Switching architecture comprising two switch fabrics

Abstract: A Switching architecture comprising a first and a second Switch Fabrics (10, 20) including a switch core (15, 25) located in a centralized building and a set of SCAL elements distributed in different physical areas. Each SCAL element respectively comprises a SCAL Receive element (11-i) and a SCAL Xmit element (12-i) for respectively permitting access to a corresponding input and output port of the switch core. The Port Adapters (30; 31) are distributed at different physical areas and each one is connected to the first and second Switch Fabric via a particular SCAL element so that each Switch core (15, 25) receives the sequence of cells coming from any Port adapter and conversely any Port adapter may receive data from any one of said first or second switch core. There are arranged means (15, 100) for assigning a particular Switch core to any Port adapter for the normal traffic of cells and for reserving the other switch core to Backup or maintenance traffic situations. To achieve this each switch core is fitted with a masking mechanism which uses the value loaded into a Mask register for altering the bitmap value which is normally used inside the switch core for controlling the routing process. Since the Mask registers in the two switch cores are loaded with complementary values, this permits a perfect distribution of the cells via one and only one SCAL Xmit element towards any Port Adapter. Preferably the Mask mechanism can be controlled by a special control field located into the cell, or when maintenance of backup conditions are planned.

Description: Technical Field of the invention The invention relates to telecommunication and more particularly to a switching architecture comprising a first and second Switch Fabrics respectively including a switch core and distributed Switch Core Access Layer (S.C.A.L.) elements. Background art. Switching technology has made a great use of the traditional crossbar switches, the latter allowing high rates (up to 100 Gigabits and even more). Crossbar switches however present the so-called «head of line blocking» drawback which seriously limits the efficiency of the switching process. Additionally, crossbar switches are not suitable for handling small message of data, such as those used in Asynchronous Transfert Mode (A.T.M.) techniques. New switching techniques are thus required for handling the small packets of data used for transporting data, voice or video over the telecommunication networks. In this respect shared buffer switches have prove to be a very interesting direction of research for the design of the switching architectures of the future. Indeed, shared buffer switching technique permits to associate a high switching rate while handling perfectly the small packets of data. Basically, in this shared buffer switching technique, a central buffer is used for storing the messages which are received from the input ports of the switch and before they are rerouted towards the appropriate output ports. Each output port of the switch is associated with an output queue which stores the succession of addresses corresponding to the location of the different messages loaded in the buffer, prior to their extraction and delivery to the considered output port. The queuing process is located at the output level of the switch, what eliminates the «head of line blocking» drawback and also facilitates multicasting possibilities. Shared buffer Switches can be more enhanced by means of speed expansion and port expansion mechanisms. Examples of such shared buffer techniques can be found in the non published European applications n° 97480057.5, 97480056.7, 97480065.8, 96480129.4, 96480120.3 assigned to IBM which are herein incorporated by simple reference. Additional prior art documents relating to shared buffer switching techniques can be found in non published European applications n° 97480100.3, n° 97480098.9, n° 97480101.1, n° 97480099.7, n° 98480007.8 and n° 98480006.0. In spite of their interesting possibilities, the use of shared buffer switches are obviously limited by the physical size of the shared buffer, thus limiting the number of cells which can be simultaneously processed. Speed performance and the complexity of the circuit drastically limits the size of the shared buffer in a single electronic module. Another problem is the possibility to improve the availability of the switch, particularly when maintenance operations are being planned. In this respect it is highly desirable to continue the routing of the cell even when a maintenance operation is being performed on one part of the switching architecture, e.g. a card replacement or a system upgrade. At last, since the huge development of the telecommunication and the transactions in the telecommunication world tend to increase the need for introduction of fault tolerance mechanism in a switching system. This is particularly true when the switch has to be operated in the satellite or other critical application. In these applications it is essential that the routing process be maintained, even with a reduced performance, when a part of the switching mechanism is rendered non operational. Fault tolerance mechanisms are generally expansive in terms of number of electronic modules, at least because, generally, that number is multiplied by two. It is thus highly desirable that the additional components brought to the architecture also participate to the overall performance of the switching architecture during the normal data communication, and not only when a breakdown occurs. Publication entitled "The MainstreetXpress Core Services Node - A Versatile ATM Switch Architecture for the Full Service Network" (Rathgeb E P et al.) IEEE Journal on Selected Areas in Communications, vol. 15, no. 5, June 1997, pages 795-806, describes how the MainstreetXpress core services node, to fully exploit the potential of the ATM architecture, addresses the problems of versatility and modularity in supporting services and protocols, and provides independent scalability of data throughput and control performance over a wide range, with reliable features adaptable to the respective application scenario. The MainstreetXpress Core Services node, which has evolved to a mature central office ATM switch, provides a future-proof architecture incorporating all of the features required in the B-ISDN. Summary of the invention The object of the present invention is to provide a buffer expansion architecture permitting to efficiently combine the buffering possibilities of a first and a second individual switching module. Another object of the present invention is to provide a switching architecture based on shared buffer individual switches and which can be still operated when a maintenance operation is being performed on one part of the switching architecture, e.g. a card replacement or a system upgrade. Particularly, loss of cells, erroneous duplications of cells, misordering of cells and excessive delay variations should be prevented. It is another object of the present invention to provide a fault tolerant switching architecture based on a first and a second switching structure which advantageously uses the resources of both structures in the normal operations, and which can still be used , with possibly reduced performance when a maintenance operation is performed in one structure. It is a future object of the present invention to provide a continuous available switching architecture permitting continuous checking of each switch fabrics of the two sides and which uses very simple Port Adapters designed to operate at the normal rate. It is a further object of the present invention to provide with a Switch architecture that easily permits two elementary switch fabric to be aggregated in order to provide a continuous availability of the overall architecture. These and other objects are provided by the switching architecture in accordance with the present invention which is defined in the set of claims. Basically a Switching architecture according to the invention comprises a first and a second Switch Fabrics including a first and second switch cores located respectively in a centralized building and a set of Switch Core Access Layer (SCAL) elements distributed in different physical areas. Each SCAL element respectively comprises a SCAL Receive element and a SCAL Xmit element for respectively enabling access to a corresponding input and output port of the switch core. The Port Adapters are distributed at different physical areas and each one is connected to the first and second Switch Fabric via a particular SCAL element so that each Switch core receives the sequence of cells coming from any Port adapter and conversely any Port adapter may receive data from any one of said first or second switch core. The invention enables that, for each Port adapter, one particular Switch Fabric be assigned for the normal traffic of data while the other is reserved for Backup or maintenance traffic situations. To achieve this each switch core is fitted with a masking mechanism which uses the value loaded into a Mask register for altering the bitmap value which is normally used inside the switch core for controlling the routing process. Since the Mask registers in the two switch cores are loaded with complementary values, this permits a perfect distribution of the cells via one and only one SCAL Xmit element towards any Port Adapter. Preferably the Mask mechanism can be controlled by a special control field located into the cell, thus permitting the introduction of test cells in the architecture for testing the backup path. Description of the drawings Figures 1 and 2 illustrate the basic architecture of the invention using two shared buffer switching structures. Figure 3A illustrates a synoptic view of an internal structure of a self-routing switching module that can be used in the preferred embodiment of the invention. Figure 3B illustrates a block diagram of the mask circuit used for performing the filtering control field process according to the invention. Figure 4 is a flow chart describing the Filtering Control Field process which is performed within each Switch core. Description of the preferred embodiment of the invention With respect to figure 1 there is shown a switch fabric structure capable of continuous availability. Switch subsystem 1 is composed of two individual and identical switch Fabrics 10 and 20 that conforms, in the preferred embodiment of the invention, to the teaching of the above mentioned European patent applications. However, it should be noticed that the concept of the invention can be used with other embodiments of switches based on shared buffer, particularly switch fabrics that include queuing resources at the output of the switching components. Each switch fabric 10 (resp. 20) includes a switch core 15 (resp. 25) which is generally located in one centralized building, and a set of Switch Core Access layers (SCAL) elements 11 and 12 (resp. 21 and 22) which are located at different points in accordance with the topology and the particular location of the telecommunication lines. It should be noticed that the architecture that is shown in figure 1 corresponds to one particular port and that the real structure should be considered with the number of different ports. In the example, the receive side of the Switch Fabric is referenced with respect to the port i, and consequently the SCAL receive element is referenced 11-i (resp. 21-i) which corresponds to the input port i, attached to the port adapter 30-i. Figure 1 shows the architecture with respect to a output port j, and consequently SCAL Xmit element 12-j (resp. 22-j) corresponds to the output port j attached to the port adapter 31-j. From a functional point of view, a Port Adapter Receive 30-i produces a sequence of cells, which are simultaneously transmitted to the corresponding SCAL receive element 11-i in the Switch Fabric 10 through the link 32-i; and to the SCAL receive 21-i in the Switch Fabric 20 through the link 33-i. Generally speaking each SCAL receive element 11-i and 21-i is connected to their corresponding switch core via a set of n serializing links 13-i and 23-i permitting the communication between the different buildings wherein the switching structure is located. Similarly the two Switch Cores 15 and 25 are respectively connected to their corresponding SCAL transmit part 12-j and 22-j, via a set of n serialized links 14-j and 24-j. The cells which are produced by the two SCAL Xmit elements 12-j and 22-j are respectively transported through the links 34-j and 35-j to the Port Adapter (Xmit) 31-j. It appears that such an architecture permits the switching of the cell when one of the two Switch Fabrics 10 or 20 is down, thereby allowing continuous availability of the overall Switching architecture. However, it also appears that the PORT Adapter (Xmit) should be capable of receiving twice the flow of cells transmitted by one Switch core, what increases the complexity of the adapter. Another solution could be to incorporate in the Port adapter (Xmit) a mechanism to cancel all the cells received from one side, the mechanism processing the cells coming from the other side. This alternate solution however entails a strong disadvantage since it leads to a complex switching system that eventually convey only half of the traffic being switched and, additionally, it might occur that the side which is not normally used is also subject of a failure - generally called a hidden failure -, what results in a general breakdown of the Switching architecture when the normally used switch falls in a breakdown state. With respect to Figure 2 the structure of the switching architecture is illustrated. Basically, the invention uses a repartition of the output ports of the Switch Subsystem 1 , and the Xmit Adapter elements in two groups: a group LEFT that will be assigned to SWITCH Fabric 10 (Left) and will normally receive the traffic cells from the latter; a group RIGHT that is affected to the SWITCH Fabric 20 and which normally receives the traffic cells from the latter. In the preferred embodiment of the invention, the continuous availability is achieved with two separate Left and RIGHT Switch Fabrics. However, it should be noticed that the system could be enhanced more by increasing the number of switching paths. In the figure the Port adapter Xmit 31(m) is assigned to the Left group while the Port adapter Xmit 31(n) is included in the RIGHT Group. Additionally, each Switch Fabric path, e.g. Switch Fabric 10, comprises SCAL Xmit elements that are divided into two groups - so called ACTIVE and BACKUP - corresponding to the repartition previously made of the PORT ADAPTER Xmit elements. More particularly a PORT Adapter Xmit 31 (m) - that belongs to the LEFT group (assigned to the left switch path) is physically connected to a corresponding SCAL Xmit element 12(m) that belongs to the ACTIVE group of the Switch Fabric 10; and is further physically connected to a corresponding SCAL Xmit element 22(m) which belongs to the BACKUP group of the SWITCH Fabric 20. Similarly, the PORT Adapter Xmit 31(n) that belongs to the RIGHT group assigned to the RIGHT Switch path is physically connected to a corresponding SCAL Xmit element 22 (n) that belongs to the ACTIVE group of the SWITCH Fabric 20; and is further physically connected to a corresponding SCAL Xmit element 12(n) which belongs to the BACKUP group of the Switch Fabric 10. The above described repartition of the PORT Adapter Xmit elements permits an effective and simultaneous operating of the switch cores 15 and 25. During the normal traffic, the repartition between the Port Adapter Xmit, decided by the Node manager, results in that the cells that are to be propagated to the Port Adapter Xmit 31(m) (belonging to the Left group) will be conveyed through Left Switch Core 15, to the SCAL Xmit element 12(m), while the same cell is duplicated by the PORT Adapter Receive 30 (i) and will be dropped by the Switch core 25. Conversely, the cells that are to be propagated to the Port Adapter Xmit 31(n) (belonging to the Right group) will be conveyed through Right Switch Core 25, to the SCAL Xmit element 22(n), while the same cell being duplicated at the level of the PORT Adapter Receive 30 (i) will be dropped by the Switch core 15. Therefore, the cells are being distributed through the two Right and Left Switch Fabrics in accordance with the repartition that was determined by the node manager. This is achieved as followed: In the best mode of the invention, the cells which are generated by the PORT Adapter Receive 30i are duplicated on the two serial links 32(i) and 33(i). It should be noticed that this duplication is involved up to the input of the Switch Core 15 and 25. This is very important because the two cores must receive the same cells so as to make sure that the control cells are also simultaneously received. In the European patent applications mentioned above, there is described that the switch core uses routing table which provides with the bitmap information that is used inside the core for controlling the routing process. That bitmap information is generally read from the contents of Control Routing tables that can be updated by means of Control cells. This is the reason why it is important, in this invention, that the same cells arrive at the input of each Switch Core. More particularly, as described in the above mentioned patent applications, PORT Adapter receive 30(i) generates a cell which comprises a Switch Routing Header (SRH) and a payload; the latter SRH including a two-bytes Routing Label characteristics of the destination of the cell, that is to say the Port Adapter Xmit in case of Unicast connection or the set of Port Adapter Xmit in case of multicast connection, as well as a one-byte Cell Qualifier. The cells is then received by the local SCAL Receive element 11(i) and 21(i), which latter introduces an additional set of two bytes that will be affected to the location of the bitmap that will be used for controlling the internal routing process inside the switch core 15 and 25. The cells are then remotely transported to the centralized Switch Cores 15 and 25, the latter comprising a Routing Control Device that uses the Routing Label for address a Routing Control table that provides with the appropriate two-bytes bitmap information that can then be used for the internal routing process inside the switch cores. In the teaching of the present invention, an additional mask mechanism is used inside each Switch core, for processing the bitmap provided by the Routing Control device before the latter is used for controlling the routing process. Fig. 3A is a schematics view of the structure of a Switch core element forming the basis of the Switch core 15 or 25. A set of 16 input ports can provide cell into a Cell storage 1 via 16 corresponding routers 2-i. The cells can be extracted from the Cell storage and transferred to the output ports via 16 Selectors 3-i. When a cell is entered into the Switch element, a Free buffer Addresses is retrieved from a Free Buffer Address Queue 5, and the incoming cell is routed to the appropriate location within the Cell storage defined by the address that was extracted. Simultaneously, the Switch routing header is extracted from the incoming cell and transmitted through a bus 6-i to a Mask circuit 100. The latter Mask circuit 100 uses the SRH value provided in order to generate a corresponding 2 bytes mask value which will be provided to a set of 16 gating systems (only gating 7-1 and 7-16 being illustrated). At the input of the gating systems 7-i, via bus 8-i, there is presented the address where the cell is being loaded within Cell storage 1. That address is loaded into one or more Output Queues 9-1 to 9-16 corresponding to the appropriate output where the cell will have to be routed. When the cell is a multicast cell, the Mask circuit 100 provides to Count circuit 110 the number of duplication of the cell via bus 10. The outgoing process involves the reading of an address in the Output Queue 9-i, corresponding to the address of storage of the cell within Cell storage 1. This address is then presented to the Selector 3-i and the cell can be shifted to the appropriate output i. Bus 13 permits the transmission of that address to the Count circuit 110 which performs a decrementation operation corresponding to a single duplication of the cell. When the cell has been outputted to every appropriate output ports, the result of the decrementation is zero, what releases the address which becomes available for the storage of a new cell. At that moment, the released address can be loaded into the Free Address QUEUE 5 under control of circuit 111. With respect to Figure 3B there is shown the preferred embodiment of the Mask circuit 100 which is used for performing the steps 403 to 409 of the Filtering Control Field process of figure 4. Basically, the SRH is loaded into a Register 101 via a bus 6-i, and a Filtering Mask Register 102 has been loaded by the control processor at the initialization of the circuit and contains the two-bytes mask that characterizes the distribution of the output ports in the two Left and Right groups. The detection of the Control packets (corresponding to step 403 of figure 4) is performed by circuit 104 which processes the contents of the bitmap field of Register 101. The Filtering Control Field in the Cell Qualifier Field is transmitted to the Filtering Device 103 with the value of the bitmap and that of the Filtering Mask register 102. The Filtering device can then provide with the residual bitmap that will be used by the gating circuits 7-1 to 7-16 so that the address of the cell being stored in Cell storage 1 is loaded into the appropriate Output Queues 9-i in accordance with the bitmap being carried by the cell, and processed in accordance with the Mask register contents. This achieves the cell routing in accordance with the Left and Right repartition of the output groups that were decided by the Node manager. With respect to figure 4 there is shown the Distribution process that is used for permitting the control of the two Left and Right Switch Cores. This process is being performed in every Switch Core by Mask circuit 100 as shown in figure 3B. The process in accordance with the present invention initiates with step 401 where the Routing label which is contained inside the SRH is extracted from the arriving cell. Then, in step 402, the Routing Label is used for addressing the Routing Table located inside the Switch Core, in order to extract the bitmap which is characteristics of the distribution of the considered cell to the appropriate output ports. It should be noticed that, since the same cell is received by the two Switch Core 15 and 25, step 402 causes the same value of bitmap to be extracted. In step 403 a test is performed in order to determine whether the extracted bitmap is equal to all zero, which is, by convention, characteristics of a control cell which is destined to the local internal processor and so must be extracted from the flow of data in step 404. If the cell is not a control cell, the process proceeds to step 405 where a Filtering Control Field process is initiated. For that purpose , the above mentioned Cell Qualifier comprises two particular bits forming a Filtering Control field which are characteristics of the nature of the filtering operation which is desired for this specific cell. If the Filtering Control field is characteristics of a Direct Filtering operation, as determined by the step 406, then the process proceeds with step 407 where the Bitmap which was previously extracted is logically ANDed with the contents of the Filtering Mask register shown in Figure 3. As mentioned above (with respect to Figure 3), the Filtering Mask register is designed to store a Mask value which is composed, in the preferred embodiment of the invention, of sixteen-bit, with each bit corresponding to one particular output port of the considered Switch core. At the initialization of the Switching architecture, as explained above, the Port Adapter Xmit were distributed in two RIGHT and LEFT Groups. Since the node Manager is aware of the topology of the Switching architecture, and particularly the physical correspondence between each Port Adapter Xmit - either Left or Right - and the output ports of each Switch core, the value of the mask can be determined as follows: for Switch core 15: for each output port the corresponding bit of the mask is set to one when the SCAL Xmit element 12 has been determined to be active, that is to say is designed to transmit the normal flow of data to the considered Port Adapter Xmit. In other words, the bit of the mask that is assigned to a given output port of the Switch core 15 is set to one when the Port Adapter Xmit LEFT receives the cell from that specific port. On the contrary, when the SCAL Xmit element is known to be BACKUP, the corresponding bit is set to a zero. Conversely, for Switch core 25, the bit of the mask that is assigned to a given output port of that Switch core is set to a one when the Port Adapter Xmit Right receives the cell from that specific port. On the contrary , when the output port communicates with a SCAL Xmit element known to be BACKUP, the corresponding bit is set to a zero. It should be noticed that the Filtering Mask Register of the two Switch cores 15 and 25 always contain Mask values that are complementary, so as to ensure that a perfect distribution of the cells through the two Switch cores. With respect to the process of Figure 4 again, when the test of step 406 shows that the nature of the Filtering Control Field is not characteristics of a direct filtering operation, the process proceeds with step 411. When the test of test 411 shows that the nature of the Filtering Control Field is characteristic of a reverse filtering operation, the process proceeds with step 408. Step 408, the Bitmap which was previously extracted is logically ANDed with the inverted value of the Filtering Mask Register. When the test of step 411 shows that the nature of the Filtering control Field is not characteristics of a reverse filtering operation the process proceeds to step 409 through step 410 (No operation). Step 409 is performed at the completion of either step 407 or 408 or 410. The residual bitmap being masked in accordance with the above description is then used for controlling the internal routing of the cell inside the two Switch cores, and also permits the appropriate duplication of the cells in case of multicasting. It therefore appears that, in case of a cell carrying a Direct filtering command in its Filtering Control Field, that cell which is duplicated and provided to both Switch Fabric paths, is only transmitted (thanks to step 407) to the appropriate SCAL Xmit element which was determined by the node manager to be active. Conversely, for a Cell carrying a REVERSE filtering command in its Filtering Control Field, that cell which is duplicated and provided to both Switch Fabric pats, is only transmitted (thanks to step 408) to the appropriate SCAL Xmit element which was defined to be BACKUP. In the present invention, the DIRECT Filtering Command is used for the normal traffic of data while the REVERSE Filtering Command is reserved for testing purpose of the Backup path in order to prevent the dramatic disastrous caused by hidden failures. To achieve this, with the Switching structure of the invention, testing cells can be periodically injected in the structure and are then transmitted through the backup Switch path in order to ensure the good operating of the latter. This entails the important advantage to allow a whole testing of the backup components of the Backup path, including the Backup SCAL Xmit elements and all the Backup links, to make sure that, when a breakdown will happen on one side of the Switching architecture, the only remaining Switch core will be able to switch the totality traffic of cells. Additionally, since the two Switch Cores are based on a output shared buffer architecture, as shown on figure 3, the active and backup path arrangement in accordance with the present invention permits to virtually increase the size of the shared Cell storage as far as this Cell storage is used by a lower number of output ports. This strongly improves the overall performance of the switch. The invention is enhanced by an additional mechanism which permits to avoid that two cells, a first DIRECT cell carrying normal and a second REVERSE testing cell, simultaneously arrive at the same Port Adapter Xmit. This is achieved by means a control bus 40-j in Fig. 1 connecting the SCAL Xmit element 12-j , belonging to the Switch Fabric 10, to the SCAL Xmit element 22-j belonging to the Switch Fabric 20. Therefore, two corresponding ACTIVE and BACKUP SCAL Xmit element which are connected to a same Port Adapter Xmit can communicate via the same control bus 40-j. Bus 40 has two main functions: a first function consisting of the synchronization of the cell clock of the Backup SCAL Xmit element 22m for instance with that of the clock of the ACTIVE SCAL Xmit element 12(m). In other words, the two links 34-m and 35-m have cell clocks which are synchronized. The characteristics of transmission of the two busses 34-m and 35-m ensures that the synchronism is maintained up to the input of the Port Adapter Xmit 31-m, what is generally the case when links 34-m and 35-m have the same physical length. When the Backup SCAL Xmit element 22-m wishes transmit a cell on bus 35-m, the latter sends a request on bus 40m to its associated ACTIVE Scal Xmit element 12-m. That request is received by ACTIVE Scal Xmit element 12-m which inhibits the transmission of the cell which would normally be transmitted at the next cell cycle. Concurrently ACTIVE Scal Xmit element 12-m produces an Acknowledge signal to its associated Backup Scal Xmit element 22-m on bus 40-m, what informs the latter that it is allowed to use the link 35-m to convey the pending cell at the next cell cycle. Each of the above two ACTIVE and BACKUP mechanisms are enabled in response to the contents of a register defined, under control of the Node manager, the appropriate status of the SCAL Xmit element. The mechanism thus prevents the simultaneous arrival of two cells at the same Port Adapter Xmit and entails a Substantial advantage since the Port Adapter Xmit can be designed to support only its nominal throughput. Without that mechanism it would have been necessary to use adapters being capable of supporting at least twice the nominal throughput, what would have strongly increased the cost and complexity. Now it will be described how the architecture permits easily maintenance operation, such as card replacement or system upgrade on one of the two Left or Right side, without loss of cells, erroneous duplication of cells, disordering of cells and excessive cell delay variations. It should be noticed that this requirement is particularly difficult to satisfy in the case of a very large switching architecture where the different constituents, the Switch Cores and the different SCAL elements are located in different buildings. Port Adapter receive 30-i generates, as shown in the figure 2, a sequence of cells cell(1)-cell(2)....cell(n) (only three cell being illustrated), which are duplicated and simultaneously transmitted to the two SCAL Receive elements 11-i and 21-i. It should be noticed that that sequence of cells may well include a sub sequence of ordered cells which are representative of a particular source of data, e.g. voice or image, and which must absolutely be transported to the appropriate Port Adapter Xmit, 31-m for instance, and delivered with the correct order and cell delay variation even if the normal direct traffic is switched to the backup path. This is achieved by the process of the invention which comprises the following steps: During the normal state, and in every components of the Switching architecture, there are data cells with the Filtering Control field characteristics of Direct Filtering and data cell with the Filtering Control field characteristics of Reverse Filtering and filling cells (empty cells) which are generated in the interconnecting link when there is no data cell to be transmitted. Assuming that the objective of the operation is to withdraw the Active SCAL Xmit element 12-m which is located in the Left Switch Fabric 10 for maintenance operation. In a preliminary step, the node manager sends to all the components located into the Right Switch Fabric and the Left Switch Fabric the relevant parameters corresponding to the new configuration obtained after the path switch-over. At the same time the Node manager sends to all the Port Adapter Receive a particular command to stop the transmission of the test cells carrying the Reverse Filtering Control field. After a predetermined period fixed in order to let all the remaining Reverse Filtering Control field be transmitted to their appropriate destination, the process basically begins with the transmission of a physical Left Switch Over Request (L_SOV_R) signal to all the components located in the two Switch Fabrics 10 and 20. Depending of the physical transmission characteristics the different SCAL - including the Receive and Xmit elements - receive that information at substantially different instants because of the existence of different propagation delays through the different links. It should be noticed that, generally speaking, the SCAL elements can be located in a different physical area with respect to the centralized Switch cores and the distance may well exceed several hundred of meters. Nevertheless, the SCAL receive elements 11-i and 21-i and the SCAL Xmit elements 12-i and 22-i receive that L_SOV_R signal at the same instant since they are located in the same back plane or physical entity. From the reception of the L_SOV_R signal, each component of the Switch Fabrics 10 and 20, excepted Backup SCAL Xmit element 12n and Active SCAL Xmit element 22n, initiates a particular process. The functional state of the latter Backup SCAL Xmit element 12n and Active SCAL Xmit element 22n will not involve the process which is now described. As mentioned above, the following components of the switching architecture performs an appropriate process for achieving the switch subsystem reconfiguration: the SCAL receive elements 11-i and 21-i, the SCAL Xmit element 12-i and 22-i, the Switch core 15 and 25 and the different control processors used in the architecture. However, the different processes can be grouped in three main phases which are consecutively performed. A first phase comprises the individual processes for achieving the merging of all the traffic of cells which was previously balanced between the two switch fabrics 10 and 20 so that the latter is exclusively flowing in the second one, that is fabric 20. That first phase basically involves the hardware components of the switch architecture. Following the first phase, a second phase groups the individual processes achieving the installation of the new configuration with the parameters which have been provided by the Node manager during the above mentioned preliminary phase. A third phase completes the overall switch subsystem reconfiguration achieves the load balancing between the two switch fabric 10 and 20 with the new configuration which will permit the particular maintenance operation on the system architecture. I. First phase for traffic merge of the cells Each element involved in the first phase initiates, as described below, its individual process from the monitoring of the L_SOV_R signal since the maintenance operation involves the left switch fabric 10. However, should the latter involves the right switch fabric 20, the R_SOV_R (right switch over request) would have been involved. The following description will be made with the assumption that the maintenance operation is made on the left switch fabric, and therefore, all the elements of the first phase monitor the raising edge of the L_SOV_R signal. The individual process performed in Active SCAL Xmit element 12-m begins with the detection of the raising edge of the L_SOV_R signal, the latter switches to a state where any request from the associated Backup SCAL Xmit element 22-m will not be acknowledged. This obviously causes the locking of the output of Backup SCAL Xmit element 22-m. This locking of Backup SCAL Xmit element 22-m will be useful during the next processing steps in order to avoid cell disordering. The individual process performed by SCAL Receive element 11-i also begins with the raising edge of L_SOV_R signal. On detection of the latter, there is produced a Disable input signal which prevents the input of the cells which come from Port Adapter Receive 30-i. Simultaneously, the latter replaces the production of the filling cells by a production of particular cells, so called Dummy cells, which have a specific representation. In the preferred embodiment of the invention, the Dummy cells have a particular SRH value characterizing that cell, and different from the value of the SRH which is assigned to the filling cells, because it is absolutely necessary to distinguish the two cells. Obviously, SCAL Receive element 11-i produces the Dummy cells only when, normally it would have been produced the filling cells, that is to say when no normal data cell are to be transported. After a certain period, the link 13-i only transmits Dummy cells which are inputted into Switch core 15. On the occurrence of the raising edge of L_SOV_R signal, the SCAL Receive element 21-i initiates its own appropriate process which modifies the SRH of every incoming cell in order to modify the two bits of the filtering control field from the Direct Filtering Command value, to the No_filtering Command value. Simultaneously its replaces the production of the filling cell by a production of particular cells, so called Dummy cells, as described above. It should be noticed that the first cell comprising a No_filtering command in its filtering control field, or the first Dummy cell occuring, will be transmitted on the bus 23-i only when all the cells which were possibly in the intermediate queues of the SCAL have been transmitted. This mechanism prevents the interleaving of a cell comprising No_filtering command with one that comprises direct filtering command. Switch cores 15 and 25 monitors the raising edge of the L_SOV_R signal and performs a similar processing. Considering Switch core 15 for instance, each input port i is associated with a detector which is designed for detecting the arrivals of a Dummy cell or of a cell characterized by a No_filtering command in the filtering control field. When the last of the sixteen detectors being associated with its corresponding input port of Switch core 15 has made that detection, Switch core 15 replaces the production of the filling cell by that of Dummy cell as required, that is to say when the output queues comprised in the Switch core 15 have delivered the useful data cells. It should be noticed that the process is independently performed with respect to the output ports. Therefore, after a certain period, it appears that Switch core 15 continuously produces Dummy cells on each output port and, on the other side, the Switch Core 25 continuously produces on the output ports Dummy cells and cell comprising No_filtering command. Simultaneously with the detection of the last of the sixteen detectors in a Switch core, an internal control signal is transmitted to the control processor therein located. That signal Informs the control processor that the traffic within the considered Switch core is such that the configuration parameters can be changed without prejudicial damage on the traffic. From that instant, control processor may initiate the processing of the second phase, that is to say, the installation of the new configuration. It should be noticed that that mechanism could be advantageously used when the switching architecture is based on a port expansion structure as described with details in pending European patent application n° 97480100.3 (IBM Docket FR997047), entitled «Method and apparatus for managing contention in a self routing switching architecture in a port expansion mode» and assigned to the Assignee of the present application, and which is therein incorporated by simple reference. When in port expansion architecture, the detection of last of the sixteen detectors in a same switch core is reported by means of a ALL_Detect signal to the Arbitration mechanism that manages the fan-in circuit merging the cells of two corresponding switch cores. From that instant, the Arbitration mechanism modifies its arbitration algorithm so that the corresponding fan-in circuit receive the cells coming from the opposite Switch core. As soon as the Arbitration mechanism receives the second ALL_Detect signal from the opposite Switch core, it switches back to the normal arbitration algorithm. This prevents spurious Dummy Cells occurrence. When SCAL Xmit Active element 12-m detects the arrival of the first Dummy cell which is normally preceding an entire flow of dummy cells and when, additionally, no cell to be transmitted remains in its queuing buffer, the latter switches back to a state wherein it will acknowledge the requests of the associated backup SCAL Xmit element 22-m. This obviously results in the unlocking of the output of SCAL Xmit backup 22-m, but has the strong advantage of preventing any misordering. This is also reported to the internal control processor located into the SCAL active Xmit element. The latter processor is then made aware that the processing of the second phase can be initiated without any loss of data. The last component involved in the first phase is the backup SCAL Xmit element 22-m which, on detection of the L_SOV_R signal, performs the unique step of checking whether it has been unlocked by the SCAL Xmit active 12-m. The result of that checking is reported to the internal processor that is located within the SCAL Xmit element 22-m and the latter is then made aware that the second phase may be initiated without any loss of data. II. Second phase for new configuration installation The second phase is initiated at the expiration of a certain period after the detection of the rising edge of L_SOV_request signal, what signal is, as mentioned above, monitored by any components within the switching architecture. Each control processors located in one of the two switch cores 15 or 25 verifies the reception of the above mentioned internal control signal generated in response to the detection by the last of the sixteen detectors of the Dummy cells and/or no_Filtering cells. If this test fails, an error condition is reported to the Node manager. In every Switch core 15 or 25, the internal processor modifies the value of the Filtering Mask Register in accordance with the new configuration parameters that are provided by the Node manager during the preliminary phase. This allows the modification of the repartition of the Port adapters in the group left and Right. Since the maintenance operation is assumed to occur on the SCAL Xmit element 12-m, the corresponding Port Adapter Xmit 31-m must be reassigned to the Right Group in order to receive its data from the Switch Fabric 20. It should be noticed that, thanks to the phases that occurred before, the contents of the Mask Registers can be changed without problem since, in the left switch core 15 only Dummy cells are being processed and, in the second Right Switch core 25, only cells bearing a no_filtering command are routed without involving the filtering control process. Similarly, each control processor being located into the active SCAL Xmit element 12-m verify to have received the internal control signal that was generated in response to the detection of the first Dummy cell. If this test fails, en error condition will be reported to the Node manager. Then, each internal processor modifies the internal parameter defining status ACTIVE/BACKUP of the considered SCAL Xmit element, as defined above. From this instant, the latter is assigned and operates in its new configuration, and the direction of control signal on bus 40 m is set in accordance with the new configuration. For instance, since the maintenance operation is being envisaged on SCAL Xmit element 12-m, the latter is set to a Backup state. Similarly, each control processor being located into the backup SCAL Xmit element 22-m verify to have received the internal control signal generated when it has been unlocked by the SCAL Xmit active 12-m. If this test fails, en error condition will be reported to the Node manager. Then, each internal processor modifies the internal parameter defining status ACTIVE/BACKUP of the considered SCAL Xmit element, as defined above. From this instant, the latter is assigned and operates in its new configuration, and the direction of control signal on bus 40 m is set in accordance with the new configuration. For instance, since the maintenance operation is being envisaged on SCAL Xmit element 12-m, the SCAL Xmit element 22-m is set on a complementary active set. When the expiration of the period of the first phase occurs, SCAL Xmit backup 12-n and SCAL Xmit backup 22-n, which were not involved during the first phase, update their active/backup status according to the parameter provided by the Node manager during the preliminary phase. The second phase then completes, and it becomes possible to perform any maintenance operation on the switch fabric left since all the traffic is routed through the right side. In particular the SCAL Xmit element 12-m may be unplugged and replaced by another one. It should however be noted that, during that maintenance operation, the right switch path is completely unprotected since a failure might happen on any of its components. Since the maintenance operation might takes a long time, this is a serious drawback. The invention improves that situation by providing an optional third phase which strongly improves the efficiency of the subsystem reconfiguration. The third phase permits the reconfiguration of the subsystem in a state where the maintenance operation will be permitted on one particular element, while the remaining components of the subsystem will still be used in a load balancing and protected mode. III. Third phase Traffic split The second phase causes a new assignment of elements to the right and left Groups. Therefore the lists of the components belonging to the Right Group (referenced by index m), and that belonging to the Left Group (re. index n) may have changed, at least from one unit. The third phase is then necessary to complete the load balancing and protected mode. The third phase is triggered by the failing edge of the L_SOV_R. From the reception of the falling edge of the L_SOV_R signal, each component of the Switch Fabrics 10 and 20, excepted Backup SCAL Xmit element 12-n and Active SCAL Xmit element 22-n initiates a particular process. The functional state of the latter Backup SCAL Xmit element 12-n and Active SCAL Xmit element 22-n will not involve the process which is now described. The individual process performed in the backup SCAL Xmit element 22-m begins with the detection of the falling edge of the L_SOV_R signal, the latter causing the locking of the SCAL Xmit active element 12-m via bus 40-m. The individual process performed by the SCAL Receive elements 11-i and 21-i also begins with the detection of the falling edge of the L_SOV_R signal. On detection of the latter, there is produced a Enable input signal which allows the reception of the cell on the port adapter 30-i. From that instant, the cells which are received from Adapter 30-i are no longer modified. The production of the Dummy cells is being replaced by that of the normal Idle cells (filling cells). Switch cores 15 and 25 monitor the falling edge of the L_SOV_R signal. Considering Switch core 15 for instance, each input port i is associated with a detector which is designed for detecting the arrival of idle cells or of cells being characterized by direct filtering command in the filtering control field. When the last of the sixteen detectors that are associated with its corresponding Input port of Switch core 15 has made the last detection, Switch core 15 replaces the production of Dummy cells by that of Idle cells as required. Simultaneously with the detection of the last of the sixteen detectors in a Switch core, an internal control signal is transmitted to the control processor therein located, to inform the latter that, at that level, the third phase is completed. When SCAL Xmit Backup element 22-m detects the arrival of the first Idle cell or the cell with Direct filtering Command in the Filtering Control field, and when, additionally, no cell to be transmitted remains in its queuing buffer, the latter removes the lock command to the SCAL Xmit element Active 12-m from the bus 40-m. This detection is reported to the internal control processor which is located in the Backup SCAL Xmit element 22-m to inform the latter that, at that level, the third phase is being completed. The last component involved in the third phase is the Active SCAL Xmit element 12-m which, on detection of the falling edge of the L_SOV-R signal, performs the unique step of checking whether it has been unlocked by the SCAL Xmit backup 22-m. The result of that checking is reported to the local control processor that is included into SCAL Xmit element 12-m, to inform it that, at that level, the third phase is being completed. Therefore, the subsystem is then set to the situation where the Port adapter in the Right group receives the data from the Right Switch Fabric 20 and the Port adapters of the Left Group receive their data from the Left Switch Fabric 10. It has been explained with numerous details the different steps of procedure for permitting an maintenance intervention on the SCAL Xmit 12-m. As described above, the procedure involves the preliminary phase wherein the Node manager sends to all the involved components the new configuration, followed by the three consecutive phase allowing the traffic merge, the installation of the new configuration and then traffic split. Now it will be quickly described the case where the maintenance intervention occurs on the Switch core 15, or on the SCAL receive element 11-i. Regarding the Switch core 15 , the preliminary phase and the first traffic merging phase remains unchanged. In the second phase is however modified. The second phase is however slightly modified so as to permit all the traffic to be concentrated through switch core 25. To achieve this, during the determination of the repartition of the Port Adapter Left and Right by the internal processors located in the Switch core 15 and 25, the Left Group will be declared to be empty whereby no cells will be conveyed through Switch core 15. The filtering Mask Register is thus set to a All_zero value in Switch core 15, and that in Switch core 25 is fixed to a All_one value. After the completion of the third phase, the cells which transport a Direct Filtering Command in the Filtering Control Field will be conveyed through the Switch Core 25. An intervention on Switch core 15 is then made possible without altering the traffic of normal data cell. Regarding now the manual intervention on the SCAL Receive element 11-i, the preliminary phase remains unchanged with respect to the procedure described for the maintenance of SCAL Xmit element 12-m. The first phase of traffic merge is initiated, as described above. Then the second phase of new configuration installation is involved with the following modification: the SCAL receive element 11-i is set to a Disable mode which will be used in the following third phase, and the SCAL Receive element 21-i is set in a SIMPLEX mode. Therefore, during the third split traffic phase, after detection of the falling edge of the L_SOV_R signal, SCAL Receive element 11-i , previously set in a DISABLE mode, continues to disable the reception of the cells from the Port Adapter 30-i but replaces the production of the Dummy cells by a continuous flow of Idle cells (filling cells). Similarly, the SCAL receive element 21-i, previously set on SIMPLEX mode, continues to modify the two bits of the filtering control Field of the incoming cells from the direct Filtering Command value by the No_filtering command value. Simultaneously, it replaces the production of the Dummy cell as filling cells by Idle cells.

Claims: Switching architecture comprising : a first and a second Switch Fabric (10, 20) including a first and second switch core (15, 25) respectively located in a centralized building and a set of Switch Core Access Layer (S.C.A.L.) elements , further referred to as SCAL elements, distributed in different physical areas; each SCAL element respectively comprising a SCAL Receive element (11-i) and a SCAL Xmit element (12-i) for respectively enabling access to a corresponding input and output port of one of said switch cores; a set of Port Adapters (30; 31) distributed at different physical areas, each being connected to said first and second Switch Fabrics via a particular SCAL element so that each Switch core (15, 25) receives the sequence of cells coming from any Port adapter and conversely any Port adapter may receive data from anyone of said first or second switch cores; means (100) for assigning to any Port adapter a particular Switch core for the normal traffic of cells and the other Switch core for Backup or maintenance traffic. each of said first and second switch cores (15, 25) further comprising: means for extracting a Switch Routing Header (SRH), further referred to as SRH, from a cell entering into said switch core; means for addressing a routing table with the contents of said SRH for getting a bitmap value characterizing the output ports to which the cell should be routed; a masking mechanism (100) for altering the value of said bitmap in accordance with the contents of a MASK register characterizing the normal repartition of the traffic of cells through the different Port Adapters; means for controlling the routing process with the altered value of the bitmap; whereby the normal traffic of data is only directed to the SCAL X mit elements which are considered ACTIVE while the other SCAL Xmit elements are reserved for BACKUP or maintenance traffic situations. Switching architecture according to claim 1 characterized in that said masking mechanism is controlled in order to change the normal traffic of data in case of maintenance or backup conditions. Switching architecture according to claim 2 characterized in that said masking mechanism is controlled by the value of a a Filtering Control field which is extracted from the SRH contained in every entering cell, said Filtering Control field comprising a Direct Filtering Operation command reserved for controlling the masking mechanism in normal situations, and a Reverse Control field for controlling the routing of the cell including this command to the Port adapter via the Backup SCAL Xmit element attached to that Port adapter. Switching architecture according to claim 3 characterized in that it comprises means for periodically introducing testing cells having a Reverse control field in order to check the good operation of the backup Xmit SCAL element. Switching architecture according to claim 4 characterized in that each corresponding ACTIVE and BACKUP SCAL Xmit element are connected via a control bus (40-j) which is used for preventing the simultaneous transmission of cell by two corresponding ACTIVE and BACKUP SCAL Xmit elements attached to a same Port adapter. Switching architecture according to claim 5 characterized in that it further comprises: means active during a preliminary phase before the occurrence of any planned maintenance operation on a particular Active SCAL Xmit element for stopping the transmission of test cells and for loading a new configuration of traffic through the architecture which excludes said particular element from the contribution to the routing process; means for performing a first phase for achieving the merging of all the traffic of cells which was previously balanced between the two Switch Fabrics (10, 20) so that the latter is exclusively flowing through said second switch Fabric; means for performing a second phase of achieving the installation of the new configuration with the new parameters; means for performing a third phase for completing the overall switch reconfiguration achieving the load balancing between the two Switch Fabrics except the element subject of the maintenance operation, thereby preventing any loss of cells. Switching architecture according to claim 1 characterized in that the MASK registers of the two Switch cores contain complementary values in order to ensure a perfect distribution of the cells in normal situations.

Inventors: BLANC ALAIN, BREZZO BERNARD, GOHL SYLVIE, ROBBE JEAN-CLAUDE, SAUREL ALAIN, BLANC, ALAIN, BREZZO, BERNARD, GOHL, SYLVIE, ROBBE, JEAN-CLAUDE, SAUREL, ALAIN
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1998
Main classification: H04L12
IPC codes: H04L12