Abstract:
In an ATM switching module, expansion interfaces are connected in pair to an ATM switch to respectively operate in active and standby states. When another ATM switching module is installed and a connection is established between an expansion interface of standby state and the newly installed ATM switching module, a process sets the standby state expansion interface in hot-standby state, and holds off incoming ATM cells destined for the hot-standby state expansion interface until the connection is ready to accept cell traffic, whereupon the hot-standby state expansion interface is switched to active state, whereby the ATM switch and an ATM switch of the another switching module constitute a multi-stage configuration. The processor is arranged to update the routing table according to internal state of the ATM switch so that, when a call is to be established for an incoming ATM cell, the header of the cell contains a plurality of fields containing routing data respectively associated with different stages of the multi-stage configuration.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to ATM (asynchronous transfer mode) switching technology, and more specifically to system expansion using ATM switching modules of different types without losing cells during cutover. 
     2. Description of the Related Art 
     ATM switches have been developed for high speed network applications such as broadband ISDN. For ease of system expansion, an ATM switch is usually constructed of N×N configuration to be used as a basic building block to implement a switching system of desired size. To ensure non-blocking operation during system expansion, N/2 ports of the switch are preassigned to expansion switching modules. Since the preassigned ports are normally reserved as idle ports, utilization efficiency of the input ports of a switch is low for applications if the number of subscribers is smaller than N2. This is particularly true when the operating speed of subscriber is low. In some cases, subscriber&#39;s per-port usage is much lower than the processing capacity of the switch. if this fact is taken into account with the link capacity of expansion interfaces, non-blocking operation can still be realized with switches whose preassigned ports for expansion are smaller than N/2, particularly in switching nodes at the edge of a public switched telephone network. This prior art technique is described in Japanese Laid-Open Patent Specifications Hei-4-273739 and Hei-7-115421. 
     In addition, two approaches are currently available for system expansion, one involving the use of a number of identical basic switching modules, while the other using a large-sized switching module as an intermediate stage. In these techniques, precautions are taken against losing cells during cutover of the expansion units. However, it is impossible to incorporate the large-sized switching module in an expansion process in which the basic switching modules are exclusively used without causing cell loss, since the fixed number of preassigned ports allows no freedom to flexibility in combining different types of switching modules. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an ATM switching module that can be used with a different type of switching module during system expansion without causing cell loss. 
     According to a first aspect of the present invention, there is provided an ATM switching module comprising a plurality of line interfaces, each line interface having a routing table for receiving an ATM cell, making a search through the routing table for routing data according to a connection identifier contained in the cell, and updating header of the cell according to the routing data. An ATM switch is provided for routing the ATM cell according to the updated header. A plurality of pairs of expansion interfaces are provided such that the expansion interfaces of each pair being connected together to the ATM switch to operate in active and standby states, respectively. A processor is responsive to another ATM switching module being installed and a connection being established between one of the expansion interfaces of standby state and the Installed ATM switching module, for setting the standby state expansion interface in hot-standby state, holding off incoming ATM cells destined for the hot-standby state expansion interface until the connection is ready to accept cell traffic, and switching the hot-standby state expansion interface to active state, whereby the ATM switch and an ATM switch of the newly installed switching module constitute a multi-stage configuration. The processor is further arranged to update the routing table according to internal state of the ATM switch so that, when a call is to be established for an incoming ATM cell, the header of the cell contains a plurality of fields containing routing data respectively associated with different stages of the multi-stage configuration. 
     According to a second aspect, the present invention provides a method for expanding a communications network, wherein the network comprises a plurality of ATM switching modules, each of the modules comprising a plurality of line interfaces, each line interface having a routing table for receiving an ATM cell, making a search through the routing table for routing data according to a connection identifier contained in the cell, and updating header of the cell according to the routing data, an ATM switch for routing the ATM cell according to the updated header, a plurality of pairs of expansion interfaces, the expansion interfaces of each pair being connected together to the ATM switch and operating in active and standby states, respectively. The method comprises the steps of (a) Installing an ATM switching module as an additional unit, (b) establishing a connection between one of the expansion interfaces of standby state and the installed ATM switching module, (c) setting the standby state expansion interface to hot-standby state and holding off incoming ATM cells destined for the hot-standby state expansion interface until the connection is ready to accept cell traffic, (d) switching the hot-standby state expansion interface to active state, whereby the ATM switch and an ATM switch of the another switching module constitute a multi-stage configuration; and (e) responsive to an incoming ATM cell, updating the routing table according to internal state of said ATM switch so that the header of the cell contains a plurality of fields containing routing data respectively associated with different stages of the multi-stage configuration. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in further detail with reference to the accompanying drawings, in which: 
     FIG. 1 is a block diagram of an ATM basic switching module according to the present invention; 
     FIGS. 2A to  2 D are schematic block diagrams of a system configuration when the system undergoes successive stages of expansion according to a first embodiment of the present invention; 
     FIG. 3 is a flowchart of the operation of a master processor when the system undergoes expansion according to the present invention; and 
     FIGS. 4A to  4 E are schematic block diagrams of a system configuration when the system undergoes successive stages of expansion according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1, there is shown an ATM switch module according to the present invention. This switch module is used as a basic building block to construct a multistage ATM switching system, in which one of the switching modules functions as a master switching module when a new switching module is installed. 
     The basic building block comprises an ATM self-routing switch  1 , a plurality of line interfaces  2 , and a plurality of pairs of expansion interfaces  3 ,  4 . A processor  5  receives signals from the line interfaces  2  and expansion interfaces  3 ,  4  and has access to the internal status of switch  1 . 
     Each line interface  2  has a routing table whose contents are updated by the processor  5  according to the internal states of the switch  1 . When the line interface  2  receives an ATM cell from associated subscriber terminal, it reads its VPI/VCI (virtual path identifier/virtual channel identifier) of the cell header and makes a search through its routing table for information necessary for routing the received cell and updates the VPI/VCI values according to the routing information. This ATM cell is launched into the self-routing switch  1  where it is self-routed to a desired output port according to the routing information contained in the cell. Each line interface  2  further includes a buffer for putting incoming cells in a queue when a backpressure signal is received. 
     Expansion interfaces  3  and  4  are connected together to the switch  1  to form a pair of active and standby units, while they are respectively connected to corresponding expansion interfaces of other basic switch module via cable links  6 . When a cable link is switched to an active state during a module expansion phase, the expansion interface is arranged to transmit a backpressure signal toward a call request source in response to a backpressure-set command signal from the master switch module and clears the backpressure signal when a backpressure disconnect command is received from the master switch module. The purpose of the backpressure signal is to hold incoming ATM cells in the queuing buffer until a new route is established during the module expansion phase in order to prevent them from being corrupted or lost by switchover operation. 
     To add flexibility to the ATM switching module, universal slots are provided for mounting the line interfaces  2  and the expansion interfaces  3 ,  4 . Although these interfaces have different number of input and output terminals and circuit functions, the universal slots have different sets of terminals respectively adapted for the interfaces  2  and the interfaces  3 ,  4 . When they are inserted to the universal slots, they are connected to the appropriate terminals. 
     The capacity of an ATM switching system can be expanded by installing a new switching module and adding expansion interfaces to the existing modules. Connecting cables are laid down between the new expansion interfaces of the existing modules and all expansion interfaces of the new switching module. 
     Once the existing and new switching modules are wired together, the processor  5  of the master module takes the responsibility to perform a “cutover” of all the new expansion Interfaces in order to place them into service according to the system configuration of active/standby maintenance plan. 
     For this cutover operation, control interfaces  7  are connected to the processor  5  of the master switching module to exchange control messages with other switching modules via control links  8 . 
     FIGS. 2A to  2 D illustrate a series of phases involved when a new switching module is installed in a two-unit ATM switching system as shown in FIG.  2 A. 
     In FIG. 2A, it is shown that switching modules,  10 A and  10 B are interconnected by an active link indicated by a thick line “A” terminating at both ends to active interfaces  3  and by a standby link indicated by a dotted line “S” terminating at both ends to standby interfaces  4 . In this system configuration, ATM cells transported across two switching modules are routed according to three physical addresses inserted respectively to PA 0 , PA 1  and PA 2  fields of their cell header. 
     For a given two-stage connection, the physical address of a cell in the PA 0  field indicates an outgoing line port of the switching module to which the cell is directed, the physical address of the cell in the PA 1  field indicates routing information used by the second stage of the connection, and the physical address of the cell in the PA 2  field indicates routing information used by the first stage of the connection. These routing information of an ATM cell are set according to the routing table of a line interface where its cell header is updated. 
     If the connection involves only one switching stage (i.e., intra-module call), the PA 0  field contains no data, the PA 1  field contains data indicating an outgoing line port and the PA 2  field contains routing data to be used by the local ATM switch. 
     In FIG. 2B, a new switching module  10 C is installed and new expansion interfaces are installed in the existing modules  10 A and  10 B. As a result, the system has expansion interfaces indicated by letters “a” to “I”, with the interfaces “a” and “e” being interconnected by existing active link “A” and interfaces “b” and “f” being interconnected by existing standby link “S”. Cables are laid as standby links “S” between interfaces “c” and “k”, “d” and “I”, “g” and “I”, and “h” and “j”. 
     In FIG. 2C, the switching module  10 A functions as a master module and its processor takes the responsibility to perform a cutover operation. 
     The cutover operation of the processor  5  at the master module proceeds according to the flowchart of FIG.  3 . 
     A cutover operation begins with step  30  where the master processor transmits a reconfiguration notification to modules  10 B and  10 C. At step  31 , the master processor sends an active/standby setting command from module IOA to new switching module  10 C. This command message indicates the links to be set “active” or “standby” according to the system&#39;s maintenance plan. At step  32 , the processor sends a “hot-standby” command to modules  10 B and  10 A and proceeds to set its own expansion interfaces “b”, “c”, “d” in a hot-standby state. In response, the processor at module  10 B sets its interfaces “f”, “g”, “h” in a hot-standby state and the processor at module  10 C sets all of Its expansion interfaces in a hot-standby state. The hot-standby state of an expansion interface is one in which the internal state of the Interface is active and ready to accept ATM cells, but no connection is established from the ATM switch  1 , while the active state is one in which the expansion interface is accepting flow of ATM traffic from the ATM switch  1  for processing. 
     In this way, all links set to the standby state in FIG. 2B are set to the hot-standby state as indicated by solid lines in FIG. 2C Note that the ATM traffic carried by the active link “A” between expansion interfaces “a” and “e” is not affected during this operation. 
     When the hot-standby setting operation is completed at a modules  10 B and  10 C, their processor returns an acknowledgment message to the master module  10 A. The processor at master module responds to the acknowledgment messages at step  33  and proceeds to step  34  to send an active/standby switchover command to modules  10 B and  10 C and sets its own expansion interfaces according to the active/standby command message previously sent. The purpose of this switchover command message is to switchover all the expansion interfaces according to the active/standby setting previously commanded at step  31 . In response to this switchover command message, switchover operation proceeds in each of the switching modules. 
     Following the transmission of the switchover command message, the processor at master module (or master processor) proceeds from step  34  to step  35  to send a backpressure-set command message to modules  10 B and  10 C in order to direct their extension Interfaces, which are in the process of being switched over to active state, to apply a backpressure signal to their input port. “Backpressure” is used in the ATM switching technology when it is desired to exert a backward pressure on incoming traffic to prevent it from entering the ATM switch so that the switch is prevented from being overloaded. 
     As a result, if a connection is established from a line interface through the associated ATM switch  1  to one of these extension interfaces, the backpressure signal of that interface will be transmitted to the line interface and the incoming ATM cells will be stored in the buffer of that line interface. This hold-off condition continues until the connection established by the hot-standby expansion interfaces and their associated link are completely switched over to active state and ready to accept incoming traffic. 
     When the switchover setting operation is completed at modules  10 B and  10 C, their processor returns a switchover complete message to the master module  10 A. The master processor responds to the returning switchover complete messages at step  36  and proceeds to step  37  to send a backpressure-disconnect command to modules  10 B and  10 C to direct their switched active interfaces to disconnect the backpressure signal. The hold-off condition is released and the switched-over active expansion interfaces and their associated links accept traffic. In this manner, ATM cells within the ATM switch  1  are not corrupted or lost when the hot-standby links are switched over to active state. 
     When the hot-standby links are finally switched over to the active and standby states which were set in the system according to the command message of step  31 , the link states of the system will be reconfigured as shown in FIG.  2 D. In this configuration, the expansion interfaces “b” and “d” are connected by active links to interfaces “f” and “I”, and expansion interface “h” is connected by an active link to interface “j”, and other expansion interfaces are set in standby state. 
     A second embodiment of the present invention is shown in FIGS. 4A through 4E. In the second embodiment, three existing basic switching modules  10 A,  10 B and  10 C are interconnected to form a multistage network as shown in rig.  4 A in which links  41 ,  42 ,  43  are in standby state, Interconnecting expansion interfaces “b” and “f”, “d” and “j”, and “f” and “I”. 
     According to this modification, a large-sized expansion switching module  11  is used for interconnecting the existing modules as illustrated in FIG.  4 B. Expansion switching module  11  comprises a control interface  50 , a plurality of expansion interfaces  51  through  56 , an ATM switch  57  and a processor  58  whose function is similar to the processor of modules  10 B and  10 C. Control interface  50  is connected by an active link to the control interface  7  of the master module  10 A. During the initial phase of expansion, standby links are manually established by rewiring the existing standby links  41 ,  42  and  43  to the interfaces  52 ,  54  and  56  as new standby links  41 A,  42 A and  43 A. 
     In FIG. 4C, a cutover operation of the laid-down standby links is initiated by the master processor according to the flowchart of FIG.  3 . 
     In FIG. 3, the master processor sends a system reconfiguration notification to all other switching modules including the expansion module  11  (step  30 ). At step  31 , the master processor sends an active/standby setting command to the expansion switching module  11 . This command message indicates the links to be set “active” or “standby” according to the system&#39;s maintenance plan. At step  32 , the processor sends a “hot-standby” command to modules  10 B,  10 A and  11 , and proceeds to set its own expansion interface “b” in hot-standby state. In response to this command, expansion interfaces “f” and “j” are set in a hot-standby state and corresponding interfaces  52 ,  54  and  56  of expansion module  11  are set in hot-standby mode. 
     When the hot-standby setting operation is completed at modules  10 B,  10 C,  11 , their processor returns an acknowledgment message to the master module IOA. The master processor responds to the acknowledgment messages at step  33  and proceeds to step  34  to send an active/standby switchover command to modules  10 B,  10 C and  11 , and sets Its own expansion interfaces according to the active/standby command message previously sent. The purpose of this switchover command message is to switchover all the expansion interfaces according to the active/standby setting previously commanded at step  31 . In response to this switchover command message, switchover operation proceeds in each of the switching modules. 
     Following the transmission of the switchover command message, the processor at master module proceeds from step  34  to step  35  to send a backpressure-set command message to modules  10 B,  10 C,  11  In order to direct their extension interfaces to apply a backpressure signal to their input port. As a result, when a connection is established from a line interface through the associated ATM switch  1  to one of these extension interfaces, the backpressure signal of that interface will be transmitted to the source and the incoming ATM cells will be stored in the buffer of that line interface. This hold-off condition continues until the new active line interfaces and their associated link are ready to accept incoming traffic. 
     When the switchover setting operation is completed at modules  10 B,  10 C,  11 , their processor returns a switchover complete message to the master module  10 A. The master processor responds to the returning switchover complete messages at step  36  and proceeds to step  37  to send a backpressure-disconnect command to modules  10 B,  10 C,  11  to direct their switched active interfaces to disconnect the backpressure signal. The hold-off condition is released and the switched-over active expansion interfaces and their associated links accept incoming traffic. 
     When the hot-standby links are finally switched over to the active and standby states which were set in the system according to the command message of step  31 , the link states of the system will be reconfigured as shown in FIG. 4D, In which inter-module ATM cells are transported by expansion Interfaces “b”, “f”, “j”,  52 ,  54  and  56 . 
     FIG. 4E shows the final stage of the system expansion. In this stage, standby links  44 ,  45  and  46  are manually rewired and new standby links  44 A,  45 A and  46 A are established to expansion units  51 ,  53  and  55  of the expansion module. Therefore, all inter-module calls are routed through the expansion module  11 . 
     For a given three-stage connection, the physical address of a cell in the PA0 field indicates routing information to be used by the third stage of the connection, the physical address of the cell in the PA1 field indicates routing information to be used by the first stage of the connection, and the physical address of the cell in the PA2 field indicates routing information to be used by the second stage of the connection, i.e., the expansion module  11 .