Memoryless Multistage Concentration Network

THE ABACUS SWITCH 212

7.6.1 Memoryless Multistage Concentration Network

One way to scale up the abacus switch is to reduce the time spent on Ž traversing cells from the uppermost link to the rightmost link in an RM see . Fig. 7.2 . Let us call this time the routing delay. In a single-stage abacus switch, the routing delay is N q LM y 1, which limits the switch size, because it grows with N. To reduce the routing delay, the number of SWEs that a cell traverses in an RM must be minimized. If we divide an MGN into many small MGNs, the routing delay can be reduced. Figure 7.18 shows a two-stage memoryless Ž . multistage concentration network MMCN architecture that can implement Ž . a large-capacity abacus switch. It consists of N IPCs, J s nrM MGNs, and Ž . Ž . K s NrM concentration modules CMs . Each MGN has K RMs, and each RM has n input links and LM output links. Each CM has J = LM input links and LM output links. Fig. 7.18 A two-stage memoryless multistage concentration network. ENHANCED ABACUS SWITCH 213 After cells are routed through the RMs, they need to be further concen- trated at the CMs. Since cells that are routed to the CM always have correct output group addresses, we do not need to perform a routing function in the CM. In the CM, only the concentration function is performed by using the priority field in the routing information. The structure and implementation of the RM and the CM are identical, but the functions performed are slightly different. Recall that each group of M output ports requires LM routing links to achieve high delay᎐throughput performance. The output expansion ratio of the RM must be equal to or greater than that of the CM. If not, the multicast contention resolution algorithm does not work properly. For example, let us assume that N s 1024, M s 16, and n s 128. Consider the case that there are 16 links between an RM and a CM, while there are 20 links between a CM and an SSM. If all 128 cells of MGN 1 are destined for output group 1 and no cells from other MGNs are destined for output group 1, the feedback priority of CM 1 will be the priority of the address broadcaster, which has the lowest priority level. Then, all 128 cells destined for output group 1 are cleared from the IPCs of MGN 1, even though only 20 cells can be accepted in the SSM. The other 108 cells will be lost. Therefore, the output expansion ratio of the RM must be equal to or greater than that of the CM. Let us define n as the module size. The number of input links of an RM is n, and the number of input links of the CM is J = LM. By letting n s JM, the number of input links of the CM is of the same order as the number of input links of the RM, because we can engineer M so that L is close to one. Ž . In the MMCN, the feedback priorities FPs are extracted from the CMs and broadcast to all IPCs. To maintain the cell sequence integrity from the same connection, the cell behind the HOL cell at each IPC cannot be sent to the switch fabric until the HOL cell has been successfully transmitted to the Ž . desired output port s . In other words, the routing delay must be less than one cell slot. This requirement limits the MMCN to a certain size. Cells that have arrived at a CM much carry the address of the associated Ž . output group either valid cells or dummy cells from the RM’s AB . As a result, there is no need of using the AB in the CM to generate dummy cells to carry the address of the output group. Rather, the inputs that are reserved for the AB are replaced by the routing links of MGN1. Thus, the routing Ž .Ž . delay of the two-stage MMCN is n q J y 1 LM q LM y 1, as shown in Figure 7.19, which should be less than 424. Therefore, we have the following Ž .Ž . equation by replacing J with nrM: n q nrM y 1 LM q LM y 1 - 424. Ž . 2 This can be simplified to n - 425r 1 q L . Thus, N s Jn s n rM - 2 Ž . 2 425 rM 1 q L . Table 7.3 shows the minimum value of L for a given M to get a maximum throughput of 99 with random uniform traffic. Clearly, the smaller the group size M, the larger the switch size N. The largest abacus switch can be obtained by letting M s 1. But in this case the group expansion ratio L must be equal to or greater than 4 to have THE ABACUS SWITCH 214 Fig. 7.19 Routing delay in a two-stage MMCN. TABLE 7.3 The Minimum Value of L For a Given M M 1 2 4 8 16 32 L 4 3 2.25 1.75 1.25 1.125 satisfactory delay᎐throughput performance. Increasing the group size M reduces the maximum switch size N, but also reduces the number of Ž 2 . Ž 2 2 . feedback links N rM and the number of SWEs LN q L Nn . Therefore, by engineering the group size properly, we can build a practical large-capac- ity abacus switch. For example, if we choose M s 16 and L s 1.25, then the maximum module size n is 188, and the maximum switch size N is 2209. Ž . With the advanced CMOS technology e.g., 0.25 ␮m , it is feasible to operate Ž . at OC-12 rate i.e., 622 Mbitrs .Thus, the MMCN is capable of providing more than 1 Tbitrs capacity.

7.6.2 Buffered Multistage Concentration Network