation
 configuration server 103a functioning as the active system is detected as
 being in the fault status due to the overload of the line, the active and
 standby systems are switched over based on the construction and the
 processes explained in FIGS. 3-8, whereby the ATM switch 101 is connected
 to the LAN emulation configuration server 103b.
 A function of the server registration table LES1/BUS1 is thereby assigned
 to the LAN emulation server 401, and the loads of the LAN emulations of
 the whole network are decentralized. At this time, if a sum of the loads
 of LES1/BUS1 and LES3/BUS3 falls within an allowable range of the LAN
 emulation server 401, it follows that the LAN emulation normally operates.
 Note that the embodiments discussed above have dealt with the construction
 in which the LAN emulation configuration server (LECS), the LAN emulation
 server (LES) and the address set server (BUS) for the LAN emulation, are
 connected to the ATM switch 101 but may also be, as functional units,
 built in the ATM switch 101.
 According to the present invention, it is feasible to provide the system
 capable of avoiding the fault without making the line dual by the local
 dual structure on the premise that the LAN emulation system is provided
 with the dual LAN emulation servers.
 It is apparent that, in this invention, a wide range of different working
 modes can be formed based on the invention without deviating from the
 spirit and scope of the invention. This invention is not restricted by its
 specific working modes except being limited by the appended claims.