Cross-Stuck and Toggle-Stuck Fault Detection A TS fault can Vertical-Stuck and Horizontal-Stuck Fault Detection VS and

A FAULT-TOLERANT MULTICAST OUTPUT-BUFFERED ATM SWITCH 173 Ž . Fig. 6.25 A cell loss example due to a horizontal stuck-at-1 or -0 fault at SWE 4, 2 , Ž . Ž . denoted HS 4, 2 . 䊚1994 IEEE. misrouted cells in the switch modules and thus providing on-line fault detection capability. In addition, the MPMs and the ABs are assumed to be able to generate test cells to locate faulty SWEs once a fault is detected by the FDs. The fault diagnosis and system reconfiguration can be carried out at any time when needed by retaining users’ cells at the IPCs and feeding test cells to the switch fabric. Because the test can be completed within a couple of cell slots, the overhead will not affect the switch’s normal operations. The FDs in MTTs or OPCs of the k th SM examine the k th bit of a Ž . flattened address zero or one to determine a misrouted cell. They also examine the source address in the test cell to locate a faulty SWE.

6.4.2.1 Cross-Stuck and Toggle-Stuck Fault Detection A TS fault can

be detected by monitoring cells routed to MTTs or the OPCs. If a cell routed to an MTT or OPC has a FA of all zeros, it is considered a misrouted cell. This is because any cell with all zeros in the FA should not be routed to the KNOCKOUT-BASED SWITCHES 174 south. As shown in Figure 6.23, when cell X4 is misrouted to the second output, the associated FD detects this misrouted cell because its FA is all 0s. While the TS fault can be easily detected online, the CS fault cannot be detected online. This fault will not contribute to cell-loss performance degradation if it is not located in the last column. Even if it is in the last column, we do not care about the CS fault, since this kind of sticking is Ž . exactly the action we will take for fault isolation details are explained later .

6.4.2.2 Vertical-Stuck and Horizontal-Stuck Fault Detection VS and

HS faults can be easily detected by checking if the outgoing signal from the SWE array is always one or zero. For instance, as shown in Figure 6.24, the Ž . signal from the second output link is always one or zero , which indicates that one of the SWEs in the second column must be a VS. Once a VS fault Ž . occurs, e.g., at SWE 4, 2 , all the SWEs below the faulty one remain at the cross state, so that the s-2-0 and s-2-1 faults will display at the south output of the faulty column. Cells that reach the south outputs are either valid cells from the user or empty cells from the ABs. They do not have the pattern of Ž . all zeros or all ones in their headers consisting of the FA and priority fields . So, if a cell appears at a south output and has a pattern of all zeros or all ones, it must be a VS fault. Figure 6.25 shows that the signal from the fourth discarding output is Ž . always one or zero , and that there must be an SWE HS fault in the fourth Ž . row. Once a HS fault occurs, e.g., at SWE 4, 2 , all the SWEs on the right of the faulty one remain in the cross state, so that the s-a-0 and s-a-1 faults will display at the east output of the faulty row. Cells that reach the east outputs can be valid cells from the user, empty cells from the ABs, or idle cells from the IPCs. Let us assume the address field of the idle cells is set to all zeros Ž . while the priority field is set to all ones corresponding to the lowest priority . So, if a cell appears at an east output and has a pattern of all zeros or all ones, it must be a HS fault.

6.4.3 Fault Location and Reconfiguration